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89 8.1 Construction Review for Room Acoustics As with many aspects of construction, some components of the architectural finishes that affect PA system speech intelligibility will require close and careful inspection during instal- lation, but other components may only require a general review of construction practices or spot checks to document general conformance with the specifications. Ultimately, a properly designed and installed PA system within a suitably designed acoustic space can be fine-tuned during commissioning. Any acoustical treatments for spaces should be reviewed for proper submittals and installa- tion. Examples of important elements and potential problems applicable to any acoustical project follow: â¢ Vibration-isolated ceilings and walls. Some ceilings and walls may require vibration isolation to provide noise control for noisy or vibrating equipment and maintain low ambient noise con- ditions. Proper selection and installation of isolators or resilient channels is crucial. Springs are often incorrectly sized, with a âbigger is betterâ mentality that results in a stiff spring that provides little or no vibration benefit. Ceilings must be held off the walls (and caulked if neces- sary for fire and/or sound isolation). Soffits, suspended lights, wall-mounted brackets and other items need to be designed and coordinated to avoid short-circuiting the ceiling or wall isolation. â¢ Sloped ceilings or walls. These elements may be incorporated to minimize flutter echoes or other reflections that interfere with speech intelligibility. â¢ Acoustical ceilings and acoustically absorptive surfaces. These provide specific and impor- tant amounts of absorption. In general, more absorption than specified can be beneficial because it tends to reduce the reverberation timeâtoo little can undermine speech intel- ligibility goals. Proper distribution is also importantâmost materials should be uniformly spread across the available surfaces. 8.2 PA System Bid Process The specifications for the PA system and associated acoustical materials related to architec- tural finishes and sound isolation elements will have the appropriate information related to performance and treatment areas. Specific to the PA system, the specification will have the fol- lowing information: â¢ Equipment electrical and power requirements â¢ Computer language requirements â¢ Equipment compatibility information with existing systems â¢ Interface integration requirements C h a p t e r 8 Construction Phase
90 Improving Intelligibility of airport terminal public address Systems â¢ Acoustical performance requirements â PA sound levels at listener positions â Off-axis sound levels â PA sound level uniformity â STI performance â¢ Loudspeaker specifications â¢ Loudspeaker spacing and placement (when crucial or unusual) â¢ Environmental conditions â Temperature, humidity, etc. â Nominal targeted room acoustics design goals (e.g., reverberation time) â Nominal targeted daytime ambient noise conditions design goals â Nominal room dimensions and finish treatments â¢ Measurement method (by reference or described) Other features that can be considered include system design and components that support built-in PA system component health monitoring and which include effective built-in test capa- bility. See Appendix G for sample text related to speech intelligibility. 8.3 PA System Installation 8.3.1 Submittals During the project construction phase, the architectural finishes and PA system installation need to be coordinated with the base building schedule as well as with any other cabling instal- lations for electrical, Ethernet/IT, security, and other A/V systems. Typically shop drawings and product information cut-sheets will be prepared by the drywall, ceiling, or finish subcontractor or PA system installer (integrator) and submitted for review. The general contractor or airport representative must have personnel with knowledge and ability to review and comment on those submittals. The PA system installer should provide an Operations Manual that covers, at a minimum, how to make PA-system-level changes and how to operate the paging stations. Guidance and background information to understand the PA system infor- mation is provided by others, for instance in Audiovisual Best Practices (Cape and Smith 2005). 8.3.2 Shop Tests Each component must be tested and verified that it meets the requirements, whether it be a performance test for power capacity or a simple verification that the correct part has been obtained and installed. Best practices (Cape and Smith 2005) call for all equipment to be shop tested. An ownerâs representative can be present during these tests: â¢ Proper function of the equipment â¢ Shop mock-up of system â¢ Loading and testing of all software â¢ Preset adjustments 8.3.3 Onsite Testing Best practices call for preliminary testing to be performed before system commissioning to determine if the system is substantially complete and proper connections have been made. These tests can include â¢ Powering on all equipment and verifying the functions of all components â¢ Verifying signal paths for all field-terminated wiring
