When someone speaks of reverberance, it might bring to mind Phil Collins’ heavy use of reverb in In The Air Tonight, or Led Zeppelin’s in When The Levee Breaks. Yes, both are good examples of “reverb”. But when we’re talking about architectural acoustics, that’s not exactly what we mean when it comes to the reverberation time within an acoustic space.
Defining reverberance
Defining “reverberation time” or “RT” allows acoustic consultants to measure and predict sound within a space. They can then devise solutions to improve the “RT”, if needed.
The equation frequently used to estimate “RT” is Sabine’s equation:
RT60 = 0.161 x Volume / (average absorption coefficient x area x 1 metre)
This essentially measures how long it takes for sound to decay (fade) by 60 dB (or 99.999% of its energy). For those interested in exploring the maths further, there are several articles in acoustic journals including a review of Eyring’s equation, and a revisit of Sabine and Eyring.
What does mean for us?
Since “RT” is the amount of time it takes for sound to fade within a space, the higher the “RT”, the more reverberant the space. You can get an idea of different reverberation times by clapping your hands in the bathroom and then doing the same in the bedroom. The difference is to do with the sound absorbency of the space. Bathrooms have a lot of hard surfaces, so sound “bounces” around and echoes. Bedrooms usually have carpet and other soft furnishings which absorb sound.
This is why you can often improve acoustics in a room by adding absorptive materials like a plush sofa, carpets, or wall furnishings.
Is reverberance good or bad?
Reverberation time can be a problem or a non-problem depending upon the space and its use.
For a social space like a restaurant, a high “RT” can be problematic. This is because people talking raises the background sound level, and with added reverberation, the sound is amplified. When it is difficult for people to hear each other, they speak even louder, further increasing the sound level. We’ve all been in such restaurants, I’m sure. Including soft furnishings can reduce the RT in these situations.
But a high “RT” is not always a bad thing. For example, gothic churches have a high “RT”, which is great for choral singers*. It all depends upon the context.
In contrast, low “RT” spaces are acoustically treated spaces like recording studios, where reverberation would negatively impact the recordings. Sometimes a low “RT” can be a problem too—for example, too much absorption can impact speech privacy.
Can sound masking help?
In some high “RT” situations, increasing soft furnishings is not an option. Sound masking can help where there is reverberance in an atrium or the background sound level is too low. Adding a background sound can reduce the appearance of reverberation, and reduce the negative impact for those working within the space. Similarly, where there is too much absorption and speech privacy is compromised, adding sound masking can improve the situation.
*Check back next week for our blog about choral singers.