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70 7.1 Introduction PA system design involves many factors, including not only the electronic components of that sys- tem, but also the space in which loudspeakers will be installed. This chapter provides (1) an overview of PA system design for the lay reader and (2) an understanding of the site-specific issues encountered at airports. The purpose of a PA system in an airport is to broadcast information, paging, announce- ments, and emergency messages to a large audience, including travelers, airport employees, TSA and other security employees, and emergency response personnel. The announcements can be live, digi- tally stored, text-to-voice, or prerecorded. The goal of a good PA design is intelligibility of messages under various conditions. Appendix G provides a detailed description of PA system components. Sound coverage furnished by loudspeakers is typically grouped in predefined zones within which there are multiple loudspeakers. A zone can be defined by several factorsâfunction, size, acoustical environment, and location (e.g., airside, landside, and TSA). The PA system should be able to address individual zones or multiple zones and broadcast locally generated live announcements such as at baggage or gate areas. The primary objective of the PA system is to deliver this information with adequate levels of intelligibility. The biggest challenge facing an airport PA system designer is to develop a system that can broad- cast the announcements (signal) at an adequate level above the ambient noise environment (noise); thus, the system requires a high signal-to-noise ratio (SNR). The loudspeaker selection (including size, sensitivity, directional characteristics, location, orientation, and quantity) plays an important role in maximizing SNR. The primary goals of loudspeakers are to focus sound directly to the listenersâ ears, while minimizing sound energy projected onto walls, ceilings and other acousti- cally reflective surfaces. This must be done within the context of budget, aesthetics, availability of loudspeaker mounting points, and the acoustics of the space in which speakers are installed. Assuming the acoustical design of the terminal space in which a PA system is installed does not compromise the systemâs function, the design of the PA system is crucial to the ultimate intelligibility of announcements. (Refer to Chapter 4 for discussion on the physical factors and Chapter 6 for discussion on architectural design.) 7.2 Terminology and Components 7.2.1 Terminology The following terms are used to understand and define a PA system. A well-designed system should provide the following: â¢ Intelligibility. The goal is to achieve easy comprehension of the spoken word. â¢ Stability. The announcements broadcast over the PA system should be free of feedback and spurious pick-up. Feedbackâthe endless cycling of loudspeaker output back into the C h a p t e r 7 Public Address System Design
public address System Design 71 microphone inputâis the result of improper loudspeaker location and insufficient electronic gain control. Pick-up of unwanted outside signals can be caused by an aging system or poor installation. In the case of poor installation, the audio signal cables act as an antenna to pick up and amplify signals from outside the PA system. This can be resolved by using proper ground- ing and shielding techniques and minimizing cable loops that promote electromagnetic induction of signals into the system. â¢ Clarity. Freedom from distortion or noise. Distortion mixed with noise hinders speech intel- ligibility, especially under low SNR conditions. â¢ Linearity. The PA systemâs output at the listening position should vary in direct proportion to the sound source. A linear system provides high-quality reproduction (fidelity) of the input sound. A system that does not do this is nonlinear. â¢ Naturalness. The PA system should sound balanced and natural. Given that a PA system is primarily a means for broadcasting the spoken word, the range of frequencies important to understanding speech (nominally 200 to 4,000 Hz) will be present without some frequencies being predominant or lacking. â¢ Adequate sound level. The amplitude of the sound signal is a measure of loudness and is usu- ally measured in decibels (dB) of sound pressure level (SPL). The PA system should be loud enough to be heard in the area served without being objectionably loud. â¢ Uniform sound coverage. In the region served by each loudspeaker zone, the entire area should receive evenly distributed sound levels. Neither hot spots where sound is noticeably higher, nor dead zones where sound is absent are desirable. Ideally, the uniformity is about Â± 1 dB. â¢ Adequate ratio of direct-to-indirect sound. Direct sound travels from the loudspeaker directly to the listenerâs ears. Indirect sound is reflected off one or more surfaces before it reaches the listener. Too much indirect sound interferes with the clear understanding of speech. Echo and reverberation are examples of indirect sound that can compromise intelligibility. â¢ Adequate SNR. The PA system sound level must be sufficiently above the ambient noise level to achieve intelligibility. Ambient noise sources include HVAC systems, aircraft operations, human activity, concession mechanical equipment, TVs, escalators, and people movers. 7.2.2 Components Specification of appropriate component products is an essential part of design for intel- ligibility. Sound Reinforcement Engineering (Ahnert and Steffen 2000), Advanced System Gain Structure (McGregor 1999), Sound System Engineering (Davis and Patronis 2014), and Handbook for Sound Engineers (Ballou 2012) are good resources. The PA system components that affect intelligibility include the microphones, headend electronics, and loudspeakers (see Figure 7-1). All of these components are subject to bandwidth distortion, which can diminish intelligibility in the presence of noise. Any component in the signal pathâfrom input to loudspeakerâcan introduce distortion in the form of nonlinearity between input and output. Generally, purely electronic components (such as headend electronics and power amplifiers) maintain the best one-to-one relationship between input and output. Components introducing the greatest nonlinearity are usually elec- tromechanical transducers (such as microphones and loudspeakers). Professionally prepared Figure 7-1. Typical PA system signal path.
