Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
28 4.1 General Acoustical Principles This chapter presents and discusses the acoustical principles that affect the speech intelligibil- ity of PA systems in enclosed spaces such as airport terminals. To present background infor- mation to understand these principles, this section introduces the basic concepts of acoustics, including how sound is measured. The general design principle for room acoustics is to provide a diffuse sound field in which strong echoes are not present and the reverberant field is more dominant than the direct sound from individual sound sources. As illustrated in Figure 4-1, six physical factors interact with one another to affect speech intel- ligibility. Volume and shape directly influence the reverberation and reflections and echoes in a space. In turn, reverberation and reflections and echoes influence the ambient noise. Announce- ment quality is an important and often overlooked factor that is influenced by PA system design. PA system design is influenced by reverberation, reflections and echoes, and ambient noise. 4.2 Spatial ConsiderationsâVolume and Shape Room volume and shape strongly influence the basic acoustical properties of each space. The important factors in this regard are overall size and shape, the orientation of room surfaces, and how reflective or acoustically absorptive the room surfaces are. Volume and reverberation time are related, and the room shape can create strong reflections, resulting in echoes that can degrade intelligibility. Building type and space programming generally dictate overall room volume and shape. More control and flexibility are generally feasible for ceiling heights, shape of ceilings, and design of inte- riors. Conventional rectangular shapes and flat surfaces are generally straightforward to control acoustically, but curved surfaces, which are challenging, are discussed further in Section 4.4. Ceiling height is a key factor in acoustical design and PA design. Consider the following basic height groupings: â¢ âLowâ ceiling height: less than 13 feet â¢ âMediumâ ceiling height: 13 to 24 feet â¢ âHighâ ceiling height: >24 feet Table 4-1 shows the number of spaces encountered during the acoustic field studies for each of the basic ceiling height groupings. The average ceiling height for all of the spaces tested was 24 feet. Figure 4-2 shows the measured STI plotted against the median ceiling height with the dashed lines showing the divisions between the low, medium, and high ceiling groupings indicated above. Despite some outliers, the general trend indicates a decrease in the STI with increasing C h a p t e r 4 Physical Factors Affecting PA Intelligibility
physical Factors affecting pa Intelligibility 29 Figure 4-1. The six physical factors that affect PA system speech intelligibility. Number of Test Spaces for Each Ceiling Height Grouping Low <13 feet Medium 13 to 24 feet High >24 feet 12 spaces 19 spaces 15 spaces Table 4-1. Categorization of field study locations by ceiling height. Figure 4-2. STI plotted against median ceiling height. STI target 0.50 for daytime ambient conditions.
30 Improving Intelligibility of airport terminal public address Systems ceiling height. For STI 0.60 or better, the average ceiling height was 16 feet with a range of 8 to 34 feet. For STI 0.50 or better, the average ceiling height was 17 feet with a range of 8 to 40 feet. This data demonstrates that, for most ceilings less than 10 feet high, an STI of 0.50 should be achiev- able, whereas when ceiling height is greater than 10 feet, it is more challenging to achieve an STI of 0.50, even though most test results indicate that an STI of 0.50 is achievable up to 24 feet. The data indicate that where ceilings are over 24 feet high, it is very challenging to achieve an STI of 0.50. 4.3 Reverberation Reverberation refers to the persistence of sound in an enclosed space due to reflections. Some reverberation is inevitable and actually necessary, but excessive reverberation can be detrimental to intelligibility. Reverberation is measured as the decay of sound with time. The commonly used measure of sound decay is the RT60. The RT60 is defined as the time (in seconds) it takes for sound, after it is stopped in an enclosed space, to decay by 60 dB. Figure 4-3 graphs reverberation time. The basic formula for RT60, as shown in Equation 3, is a function of volume and effective acous- tical absorption. Various other formulas are used to calculate reverberation time, and all of these formulas address nominal geometry and acoustical absorption conditions. However, this formula, developed by Wallace Clement Sabine, was the first and is still useful for general guidance. Given that the RT60 is proportional to volume, the larger the volume, the higher the reverberation time for a fixed amount of absorption. In larger spaces, the sound strikes the surfaces (where it would be absorbed) less frequently, reducing the decay time and thereby increasing the reverberation time. Given that volume and surface area do not increase in the same proportion, as spaces increase in volume, there is proportionally less surface area to treat, and to achieve the same reverberation time, more surface area needs to be treated or else the surface area needs to be treated with more effective absorptive material. The Sabine formula assumes uniform distribution of absorption for a âliveâ room. Several other algorithms are in common use. One of these, the Eyring formula, assumes uniform distribution for a âdeadâ roomâthat is, a room with a very high level of acoustical absorption; this is not typically applicable to airport spaces. The Fitzroy formula has a distinct advantage in that it evaluates the three axes of a room individually (i.e., northâsouth, eastâwest, floorâceiling) and then combines these to determine the overall contribution. The Arau-Puchades method is similar to the Fitzroy Figure 4-3. Reverberation time (RT60). So un d Pr es su re L ev el (d B) Reverberation Time Time (seconds)
