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Ideal HOW Acoustics

Feb 2, 2011 3:49 PM, By Bob McCarthy

How variable acoustics can create a continuously adaptive sound environment for contemporary houses of worship.

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A section view showing locations of speakers and microphones for a reverberation enhancement system. The overhead speakers are ceiling-mounted and the mics are hung down from the ceiling. The lateral speakers are wall-mounted.

A section view showing locations of speakers and microphones for a reverberation enhancement system. The overhead speakers are ceiling-mounted and the mics are hung down from the ceiling. The lateral speakers are wall-mounted.

One of the most famous lines in American film is found in the movie The Graduate, when young Dustin Hoffman is told, "I've got just one word to say to you about the future: Plastics." Let's move to the present and let's limit our focus to sound in multipurpose spaces such as theaters and houses of worship. I've got just two words (and a hyphen) to say to you about the future: Electro-acoustic architecture.

What would be the ideal room acoustics in a contemporary house of worship? Of course the normal answer would be, "It depends." Now we have a better option. The best answer is acoustics that don't depend. The best answer is acoustics that can adapt in realtime to the current needs. If the choir is singing, give us a cathedral. If the band is rocking, let's keep it tight in the low-end, and if there is a speech in progress, let's give the voice just enough support to be easily heard without excessive reverberation. Is there any kind of hall that has these contradictory acoustical properties? Yes—a hall with variable acoustics. Halls with variable acoustics are a tiny minority at the present time. Houses of worship are a small minority of that minority. But in the not-too-distant future, variable acoustics will be commonplace for houses of worship.

Variable acoustics come in two forms: mechanical and as electro-acoustic enhancement systems. The mechanical version has been used effectively in the construction of many of modern era symphony halls. These halls look like a typical symphony hall on the inside, but they contain more hidden doors, chambers, curtains, and secret passages than Hogwarts School of Witchcraft and Wizardry. Such a hall can have its reverberation time extended by opening chambers or conversely have it reduced by dropping in curtains or reversing doors from a reflective to an absorbent side. A modern symphony hall can be optimized for classical one night (1.8 seconds of reverberation time), for pipe organ the next (5+ seconds), and then bring in a lecture presentation (1.0 seconds).

Adding mechanical variable acoustics is a huge up-front cost to a modern symphony hall, but it is orders of magnitude cheaper than building three halls. The operational and maintenance costs are substantial as well, but this is more than made up for by the fact that the variable hall can be booked with a much wider variety of acoustically successful events than any hall with static acoustics. This means more bookings and fewer dark days. The variables at play here are volume and the amount of absorption.

Increased volume (for a given amount of absorption) will increase the reverberation. Increased absorption (for a given volume) will decrease the reverberation. The variable acoustic hall in its "standard" configuration for symphonic music has a large proportion of exposed reflective surfaces to create an even decay from all directions. These surfaces cannot be made more reflective, so in order to increase the reverberation, we must increase the volume. Doors are opened to allow the sound into highly reflective coupled chambers that increase the volume of air in the space, thereby extending the decay time.

Conversely, if we want to reduce the reverberation, it is much more practical to increase the absorption. This is done by dropping in curtains or turning panels over to their absorptive side or other mechanical options. The contemporary house of worship does speech, organ, rock, choir, rap, and more. Therefore variable acoustics is a natural fit. But, unlike the concert hall, the HOW does not do speech one night and choir the next. The entire range of musical genres and speech may all occur in an hour or less. How many volunteers will it take to operate the reversible doors, open and close chambers, and drop down curtains for each different part of the service? Do you think this might be just a bit distracting to one's spiritual contemplation?

Obviously this is not practical, which brings us to the second option in the world of variable acoustics: electro-acoustic architecture (also known as acoustic enhancement systems, artificial reverberation, reverberation enhancement, and other names). With electro-acoustic architecture, you can change the room's response from voice-optimized to organ-ready in a single second, without any visual clue and without a labor call.

Whereas the mechanical variable acoustics began in the middle acoustical ground and added volume or absorption to go one way or the other, the electro-acoustic technology can only add reverberation, not reduce it. Therefore the electro-acoustic versions want to start from a relatively dry room and then add from there. A well-designed system can at least double the reverberation time of the physical room and possibly extend it further from there. This is achieved by electronically simulating the effects of adding volume and decreasing absorption.

How electro-acoustic architecture works (simplified)

As the name implies, the acoustic response is going to be enhanced electronically. There are three major ingredients: microphones to simulate the sound going to the walls, speakers to simulate the reflections coming off the walls, and digital signal processing to manage the character of the reverberation created and possibly to create additional electronic reverberation. All of this is added on top of the original acoustical properties of the space. Reduced to its simplest form, we have the following signal path: (A) acoustic source to (B) mic to (C) processing to (D) speaker, which then returns to the mic and around we go again and again. This is similar to the natural acoustic path: (A) acoustic source to (B) wall, (C) wall, (D) wall, and on and on. This contrasts to the reverberation device you might have out at front of house, which creates all its reverberation inside the processor and then sends it out to the speaker system.

Now let's dig deeper and get some more detail about what it is going to take to create a plausible multidirectional reverberation tail in a dry room. If we just add reverb into the speaker system, we have reverberant speakers in a dry room. The more reverb we add the more out of scale the two worlds become. I like to put it to the "singing in the shower" test. Electronic reverb puts the singer in the shower, but the audience is still dry. This could make folks feel a bit uncomfortable.

If the reverberation is created in the room by either physical acoustics or electro-acoustic architecture, then the audience and the performer are all in the shower together. Much better, eh?

The key ingredients that separate reverberation created in the room from your front-of-house electronic reverberation are that the in-room reverberation is initiated by any sound source, anywhere in the room, and that the reverberation surrounds you. Your FOH reverb affects only the performer(s) and only arrives from the direction of the speakers, which are typically in front of you. How about if we add some reverb to the surround speakers? Nice try, but surround speakers are too few and far between to fool anybody into believing that they are in a reverberant space rather than hearing a few isolated effects sources.

Reverberation enhancement systems use multiple mics placed around the stage and audience areas as well as large number of speakers along the walls and ceiling. The density of the speaker placement has to be high enough to make the listeners unable to localize individual elements, thereby creating the unified reflective character of a wall. The quantity of mics has to be high enough to give us lots of return portals into the speakers so that we can have enough recirculated energy to extend the reverberant energy without causing instability and feedback. The quality of all components must be high enough so that no clues are given to listeners that the sound is coming from speakers or mics.

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