Sound level meter
A sound level meter is used for acoustic measurements. It is commonly a hand-held instrument with a microphone. The best type of microphone for sound level meters is the condenser microphone, which combines precision with stability and reliability. The diaphragm of the microphone responds to changes in air pressure caused by sound waves. That is why the instrument is sometimes referred to as a sound pressure level meter. This movement of the diaphragm, i.e. the sound pressure, is converted into an electrical signal. While describing sound in terms of sound pressure, a logarithmic conversion is usually applied and the sound pressure level is stated instead, in decibels, with 0 dB SPL equal to 20 micropascals.
A microphone is distinguishable by the voltage value produced when a known, constant root mean square sound pressure is applied. This is known as microphone sensitivity. The instrument needs to know the sensitivity of the particular microphone being used. Using this information, the instrument is able to accurately convert the electrical signal back to sound pressure, and display the resulting sound pressure level.
Sound level meters are commonly used in noise pollution studies for the quantification of different kinds of noise, especially for industrial, environmental, mining and aircraft noise. The current international standard that specifies sound level meter functionality and performances is the IEC 61672-1:2013. However, the reading from a sound level meter does not correlate well to human-perceived loudness, which is better measured by a loudness meter. Specific loudness is a compressive nonlinearity and varies at certain levels and at certain frequencies. These metrics can also be calculated in a number of different ways.
The world's first hand-held and transistorized sound level meter was released in 1960 and developed by the Danish company Brüel & Kjær. In 1969, a group of University researchers from California founded Pulsar Instruments Inc. which became the first company to display sound exposure times on the scale of a sound level meter, as well as the sound level. This was to comply with the 1969 Walsh-Healey Act, which demanded that the noise in US workplaces should be controlled. In 1980, Britain's Cirrus Research introduced the world's first handheld sound level meter to provide integrated Leq and sound exposure level measurements.
Classification
Types
The IEC 61672-1 specifies "three kinds of sound measuring instruments". They are the "conventional" sound level meter, the integrating-averaging sound level meter, and the integrating sound level meter.The standard sound level meter can be called an exponentially averaging sound level meter as the AC signal from the microphone is converted to DC by a root-mean-square circuit and thus it must have a time constant of integration; today referred to as the time-weighting. Three of these time-weightings have been internationally standardized, 'S' originally called Slow, 'F' originally called Fast, and 'I' originally called Impulse. Their names were changed in the 1980s 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. This is 20 times the base 10 logarithm of the ratio of 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 the 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 averaging sound level meter, which gives a snapshot of the current noise level, is of limited use for hearing damage risk measurements; 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 the unit of sound was historically described in decibels, the exposure is most often described in terms of sound exposure level, the logarithmic conversion of sound exposure into decibels.
Personal noise dosimeter
A common variant of the sound level meter is a noise dosemeter. However, this is now formally known as a personal sound exposure meter and has its own international standard IEC 61252:1993.A noise dosimeter or noise dosemeter is a specialized sound level meter intended specifically to measure the noise exposure of a person integrated over a period of time; usually to comply with Health and Safety regulations such as the Occupational Safety and Health 29 CFR 1910.95 Occupational Noise Exposure Standard or EU Directive 2003–10/EC.
This is normally intended to be a body-worn instrument and thus has a relaxed technical requirement, as a body-worn instrument—because of the presence of the body—has a poorer overall acoustic performance. A PSEM gives a read-out based on sound exposure, usually Pa²·h, and the older 'classic' dosimeters giving the metric of 'percentage dose' are no longer used in most countries. The problem with "%dose" is that it relates to the political situation and thus any device can become obsolete if the "100%" value is changed by local laws.
Traditionally, noise dosemeters were relatively large devices with a microphone mounted near the ear and having a cable going to the instrument body, itself usually belt worn. These devices had several issues, mainly the reliability of the cable and the disturbance to the user's normal work mode, caused by the presence of the cable. In 1997 following a UK research grant an EU patent was issued for the first of a range of devices that were so small that they resembled a radiation badge and no cable was needed as the whole unit could be fitted near the ear. UK designer and manufacturer, , introduced the , which was the world's first truly wireless noise dosimeter. Today these devices measure not only simple noise dose, but some even have four separate dosemeters, each with many of the functions of a full-sized sound level meter, including in the latest models full octave band analysis.
Classes
IEC standards divide sound level meters into two "classes". Sound level meters of the two classes have the same functionality, but different tolerances for error. Class 1 instruments have a wider frequency range and a tighter tolerance than a lower cost Class 2 unit. This applies to both the sound level meter itself as well as the associated calibrator. Most national standards permit the use of "at least a Class 2 instrument". For many measurements, it is not necessary to use a Class 1 unit; these are best employed for research and law enforcement.Similarly, the American National Standards Institute specifies sound level meters as three different Types 0, 1 and 2. These are described, as follows, in the Occupational Safety and Health OSHA Technical Manual TED01-00-015, Chapter 5, OSHA Noise and Hearing Conservation, Appendix III:A, "These ANSI standards set performance and accuracy tolerances according to three levels of precision: Types 0, 1, and 2. Type 0 is used in laboratories, Type 1 is used for precision measurements in the field, and Type 2 is used for general-purpose measurements. For compliance purposes, readings with an ANSI Type 2 sound level meter and dosimeter are considered to have an accuracy of ±2 dBA, while a Type 1 instrument has an accuracy of ±1 dBA. A Type 2 meter is the minimum requirement by OSHA for noise measurements and is usually sufficient for general-purpose noise surveys. The Type 1 meter is preferred for the design of cost-effective noise controls. For unusual measurement situations, refer to the manufacturer's instructions and appropriate ANSI standards for guidance in interpreting instrument accuracy."
Measurements
Labels used to describe sound and noise level values are defined in the IEC Standard 61672-1:2013 For labels, the first letter is always an L. This stands for Level, as in the sound pressure level measured through a microphone or the electronic signal level measured at the output from an audio component, such as a mixing desk. Measurement results depend on the frequency weighting, and time weighting applied.Frequency weighting
The second letter indicates the frequency weighting. "Pattern approved" sound level meters typically offer noise measurements with A, C and Z frequency weighting.Z-weighting represents the sound pressure equally at all frequencies. A-weighting, weights lower and higher frequencies much less, and has a slight boost in the mid-range, representing the sensitivity of normal human hearing at low levels. C-Weighting, more sensitive to the lower frequencies, represents what humans hear when the sound is loud.
The IEC 61672-1:2013 mandates the inclusion of an A-weighting filter in all sound level meters, and also describes C and Z frequency weightings. The older B and D frequency weightings are now obsolete and are no longer described in the standard.
In almost all countries, the use of A-weighting is mandated to be used for the protection of workers against noise-induced hearing loss. The A-weighting curve was based on the historical equal-loudness contours and while arguably A-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-weighting is the only weighting mandated by the international standard, the frequency weightings 'C' and 'Z' being options.
Originally, the A-weighting was only meant for quiet sounds in the region of 40 dB sound pressure level, but is now mandated for all levels. C-weighting is however still used in the measurement of the peak value of a noise in some legislation, but B-weighting – a halfway house between 'A' and 'C' has almost no practical use. D-weighting was designed for use in measuring aircraft noise when non-bypass jets were being measured; after the demise of Concord, these are all military types. For all civil aircraft noise measurements, A-weighting is used, as is mandated by the ISO and ICAO standards.