In our previous reverberation time measurement tutorial an impulse response, created by bursting a balloon, was used as the measurement signal. This is a quick and simple method of carrying out a reverberation time measurement, but may not be the most accurate method. In this tutorial we will look at an alternative method that can provide improved results.
When bursting a balloon the majority of energy released is in the mid to high frequency region, which can mean that low frequency results (under 500Hz) are quite variable and unreliable. In order to create more sound energy at low frequencies the use of a loudspeaker can increase signal level, while also reducing the variability of sound energy across the frequency spectrum.
Although loudspeakers are capable of playback at high sound pressure levels, they are not suited to the output of very short impulses, which in this case are needed for the calculation of reverberation time. To get around this a sine sweep may be used, containing the same output energy at every frequency within the range generated over the period of a few seconds. This sweep can then be processed after recording to produce an impulse response, through a process known as deconvolution. The deconvolved impulse can then be treated the same as in the previous tutorial, and processed through Room EQ Wizard to give the reverberation time.
A loudspeaker of some kind will need to be used to output the sine sweep, this should be of adequate size and power to produce a signal level that is considerably higher than the background noise in the space you are trying to measure. In order to get a reliable result at relatively low frequencies (<500Hz) the speaker must be large enough to generate sufficient levels of low frequency sound.
The sine sweep used in this measurement must be generated by the ReaVerb VST found in Reaper. This is because the deconvolution process requires both the original sweep and the recorded sweep to be of exactly the same length. This also means that the signals must be played out and recorded simultaneously into Reaper.
Before starting a project ensure that the buffer setting on the audio device driver are set as low as possible to minimise recording latency.
To begin create 3 new audio tracks.
The first track will be a utility track that is only used to generate and decode the test signal for the impulse response. Insert the “ReaVerb” VST plug-in using the “FX” button.
Under “Impulse Generation” select “Add” > “File” (If a pop up box appears, close it). Now you can use the “Generate Test Tone…” button under “Impulse Generation Utilities” to produce your test tone.
Choose a file name, as well as the sweep length, sample rate, and bit depth. A longer sweep length will improve the signal-to-noise ratio, so 10s is a good length. A sample rate of 44.1kHz or higher will be fine, but make sure to select a bit depth of 24bit to be compatible with Room EQ Wizard.
Once exported, re-import the file to track 2 of your project. This will be the track used for signal output.
Right-click the record button on the transport bar to select ‘Record mode: time selection auto punch’. This ensures that the sweep and the recorded impulse-response audio files end up the same length as each other.
The third track should be named according to the room or acoustic environment you will be measuring, this is where the impulse response file will be recorded.
Select the appropriate input and arm the track for recording.
You are now ready to playback and record the sweep simultaneously.
With the sine sweep recorded, use the “Deconvolve…” function in ReaVerb to first select the recorded sine sweep, then the generated test tone, and finally where you would like the created impulse response to be saved.
This impulse response can now be imported into Room EQ Wizard for the calculation of reverberation time, as in part 1 of this tutorial.