Thomas Trask is an acoustical engineer and senior associate at Newcomb & Boyd, an Atlanta, Ga.-based multidiscipline consulting and engineering firm providing innovative solutions for facility design, construction and maintenance. His acoustical engineering responsibilities have included acoustical analysis of performing arts centers, museums, laboratories, houses of worship, data centers, commercial buildings, production studios, high-rise residences, hospitals, academic buildings, and judicial facilities.
What piece of the building design puzzle pertains to acoustical design?
Typically an acoustical engineer is tasked to do an acoustical design for a few specific spaces in an entire facility. It can be anything from every space to only one or two spaces. Mainly it will be the most critical spaces in respect to clarity, such as a lecture space. It also depends on what you are trying to achieve: Are you trying to keep noise from getting into or from getting out of the space? Or are you trying to have the noise in the space meet a certain quality or quantity? Low background noise is beneficial in a learning environment, whereas an open office environment prefers a higher level to mask the conversations. Mainly, acoustical designs are for spaces that need critical engineering analysis on the acoustic end, such as an auditorium, sports venue, or another big, reverberant space where the role of audio reception is of importance to the occupants.
What is the tie-in with fire and life safety designs?
In the past, the fire alarm industry primarily focused on audibility requirements, assuming that if the sound was loud enough, it would be sufficiently intelligible. With the increasing use of voice messages for controlled and staged emergency evacuation, intelligibility now plays a role. The first objective standards for speech intelligibility in the context of fire and evacuation were introduced as an appendix to NFPA 72, 2002. This intelligibility requirement is intended to help ensure that the messages from voice evacuation and fire systems can be heard and understood by the occupants of a building.
Although a specific measure of intelligibility is noted, but not currently specified, by NFPA 72, the Code’s Annex recommends the use of International Electrotechnical Commission (IEC) 60849 and a Speech Transmission Index Public Address (STIPA) of 0.50 or Common Intelligibility Scale (CIS) measurement of 0.70. CIS = 1+log10 (STIPA). For example: A voice communication that comes over the alarm system says to evacuate. From a design standpoint, the code says that voice communication – whether it’s prerecorded or a live person – has to meet a CIS level of 70 percent voice/speech intelligibility. Because the code doesn’t mandate proof that this will be met during the design phase, it is left up to the local Authority Having Jurisdiction (AHJ) official to require a measureable quantity.
To an acoustical engineer, 70 percent is still a very marginal measure of intelligibility. We’d like 90 percent, especially for clarity and comprehension. If you were having a phone conversation and could only understand 70 percent of it, would that be adequate?
How is the proper CIS level calculated or determined?
Intelligibility, by definition, is difficult to quantify. Right now, it is calculated only in instances where some authority mandates it or it’s stated in a job’s RFP. When this does happen, it brings everyone on a level plane, knowing that they now have to do an acoustical assessment when they are doing the fire protection design.
Of the places that have adopted the NFPA 72 code and require intelligibility measurement, the IEC 60849 code provides a procedure to measure the CIS levels. In reality, such places have typically been limited to airports, convention halls, and sometimes sport centers/stadiums. It’s usually instances where you have large groups of people at any one time. That makes a lot of sense from a safety perspective.
The question becomes, when do you measure it? You start with a reference signal that you are measuring against. In an airport concourse, for example, do you measure it when there is a large group of people present or when it’s empty? Ideally, the code prefers that testing be conducted while occupancy is near typical levels, but the AHJ will be the final arbiter. While instrumentation is readily available to conduct intelligibility measurements for life safety systems, only qualified staff are currently allowed to conduct the actual measurements.
As an acoustical engineer, how do you design to that standard?
For the most part, acoustics is not considered to be life safety, life structural or fire safety. However, an acoustical engineer would look at the architectural design of the space. That is going to have the greatest impact on the intelligibility of the room once you get past the device placement. These factors are usually going to be the quantity and types of finishes that go in the room. Background noise can have an effect on it, but usually the fire enunciators are capable of providing a signal high enough to overcome background noise in most spaces. The only time you would not be able to do that is in a large space like a stadium, where it would work better if the annunciation/evacuation system is tied into the house sound system, which uses large, professional-grade loudspeakers. Typically, the fire system is a separate entity.
Placement of devices, on the other hand, depends upon the room size and the ceiling height. For a low ceiling, you place them closer together. But for a high ceiling, the typical approach is to put more devices into that space in hopes that the extra devices will make up for the poor acoustic design. The intelligibility from an acoustics/voice standpoint in the room design, meaning the volume, the finishes and background noises in the space, all have proportional implications.
Do more units offer greater intelligibility?
Not necessarily, but if you have them closer to people, then it can because you don’t have to drive the signal as loud and still retain a sufficient signal-to-noise ratio. It’s somewhat analogous to headphones; when you put headphones on, you will hear a lot more clearly. This approach will require more speakers in order to maintain decent sound level uniformity over the speaker’s coverage area without having to overdrive the signal to meet audibility requirements. But this design approach is better able to compensate for acoustically challenged spaces that typically exhibit poor intelligibility due to the space’s inability to absorb sound as it propagates around the room. If you put acoustically absorptive finishes in the room, then this sound is more likely to get absorbed. By the time it arrives back to the people, it will be at such a level that it won’t matter anymore.
Are there any tools that you use to help with design issues?
There are 3D modeling programs typically used by audiovisual professionals that assist in predicting a number of acoustic attributes within a defined space, including intelligibility, but these programs have not traditionally been applied to emergency evacuation systems. For this to happen, life safety manufacturers will have to begin to offer modeling data for their speaker devices.
How does the issue of intelligibility differ in a healthcare setting?
Healthcare is a bit more acoustically challenging because of the desire for microbial-resistant finishes, which typical sound absorptive materials, such as fiberglass, do not possess. However, proper space planning and the introduction of new “green” sound absorptive materials can help to mitigate distracting noise that occurs from activities and equipment.
A joint sub-committee, the Acoustical Society of America (ASA) and the Institute of Noise Control Engineering (INCE), is trying to pass a guideline: Sound and Vibration Design Guidelines for Hospitals and Healthcare Facilities. This guideline is not directed to fire and life safety A/V devices and does not mention NFPA 72 or voice emergency systems. It primarily addresses the criteria that would make the facility most beneficial to the patient by reducing the noise level while they are trying to recuperate.
Tags: A/V Notification, Q&A
Posted in Audible/Visible Notification, Q & A : Ask the Expert, Spring 2010
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