72 Improving Intelligibility of airport terminal public address Systems prerecorded announcements will have adequate sound level and be free from noise and distor- tion. Loudspeaker selection is critical in PA system design because variation in output over a loudspeakerâs frequency range introduces distortion, which diminishes intelligibility, particu- larly in the presence of noise. Thus, when specifying equipment, it is important to use high- quality, commercial-grade components for the loudspeaker and microphones. Consumer-grade or home hi-fi components have no place in airport PA systems. Components within a PA system must be properly matched to ensure that their imped- ance and signal levels are matched to the other components in the system. In simple terms, a deviceâs impedance is its opposition to current flow between components. Proper impedance- matching maximizes power transfer between components. Mismatched impedance makes this power transfer inefficient and will introduce signal loss as the sound signal moves from one mismatched component to the next. Signal loss then increases the likelihood of a poor signal and distortion at the loudspeaker due to poor signal level and poor gain settings. In modern PA systems, impedance matching is less of a problem because inputs are typically actively balanced. When all components are obtained from one manufacturer, it is more likely that the individual components are properly matched. The PA system should be correctly configured at every stage of its operation. Overdriving inputs with a signal that is too strong can cause output clipping, which will introduce distortion and diminish intelligibility. Clipping is a type of waveform distortion that occurs when the signal is cut or âclippedâ because the device has reached the limit of its ability to transmit the full signal power. Clipping in the digital signal processor (DSP) can occur when the signal is driven beyond its digital capability to perform the signal processing functions easily. Clipping can also occur when the amplifier is overdriven beyond its maximum capability, causing the signal broadcast through the loudspeaker to no longer match the original input. This can also cause perma- nent damage to the loudspeakers. For example, when a microphone is abruptly turned off or disconnected, a crunchy, static-like chirp is heard at the loudspeaker; this chirp can cause the loudspeaker cone to break. 7.3 Microphones The microphone converts acoustic signals to electrical signals. In airport PA systems, the acoustic signal is the human voice. Quality microphones are rugged and robust with a smooth, linear response, typically Ã© 72 dB, in the speech frequency range between 200 Hz and 4,000 Hz. Two types of microphones are used in airport PA systems: omnidirectional and unidirec- tional. The unidirectional microphone is most sensitive to sound arriving from one particu- lar direction and is less sensitive at other directions. This gives the unidirectional microphone a higher gain-to-feedback ratio, which maximizes performance and intelligibility. A cardioid microphone is an example of a unidirectional microphone and has a heart-shaped response about its main axis (see Figure 7-2). This pick-up pattern is the most sensitive at 0 degrees (on-axis) and is least sensitive at 180 degrees (off-axis). Independent of microphone selection, feedback rejection can also be improved with proper loudspeaker placement at the microphone location. If the design can avoid loudspeakers above the microphone location, omnidirectional microphones can be used. The omnidirectional microphone has a lower gain-to-feedback ratio, which is undesirable, but the following desirable attributes explain its common use in airport PA systems: â¢ Lower distortion. â¢ Smoother off-axis coloration.
public address System Design 73 â¢ Simpler training. Less microphone technique training is required, because there is no prox- imity effect in omnidirectional microphones. A common, but ineffective, microphone tech- nique is to speak with the microphone right up to the speakerâs lips, which disproportionately increases the low-frequency (bass) response of the microphoneâthis can then cause the microphone signal to overload or distort, or, in the case of automatic gain systems, the micro- phone gain is lowered in response to the strong bass response. Airport paging microphones have a âpush-to-talkâ feature which keeps the microphone muted until the talker is ready to make an announcement. A good push-to-talk microphone has a reliable, high-quality, long-lasting switch. The button is mounted on the microphone or on the paging station, depending on the type of microphone used. 7.3.1 Handheld Microphones Handheld microphones are used in airport PA systems because they are convenient for users. Such microphones are particularly useful to improve intelligibility for live announcements in noisy, reverberant environments (e.g., baggage claim or gate areas). Handheld microphones are found at wall- or desk-mounted paging stations where the microphones are conveniently located and readily accessible to gate and baggage agents. Handheld microphones should have a frequency response designed for voice communications. The limiting factors that affect paging microphone performance are microphone sensitivity and frequency response. The microphone output should match the DSP input in level and impedance to maintain good signal quality. Handheld microphones typically have a clip or hook for mounting to the wall station. One manufacturer offers a microphone with a magnet to hold the microphone on the wall station. Besides serving as the microphone input, the paging stations also have a keypad for access control and routing. The pushbutton on the side of the microphone activates it and engages the PA system. Figure 7-3 shows a push-to-talk microphone. Figure 7-2. Polar response pattern.
74 Improving Intelligibility of airport terminal public address Systems 7.3.2 Other Paging Microphone Types Desktop (gooseneck) paging microphones, which are similar to handheld microphones in performance and function, are used where podium or desk stations are more convenient for paging or where vertical surfaces for the handheld microphone mounting plates are not avail- able. The desktop and gooseneck microphones also have pushbuttons, typically on the base or paging station, to activate the microphone and engage the PA system. Figure 7-4 shows a desktop paging microphone. Photo Credit: J. Lewitz Figure 7-3. Push-to-talk microphone. Photo Credit: Wikimedia Commons Figure 7-4. Desktop paging microphone.
