Note: In last month’s FRONT of HOUSE (Feb. 2021, page 17), we offered an introduction on architectural acoustics, with a focus on the topic of room acoustics (excluding sound isolation and noise control). The article touched on the various types of architectural acoustics treatments, and the interaction between room and sound system for modern churches and pop/rock music venues. This month, we dig a bit deeper into architectural acoustics. —ed.
Large and Small Spaces
While there are some similarities between large and small rooms, the two present very different primary acoustical issues. Most FRONT of HOUSE readers work in what acoustical consultants call large rooms, such as churches and live-music venues. These spaces, if untreated, are subject to excessively long echoes and reverberation. These days, there are numerous YouTube videos on acoustics (some good, many not-so good) with the majority focusing on soundproofing (sound isolation) and acoustical treatment of small rooms, such as home theaters, music practice rooms and recording studio-type spaces. The main concerns in such environments tend to be near-field monitor acoustic issues and standing waves, as these spaces are typically too small to create long echoes/delays.
Acoustical Reverberation
Acoustical reverberation is a persistence of sound after the direct sound is produced. Reverberation is created when a sound is reflected, causing numerous echoes/reflections to build up and then decay, as the sound is absorbed by the surfaces of objects in a venue. Reverberation time (RT or T60) is defined as the time it takes the sound level to decrease by 60 dB after the source of sound has stopped radiating acoustical energy.
A certain amount of reverberation is desirable, as it makes speech and music sound natural. Too much reverberation and echoes are undesirable if reflections arrive at the same time a new word is uttered and interfere with its clear perception. Classical music sounds better with more reverberation, but the rapid tempo of rock music and speech tends to require a lower reverberation time. Thus, there is no single reverberation time that is perfect for all uses of a given room.
Various authors have made recommendations on appropriate reverberation times for different types of uses in public meeting and worship spaces. Recent trends, particularly in the design of churches for electronic music, have driven the desirable reverberation times in large church spaces downward (requiring acoustic treatment), since reverberation can be added back electronically as needed.
The Forum
I spoke with experts who specialize in the design of large room acoustics. The respondents were: Neil Thompson Shade, the president/principal consultant of Baltimore-based Acoustical Design Collaborative, Ltd.; and Dr. Richard Honeycutt, a noted author and the principal consultant at EDC Sound Services, Lexington, NC. The topics included: advice regarding various types of acoustical treatments; optimizing and controlling reverberation and long echoes; designing for more control of the loudspeaker array/room acoustics interaction; and how to avoid over-exciting room acoustics by proper array/speaker selection.
Neil Thompson Shade Replies:
An important distinction with reverberation is the duration of the sound decay, measured in seconds, and is called the reverberation time, and the reverberant sound level, measured in decibels. While the duration of the sound decay is important, the reverberant level is often overlooked. The ratio of the direct-to-reverberant (D/R) sound is a critical factor for speech intelligibility and music clarity. When the ratio is positive, the direct sound is greater than the reverberant sound. Negative values imply the reverberant sound is greater than the direct sound.
Reducing reverberation time involves applying acoustic materials to a room. Changing the D/R ratio can be achieved either through acoustic materials or selecting loudspeakers with controlled directivity to limit sound radiation on wall or ceiling surfaces.
The interaction of loudspeakers and room acoustics depends on the reverberation time and level, potential undesirable sound reflections, and the loudspeaker locations relative to the room surfaces. Reverberant rooms require more directional loudspeakers to minimize sound radiation on wall and ceiling surfaces, which requires larger loudspeakers. Second, unavoidable sound radiation on nearby room surfaces due to loudspeaker positioning can result in comb filtering. Third, loudspeaker position near room surfaces may affect the directivity of the device, particularly at low frequencies.
Sound absorptive materials on the walls or ceiling may be required to control sound reflections, both from surfaces distant and near the loudspeakers. Rear walls are a prime candidate for sound absorptive treatment. Depending on the desired reverberation time, or the potential for ceiling sound reflections, acoustic treatment may be required on some portion of the ceiling. In most venues, a significant amount of sound absorption can result from from the audience itself or from padded, fabric upholstered seating. Room surfaces near the loudspeakers can benefit from acoustic treatment to reduce early reflections that can combine with the direct sound causing a comb filter response at the listener. The loudspeaker directivity pattern and location should be reviewed to determine potential room surfaces that may require acoustic treatment. Note that treatment for comb filtering will also help reduce reverberation.
While not a room finish treatment, an effective means of controlling the interaction of loudspeakers and room acoustics is to select loudspeakers based on the coverage requirements of the audience and to aim the loudspeakers at the audience.
A variety of acoustic materials are available from manufacturers. The most generic are fabric-wrapped fiberglass panels, available in 0.5- to 4-inch thickness as shown in Fig. 1A. These panels tend to have increased sound absorption as a function of frequency and thickness, although four-inch thick panels provide almost unity absorption even to 125 Hz.
To complement the appearance of standard and specialty acoustic panels, many manufacturers provide matching sound reflective panels of plywood or gypsum board, as shown in Fig. 1B. These panels can be provided in the same fabrics as the other room panels to create a cohesive monolithic appearance, rather than a “bolted-on” appearance that results when acoustic panels are typically placed on room surfaces.
The third specialty panel type — sound absorptive/diffusive panels — comprise a fiberglass core with a proprietary perforated physically hard finish face such as 1/8” thick plywood or Masonite. Sound is diffused (scattered) by the perforations which expose the fiberglass, creating a discontinuous acoustic impedance across the panel face, as shown in Fig. 1C.
Richard A. Honeycutt, Ph.D. Replies:
Apply acoustical absorption to the back walls and any balcony faces to make sure the speakers are not hitting hard surfaces head-on with direct sound. Absorption should be effective to the bottom of the main speakers’ frequency response: 1” or even 1.5” fuzz seldom does the trick — you may need at least 2-1/8” material. Absorption is often preferable to diffusion, since diffusion simply redirects sound, and you have to be very careful where it gets redirected to; you can easily just move the problem rather than solving it. Diffusion does often help on parallel side walls near the stage to prevent flutter echoes.
If using line arrays, remember that most of these have a very wide horizontal coverage pattern. Make sure that no seats receive sound from more than one array with over a 50-ms delay (as shown in Fig. 2). This means positioning the line arrays carefully — not too far apart from side to side. Carefully model the room with the speakers in the model, and make sure the predicted RT is low enough for the program material. The RT map should not be “spotty,” but smooth. Gross irregularities can indicate undesirable artifacts. Also look at the “echo speech” and “echo music” results to spot any slap or flutter echoes.
Look at one of the standard texts (Beranek, Kinsler/Frey, Honeycutt) for recommended RT values for specific program material. Make sure there are no reflection paths from the speakers to any seats having a delay greater than 50ms in the reflected path, unless the reflections are sufficiently attenuated. And as mentioned earlier, analyze RT and echo paths carefully using EASE, CATT or ODEON, another excellent modeling program shown in Fig. 3.
David K. Kennedy operates David Kennedy Associates, consulting on the design of architectural acoustics and live-sound systems, along with contract applications engineering and market research for loudspeaker manufacturers. He has designed hundreds of auditorium sound systems. Visit his website at www.immersive-pa.com.
