Course:PHYS341/Archive/2016wTerm2/MusicWithHugeReverbTimes

Music with Huge Reverb Times

Natural reverberation occurs when a sound wave radiates from its source and then reflects off the surfaces surrounding it, creating a secondary echo sound. A perceiver hears this reflection before, after, or simultaneously to the source sound. Early reflections are a set of more distinctly separate echoes which return to the listener just before the less distinct "decay", which accounts for most of a huge reverb sound. "Reverberation time" refers to the entire duration of reverb sound, until the sound pressure level drops 60 dB or a millionth of the sound's initial volume - this is usually measured in seconds. Environments that reflect enough sound to be considered to have long or "huge" reverberation times are made of hard materials such as rock and metal - such surfaces are the most reflective. Hence environments like large halls, stadiums and churches as well as artificial reverberation units provide the reverb times necessary to qualify (1.5 seconds and up).

Reverb or Space Signature

Every space has a "signature" - or a reverb that's unique to a specific space. Music with huge reverb times is defined by its dense and long space signature. Here are some of the fundamental factors that make up that signature:

All rooms (and any other reflective environments) have a series of modes, which are frequencies at which the room naturally resonates. When a wavelength is equal to one half of any of the distances between two parallel surfaces, or any whole number multiple thereof, a standing wave occurs. Standing waves are a phenomena in which a sound wave hits and reflects off of a boundary in such a way that it’s perfectly in phase with itself, causing a rise in volume of the frequencies corresponding to the wavelength. Since rooms tend to be made with parallel walls of hard, reflective materials, standing waves are unavoidable. A room's modes then are defined by the distances between any walls, the floor, the ceiling, or any other surface. The lowest mode will correspond to the longest distance between two walls in the room; for example, if a room is 8'x10'x8', the lowest (first) mode will have a wavelength, and so on. Figure 1 is a diagram of various room modes between two walls. The second mode will correspond to the frequency of a 2.5' wavelength, and so on.

The first 3 modes between two hard walls. Source: Wiki Commons

A space's signature isn't only a result of its modes - there are many other aspects. Early reflections are the sound that reaches a perceiver before the decay of the reverb, around 50 - 80 milliseconds. This part of the reverberation tail orients a sound in a listeners perceptual field - if the sound source is to the left, the early reflections on the left will be heard first, giving the brain the necessary information to locate the sound. The late reflections, or the decay, is the rest of the time until the sound energy is has been fully absorbed into the environment. In music with huge reverb times, this aspect is essential - music composed with heavy reverberation in mind will always rely on a long, dense decay. Figure 2. shows a Time x Amplitude graph of a sound in a highly reverberant space, and divides the waveform into the source, the early reflections and the decay.

Fig. 2: Time x Amplitude Graph of a sound with long reverb tail. Source: Own work

Figures 3 and 4 show two different modelled spaces in Time x Amplitude graphs from the Waves IR-1, a digital reverb plug-in that mimics specific space signatures as precisely as a computer possibly can. It uses Impulse Response technology, where a sweep of all the audible frequencies is played from a speaker in a space and the resulting reverberation is measured with microphones. The measurements are then put into an algorithm that gives a model of the space. In the diagram, you can see how the room modes affect the consistency of the amplitude of the decay. There are billions of air particles at play in any given space with variables of velocity and temperature, so even the most powerful computers cannot reproduce the effect of a natural room.

Fig. 3: Time x Amplitude graph of a bath house's space signature, generated with Waves IR-1 from their IR library Source: Own work.

Fig. 4: Time x Amplitude graph of an Impulse Response of a Russian Synagogue, generated with Waves IR-1 from their IR library. Source: Own Work

How Sounds Behave with in Environments with Huge Reverb Times

In music with huge reverb times, percussive sounds — drums, wood blocks and tambourines, etc -- are often foregone. reflections from repeated percussive sounds in highly reverberant spaces cause a buildup of sound waves within the space. This situation usually results in a kind of sonic mush, which is why music composed for environments with no reverberation (smaller spaces, outdoors) tends to be more rhythmically focused and complex

As a condition of the environments available to them, composers of music with huge reverb times tend to use mostly sustained sounds that will be supported by the reverberation of the space. Sustained sounds, such as the sound of organs or bowed instruments, function differently because they feed energy into the space for the duration of the sustain, until, for example, the violin player stops bowing. This results in a buildup of sound energy until the input rate equals the absorption rate, and then the decay of the reverb. Reverb will build up on top of the source sound as the source excites the modes of the large space or electronic device. A common example of music composed for huge reverb times is music for the pipe organ, which is often supported/ inhibited by the 2 or more second reverberation of a church or cathedral.

An Example: Deep Listening Band

The Deep Listening Band, an experimental project of David Gamper, Pauline Oliveros, Stuart Dempster, recorded in the Dan Harpole Cistern, a structure at Fort Worden Park in Washington State. The empty cistern, which at one time held 2 million gallons of water, is 200 feet in diameter and 14 feet deep, giving the environment a truly huge reverberation time of 45 seconds.The music the trio composed there was consisted entirely of long, sustained notes, as percussion instruments would not be discernable in the "uniform acoustic" of the space. Dempster says of the experience: “playing in the cistern teaches two things to a musician: one is having a secure tonal center, and the other is intonation. If the attack is unstable, the reverberation captures all of that instability. If intonation is not accurate, the cistern lets one know... When one stops a mistake in a normal playing environment, the mistake has the decency to stop—not so with the cistern!”

See Also

• Reverberation
• Deep Listening Band
• Room modes