Sound level meter

Sound level meters measure sound pressure level and are commonly used in noise pollution studies for the quantification of almost any noise, but especially for industrial, environmental and aircraft noise. However, the reading given by a sound level meter does not correlate well to human-perceived loudness; for this a loudness meter is needed. The current International standard for sound level meter performance is IEC 61672:2003 and this mandates the inclusion of an A-frequency-weighting filter and also describes other frequency weightings of C and Z (zero) frequency weightings. The older B and D frequency-weightings are now obsolete and are no longer described in the standard.

A-frequency-weighting

In almost all countries, the use of A-frequency-weighting is mandated to be used for the protection of workers against noise-induced deafness. The A-frequency curve was based on the historical equal-loudness contours and while arguably A-frequency-weighting is no longer the ideal frequency weighting on purely scientific grounds, it is nonetheless the legally required standard for almost all such measurements and has the huge practical advantage that old data can be compared with new measurements. It is for these reasons that A-frequency-weighting is the only weighting mandated by the international standard, the frequency weightings 'C' and 'Z' being optional fitments.

Originally, the A-frequency-weighting was only meant for quiet sounds in the region of 40 dB SPL, but is now mandated for all levels. C-frequency-weighting however is still used in the measurement of the peak value of a noise in some legislation, but B-frequency-weighting - a half way house between 'A' and 'C' has almost no practical use. D-frequency-weighting was designed for use in measuring aircraft noise, when non-bypass jets were being measured and after the demise of Concord, these are all military types. For all civil aircraft noise measurements A-frequency-weighting is used as is mandated by the ISO and ICOA standards.

Exponentially averaging sound level meter

The standard sound level meter is more correctly called an exponentially averaging sound level meter as the AC signal from the microphone is converted to DC by a root-mean-square (RMS) circuit and thus it must have a time-constant of integration; today referred to as time-weighting. Three of these time-weightings have been standardised, 'S' (1s) originally called Slow, 'F' (125 ms) originally called Fast and 'I' (35 ms) originally called Impulse. Their names were changed in the 1980's to be the same in any language. I-time-weighting is no longer in the body of the standard because it has little real correlation with the impulsive character of noise events.

The output of the RMS circuit is linear in voltage and is passed through a logarithmic circuit to give a readout linear in decibels (dB). This is 20 times the base 10 logarithm of the ratio of a given root-mean-square sound pressure to the reference sound pressure. Root-mean-square sound pressure being obtained with a standard frequency weighting and standard time weighting. The reference pressure is set by International agreement to be 20 micropascals for airborne sound. It follows that the decibel is in a sense not a unit, it is simply a dimensionless ratio—in this case the ratio of two pressures.

An exponentially integrating sound level meter, giving as it does a snapshot of the current noise level, is of limited use for hearing damage risk measurements and an integrating or integrating-averaging meter is usually mandated. An integrating meter simply integrates - or in other words 'sums' - the frequency-weighted noise to give sound exposure and the metric used is pressure squared times time, often Pa²·s, but Pa²·h is also used. However, because sound was historically described in decibels, the exposure is most often described in terms of sound exposure level (SEL), the logarithmic conversion of sound exposure into decibels.

Note: in acoustics all 'levels' are in decibels.

L_eq: Equivalent continuous sound level

For various reasons even SEL is not much used in industrial noise measurement—instead the time-averaged value is used. This is the time average sound level or as it is usually called the 'equivalent continuous sound level' with the correct symbol of LAT although commonly abbreviated as L_eq,. This is 20 times the base 10 logarithm of the ratio of a root-mean-square sound pressure during a stated time interval to the reference sound pressure and there is no time constant involved. To measure LAT an integrating-averaging meter is needed; this in concept takes the Sound Exposure and divides it by time and takes the logarithm of the result.

Short L_eq

A variation of LAT is "short L_eq" where very short L_eq values are taken in succession, say at 1/8 second intervals, each being stored in a digital memory. These data elements can either be transmitted to another unit or be recovered from the memory and re-constituted into almost any conventional metric long after the data has been acquired, using either dedicated programs or standard spreadsheets. Short L_eq has the advantage that as regulations change, old data can be re-processed to check if a new regulation is met. It also permits data to be converted from one metric to another in some cases. Today almost all fixed airport noise monitoring systems, which are in concept just complex sound level meters, use short L_eq as their metric, as a steady stream of the digital one second L_eq values can be transmitted via telephone lines or the Internet to a central display and processing unit.

Until 2003 there were separate standards for exponential and linear integrating sound level meters, (IEC 60651 and IEC 60804—both now withdrawn), but since then the combined standard IEC 61672 has described both types.