public address System Design 75 7.4 Headend Electronics Headend electronics are components that constitute the âbrainsâ of the PA system. Headend equipment constitutes the control center where most of the functional aspects of the PA system are established and stored. The main components of the headend electronics are the digital signal processor (DSP) and the power amplifiers. This equipment is typically rack mounted in a telecom or server room. In a large airport, the headend may be in a central location, feeding audio signals to audio power amplifiers in satellite equipment rooms to minimize loudspeaker line power loss. Long loudspeaker lines waste amplifier power and are costly. Typically, loudspeaker line loss should be kept under 0.5 dB. A 3 dB loss would represent a loss of half the power over the length of the line, so loud- speaker line loss is an important design consideration. Loudspeaker line loss is determined pri- marily by the impedance of the connected loudspeaker load, the length of cable, and the cable size. Line loss can be compensated for by using larger gauge (AWG) cables, but this is also a cost consideration. Shortening the loudspeaker lines by remotely locating the amplifiers, if possible, is the most cost-effective strategy. Minimize loudspeaker line power loss. This can be done through specification and layout. An important part of the headend DSP is the processing of the information from the ambient- noise-sensing microphones. Some systems include ambient-noise-sensing microphones to mea- sure the noise environment in each zone. The DSP in the headend uses that information to temporarily add gain to the loudspeaker input signal when the ambient noise levels increase. 7.4.1 Digital Signal Processor The digital signal processor (DSP) makes changes to audio signals. The functions of the DSP include â¢ Pre-amplification â¢ Compression â¢ Limiting â¢ Equalization â¢ Delay â¢ Combining â¢ Routing â¢ Switching â¢ Gain staging The DSP is used to select, combine, route, filter and otherwise process audio signals (including basic functions of calibration, level-setting, delay, and equalization) before amplification. Device latency is the time it takes to transmit the digital signal through the DSP, including the A/D (analog to digital) and D/A conversion. Device latency should be considered in the PA system design and product selection. Excessive audio delay anywhere in the PA system hinders intelligibility, but can be controlled through specification and component selection. More detailed explanations of each DSP function are available in other sources such as Sound Reinforcement Engineering (Ahnert and Steffen 2000), Advanced System Gain Structure (McGregor 1999), and Handbook for Sound Engineers (Ballou 2012). Key operations that relate to speech intelligibility are included here: Pre-amplification During pre-amplification low-level microphone signals to be processed by the DSP are ampli- fied. This stage electronically amplifies a very weak signal (for example from a microphone or
76 Improving Intelligibility of airport terminal public address Systems pick-up) and transmits it to the DSP. Balanced gain structureâstarting with the pre-amplification stage and throughout the entire PA system including the DSPâis important to optimize speech intelligibility. Compressors and Limiters The compressor, a component of the DSP, narrows the difference between the softest and loudest sounds passing through the PA system. The compressor does this by compressing the dynamic range of the audio signal, thereby essentially reducing the volume of loud sounds and amplifying quiet sounds. Large swings or extreme peaks in the PA signal level are detri- mental to intelligibility. If the PA is too loud that can be annoying or distracting, but if it is too soft, the intelligibility would be lost in the ambient noise. Such issues can be controlled through specification, component design and PA system optimization. A limiter is a compressor with a high compression ratio and a fast attack time. The attack time determines how quickly the compressorâs gain reduction reacts to changes in the input signal level. A limiter is intended to âlimitâ peak levels in the audio signal. Compression is sometimes built into the paging microphone circuitry. Equalization (EQ) Equalization increases or decreases the level of different frequencies in the PA signal. Equal- ization is performed by digital electronic equalizers within the DSP component. A basic type of equalization is the bass/treble control in a home stereo system. In the DSP, the equalizer performs more complex frequency-response adjustments to tailor the frequency response of the PA system to improve sound quality and intelligibility. Examples of EQ use for speech intelligi- bility would be to emphasize and smooth the frequencies useful for understanding speech or to compensate for frequency-response anomalies in the loudspeakers or room response. Different equalization is necessary for different signal sources and zones such as loudspeaker zones, local announcements, prerecorded announcements, and background music. Audio Delay In some situations, it is necessary to delay an audio signal in time to synchronize arrival of signals between loudspeakers at different distances from a listener. Sound travels at a fixed rate of speed. Speech intelligibility is affected if one loudspeaker broadcasts a signal sooner than a later arriving signal that has to travel from across a large room. Audio delay can be necessary to syn- chronize arrival of signals between loudspeakers and improve speech intelligibility. The need for audio delay can be identified during design and optimized during installation/commissioning. Combining, Routing, and Switching Combining, routing, and switching signals includes collecting signals from different sources, directing them to desired zones, and switching between sound sources. This process is done in the DSP on direction from the users. For example, the curbside message âactive loading and unloading onlyâ is routed to the power amplifiers serving the curbside loudspeakers in the DSP. Another example is when an emergency page is to be broadcast; a signal from security would switch from normal paging announcements to the emergency message. Gain Staging Gain structure is an important software control function within the DSP and affects overall intelligibility. Gain stages are the points in the signal chain where level adjustments are made. This is important for system calibration which sets overall PA system sound levels. Noise and distortion can occur if levels are not properly balanced in the DSP. Gain staging and structure are established during PA system design.