Construction phase 91 â¢ Configuring and testing the functionality of the PA system â¢ Balancing the PA system components â¢ Configuring basic PA announcement settings â¢ Configuring basic zone EQ settings â¢ Verifying any communications services that are integrated with the PA system (e.g., tele- phone, Ethernet, fire, and others) â¢ Checking loudspeaker polarity â¢ Verifying loudspeaker line impedance â¢ Checking hum and noise level â¢ Verifying acoustical and electrical frequency response â¢ Verifying signal-to-noise ratio (SNR) Document initial tests and adjustments, including numerical values of relevant equipment settings, for reference during the system acceptance testing. 8.4 Site Reviews and Inspections The three tests presented in the following subsections (8.4.1 through 8.4.3) are also done during commissioning to verify operation and optimize the system, but it is generally expected that installers will have performed the basic preliminary work to demonstrate for themselves that they have installed the system correctly. 8.4.1 Balancing the System Balancing the system means that all inputs and outputs to the headend of the PA system are at the same or comparable levels. Once the PA system outputs are balanced (equal) at all zones, the overall PA system sound level can be set for each input so that there are no obvious fluctuations in level between different inputs or interfaces. There are three elements of the PA system, and each element can require adjustments for the system to function properly: â¢ Input signal. Each audio input is considered, along with the native input level and condition- ing or gain required. An equalized reference curve and relative levels for each input source is matched for all loudspeaker zones so that all inputs are generally presenting same signal level to the headend and loudspeakers. Gain adjustments at the input signal allow the system to maximize the SNR. It is good practice to have separate controls for level adjustments and equalization for each of the inputs. At airports, these inputs and system interfaces are typically as follows: â AODB (airport operational database) â FIDS (flight information display system) 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 â¢ Amplifier (headend) gain settings. These gain settings typically start off at the neutral setting to maximize the signal-to-noise ratio (SNR). Each element of the PA system can inject noise native to the electronics of each element. Although this kind of noise has, in recent years, typically been reduced because of improvements to the electronic components, it is still best practice to hold gain adjustments to a minimum at the headend. All gain settings should be set to avoid distortion. Proper gain structure should be optimized throughout the system to
92 Improving Intelligibility of airport terminal public address Systems minimize noise and signal distortion. Usually distortion is caused by overdriving the micro- phone preamplifier (e.g., the headend gain is too high). Normally, an input attenuator (pad) is implemented at the headend amplifier to reduce the level at this point, which allows the level control to be provided at the second gain stage. (For more details, see McGregor 1999.) â¢ Zone gain settings. Given that each ADS has slightly different acoustical properties and back- ground conditions, adjustments at individual loudspeaker zones may be necessary during commissioning and optimization. 8.4.2 Setting PA Announcement Levels The overall sound pressure levels in each loudspeaker zone should be around 72 to 78 dBA at the nominal listening height (3 to 5 feet above the floor) or as indicated in the specifications. 8.4.3 Loudspeaker Zone Frequency Response Equalization The level from the microphone in each loudspeaker zone is the first area that is checked and adjusted before starting frequency equalization or any other adjustments for that zone. Typical tasks to equalize each loudspeaker zone are as follows: â¢ Document the background. Measure and document the background (or quiet ambient) noise level (dBA) and the frequency spectrum in one-third octave bands from 125 Hz to 6,300 Hz. â¢ Measure the Initial EQ. Broadcast a pink noise (equal level in each one-third octave frequency band) signal through the loudspeakers. As needed, make level adjustments to the pink noise signal so that the measured broadcast signal is approximately 10 dB above the background level in each frequency band from 125 to 6,300 Hz. Adjust the equalization to attenuate sharply the PA system frequency response below 125 Hz at approximately 6 to 12 dB per octave. Roll off (attenuate) the frequency response above 6,300 Hz at 6 dB per octave. This attenuation filters out frequencies that are not important for speech intelligibility. â¢ Adjust the Zone EQ. Play pink noise through the loudspeakers in each zone, set the EQ level, and spatially equalize for that ADS area. â Set the live announcement level. â Talk into the microphone (live announcement) and measure the sound pressure level in that loudspeaker zone. The level should be measured on-axis with the loudspeaker. Con- firm that the microphone gain has been set so that the measured sound pressure level falls between 72 and 78 dBA, depending on the acoustical conditions and specification require- ments. These levels should be maintained throughout the zone. â After the live announcement level has been set up, play pink noise through the microphone via âtalk-boxâ at the same level as the reference level announcement. â Adjust the frequency band equalizer so that the spatially averaged sound is a nominal match for pink noise; however, note that it is not usually advisable to increase the gain for any individual one-third octave band by more than 2 dB. â¢ Readjust the sound pressure level. After frequency equalization, the loudspeaker zone overall sound pressure levels will typically require adjustment to reset back to the reference level. Sound pressure level variations around the entire loudspeaker zone should be noted at this time. â¢ Troubleshoot as needed. Refer to Section 9.4.5, which presents troubleshooting tips.