physical Factors affecting pa Intelligibility 31 formula in that three axes are analyzed; however, it uses a different variation for combining them. Either of the latter two methods are attractive for design studies because they allow one to isolate particular room surface pairs and study design changes. The acoustical absorption of air starts to come into play at frequencies of 2,000 Hz and higher and may be necessary for complex spaces where the reverberation time is difficult to control, because it can be factored into design calcula- tions in addition to the room finishes. Reverberation time is the differences in the sound-absorbing characteristics of common room finishes. Equation 4-1. RT60 (Sabine formula). ( ) =RT seconds 0.049 V A60 V = room volume (ft3) A = effective acoustical absorption (sabins) = [S1a1 + S2a2 + S3a3 + . . . . . . + Snan] S = surface area (ft2) a = average absorption coefficient of surface material The larger the volume of the space, the more surface area must be covered with acoustical absorption to maintain the RT60 within an acceptable range. The acoustical absorption in the space is the total absorption provided by the interior room finishes. In some cases, an entire surface may be an opening into the adjacent space (e.g., the dividing plane between a low-ceiling gate hold area and the adjacent higher ceiling walkway. The absorption is calculated by multi- plying the surface area of interior room finishes by their respective absorption coefficient and summing the total absorption in the room (expressed in sabins). When reverberation time is not controlled, the following can result: â¢ The persistence of reverberant sound in the space has a masking effect on later-occurring sounds. â¢ In the case of announcements, the masking effect results in overlapping speech syllables, which can sound garbled and unintelligible to the listener. â¢ Reverberation has the undesired effect of increasing room ambient noise (since the reverberant noise tends to build up more in spaces with longer reverberation times). â¢ Higher ambient noise degrades the SNR of the announcement level, which further undermines the intelligibility of announcements. Caution: Spaces with high reverberation cannot simply be âcompensated forâ by boost- ing the signal level of announcements. In fact, this can have the opposite effect and further degrade speech intelligibility by exciting more of the reverberant field. Refer to the discussion on SNR in Section 3.4 and the discussion on designing PA systems in large reverberant spaces in Section 7.8. Reverberation time is frequency-dependent. Typically, it is evaluated over the audible frequency range with particular attention to the behavior at 500 Hz and 2,000 Hz, which are important fre- quencies with respect to speech intelligibility. Low-frequency reverberation can also be an impor- tant factor in airport spaces and needs to be considered when determining how the PA system should be equalized (see Section 8.4.4). A sample RT60 chart is shown in Figure 4-4, comparing a well-controlled baggage claim area with a poorly controlled concourse area. In Figure 4-4, the high ambient noise levels and the limitations of the airport PA systems did not allow a full 60 dB test. This is a common issue, and, in these cases, the initial trend is extended to estimate the reverbera- tion time, as shown in the figure. As mentioned in Chapter 1, few previous studies were found in the literature review specific to airports, although some researchers have investigated large public spaces and various measures
32 Improving Intelligibility of airport terminal public address Systems of intelligibility. Studies indicate that speech intelligibility is strongly correlated to reverberation time, and, although successful examples have been identified in the literature with reverberation times up to 2.4 seconds, reducing the reverberation time below 1.9 seconds showed the strongest effect on increasing STI. The results of the acoustic field studies confirm how critical it is to control reverberation time for speech intelligibility in the design of airport terminal spaces. In fact, other than good PA system design, an adequate reverberation time and low ambient noise levels are the two most important parameters for achieving good intelligibility. Passengers with visual impairment also rely on the acoustic characteristics of spaces to orient themselves within those spaces, so a total reduction of reverberant conditions would not be helpful for those passengers. Table 4-2 compares the measured reverberation times as a function of terminal space type. Figure 4-5 presents all of the reverberation time data at 2,000 Hz from 45 spaces at 6 airports grouped by ceiling height. The strong association between RT60 and STI is evident with the STI dropping off substantially with increased reverberation time. Of all the case studies, only a few achieved STI 0.50 intelligibility with reverberation over 2.0 seconds. Most industry survey respondents (69%) judged high reverberation to be one of the most important factors hindering passengersâ ability to understand PA announcements. Based on all of the field measurements, the average reverberation time at 2,000 Hz measured 1.6 seconds. In many cases, spaces with reverberation times less than 1.6 seconds were also near or better than the STI 0.50 target. Good acoustical practice would provide lower reverberation Figure 4-4. Comparison of reverberation time measurements.
