Sound level meter

A sound level meter is a measuring instrument for the determination of sound pressure levels. They indicate level value in decibels ( dB), together with the indication of the level; particular frequency and time weightings are displayed. Sound level meters are used in applications where a quantification of noise is necessary, such as in the evaluation of aircraft noise, engine noise, environmental noise and noise at work.

Sound level meters are standardized in IEC 61672:2003 (EN 61672-1:2003-10 ). It requires that at least one A- frequency filter must be present. Other frequency filters are also described.

Sound level meters are basically comprised of a measuring microphone with omnidirectional polar pattern with a preamplifier, an evaluation unit and a display. You have to go case with a cone shape to the microphone that reduces reflection of the sound on the device. The microphone converts the sound pressure measurement into a corresponding analog electrical voltage. The preamplifier with range switch brings the tension in the working range of the detector and the display. This frequency and time weightings are applied. Modern sound level meters are equipped with a digital display, many older models have analogue displays. The display shows the level of the time-and frequency-weighted rms value as an analog pointer deflection or digital can dar. by numerical values ​​or LED columns Moreover, sound level meter with another frequency band filters ( octave or third-octave band ) and data stores to be equipped.

Term review

The term evaluation is carried out at time of fluctuating sound level for averaging the measured value. Standardized are the time reviews " fast" (F = fast, Valuation 125 ms), " slow ( S = slow, rating time 1 s ), and pulse ( I = impulse, evaluation time 35 ms). The I- term evaluation in the international standardization is no longer included as it correlates little with the actual pulse character of the sound events. additional term evaluation occurs when Taktmaximal method.

Frequency

In most countries, A- frequency weighting is used in noise measurements in the workplace. The A- frequency curve is based on the historical aurally volume; although it is not a general representation of the spectral hearing sensitivity, it has prevailed almost universally in standards and other regulations. Thus historical and current data can be compared. The frequency Reviews "B". "C" and "Z" are optional.

The A-weighting was originally intended only for soft sounds, such as in 40 -dB sound pressure level range, but it is now applied in all level ranges. The C- frequency weighting is applied to the acoustic measurement of aircraft noise. The application of B- frequency weighting is nowhere prescribed. The D- frequency rating has been designed for the measurement of aircraft noise at the time, were measured as aircraft without pressurization. Since the suspension of air traffic on the Concorde this concerns only military aircraft.

Exponentially medium sound level meter

The standard sound level meter should be more accurately referred to as exponential medium sound level meter, because the AC signal is converted from the microphone through an RMS circuit in a DC signal, so there's an integration time constant, which is now referred to as term evaluation.

The output of the effective value circuit has a linear voltage and passed through a logarithmic conversion to produce a linear response in decibels (dB).

An exponentially medium sound level meter is of limited use for hearing protection measurements because it provides only single values ​​of the current noise level. For this application, an integrating sound level meter is usually prescribed. An integrating sound level meter integrates the frequency- weighted level measurement to reproduce the sound exposure. The purpose unit used is pressure squared times time: was usually Pa2 times s, but also Pa2 times h Because sound but historically always referred to in decibels, the exposure is usually expressed as Sound Exposure Level (SEL ), the logarithmic conversion of sound exposure in decibels.

LAT or Leq: Equivalent continuous sound

The area to be entered in dB sound exposure level is rarely used in noise measurements. Instead, a time average, the continuous energy equivalent sound level is used. He is represented formally as LAT and is described in section 3.9 of IEC 61672-1. In this section, both the common and the proper abbreviations acoustic definitions according to ISO. For historical reasons, LAT is normally referred to as Leq. A time constant does not matter. To measure the LAT, an integral medium sound level meter is necessary. The sound exposure is divided by the time simplified, and the logarithm of the determined result.

Short Leq

An important variant of the entire LAT is the " short Leq ". This very short Leq values ​​are taken (intervals of 0.125 s ) individually and stored digitally. These data elements can be transmitted to another device either, or can even long after it has been collected, can be retrieved from the memory and converted to almost any conventional metric. The calculation can be carried out either by a special software and a spreadsheet. Short Leq has the advantage that older data can be recalculated for the investigation, the law should change. In some cases, allows the " short Leq " the data conversion from one metric to another. Currently, almost all permanently installed noise monitoring systems (which is actually only complex sound level meters ) " Short Leq " as their metric. A continuous flow of digital 1s " short Leq " values ​​can be transmitted over the telephone or Internet to a central display / processor unit. Short Leq is a function that is part of most commercial integrating sound level meters. It is referred to by different providers but, in contrast.

Short Leq is a very valuable method of data storage. He was originally an idea of the French Laboratoire National d' Essais (ref 1) and has become the most common way to store the real time history in professional sound level meters and display. The alternative method provides to generate the time history by placing samples of the exposed sound level are stored and displayed. Because this method has too many artifacts of the sound level meter, this data can not easily be combined to form an overall data set.