public address System Design 77 Audio Power Amplifiers The role of the audio power amplifier is to amplify the low-power signals from the DSP to a level suitable for driving the loudspeakers. This step is where the signal levels are matched. The power amplifiers should be sized for the wattage necessary to drive the loudspeakers to the required sound levels. When the power amplifiers are undersized or overdriven, clipping and other distortion occurs, which hinder intelligibility and can damage the loudspeakers. Audio power amplifiers should be appropriately sized to avoid distortion in the signal and degra- dation of speech intelligibility. The system should be engineered to furnish a minimum 3 dB of headroom at maximum power amplifier output. 7.4.2 Ambient-Noise-Sensing System Ambient noise conditions influence speech intelligibility and the STI. One technique to offset the effect of varying daytime ambient conditions is to use an ambient-noise-sensing system that can boost the PA signal 4 to 6 dB during periods of higher-than-normal ambient conditions. When planning implementation of ambient-noise-sensing microphones in the PA system, consider the following: â¢ Commissioning gain adjustments are typically made during low to moderate ambient conditions (quiet daytime periods during operations) â¢ STI tests are typically conducted during low ambient conditions (nighttime or after operation hours) â These quiet daytime conditions are expected to be the same or slightly higher than the nighttime ambient noise conditions during which the STI tests are done. â Based on calculations made on the ADS measurements and laboratory tests, on average, the daytime STI can be 0.20 lower than the STI measured during nighttime conditions. â¢ Ambient noise microphones can help offset some of this reduction, given that they are typi- cally programmed to increase the announcement signal as the daytime ambient noise level increases. â¢ Practically speaking, there is a limit to what the ambient-noise-sensing system can achieve. Issues such as feedback and distortion typically limit the gain that the ambient-noise-sensing system can provide. A nominal 4 to 6 dB boost can often be implemented. 7.5 Loudspeaker Type Selection Although there are many types of loudspeakers, Table 7-1 lists types commonly used at air- ports. The type of loudspeaker selected for a particular space depends on the use for which it is normally intended and for which it was designed. Some types of loudspeakers function well as a distributed system, whereas others are intended to cover a large space with a few loudspeakers. 7.5.1 Cone Loudspeakers Distributed ceiling-mounted cone loudspeakers are preferred in mid-to-low-ceiling areas (less than 24 feet). Cone loudspeakers provide the most uniform sound coverage if the proper loudspeaker density is maintained. A good rule of thumb is to space the loudspeakers at a dis- tance equivalent to the floor-to-ceiling height. Uniform sound coverage contributes to good intelligibility by maintaining uniform PA sound levels in the listening plane. The listening plane is an imaginary horizontal surface located at the listenerâs ear height. Distributed loudspeakers in a low-ceiling space can be operated at lower audio power levels. This improves intelligibility, especially in reverberant spaces. Distributed ceiling-mounted
78 Improving Intelligibility of airport terminal public address Systems loudspeakers in high ceiling spaces are less desirable because they have to be operated at higher sound levels. This does not automatically result in better speech intelligibility. If too few loudspeakers are spaced too far apart, uniformity of sound coverage and intelligibility are diminished. Spaces where the floor and the ceiling are both acoustically hard and reflective are problematic for distributed ceiling-mounted loudspeaker systems. Aside from the problem of reverberant buildup, the reflections between the two reflective parallel surfaces reduce intelligibility. In this case, carpeted floor or acoustical ceiling treatment should be considered so that at least one of the two opposing parallel surfaces is absorptive. Acoustically treated flat ceilings are most appropriate for ceiling-mounted cone loudspeakers. Avoid ceiling-mounted loudspeakers in concave ceilings that are not acoustically treated. Con- cave ceilings exacerbate the multi-focusing effect of the sound energy and greatly reduce intel- ligibility. Focusing of sound would be an acoustical anomaly that would result in nonuniform sound coverage. Acoustically treating the surface will diminish the focusing effect by reducing the reflections off the surface. Cone loudspeakers with a coaxial construction are necessary to optimize clarity and intelligi- bility. Loudspeakers with a single-cone construction cannot faithfully reproduce the broad band of frequencies required. Loudspeakers with a dual-cone construction use separate transducers for the low and high frequencies. Each transducer is uniquely designed for the frequencies to be reproduced. The separate cones are coaxially mounted in a single frame to synchronize arrival of sound from each cone where the frequencies cross over from low to high. Figure 7-5 shows examples of ceiling-mounted cone speakers. 7.5.2 Passive Column-Array Loudspeakers Passive column-array loudspeakers are very useful in large areas with high ceilings, areas where the speaker must reach an area quite far away (long throws), and so forth. Passive column arrays use small individual loudspeakers vertically stacked so as to interact with one another to maximize sound coverage in the horizontal plane and narrow the coverage in the vertical plane. Figure 7-6 shows an example of a passive column array loudspeaker sound field distribution. This enables the loudspeaker to maximize sound coverage in the listener plane and minimize sound coming from reflecting surfaces, thus improving intelligibility, especially in reverberant Loudspeaker Type Configuration Comment Ceiling-mounted Distributed on ceilings Mid-to-low ceilings; not suitable for concave ceilings Passive column array Distributed on walls or columns Easy to reach longer distance (high ceiling, long throw) Steerable column array Distributed on walls or columns Easy to reach longer distance (high ceiling, long throw), ability to âsteerâ coverage to the desired listening area. Wall-mounted (multiple components in a single box or enclosure) Distributed on vertical surface Similar to passive column array, but with limited sound coverage pattern control Omnidirectional/spherical Do not use Basic properties run counter to speech intelligibility needs at airports Table 7-1. Loudspeaker types and beneficial configurations.