physical Factors affecting pa Intelligibility 33 RT60 (2,000 Hz) Average Low High Arrivals 2.0 1.6 2.3 Baggage 1.5 0.6 2.8 Concourse 1.8 0.8 3.4 Food court 1.9 1.1 3.0 Gates 1.3 0.7 3.5 Ticketing 1.6 0.6 2.9 TSA 1.4 1.0 2.1 All spaces 1.6 0.6 3.4 All adequate spaces (STI 0.50 or greater) 1.1 0.6 2.9 Table 4-2. Measured reverberation times grouped by terminal space. Figure 4-5. STI plotted against reverberation time, grouped by ceiling height.
34 Improving Intelligibility of airport terminal public address Systems time, depending on the space and what is feasible given the size of the space. For more informa- tion, see Chapter 6. The average reverberation time for spaces with STI 0.50 or better was 1.1 second or less, while the average reverberation time for spaces with STI greater than or equal to 0.60 was 1.0 second or less. Thus, while RT60 is strongly tied to STI performance, the nominal guidance target is in the range of 1.1 to 1.5 seconds, preferably less. Figure 4-6 illustrates the typical surface area required to achieve an RT60 of 1.5 seconds or less, expressed as a percentage of total surface area. Guidance is set for 1.5 secondsâslightly lower than the field measurement results and more in line with best practice for acoustics. 4.3.1 Acoustical Finishes Reverberation can be controlled through the use of acoustically absorptive room surface finishes, which can be more or less effective, depending on their inherent qualities. In an airport terminal environment, surface finishes that are desirable for aesthetics and maintenance are not always those with the best acoustical properties. Finding the appropriate materials and mix of those materials can be a challenge when working to develop a design that adequately controls reverberation. However, various acoustical products exist, with an increasing number of prod- ucts available on the market that not only provide necessary control of reverberation but that can also be used to enhance spaces visually. Section 6.3 provides more information on designing acoustical finishes, the range of products that can be used, and what to look for when reviewing acoustical product data. Figure 4-6. Nominal percentage surface area necessary to achieve RT60 less than 1.5 seconds.
physical Factors affecting pa Intelligibility 35 There is a trade-off during design when deciding which materials to use and how much to use, depending on how absorptive they are. For instance, some materials are quite absorptive (e.g., 1-in.-thick acoustical panels) and could require less surface area application than a less absorptive material (e.g., Â½-in. think suspended acoustical tile). However, there is a limit to how effective a minimal amount of treatment will be, even if it is inherently absorptive. In general, it is better to use more material over a broader area than highly absorptive material over a small area. The Sabine formula (Equation 4-1) is based on an assumption of nearly uniform absorp- tion coverage. As coverage becomes more localized (that is, nonuniform), other equations must be used to calculate RT60. See Section 6.10 for more information on modeling. 4.3.2 Passengers Clothing worn by passengers absorbs sound. When determining how much reverberation will exist, the average number of travelers in a space needs to be taken into account as well as the fact that the number is constantly varying. Appendix E provides acoustical absorption data for people and comparison with acoustical absorption for standard finishes. 4.4 Sound Reflections and Strong Echoes A diffuse field is one where the reflected sound from all surfaces is higher than the direct sound from any one source. As good room acoustics design strives to provide a diffuse sound field, unwanted sound reflections and focusing can cause discernible echoes and adversely affect spaces, even when reverberation time is adequately controlled. A concave surface can focus sound, unless it has been treated with an appropriate acoustically absorptive material. Sound reflections are per- ceived as echoes when reflected sound arrives about 50 to 100 milliseconds after the original sound. For illustration, consider that sound (which travels at a speed of 1,120 feet per second) would take about 50 milliseconds to reflect from a surface about 30 feet away and about 100 milliseconds to reflect from a surface about 55 feet away. Echoes should be avoided where possible with proper PA design and architectural design. The degree to which the echo will be disturbing to