Reverberation is a major factor in how good speech or music will sound in a room. It can be thought of as echoes occurring so closely together that you don’t hear them as distinct sounds, but rather one long sound that gradually fades away. Reverberation time (RT), the time it takes for a sound to fade away, is a common measure in discussing the “ideal” acoustics for a space. A room with a long RT is said to be lively, while one with a short RT is considered to be dead. There are several factors that influence the RT of a room: volume of the room; surface area of the floor, walls, and ceiling; and how these surfaces are finished (hard, soft, smooth, rough, etc.).
A common dilemma in House of Worship (HOW) acoustics is finding a balance between the needs of speech intelligibility and some types of live music. In general, a short RT is necessary for good intelligibility. However, a long(er) RT is desirable for pipe organ and acoustic groups, such as a string quartet or choir. Electric instruments sound better in a “dead” room. A typical large space without much sound absorption would have a long RT, and the congregants might be content with the sound of the organ. But, the congregation will have difficulty understanding the spoken word. The two typical options are:
1. Leave the natural acoustics unchanged (don’t alter the wall, floor, or ceiling finishes) and rely on the loudspeaker system to focus as much direct sound to the listener as possible. But, any sound reflected off of walls, floor, and ceiling will mix with the sound going directly to the listener and affect intelligibility adversely.
2. Treat the room’s surfaces with sound absorbing material to achieve an RT that’s somewhere between what’s best for speech and acoustic instruments. But, let’s say speech needs an RT60 of .75 sec. and pipe organ sounds great with an RT60 of 3 sec. or more. A compromise RT60 of 1.75 sec. will wreak havoc with intelligibility, and most people won’t be happy with the organ sound. I.e., the acoustics will be poor for both speech and music. (RT60 is the time it takes for a sound’s pressure level to decrease 60 decibels)
A third option is to add absorption to the walls and ceiling in order to make the room dead. Speech and electronic instruments will work well, but acoustic instruments will sound weak. This is where electronic enhancement can help. Adding this absorption is a great option if:
• The room must support many different sound sources and events – contemporary and traditional services, any type of music group requiring sound reinforcement, symphony orchestra, etc.
• A room needs to sound larger than it is; e.g. a rehearsal room that needs to mimic the characteristics of the sanctuary.
• A room should sound as large as it is. A room can be treated with so much absorptive material that it feels strange to be in the room – there can be a disconnect between what our eyes and ears tell our brain. Enhancement can be used to make the room “feel” as large as it is.
Electronic Enhancement – What Is It?
Electronic enhancement could be described as electronic reverberation played through a giant surround sound system. There are really two acoustical characteristics that we’re trying to enhance – early reflections and late reflections. Early reflections are those sounds reflecting off nearby surfaces. They help the performers on stage by providing (quasi-) natural reinforcement making it easier to hear others onstage and helping to achieve some balance between the performers. You can think of this enhancement as creating a virtual orchestra shell onstage.
Late reflections, as you might guess, are sounds reflecting off surfaces much further from the source – walls, floor, ceiling, etc. within the audience area. Late reflections create a sense of envelopment (think natural surround sound).
A typical system consists of microphones over the platform to pickup the direct sound source, a processor to perform the digital “magic”, and several loudspeakers distributed throughout the space. These loudspeakers do not have to be large and powerful as they are reproducing subtle virtual reflections. This makes them somewhat easier to integrate into a room’s architecture.
Loudspeaker zones are typically created so that the platform and audience areas receive signals processed specifically for each area. For example, a zone would be created for the platform, which may need enhancement of early reflections, and audience areas, which may need enhancement of late reflections. And, zones might be created for different seating areas – main floor, balcony, under balcony, etc. Although these areas are all receiving reflected sound, they are receiving different reflections and may need differing types and amounts of enhancement.
There are three primary methods to provide the enhancement: active, passive, and hybrid.
In an active enhancement system, the original direct sound is sampled. Several channels of digitally synthesized versions of this sound are created, each with the appropriate electronic reverberation added — depending on which loudspeaker and zone receives the signal. Active enhancement can be applied to both early and late reflections. Figure 1 shows an example of this method.
There are two different methods used in a passive system. The first method uses several microphones to pick up the direct sound, similar to an active system. But, rather than synthesize a completely new signal, the passive system delays the signal by varying amounts. The amount of delay in each loudspeaker channel is continuously changing in time. Figure 2 shows an example of this method.
The second method replaces the platform microphones with a microphone at each loudspeaker location. As you probably know, if you place a microphone in close proximity to a loudspeaker, you can create a feedback loop – the loudspeaker signal is picked up by the microphone, which gets reproduced by the loudspeaker, which gets picked up by the microphone, etc. This method of passive enhancement is essentially controlled feedback. Figure 3 shows an example of this method.
Passive enhancement is typically used for late reflections.
As the name suggests, this system is a mix of both active and passive methods. Typically, active enhancement is applied to the early reflections, while passive enhancement is applied to the late reflections.
No matter which method is used, the enhancement system’s loudspeaker system can also be used for those sources that don’t require enhancement. For example, if the space has been treated to be good for intelligibility and enhancement is used for pipe organ, the processor can be programmed to mix the organ signal with the un-processed speech signal. And, the combined signal can then be fed to the distributed loudspeaker system.
Enhancement systems can, in theory, increase the RT infinitely. Typically, though, enhancement systems are used to increase the time two to three times the natural RT of the room. For example, if the natural RT is one second, you would typically use the system to increase it to no more than about three seconds. The exception is if the system is being used to mimic a much different space. For example, a small rehearsal room may need to have the RT increased much more to emulate the RT in the sanctuary. If you need to emulate different RTs for different events, presets can be created and recalled as required.
In the early 1990’s when electronic enhancement systems first became available, the biggest obstacle to electronic enhancement was cost, which made it feasible only for large projects. The size and configuration of the room will determine the number of zones and loudspeaker channels, which in turn will influence the system cost. But recently, the price has decreased to a point that some mid-size and even small projects can afford the benefits of adding an electronic enhancement system. The result is a space in which speech can be understood and music can also sound its best.