Theoretical Context

Impulse Response

In acoustics, an impulse is a short, almost instantaneous sound, similar to a clap, pop, or snap. This short impulse is used for room characterization, as a room's impulse response is used to define the space's reverberation time, clarity, bass ratio, and other characteristics. 

Reverberation Time

Reverberation is the persistence of sound after the source has stopped emitting. Reverberation time (RT) is the time it takes for sounds energy to decay by 60 dB, an inaudible level. It contributes to the loudness and fullness of the sound and is associated with the sounds richness. A higher RT indicates a more lively room while a lower RT indicates a more acoustically dead room. 

There are a handful of ways to calculate reverberation time:

• T20: T20 calculates the reverberation time using the time it takes for the energy to decay between -5 dB and -25 dB. This time is then multiplied x3 to achieve a decay of 60 dB.

• T30: T30 calculates the reverberation time using the time it takes for the energy to decay between -5 dB and -35 dB. This time is then multiplied x2 to achieve a decay of 60 dB.

• EDT (early decay time): EDT calculates the reverberation time using the time it takes for the energy to decay between 0 dB and -10 dB. This time is then multiplied x6 to achieve a decay of 60 dB. An important feature of EDT is that the calculation includes the direct sound and early reflections that T20 and T30 exclude.

• RT60 Sabine: A method of estimating the time it takes for sound to decay by 60 dB for each octave band. The RT60 Sabine Equation is calculated by dividing the overall volume of the space by the total absorption of each surface for each octave band. The absorption of each surface is defined as the surface area multiplied by the absorption coefficient of the surface's material. The following equations are used for these calculations. One downfall of this method is the opportunity for human error in the calculations. The volumes and surface areas are a best approximation based on manual measurements, which is a frequent cause of error. Additionally, there are multiple sources for finding material absorption coefficients, many of which report different values for the same material. This inherent flexibility can also be useful, however, for estimating the effects of changing materials, surface areas, and room volumes, without needing to remodel or change the physical space.