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.
149 A p p e n d i x F For more detailed discussion of individual components and these terms, refer to other sources such as Sound Reinforcement Engineering (Ahnert and Steffen 2000), âAdvanced System Gain Structureâ (McGregor 1999), Sound System Engineering (Davis and Patronis 2014) and Hand- book for Sound Engineers (Ballou 2012). â¢ Adequate sound level. The amplitude of the sound signal is a measure of loudness. It 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¢ 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 louder than the ambient noise level to achieve intelligibility. Examples of ambient noise sources include HVAC systems, aircraft operations, people activity, concession mechanical equipment, TVs, escalators, and people movers. â¢ Clarity. Freedom from distortion or noise. Distortion mixed with noise impedes speech intel- ligibility, especially under low SNR conditions. â¢ Digital signal processor (DSP). One of the headend electronics. The DSP selects, com- bines, routes, filters, and otherwise processes the audio signals before the amplification stage. The DSP includes the basic functions of calibration, level-setting, delay and equalization. (See Section 7.4.1 for a summary of the key functions. â¢ Directivity factor. In general terms, most sounds emit uniformly in all directions. When a sound source is placed on a hard surface, the sound that would have traveled down is reflected from the hard surface, effectively doubling the strength of the sound source. Similarly, a sound source in a corner benefits from at least three surfaces. A directivity factor can be assigned to each of four conditions. â 1: free field, â 2: on a flat plane or surface, â 4: at two perpendicular planes, and â 8: in a corner at three perpendicular planes. â¢ Equalization (EQ). Equalization increases or decreases the level of different frequencies in the PA signal. Equalization 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. â¢ Frequency response. All audio equipment physically responds to sound according to the frequency that it receives (microphone) or transmits (loudspeaker) or both (PA system). High-quality electronics have a flat response in their nominal operating frequency range. Quality loudspeakers have an overall response of typically (Â± 5 dB) over a broadband operating PA System Glossary
150 improving intelligibility of Airport Terminal public Address Systems 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. Quality microphones are rugged and robust with a smooth, linear response, typically Â± 2 dB, in the speech frequency range between 200 Hz and 4,000 Hz. â¢ Gain before feedback (GBF). This is a function of how much the microphone signal can be amplified before the system begins to âhowlâ or feed back into the microphone. Gain is the desired increase in power level or sound level in the audio system. Maximizing gain settings from the system and rejecting feedback improves intelligibility in the PA system when a microphone is used for live announcements. Maximizing gain settings from the system and rejecting feedback improves intelligibility in the PA system when a microphone is used for live announcements. The acoustical design of the cardioid microphone capsule minimizes the sensitivity to reflected sound and signals arriving from loudspeakers, thus improving GBF. â¢ Headend. The electronics that form the âbrainsâ of the PA system (i.e., DSP and power amplifiers). â¢ Intelligibility. The goal is to achieve easy understanding of the spoken word. 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. Because this is primarily a means of 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 predomi- nant or lacking. â¢ PA system uniformity. The uniformity of sound coverage can be documented by âwalkingâ each ADS to sample the one-third octave band spectrum once per second. Thus, some samples are taken on-axis under a loudspeaker and some are taken between loudspeakers. The uniformity can be represented graphically from this data. The data uses a pink noise input signal at a level necessary for good acoustical SNR. â¢ Polar plot. A useful way to view the directionality or uniformity of an audio transducer (micro- phone or loudspeaker). Polar plots are two- or three-dimensional plots showing the response in any 360-degree direction. (See Figure 7-2 for an example.) â¢ Power amplifier. 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 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 over- driven, clipping and other distortion occurs. This hinders intelligibility and can damage the loudspeakers. The system should be engineered to furnish a minimum 3 dB of headroom at maximum power amplifier output. â¢ Stability. The announcements broadcast over the PA system should be free of feedback and spurious pick-up. Feedback (i.e., the cycling of the loudspeaker output back into the micro- phone input) results from improper loudspeaker location and insufficient electronic gain con- trol. Pick-up of unwanted outside signals can be caused by an aging system or poor installation. If audio signal cables act as an antenna to pick up and amplify signals from outside the PA system, using proper grounding and shielding techniques and minimizing cable loops that promote electromagnetic induction of signals into the system can resolve the situation. â¢ Total Harmonic Distortion (THD). This term, used to characterize the performance of audio and power electronics, is a measure of the linearity of the components. â¢ Uniform sound coverage. In the region served by each loudspeaker zone, the entire area should receive evenly distributed sound levels. Hot spots (i.e., where the sound is noticeably higher) and/or dead zones (i.e., where the sound is very low or absent) are to be avoided or addressed. Ideally, the uniformity of sound coverage is about Â± 1 dBA.