Until 2003 there were separate standards for exponential and linear integrating sound level meter: IEC 60651 and IEC 60804, which have now been withdrawn. Since then the combined standard IEC 61672 describes both types sound level meter. This remains short Leq valuable, the manufacturer must ensure that every single short Leq of IEC 61672.

Personenlärmdosimeter

A variant of the sound level meter is the noise dosimeter. This is officially called Personenlärmexposimeter and is subject to the international standard IEC 61252:2003. Because the device is worn on the body, the technical conditions are somewhat simpler than for Sound Level Meters. A Personenlärmdosimeter provides measurement data due to noise exposure (usually in Pa2h ). Older models that specify values ​​in % dose, are no longer used in most countries. The problem with a % dose is that it refers to a political situation, and thus, any scaling at a regional adaptation of the statutory 100 % value be outdated. One of the devices currently in use is a Miniaturlärmdosimeter, which is described by many manufacturers as " doseBadge " or similar. The device is so small that it resembles a Röntgendosimeter. These small devices have the advantage that they do not affect not only the sound field, but do not interfere with the wearer in his work, and because they do not require cables, also constitute a source of danger.

LCpk: peak sound pressure level

Many national laws require that the absolute peak value is measured in either 'C' or 'Z' Frequency weighting to protect workers' hearing from sudden pressure spikes. However, the peak sound pressure level should not be confused with the maximum sound pressure level. The maximum sound pressure level is only the highest measurement value that a sound level meter measures during a measurement, and this may actually be a few decibels below the peak sound pressure level. In the European Union, the maximum allowable value of the peak sound pressure level is 140 dB (C ), which corresponds to 200 Pa pressure. The maximum sound pressure level (shown at a frequency rating ' A' and an 'F ' as LAFmax. , The C-weighted peak sound pressure level is displayed as LCpk.

Classification of sound level meters

Sound level meters are divided into two classes. These were identified in earlier standards as types. The two classes differ in the requirements for the accuracy of the measurement. Class 1 devices measure a wider range of frequencies at a lower tolerance than Class 2 equipment that cost less but. The difference affects both the sound level meter and the associated calibrator. Most national standards allow as a minimum, a sound level meter Class 2, and in many measurements, there is no practical advantage in using a device class 1 sound level meter class 1 usefully be in the areas of research and law enforcement enforcement (eg vehicle registration ) are used. New in the standard IEC 61672 is a linear minimum range of 60 dB and a Z frequency weighting. The tolerances were limited and described the inclusion of permissible measurement uncertainty in the measurement section of the standard. Thus, the probability is low that a sound level meter, which was built by the older standards 60651 and 60804, the requirements of IEC 61672:2003 met.

Type approval

The IEC 61672 Part 2 describes the type approval for the classification of sound level meters. The manufacturer shall be checked for compliance with the class the devices in a national laboratory. Existence of the devices this test, a type approval is granted. In Europe, the most famous institution is the Physikalisch-Technische Bundesanstalt ( PTB) in Germany. Can not be produced by a manufacturer that at least one product has received from his house, a type approval, the quality of the products has to be questioned. However, the associated costs are so high that a manufacturer will hardly be able to be assessed on its entire range. In the market there are many cheap devices below 200 €, who call themselves sound level meter, but until now no one can prove these devices by a type approval, that they actually meet the standard.

Sound level meter must be calibrated regularly to ensure a nearly constant sensitivity. The PTB also performs type approvals of calibrators by according to IEC 60942:2003. In April 2008, the first commercial devices have been approved in both class 1 and class 2 with the approval number PTB 1.61.4028829. These calibrators consist of a computer-controlled generator with additional sensors, to compensate for the humidity, temperature, battery voltage, and static pressure. The generator output is fed to a transducer in a Hallbzoll cavity resonator, in which the microphone of the sound level meter is used. The generated acoustic level is 94 dB ( 1 Pa) at a frequency of 1 kHz, with all frequency weightings have the same sensitivity.

International standards

The following international standards define sound level meter, Personenschallexpositionsdosimeter and their accessories. The national standards of most countries ( except the U.S. ) take this as far as possible. The national expenditure of content standards are almost the same with the European, if, after the national symbol in addition the abbreviation EN is performed in the title.

• IEC 61672: 2003 " Electroacoustics - Sound level meters " (English and French)
• IEC 61252: 1993 " Electroacoustics - specifications for personal sound exposure meters" (English and French)
• IEC 60942: 2003 " Electroacoustics - Sound calibrators " (English and French)

These international standards were of the IEC technical committee 29: Electroacoustics developed in collaboration with the International Organization of Legal Metrology (OIML ).