public address System Design 79 spaces. The columns are mounted at a low level so that the plane of sound coverage corresponds to the listenerâs ear height. Also, the horizontal energy distributions are fairly wide. 7.5.3 Steerable Column-Array Loudspeakers Steerable column-array loudspeakers have the same attributes as passive column-array loud- speakers, except that the loudspeakers in the array have individual microprocessor controls to allow the coverage to be actively or electronically âsteeredâ to direct the sound coverage toward the listeners. Maximizing direct sound (i.e., the sound that comes directly from the loudspeaker to the listener) improves intelligibility. Defining the coverage pattern to the areas where the listeners will be seated or standing is useful when the space is very large and reverberant. The sound energy is directed away from walls, windows, and similar surfaces. Unwanted reflections excite the rever- berant field and diminish intelligibility. Steerable column arrays can be mounted high on a wall or column and vertically flat against the wall because the coverage can be electronically steered down to the listeners. Figure 7-7 shows an example of array steering. Photo Credit: Wilson Ihrig Figure 7-5. Examples of ceiling-mounted cone speakers. Source: Wilson Ihrig Figure 7-6. Example of passive column array loudspeaker sound field distribution.
80 Improving Intelligibility of airport terminal public address Systems Steerable column arrays have less sound attenuation over a defined distance than standard box or passive column-array systems. This is a result of microprocessor control of the individ- ual loudspeakers in the column and their interaction with each other. Fewer steerable-column arrays are needed to uniformly cover a large area. Figure 7-8 shows an example of steerable- column array loudspeakers. 7.5.4 Horn Loudspeakers A horn loudspeaker uses an acoustic horn to increase the overall efficiency of the driving ele- ment. Because horn loudspeakers are not as directional as passive column arrays or steerable column arrays, horn loudspeakers are not as desirable indoors in large spaces. Their weather resistance makes them more suitable for curbside applications. Their high efficiency also Source: Wilson Ihrig Figure 7-7. Example of array steering. Photo Credit: Wilson Ihrig Figure 7-8. Example of steerable column array loudspeakers (circled).
public address System Design 81 supports the higher sound levels required in noisy outdoor areas such as at curbside. Figure 7-9 shows examples of horn loudspeakers 7.5.5 Undesirable Loudspeaker Applications In a large space, wall-mounted nondirectional loudspeakers (e.g., cone loudspeakers in a box) provide poorer uniformity of sound coverage than distributed column loudspeakers. Optimiz- ing uniformity of sound coverage improves intelligibility. Omnidirectional or spherical loudspeakers should be avoided because they work against the concept of maximizing the ratio of direct-to reverberant sound to maximize intelligibility. By its very nature, an omnidirectional loudspeaker delivers excessive sound energy to the reverberant field, thus diminishing intelligibility. 7.6 Loudspeaker Layout The design of a loudspeaker layout is somewhat determined by the type of loudspeaker that will best serve the space. The loudspeaker selection is typically based on the physical layout of the space. As noted above for microphones, feedback rejection can also be improved with proper loudspeaker placement at the microphone location. If loudspeakers can be avoided above the microphone position in the design, omnidirectional microphones can be used. The physical loudspeaker spacing can also be determined based on the kind of loudspeaker that might be required. Not all loudspeakers are designed to serve a broad range of acoustical environments. The acoustical environment and physical dimensions of the space will dictate what loudspeaker type is best, where the loudspeakers are mounted, how many loudspeakers are needed, and what type of loudspeaker layout configuration will be used. In a low-ceiling space, the distributed ceiling-mounted loudspeakers need to be close together to furnish uniform sound coverage and avoid âhot spots.â The number of loud- speakers and the spacing density is a function of the sound pattern broadcast by the individual loudspeakers. A loudspeaker with narrow angle of coverage will require more loudspeakers. Photo Credit: Wilson Ihrig Figure 7-9. Examples of horn loudspeaker.
