Reverberation time and room acoustics

30 October 2012 | Category: Interesting facts, How does it really work, Great Sound

You can very easily understand what reverberation time is by clapping your hands and listen to the echo and how it fades. The time it takes for it to go away is the reverberation time.

When we talk about reverberation time we often mean the T60, which is the time, in seconds, it takes for the sound to be attenuated 60 dB.

In practical terms it is the time it takes for the sound to fade completely.

 

The expression for reverberation time was defined by the American physician  W.C. Sabine, as:

 

 

Where 0.161 is a defined constant. V is the volume of the room in m3 and A is the total absorbing area in m2.

The total absorbing area is calculated as the absorption factor mulitplied with the area of the absorbents.

This will tell you that the ceiling height of the room is directly affecting the reverberation time. The higher the ceiling height, the longer reverberation time.

it also tells you that the absorbing capacity of the absorbents matters. If you double the absorption factor you can have the same reverberation time with half the area of absorbents.

In theory, that is…

First of all, every single frequency has its own reverberation time depending on the absorbing materials ability to absorb the sound waves of that specific frequency. it is as a rule of thumb easier to absorb medium and high frequencies compared to low frequencies. These low frequency reverberations can blend in and interfere with the higher frequencies in a way that the listener interpretates as a noisy environment, even if the reverberation for the mid and high frequencies are relatively short.

what does that mean?

Well, take for example noise from HVAC, fans, electrical equipment etc. Even though most of that noise are low frequent, it will be affecting conversations taking place in the room.

In addition to this:

Most rooms are not shaped in a way that makes this simple formula accurate.

Hard reflective materials in walls and floors can cause phenomena where the sound waves bounces back and forth between for example two parallel walls. You will then experience a much longer reverberation time compared to what the calculation shows.

The type of furniture used and how it is placed, together with how the absorbing material is placed will affect the reverberation time quite a lot and that is not at all taken into consideration in the simple formula above.

Therefore reverberation time is a nice way of explaining roughly how you are going to experience the sound environment of a room, but not good enough to be used to create excellent room acoustics.

An experienced acoustic engineer can get closer by  adding his/her knowledge about how the placement of the absorbers and the decoration of the room would affect the reverberation and use that to place the absorbers on the right places, but to get a better understanding they should do a simulation and look at the Speech Transmission Index, STI or more commonly used the Rapid Speech Transmission Index, RASTI.

By using software, as for example EASE by Renkus Heintz, you can make graphic maps of the STI as well as other important parameters, to show very visually how you would experience the acoustics in the different parts of the room. In fact you can even make what is called an auralization and create a sound file that illustrates how it would sound in a room with the chosen design.

 

 

 

 

 

 

 

 

 

 

 

What we say is, that using computer software, together with absorbing products that can deliver specific absorption at a given range of frequencies, can create excellent room acoustics with optimal use of the absorbing products, while relying on rules of thumbs and absorption classes or NRC only, can be a serious mistake.

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