Room Acoustic Parameters
The Room Acoustic Parameters are calculated from the energy impulse response, as we have seen it just above, and they quantify various sound attributes in the room. If somebody asks you: ‘How does this room sound?’, perhaps you could give answers like: it’s dry, dead, rich, loud, long-lasting sound, hard to listen to others speaking, blurry, etc. All these subjective judgements can be turned into objective parameters which are called Room Acoustic Parameters. A full list can be found in the ODEON User Manual, but below are listed some of the most important ones:
Reverberation Time is the most frequently used parameter in room acoustics. It is approximately the time it takes for a sound to decay and cease to an inaudible level after a loud sound source has been switched off.
The most common experience of reverberation time in a space is when making a single hand-clap. This corresponds to a loud sound source which has been ‘turned on’ and ‘off’ almost momentarily, leaving a decaying tail of sound that takes a few seconds to cease (or less, depending on the room). Reverberation time can reveal information about the size and absorption of a room. Large rooms with sound reflective surfaces have longer reverberation times, while small rooms with sound absorptive surfaces have short reverberation times. The reverberation time of a room is defined as the time it takes for the sound to be attenuated by 60 dB after the source has been switched off. This is noted as T60. However, when measuring reverberation time in practice, there is always a considerable level of audible background noise in the recording (ambient noise, electric noise in electronics, etc.) which reduces the usable dynamic range to less than 60 dB. Therefore, in a measurement, the reverberation time is calculated based on smaller decay ranges instead (10 dB, 20 dB or 30 dB), and then the obtained time is multiplied by an appropriate factor to extrapolate the time for 60dB decay.
Modern measurement methods do not use the interruption of a noise source, which causes stochastic variations of the results. Instead, the impulse response is measured with an impulse source or a sine-sweep followed by appropriate signal processing. When the impulse response is squared it becomes an energy impulse response. The backwards integrated squared impulse response is the perfect decay curve for the actual position in the room, because it is the curve that would be the result of averaging an infinite number of interrupted noise curves. Calculating reverberation time from the decay curve results in three different types of reverberation times. The following graph shows the process: