Simon and Garfunkel once sang about the sounds of silence but few actually get to experience complete and utter silence. University of Texas in Austin is home to one of the rare anechoic chambers -- a room completely lined with wedge-shaped sound absorbers that take in any vibrations in the air and dampen it. It effectively achieves almost total silence and it is so quiet in the room that after a short period of time you begin to hear your own heart beat. Apparently the quietest one ever built is housed in Washington by Microsoft’s audio lab and it is so quiet that the maximum amount of time anyone has been able to stay inside is approximately 45 minutes. This is because human beings orient themselves through the sounds we hear, so when that is completely eliminated it’s almost impossible to get your bearings.
These chambers are commonly used to conduct experiments such as testing the response of loudspeakers, microphones, and even motorcycles if the chamber is big enough. In our case, we used UT’s anechoic chamber to calibrate four Wildlife Acoustic SM4 SongMeter recorders that will be used in Wild Basin WIlderness Preserve to capture sound and monitor the sounds of wildlife. he long-term goal of the study is to monitor changes in bird and anuran communities as the surrounding landscape becomes increasingly urbanized.
In order to calibrate each recorder, we set up a loudspeaker directly 1 meter (3.28 ft) in front of each SM4 SongMeter device and played a series of pre-recorded “pink noise” sound tracks. Each recording was played for three 30-second intervals, and the process was repeated for both the Left and Right microphones. “Pink noise” is the desired noise to use when calibrating sound instruments because it mathematically corresponds to how the human ear is designed to hear sound (i.e., in octave bands). Pink noise contains equal energy per octave, so when you look at the recordings on a typical spectrogram display (a plot of sound frequency over time), you will see the sound represented as a box because all sound frequencies occur simultaneously. These calibrations are important to do prior to collecting data in the field to ensure that all recorders are collecting and labeling sounds in the same way.
Getting to experience an anechoic chamber was very cool -- and pro tip: eat before entering one as your stomach growls become very loud and evident in the chamber!
Mathew Piotrowicz, Mechanical Engineering student at University of Texas, helps calibrate the SM4 Song Meter units in the anechoic chamber.