Measurements Without Computers: Leave The PC At Home And Get Better Data

A slowly swept sine wave offers some significant advantages:

• Significantly more energy is fed to the room. This offers dramatic signal-to-noise benefits.

• It’s less likely that you will overload your recorder, since the level of a sine wave doesn’t fluctuate like a noise or impulsive stimulus.

• The simple nature of the waveform allows the recording to be done with most file formats (i.e., WAV, MP3). Complex waveforms can be significantly altered by the compression schemes used by some formats.

• It is relatively easy to set recording levels.

Beware. Since sine waves generate significant power (that’s why you get a better signal-to-noise ratio), be careful not to burn up the loudspeakers used for the test.

The Equipment
There are lots of possibilities here. My setup consists of things that I already had. GratisVolver includes the forward and inverse sweeps needed for the test. You will need to supply a few hardware items.

For my stimulus file, I created a WAV file that includes the 14-second forward sweep and a short speech track. The total duration is about two minutes.

I use a media player (i.e., iPod) to play the file into the system. As a separate unit, it eliminates the need to string long cables in the venue to excite the system. The file is loaded on to a stereo portable handheld recorder. My mic is a Crown SASS, ideal for this application since it emulates a human listener. All of this fits neatly in the SASS case.

The RIR can be analyzed with many platforms, even though the data-gathering was independent.

The data is gathered by playing the sweep file into the system and simply recording it. The 14-second log sine sweep is recorded at each desired measurement position. The speech track is used to provide a reference for what the measurement position sounded like. It can later be used as a reference to compare the convolved files to. The whole process takes about two minutes per listener location.

The Possibilities
Now the fun begins! The RIR can be opened in any acoustics measurement package that recognizes the WAV file format, which is about all of them and processed to yield the various acoustic measures.

It can be convolved with dry program material in GratisVolver. The user is free to exploit the strengths of all of these platforms (plus their ear-brain system) to analyze the data. It also allows for easy comparison of the measurement platform algorithms such as reverb time and clarity scores.

You will learn which measures are more meaningful to the way that you think about sound, and that help you draw meaningful conclusions from the data. Such files can be exchanged with colleagues for consultation, and can serve to document the before – and – after the performance of a sound system renovation.

They are also excellent for archival purposes. How would you like to have an RIR of the Fogg Lecture Hall where Sabine derived his famous equation? We could return to that space virtually and listen to it, as well as many others.

RIR testing is nothing new. We just have a few new tools that help us collect and analyze. All serve to make us better measurers and better listeners, ultimately resulting in better sounding systems.