There are many kinds of acoustics measurements, in this report we emphasize on frequency response. It usually is dB versus frequency of 20Hz to 20kHz. The measurement should also be done in an Anechoic Chamber so that only the subject under test will be measured and not the reflections of the walls i.e. room acoustics.
Modern computer aided equipment such as MLSSA are designed to be used in normal environment without the help of an anechoic chamber, but there is one limitations. First of all MLSSA uses “Impulse Response” and it is measured in relation to time.
The advantage of measurement in time is that we know when does the sound reach the microphone and where is the first reflections impulse so that we can cut it away. Reflections are created from the source hence it is always later then the source.
After cutting the reflections away, MLSSA then performs a Fast Fourier Transform (FFT) to extrapolate a “frequency response” representation of the “time” measurement.
When reflections impulse is cut away, it is important to note did it enter before the original impulse can complete without interference. Take example below.
Mic to Speaker distance 1 meter = = 2.9miliseconds
Floor to ceiling distance in a typical office is 2.36meter. Divide by 2 due to microphone place between them = 1.18meter
Hence reflection path at around 30° to the floor and up to the microphone is 2.56meter and it is 7.5miliseconds
With the information above we know that the time window for a FFT is 7.5ms – 2.9ms = 4.6ms. From MLSSA’s manual stating low frequency response extrapolation limitation is a time window of hence the answer is 217Hz lowest frequency. Any frequency below that is completely not trustable.
Now we know a normal office environment cannot measure frequency range from 20Hz even using modern computer aided equipment. Hence the idea of using an aircraft hanger with very high floor to ceiling distance might help.
Armed with the knowledge minimum frequency is 20Hz, we can work out the reflection path has to be as long as and we still need to keep microphone to speaker distance at 1meter.
Further analysis of this information shows that microphone to first boundary is
A guess of the aircraft hanger floor to ceiling height easily rules out 34.3meter because assuming any standard floors is 3meter, it will be as high as 11 stories building and a aircraft hanger is no where close to 11 stories high.
Not only does an aircraft hanger unable to assist the computer aided equipment to achieve low frequency of 20Hz, it is also cold and the results will deviate because acoustics characteristic is completely changed and does not give a representation of room temperature of 20° Celsius.
The aircraft hanger is unsuitable for acoustics measurement because it’s floor to ceiling height is not enough and that temperature variation makes result not comparable to room’s temperature results. It is not feasible to rent the aircraft hanger because it cannot achieve low frequency resolution of 20Hz.
Report prepared by Fa Cheng CHIN, B.Eng.(Hons.)Electroacoustics
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