listeners is a function of the echo level (the strength of the echo relative to the direct sound) and the delay time. Psychoacoustic research experiments have investigated disturbance due to discrete echoes in specific reverberation con- ditions. Sounds separated by 80 milliseconds are perceived as separate events, and thus, for adequate speech intelligibility, the delay should be significantly less than 80 milliseconds; indus- try practice shows that a delay less than or equal to 40 milliseconds is ideal. The longer the delay, the lower the echo must be relative to the direct sound. Some surfaces, such as those with a concave shape, have a focusing effect on incident sound that can lead to acoustical âhotspotsâ in a room that reduce intelligibility. Convex surfaces, on the other hand, can have a diffusive effect on incident sound and help to spread sound in a space. Certain shapes can have particularly strong focusing effects depending on the geometry. Shapes to avoid include any sort of concave curved wall or ceiling such as a dome, arch, oval, or rotunda, unless an adequate amount of acoustical absorption is provided to offset these focusing effects. âFlutter echoâ refers to a distinct sound reflection pattern that may occur in the presence of large flat or parallel surfaces. This might be noticed in a large hallway where sound from a hand clap, for example, reflects repeatedly off the walls or between parallel planes of acousti- cally hard ceiling and floor. The sound can be observed to âreturnâ several times before it dies out. This effect can be controlled by acoustically treating one or more of the surfaces. An alternative is to slope one of the parallel surfaces (e.g., one of the walls or the ceiling) at a 1:11 slope.
36 Improving Intelligibility of airport terminal public address Systems 4.5 Ambient and Background Noise One of the main goals in speech intelligibility is to increase the signal-to-noise ratio (SNR). The ambient noise can compromise intelligibility by decreasing the SNR. In an airport environ- ment, background noise is the steady noise that does not vary much during the day. Ambient noise is the all-encompassing noise at any moment, including the background noise and tran- sient noise. There are many sources of background noise in an airport terminal. They can be steady or constant noises, but many are time-varying or transient noises. It is important to think about how to account for this difference when considering SNR. Not surprisingly, in the industry survey, background noise was considered one of the most important factors negatively affecting passenger ability to understand PA announcements. Both this factor and reverberation time were cited as negative factors, indicating that background noise and reverberation time are recognized as influencers of speech intelligibility. This has been borne out with measurements as discussed in Section 3.8. 4.5.1 Steady Noises Examples of steady noises are airport terminal HVAC systems and escalators. These noises are relatively easy to account for in design and can be controlled so that they do not signifi- cantly impair intelligibility. For exterior curbside locations, the ambient noise is also caused by automotive traffic in the airport or on nearby busy roadways. 4.5.2 Transient Noises Transient noises are intermittent and thus, during design, are more challenging to account for due to their continually varying nature. Examples of transient noise sources include occupant activity such as people talking and moving about, television monitors, airplane activity, and roadway traffic. 4.5.3 Ambient Noise Measurements (Interior Sources) Table 4-3 summarizes ambient noise data obtained from the field studies, organized by type of space. When the results for spaces meeting the target STI value of 0.50 are compared with Terminal Area Ambient Noise Levels (dBA)â Daytime Ambient Noise Levels (dBA)â Nighttime Average Low High Average Low High Concourse and food court 64 62 71 57 52 63 Arrivals hall and gates 60 51 67 54 47 61 Baggage and curbside 62 53 71 55 46 64 Ticketing and TSA 63 56 68 55 51 66 Average of all spaces 62 51 71 55 46 66 Average of adequate spaces (spaces with STI 0.50 or greater) 61 53 71 54 46 63 Samples were short snapshots of the particular time that the space was accessed. To the extent possible, these values represent the typical conditions without nearby PA announcements or nearby transient noises. Table 4-3. Summary of interior ambient noise data by terminal area.
