1.5 SOUND ANALYSIS INSTRUMENTS/TECHNIQUES

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The basic setup

1.5.1 Noisy machines

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will be covered later, but for now we want to determine how much sound pressure they are creating at different frequencies.

See spectrogram figure on page 95, figure 4.12 Irwin and Graf

1.5.2 A Room (or not)

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The measurements are done most often in imperfect environments (perfect environments discussed later)

1.5.3 Equipment (Microphones)

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Convert sound pressure to electrical signals

A microphone has a finite area to measure the sound with. As the diameter `d' approaches the wavelength of sound, the sound becomes distorted.

When wind blows across a microphone, it generates a low frequency turbulence noise. This is often corrected by using dBA measurements, and foam balls.

Corrections must also be made for changes in temperature and altitude

1.5.4 Noise Measurement Form

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1.5.5 Equipment - Spectrum Analyzers

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Signal spectrum is a fundamental concept, these machines allow us to measure this

Some possible spectrums are,

Filters can be designed using Bode Plots. These filters can reduce or increase the amplitudes of input frequencies in certain ranges.

How filtering works

Octave band analyzers (and others) are mainly distinguished by the difference between the upper and lower cutoff frequencies.

The narrower the bandwidth of the analyzer, the more expensive the instrument

octave, 1/2 octave, and 1/3 octave analyzers are more common.

Try problems S24, S25, S26

The analyzers so far only look at specific frequencies but we can use them to look at a variety of frequencies.

A typical mode of operation for older analyzers requires some interpretation. The graph is drawn by switching each channel on for a short period of time, the voltage from a certain frequency band is plotted (this allows variations at that frequency to be seen). After some period of time the centre frequency is shifted to another value. This means the bands are wider for an octave analyzer,or the bands of the analyzer are narrower for a 1/3 octave bandwidth. The wider bands hide noise when using wider bandwidths. For example the peak near 200Hz is more obvious with the 1/3 octave analyzer. The noise appears to be a multiple of 60 (3 * 60 = 180), suggesting that it is the motor speed, driving a geared up impeller.