Tutorial: Measuring Reverberation Time

The reverberation time of a room has a distinct effect on its suitability for different tasks, in this tutorial we will look at the different methods of measuring reverberation time and what the results can show us.

The first step in carrying out a reverberation time measurement is choosing the method. There are a number of different methods available to us however some will require specialist equipment that the average person will not have access to (look out for follow up tutorials for these!). The simplest method involves creating and recording an impulse response.

An impulse response consists of a single, impulsive sound with a large amplitude such as a clap, starting pistol or balloon burst. For this tutorial we will use a balloon burst as it is the cheapest and most easily available.

When an impulsive noise is created by popping a balloon in a space sound will radiate out omni-directionally, reflecting off of the walls, ceiling, floor and other surfaces around the room. As the sound goes through a number of different reflections it decays in level until is inaudible. The time it takes for the sound to decay away is what we will be measuring. The standardised measure for this is RT60, which refers to the time taken for the sound to decay by 60dB.

Equipment required for this measurement:

Balloons
Recording equipment
Computer (PC or Mac)

Method

First, set-up your recording equipment so that it is recording to either 16 or 24bit WAV. Place it roughly in the middle of your room, with your gain level set low enough to avoid clipping when the impulse is created. Next, blow up a few balloons, the amount of air inside the balloons will have an affect on the frequency content produced when popped, so vary the size of them if you want to compare the differences.

Now start recording, pop a balloon with a pin far enough away from the microphone that it will not cause clipping. Now allow time for the sound to decay in your room as this is what you are trying to measure! After a few seconds (unless you are somewhere very reverberant) stop the recording. You now have an impulse response for the room. Do this a few times to allow for errors or other miscellaneous noise effecting the result.

Now that you have an impulse response, transfer this file to your computer (Mac or Windows). You will need to use a free piece of software called Room EQ Wizard. Before importing, cut off the silence at the beginning of the impulse file (using Audacity for example). Once you have exported your edited impulse, open up Room EQ Wizard and go to File>Import Impulse Response. You will now need to click the RT60 tab to view your impulse.

Towards the bottom of the page you will see some check boxes named:

Topt This will give you the optimal result
EDT Early Decay Time, this refers to the time taken for the sound to decay by 10dB
T20 This refers to the time taken for the sound to decay by 20dB
T30 This refers to the time taken for the sound to decay by 30dB

With only Topt selected you will see a single line representing your reverberation time. The X axis represents frequency,  displayed logarithmically while the Y axis represents time in seconds. Clicking on the graph will show you a blue cursor that displays the reverberation time at that particular frequency.

rt60 screen cap

More information on using Room EQ Wizard’s RT60 function can be found here.

Analysing Results

Now that you have some results you need to know what they show. The first thing to note is that you do not have a single time value for reverberation time, this is because your results will vary with frequency, generally decaying from a longer time at low frequencies to a shorter time at high frequencies. You may wish to use the time given at 500Hz as your single value measurement.

For most scenarios the best reverberation times are those that vary little with frequency, especially in an environment where audio mixing might take place. It is normal for the low frequencies to have a longer reverberation time as these are the hardest to control, but the aim should be to limit the time variation in the mid and high frequencies.

Different reverberation times suit different applications, for example, a typical living room may have an RT of 0.5s while an auditorium may be closer to 2.0s. A recording studio may wish to be between 0.2 and 0.5s when recording speech, but for classical music be closer to 0.8s. These variations can help to either achieve a dry signal (a short reverberation time) or to give music and speech warmth (a long reverberation time).

Controlling RTs

Having measured the reverberation of a space you may wish to modify it to suit a particular purpose, in order to do this you will need to change how absorptive the surfaces of your room are. To reduce reverberation time, hard reflective surfaces should be avoided, thick curtains and the addition of furniture can make a big difference at the lowest cost however they may not reduce the reverberation time uniformly across the frequency spectrum. To be able estimate the changes in RT60 time due to changes in room surfaces, a Sabine equation may be used that takes into consideration how absorptive each of the surfaces of the room are. This may be covered in another tutorial.

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