1.2 SOUND MEASUREMENTS

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Previous discussions centered on simple sound measurements, but actual machines are more complicated.

Sound levels are unequal in different directions.

The unequal distribution of sound energy can be expressed with a directivity factor,

A directivity Index can be helpful when the directivity factors have a wide range

Try problems S5, S6

1.2.1 Measurement Techniques

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It is impossible to measure sound at all points around a source, so some points must be selected.

One technique is to divide the surface of the sphere into a number of flat polygons.

It is best to have an anechoic environment

anechoic - an environment that (effectively) eliminates reflected noise.

In some cases only half of the measurement sphere can be used because of object size, or requirements.

The basic procedure is,

1. Select number of points to measure

2. Get list of points by selecting measurement radius, and with point table (only the top half is a hemisphere)

3. Measure sound pressure at each point (Lp)

4. Correct for points on plane if half sphere

5. Find Lp (correct for -3dB if hemisphere)

6. Find Q or DI for each point is required

try problems S7, S8, S9, S10, S11, S12