Friday, April 12, 2024

Choosing The Right Sound Level Meter

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When performing acoustic testing, choosing a proper sound level meter (SLM) testing system is the key. An SLM is commonly used in noise pollution studies for quantification of different kinds of noise, especially industrial, environmental and aircraft. However, reading from an SLM does not correlate well with human-perceived loudness, which is better measured by a loudness meter.

Types of SLMs
Integrating meter. An integrating meter is one that can measure an equivalent continuous sound level (Leq) and is required for any proper occupational or environmental noise monitoring. This meter is usually mandated.

[restrict …]Non-integrating meter. A non-integrating meter displays only the instantaneous noise level at any one time and is suitable only for spot checks and very steady noise levels.

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As per IEC standards, SLMs of the two classes, Class 1 and Class 2, have the same functionality but different tolerances for errors. Class 1 instruments have a wider frequency range and a tighter tolerance than lower-cost, Class 2 units.

ANSI specifies SLM as three different types, namely, 0, 1 and 2. Type 0 is used in laboratories, type 1 for precision measurements in the field and type 2 for general-purpose measurements. Type 1 meter is preferred for designing cost-effective noise controls.

Frequency weighting
Frequency weighting selections allow users to choose how the meter will treat sound measurements over its specified frequency range. Certified SLMs offer noise measurements with A, C and Z frequency weighting.

Fig 1
Fig. 1: A sound level meter in use

A human ear responds more to frequencies between 500Hz and 8kHz and is less sensitive to very-low-pitch or high-pitch noises. In A weighting (expressed as dBA), the meter gives more weight to certain frequencies. In other words, sound levels at certain frequencies are boosted or cut (to match how the human ear responds to sound).

In C weighting (expressed as dBC), all frequencies are treated the same (no boosting or cutting of sound level).

Z weighting (expressed as dBZ) is a flat frequency response of 10Hz to 20kHz ±1.5dB.
Frequency weighting A is the only weighting that is mandated by the international standard; frequency weightings C and Z are optional fitments.

fig 2
Fig. 2: A, C and Z frequency weightings for sound

Why frequency is important
Consider the following example. A whistle blows at about 45dB, while a thunder is at 100dB. But, a thunder has frequency of only 50Hz, whereas the whistle has a frequency of 1000Hz or more. Decibel level is not enough to understand the quality of sound; frequency is equally important. Hence, an SLM that has better response over a large frequency range is considered more accurate.

Leq (equivalent continuous sound level). In industries, noise-measurement time-averaged value (in decibels) is used instead of sound exposure level. Leq is the average sound pressure over a period of time. Unlike numeric values, decibel values cannot be averaged directly because these are logarithmic values. Hence, these cannot be added or subtracted directly.

SLMs automatically calculate Leq during and after the measurement has finished. These sample the noise level 16 times per second and then convert dB readings back into sound pressure levels, add these up, divide by the number of samples and return Leq back in dB form. SLMs that integrate such logarithmic values by taking samples at frequent intervals over a period of time are integrating SLMs.


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