B&K 2250 Light Manuel d'utilisateur télécharger pdf (Page 33)

Chapter 4

Getting to Know Your Analyzer

4.1 What is a Sound Level Meter?

A Sound Level Meter (SLM) is an instrument that is designed to measure sound levels in a

standardised way. An SLM comprises a microphone, a preamplifier, a main processor and a read-

out unit. The microphone converts the sound signal into an equivalent electric signal. The

electric signal that the microphone creates is at a very low level, so it is made stronger with the

help of a preamplifier before it is processed by the main processor.

Processing includes applying frequency and time weightings to the signal as specified by

international standards, such as IEC 61672–1, to which the analyzer conforms.

Frequency weighting adjusts how the SLM responds to different sound frequencies. This is

necessary because the human ear’s sensitivity to sound varies according to the sound’s

frequency. The most commonly used frequency weighting is A-weighting, which adjusts a

signal in a way that best resembles the human ear’s response at medium-range levels.

A-weighting is required for nearly all environmental and workplace noise measurements, and

is specified in international and national standards and guidelines. All of the analyzer’s

measurement parameters apply A- or B-weighting and a choice of C- or Z-weighting, except

for the measurement of peak levels where a single weighting (typically ‘C’) is applied. In this

case, C-weighting is used to take into account the energy present at low frequencies even when

they are not particularly annoying.

Time weighting specifies how the SLM reacts to changes in sound pressure. It is an exponential

averaging of the fluctuating signal, providing an easy-to-read value. The analyzer applies Fast,

Slow and Impulse (or ‘F’, ‘S’ and ‘I’) time weightings, which are the required weightings

according to the vast majority of international and national standards and guidelines.

The signal is processed through the weighting filters, and the resulting sound pressure level is

displayed in decibels (dB) referenced to 20 µPa on the analyzer’s screen. The sound pressure

level values are updated at least once per second.

Assessing a fluctuating noise level means getting a value for a level that is, in simple terms, the

average level. The ‘equivalent continuous sound level’, L

, is known around the world as the

essential averaged parameter. L

is the level that, had it been a steady level during the

measurement period, would represent the amount of energy present in the measured,

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B&K 2250 Light Manuel d'utilisateur Page 33