82 Improving Intelligibility of airport terminal public address Systems In a high-ceiling space, the challenges are different, because a loudspeaker that is too far from the listener will not have an adequate ratio of direct-to-indirect sound; the listener will not be able to understand announcements over the ambient noise. This is a case where a different type of loudspeaker must be considered. 7.6.1 Loudspeaker Zones An important part of the design process is to identify âzonesâ where sound coverage is desired (e.g., ticketing, concourses, gates, and baggage areas). The DSP is programmed to route specific PA signals from specific paging stations to the power amplifiers serving the loudspeakers in a desired zone of coverage. Part of the design process is to identify an acoustically distinguishable space (ADS) where the acoustics and physical characteristics of the space are fairly uniform. An ADS is distin- guished from other spaces due to acoustical, environmental, or use characteristics (e.g., rever- beration time and ambient or background sound level). Intelligibility will be maintained when PA system design elements, including sound level, equalization and loudspeaker type, location and orientation are consistent and tailored to each ADS. 7.6.2 Spatial Considerations For a well-defined space, where wall-to-wall distances and floor-to-ceiling distances are not great, the challenges are fewer. All spaces, regardless of size, require a sufficient amount of acoustical treatment on room surfaces to provide adequate absorption of reverberant sound. For very large spaces, the types of available loudspeakers that will function properly and produce acceptable intelligibility are limited. In large spaces, installing the loudspeakers closer to the passengers is advisable. This installation is done with wall- or column-mounted loudspeakers with high directional capabilities to concentrate the sound on the listeners. 7.6.3 Audio Delay Design should avoid overlapping sound coverage from loudspeakers. For instance, when a listener hears sound from two separate loudspeakers spaced more than about 40 feet apart, the delayed arrival of sound from the more distant loudspeakers creates an artificial echo, which will reduce intelligibility. In some cases, an electronic audio delay unit in the DSP can be used to synchronize arrival of sound in zones of overlapping coverage from loudspeakers spaced more than 40 feet apart. Loudspeakers must be grouped into different zones. 7.6.4 Loudspeaker Grid: Distribution Two types of distributed loudspeaker systems are found in airports: â¢ Distributed ceiling-mounted loudspeakers pointing down. Typically, these â Are found in spaces with low or medium ceiling heights (i.e., less than 24 feet) â Are on ceilings less than 24 feet high; the spacing is nominally equal to the ceiling height â Use cone loudspeakers (so called because of the âconeâ loudspeaker diaphragm and conical coverage pattern) â¢ Distributed wall- or column-mounted loudspeakers. Typically, these â Are found in high ceiling spaces and highly reverberant environments â Use loudspeaker column arrays (so called because of their construction using loudspeakers vertically stacked in a column)
public address System Design 83 Where a distributed loudspeaker system can be used, the loudspeaker grid (i.e., loudspeaker spacing in the grid) depends on several characteristics, including â¢ Ceiling height â¢ Loudspeaker directivity â¢ Loudspeaker sensitivity â¢ Distance from any one loudspeaker to a listener â¢ Acoustical conditions, including reverberation and noise From an intelligibility standpoint, the main goal of selecting a loudspeaker distribution pat- tern is to minimize the distance from a loudspeaker to a listenerâs ear, regardless of where a person stands. Such a distribution will allow for more uniform coverage. Figure 7-10 shows an example of loudspeaker coverage. The distance between the loudspeaker and the listener will determine the configuration of the loudspeaker installation. From a cost standpoint, it is neces- sary to optimize the number of loudspeakers while designing for acoustically acceptable cover- age. The type of loudspeaker selected will determine the distribution of loudspeakers. If the ceiling height is less than 24 feet, the distributed ceiling-mounted loudspeaker spacing is the same as the ceiling height. For ceilings higher than 24 feet, ceiling-mounted loudspeakers create a challenge for adequate speech intelligibility, so in this case, use distributed wall- or column-mounted loudspeakers. 7.6.5 Point Source Distribution In some cases (e.g., physical restrictions such as available mounting points or the size and shape of the space), a single point source may be the best way to provide the most uniform sound coverage. A point source is a single loudspeaker or cluster of loudspeakers projecting sound to a large space such as an atrium, large concession area, or arrivals hall. 7.7 Loudspeaker Quality The quality of a loudspeaker refers to its ability to reproduce sound, either from a record- ing or a live announcement, that is as close to the original sound as possible. Use only robust, professional-quality, reliable, loudspeakers. Loudspeakers can introduce distortions into the PA system, which can make it difficult to optimize the system for suitable speech intelligibility. The goal is to minimize the need to compensate electronically for poor frequency response or other quality deficiencies such as loudspeaker sensitivity. A low-quality loudspeaker often has low sensitivity, which then needs more amplifier power to achieve a specified sound level. This is costly, inefficient, and counterproductive to sustainability. See Appendix G for an example of the relevant excerpts of specification for loudspeakers. Figure 7-10. Example of loudspeaker coverage.
84 Improving Intelligibility of airport terminal public address Systems Quality loudspeakers have an overall response of typically Â± 5 dB over a broadband operat- ing range between 70 Hz and 15,000 Hz with a smooth, linear response, typically Â± 2 dB, in the speech frequency range between 200 Hz and 4,000 Hz. Coaxial construction, adequate magnet weight (10 oz. or more for the low-frequency reproducer), and high sensitivity (95 dB at 1 watt, 1 meter) are all attributes of a quality loudspeaker. Specify quality loudspeakers: â¢ Adequate magnet weight (10 oz. or more) â¢ Overall Ã© 5 dB over the range of 70 Hz to 15,000 Hz â¢ Smooth, linear response (Ã© 2 dB) over 200 Hz to 4,000 Hz â¢ Coaxial construction â¢ High sensitivity (95 dB at 1 watt, 1 meter) 7.8 Loudspeaker Terminal Location Considerations PA system design considerations primarily depend on two factors: ceiling heights and acoustical conditions. At airports, these two factors are consistently the same at some ter- minal locations; however, many spaces are highly variable. Loudspeaker selection should be appropriate for specific terminal functional areas. Table 7-2 summarizes guidance for selecting loudspeakers and designing layout specific to areas within the terminal. Relevant acoustical factors (ordered by terminal functional area) follow: â¢ Ticketing. Ticketing areas can have high ceilings and can be reverberant and noisy due to passenger activity. Column array-type loudspeakers should be considered to maximize SNR. â¢ TSA security checkpoint. These areas are particularly noisy because of the number of passen- gers brought together in a small area. Consider increasing the density of the loudspeakers to provide increased sound coverage and intelligibility. â¢ Long corridors. These corridors tend to be quieter than other circulation areas, unless noise from people movers is excessive. Low-ceiling areas typically use distributed ceiling-mounted loudspeakers. â¢ Gate hold areas. Gate areas typically have low ceilings and carpeted floors for which distrib- uted ceiling-mounted loudspeakers will suffice. Increase loudspeaker density in gate areas affected by outside jet noise and in areas where there are overlapping zones of loudspeaker coverage. Avoid TV audio interfering with the PA announcements. â¢ Concessions. Retail and food courts are usually noisy from passenger activity and mechani- cal noise from concession refrigeration and other mechanical equipment. High ceiling spaces should use column or array loudspeakers to maximize SNR. â¢ Large circulation areas. Acoustically large spaces, such as arrivals and departures halls, are typically noisy and reverberant. Avoid distributed ceiling-mounted loudspeakers in high ceiling spaces. Use column or array loudspeakers to maximize SNR. â¢ Baggage claim. Many of these areas have low ceilings, but carousel noise is a problem. Main- tain adequate sound level and distributed ceiling-mounted loudspeaker density. â¢ Curbside. These areas are often extremely noisy because of traffic. The almost constant drone of cars and buses is difficult to overcome especially if loudspeakers are too far apart. Outdoor conditions may dictate weather-resistant horn loudspeakers. â¢ Restrooms. Restrooms are usually small volume spaces that are often quiet and can be easily covered by a minimum number of ceiling-mounted loudspeakers. â¢ Customs/immigration. This area typically has a low ambient profile, with important PA announcements in several languages. Maximize SNR with dense loudspeaker coverage, typi- cally closely spaced ceiling-mounted loudspeakers.