physical Factors affecting pa Intelligibility 37 the entire group of measurements, the nighttime and daytime measurement results are not sig- nificantly differentâonly 1 dBA different for the average values. Furthermore, there is a small but consistent difference in measurements between some of the terminal areas. Concourse and food court areas not only had the highest average values for daytime and nighttime, the low value (background) was also substantially higher for that categoryâ6 to 11 dBA higher during the daytime and 1 to 6 dBA higher during the nighttime. This makes sense, given that the food court areas in general tended to have the highest ambient noise levels. The average daytime ambient noise level for all adequate spaces (where the dry condition speech intelligibility measured STI 0.50 or better) was 61 dBA, just slightly less than the 62 dBA average for all spaces. Perhaps a more useful grouping of the data considers only the ambient level, irrespective of the terminal use. Some airports were inherently quieter than others, so averaging across âquietâ and ânoisyâ gate areas made it difficult to capture the difference. By separating the ambient measure- ments by level, a clearer characterization was identified with the four groups shown in Table 4-4. The two noisy groupings had a higher RT60; the longer reverberation time directly increases the ambient environment about 1 dBA. The octave band spectra for each of these four groups are provided in Appendix I. 4.5.4 Ambient Noise Measurements (Exterior Sources) Control of exterior noise should be addressed by providing building facade elements in the design of the base building that adequately attenuate noise (see Section 6.6.9). Jet noise is an example of a transient noise that can be intrusive to the acoustical environment of interior spaces. Proper glazing can provide sufficient attenuation of jet noise. The acoustical metrics that describe noise attenuation of exterior shell elements such as glazing are the Outdoor-Indoor Transmission Class (OITC) and the Sound Transmission Class (STC). During the field study, measurements were performed in an unoccupied gate with airplanes taking off on a runway next to the terminal. Average noise from jets ranged from 59 to 62 dBA, with maximum noise levels of 63 to 68 dBA. Given that higher frequencies are attenuated more than lower frequencies, the potential for jet noise to degrade STI is not as high as one might expect. Figure 4-7 compares interior noise from several jets taking off with the interior ambient noise level of 55 dBA. With a design target of 59 dBA for ambient noise levels during the day, it would be help- ful to improve the building shell design to provide an additional 3 to 9 dBA noise reduction. This could require an OITC 40 rating for terminal buildings exposed to loud runway noise. It may be necessary to increase the building sound insulation target near runways to achieve the target 59 dBA ambient conditions. Table 4-4. Airport ambient conditions, grouped by sound level and spectral characteristic. Ambient Character Average Ambient Level (dBA) Average STI Average RT60 at 2,000 Hz Daytime noisy 65 0.51 1.8 Daytime quiet 59 0.51 1.4 Nighttime noisy 59 0.50 1.8 Nighttime quiet 51 0.52 1.4
38 Improving Intelligibility of airport terminal public address Systems 4.6 PA System Design One of the concepts underlying speech intelligibility is the SNRâhow does the PA system provide adequate signal to compensate for the ambient noise environment? The two primary physical factors that affect the SNR and the speech intelligibility of PA systems are reverberation time and ambient noise. If the ambient noise level is high, the PA system design and settings must be able to furnish the 10 to 15 dB SNR. If the reverberation time is overly long, it is difficult to reduce the ambient noise, and also the passengerâs ability to distinguish and understand the content of the message is decreased. A major goal in PA intelligibility is to implement a PA sound system that reproduces sound without distortion and at a level sufficiently above the background noise level. (See Chapter 7 for detailed information on which aspects of PA system design are related to speech intelligibility.) 4.7 Announcement Quality In particularly challenging environments, announcement quality is even more important. To take full advantage of good room acoustics, a well-designed sound system, and low background noise, the prerecorded announcements need to be of high quality. The industry standard for live announcements must also be consideredâcurrently the quality of live announcements varies depending on the training and speaking ability of the person making them. (Chapter 5 presents more detailed observations and suggestions on live speaking and automated and artificial voice messaging systems. Chapter 10 addresses announcement content and composition. Chapter 11 includes guidance about training.) Black curve is the ambient noise inside the gate area in the absence of the jet noise Shaded region is jet noise inside the gate area transmied via the building shell/glazing Low frequencies transmit Glazing filters mid- to high frequencies which influence STI the most Figure 4-7. Noise measured in unoccupied gate area during jet take-offs on runway adjacent to terminal building.
physical Factors affecting pa Intelligibility 39 4.8 Guidance Targets Given that SNR is one of the underlying concepts for speech intelligibility, PA systems must furnish adequate signal to compensate for the ambient noise environment. Guidance values are as follows: â¢ Design for RT60 1.1 to 1.5 seconds at 500 to 2,000 Hz to support adequate speech intelligi- bility of the PA system. â¢ Design for daytime and nighttime ambient noise levels 59 dBA or less with noise control techniques to maintain low ambient noise conditions and maximize PA system SNR. â¢ Aim for median ceiling height less than 16 feet if ceiling-mounted speakers are desired. This should be generally straightforward to achieve adequate performance without the need for substantial input from design professionals in acoustics or PA system design. Ceiling heights greater than 24 feet are not good candidates for ceiling-mounted speakers. â¢ Consider the necessary building sound insulation to achieve the target 59 dBA ambient conditions near runways.