How To Hold The Instrument; Measuring In Free Or Diffuse Sound Fields - BRUEL & KJAER 2240 Guia Del Usuario

26 Integrating-averaging Sound Level Meter Type 2240 – User Guide
When taking measurements, you will often find instructions in local regulations concerning the
particular measurement, which of course, you must always follow. This chapter provides gener-
al hints and guidelines that are useful when no specific instructions are available.

How to Hold the Instrument

All objects present in the sound field where you are taking measurements exert some influence
on the sound field and, to some extent, have an effect on the values you measure. One of the
objects present is your body, which can either reflect or block sounds. To minimise the effect of
your own presence:
1) Point the sound level meter towards the sound source.
2) Hold the instrument away from your body, at arm's length.

Measuring in Free or Diffuse Sound Fields

There are two types of sound fields you should be aware of: free and diffuse. In a free sound
field – for example, out of doors away from reflecting surfaces – sounds arrive from one
principal direction. In a diffuse sound field, such as in a reverberant room, sounds arrive ran-
domly from all angles (random incidence). Specific microphones are used in each condition to
ensure accurate measurements and a uniform response at all frequency ranges. For free-field
measurements, the sound level meter should be fitted with a microphone with a free-field
response. During diffuse-field measurements, a microphone with a random-incidence response
should be used. However, you must always follow the relevant standard's requirements to
ensure complete measurement compliance, for example: IEC standards specify the use of sound
level meters with a free-field response, while the American ANSI standards call for a random-
incidence response.
Your Type 2240 is fitted with a free-field microphone. However, if your local standards require
the use of a random-incidence response, it is possible to fit your microphone with the supplied
Random-incidence Corrector DZ 9566 (Fig.5.7).
Fig.5.7
DZ 9566 Random-
incidence Corrector
Under free-field or near free-field conditions you simply point the sound level meter towards
the sound source. However, if you have fitted the Random-incidence Corrector and want to
measure with a free-field response, you must orient the instrument at an angle of 70 – 80
sound source; if pointed directly at the source, the result will be too high, especially if high
frequencies are prominent. Conversely, if you use a free-field response in a diffuse sound field
the sound pressure will be underestimated.
°
to the