public address System Design 85 7.9 System Interfaces Level balancing is in the interface between live and prerecorded announcements. Specifically, no matter what the origin of the page, the sound level must be consistent and adequate at the listening location. All sources must be prepared, tested, and adjusted to maintain consistent level into and out of the DSP to maintain consistent levels of intelligibility. The various announce- ments must be clear and distortion free. Typical sources include â¢ AODB (airport operational database) â¢ FIDS (flight information display system) Physical Factor Challenges Condition/Challenge Reverb. Reflections and Echoes Ambient Noise Guidance Ceiling height <24 feet X Ceiling-mounted loudspeakers; loudspeaker spacing comparable to ceiling height Ceiling height >24 feet X X Loudspeaker spacing less than ceiling height Ceiling height >>24 feet X X Use wall-mounted or column array; special attention during optimization/commissioning Concave ceiling X Avoid ceiling-mounted loudspeakers unless the ceiling has high performing acoustical tile Exterior spaces X X Exterior spaces typically require durable loudspeaker materials, typically found in horn loudspeakers Few hard surfaces (<15% surface area) No special design requirements Moderate hard surfaces (<40% surface area) X X Loudspeaker spacing less than ceiling height; special attention during optimization/commissioning Many hard surfaces (>40% surface area) X X X Loudspeaker spacing less than ceiling height; special attention during optimization/commissioning; use wall-mounted or steerable column array Busy, active areas X Loudspeaker spacing less than ceiling height Multiple gate areas in the same ADS X Loudspeaker spacing within each zone less than ceiling height; >40 feet spacing between loudspeakers for each gate Mechanical equipment in public spaces X Loudspeaker spacing less than ceiling height; use higher PA signal level HVAC equipment above public spaces X Loudspeaker spacing less than ceiling height; use higher PA signal level Areas with TVs X Loudspeaker spacing less than ceiling height Areas with intrusion from exterior jet noise X Loudspeaker spacing less than ceiling height; use higher PA signal level Small volume space <<5000 cubic feet No special design requirements Very large volume space >>500,000 cubic feet X X Use wall- or column-mounted loudspeakers with high directional capabilities; special attention during optimization/commissioning Table 7-2. Guidance summary for PA system design.
86 Improving Intelligibility of airport terminal public address Systems â¢ PBX (phone system for paging) â¢ Mobile phones (for messaging to phones or smart devices) â¢ Text-to-voice â¢ Gate microphones â¢ Emergency announcements â¢ Call-in recordings (within the airport or from exterior calls) 7.10 Computer Modeling for PA System Design Software modeling brings value to the design process. Programs are available to create a 3-D model of the airport terminal and the PA system within each space. The physical space is built in the computer, surface treatments are added, and then the loudspeaker devices are entered. From this, the program evaluates the acoustics of the space and derives expected PA system parameters. This allows virtual modeling and pretesting and enables design issues to be addressed early in the process. Software modeling can help guide installation and limit costly design changes. Intelligibility can be predicted because the same computer model is used for both the acoustical design and the PA system design. The power of the model lies in the ability to quickly evaluate options based on the performance results. The following questions should be considered when selecting software modeling for PA sys- tem design: â¢ What is the processing power and speed of the software? â¢ How easy is it to construct the physical space in the computer model? â Is there a built-in drawing module? â Can it be integrated with third-party CAD programs? â¢ How easy is it to place loudspeaker devices in the model? â¢ Can SPL, STI, Reverberation Time and Uniformity be derived from the model? â¢ Is there a large database of loudspeaker devices available for the model? â¢ Is there a database of acoustical materials available? Some software programs are simplified for use in mass notification types of projects. Computer modeling is necessary to predetermine the performance expectations for the PA system design. As discussed in Chapter 6, many commercially available packages can evaluate the room acous- tics and calculate the STI from the PA system (STIPA). However, these are not all comprehensive packages for PA system design given that they typically only model the output of the loudspeakers, not the complete PA system component design. For a simple or moderately complex room, a basic PA system design program that uses simplified characteristics of the room acoustics (e.g., percentage surface area treated) may be sufficient. For complex spaces, however, a software package that can import the room acoustics model would be useful. In these cases, the designers should anticipate this and prepare for this hand-off during construction document design. 7.11 Considerations for Renovation Projects Renovation projects can take the form of partial or complete replacement of the PA system. If it is desired to keep the existing loudspeakers, a complete loudspeaker system survey is required to verify that all loudspeakers are operational and have adequate power-handling capacity. Exist- ing loudspeakers could be several decades old, in which case it would be prudent to examine them, including a listening test to assess potential deterioration Reusing existing loudspeakers presupposes that the zoning and coverage of the existing system is adequate. Loudspeaker lines must be checked for continuity. Most of the existing loudspeakers
public address System Design 87 can be used with a new or updated digital headend with very good results. Inspect all loudspeak- ers and address previous connection issues. All renovation projects will benefit from recommis- sioning of the system (see Chapter 9). 7.12 Considerations for Combining Emergency and Non-Emergency Announcements It may be desirable to use the PA system loudspeakers as part of the airportâs Emergency Alert System (EAS). This can be done, subject to local code requirements. If the PA system will be used for major mass evacuation, certain equipment conditions may be required (e.g., UL-rated loudspeakers and end-of-line monitoring and programming input logic to control priorities). Per NFPA 72, existing PA systems can be used as part of an EAS following a formal risk assessment and with approval of the authority having jurisdiction (AHJ). PA systems can be used for emergency announcements, as long as the PA system meets the code requirements for emergency use, including â¢ Meeting objective, measurable intelligibility criteria â¢ Supervised lines and other reliability code requirements â¢ Emergency power backup A combined system would require a way to switch to the emergency announcement source while muting the non-emergency announcements. This can be done digitally in the DSP. Some jurisdic- tions require an analog relay to avoid DSP programming changes (even with software password protection) or failures, which could disrupt the switchover. 7.13 Sustainability and PA Systems The EPA provides information on the sustainable management of the following electronics and lifecycle stages (EPA 2016): â¢ Raw materials acquisition and manufacturing. â¢ Purchase and use. Covers both âfirst useâ and âsecond use.â First use indicates use by the original purchaser of the product, and second use indicates when the first user no longer uses the electronic product and sells or gives the product to another person. â¢ Storage. Concerned with how long users store products when they have finished using them, thus affecting when a product is ready for end-of-life management. â¢ End-of-life management. Products at their end-of-life are managed by one of two practices: â Collected for recycling. May be subsequently reused, refurbished, or recycled for materials recovery. â Disposed of primarily in landfills. Combustible components may be collected and sent to waste-to-energy incinerators. On their website (epa.gov/smm-electronics), the EPA indicates that sustainable electronics management includes the following steps: â¢ Buy green. Purchase new equipment designed with environmentally preferable attributes. The EPA website includes a list of ways to buy greener electronics. â¢ Power consumption. The power amplifiers and headend electronics have the highest power requirements. The amplifiers are typically multichannel units with outputs ranging from 200 watts to 600 watts per channel. Using multichannel units minimizes materials compared to separate single-channel units. Use Energy Star-rated equipment, or review information at energystar.gov to evaluate equipment energy use.
88 Improving Intelligibility of airport terminal public address Systems â¢ Sourcing of materials. Although many of the materials for the enclosures and electronics can be widely sourced, all audio electronics require some small amount of rare metals, and some permanent magnets in loudspeakers may also use rare metals such as neodymium. â¢ Carbon footprint. Many of the manufacturing facilities for the circuit boards and parts within PA system components are outside of the United States. Some of the component-level manu- facturers are in the United States, but most of them are off shore. Thus, there are transportation costs involved in every level of the component chain. â¢ Reuse and donate electronics. Lengthening the service life of electronics to keep them out of the waste stream is preferable to recycling. Headend electronics have few moving parts and can be donated to non-profit organizations and schools to upgrade their PA systems. Loudspeakers can be similarly reused or donated, although the loudspeaker cones can experience wear or aging that affect performance after a long period of performance or operation in harsh environmental conditions. â¢ Recycle electronics. Electronics recycling (e-cycling) allows for the recovery of the rare metals from the electronics. To ensure responsible e-cycling, e-cycling businesses should be certified by a third-party program. The EPA has more information on responsible and sustainable e-cycling. 7.14 Induction Loops for Assisted Listening For people with hearing loss to receive the same passenger information and emergency mes- sages that other travelers would expect to hear, an assistive listening system is necessary. Because it is not practical to hand out personal receivers at the airport, an induction loop system is used to transmit an audio signal directly into a hearing aid via a magnetic field. This greatly reduces background noise, competing sounds, reverberation, and other acoustic distortions that reduce clarity of sound. The loop requires signal conditioning for the broadcast information signal, and the signal in the loop, in turn, induces a signal in a telecoil (T-coil) receiver, such as most modern hearing aids. Perimeter loops are the simplest kind of hearing loop with the area to be served surrounded by a copper cable embedded in the floor which is connected to an output amplifier. Multiple loops serve individual areas of the airport (e.g., gates and baggage claim). Loops can also be installed at point-of-sale or service counters. The feed for the system is a separate output from the DSP. Installation and equipment standards are included in IEC 60118-4.