Noise Meter / Sound Meter

General:

Noise is defined as a dangerous, annoying or disturbing sound. Thus, noise is a subjective sensation of the sounds affecting us. So, the noise as such is not directly measurable, even with a sound meter. Through the vibrations of a sound source e.g. voices, loudspeaker membranes, cars, railways, aircrafts, industrial equipment, the vibrations (pressure fluctuations) in the air are generated, which our ears perceive as sound. The frequency of the pressure fluctuations is perceived at the low frequency as low tones and at fast frequency as high ones. When the strength of the pressure fluctuations is increasing, we take it as the sound getting louder. These pressure fluctuations are recorded by the sound meter and displayed on the sound meter in a measured value in dB (decibels). The sound meter’s decibel range from 0 ... 130 is logarithmic. O dB describes the lower auditory threshold and 130 dB corresponds approximately to the human pain threshold. On the basis of the logarithmic structure, the measured values cannot be simply added, subtracted or averaged. If we have two sound sources with 50 dB each, the total volume increases only to 53 dB. A doubling of the volume always increases the total value by only 3 dB. If we move away from a sound source, we get a decrease of the volume by 6 dB when the distance is doubled.

Sound meters / noise meters are offered with different accuracy classes, different time evaluations and there are noise meters / sound meters with different possibilities of the frequency evaluation (filter). As for additional options, noise meters / sound meters with and without storage are available.

As for accuracy, the sound meter / noise meter is divided into two classes. A class 1 noise meter / sound meter has a broad frequency spectrum and a very small deviation. For class 2 sound meter / noise meter devices, the deviation over the complete frequency spectrum may be slightly higher. Since the noise meter / sound meter accuracies to be observed at different frequencies may differ, the sound meter / noise meter accuracy cannot be specified with a fixed deviation number. A sound meter / noise meter of class 1 is mainly used for measurements in laboratories or in open-field measurements. Almost all other measurements can be carried out with the class 2 sound meters / noise meters.

The time evaluation of the sound meters comprises three ranges, based on which the respective average value is formed and displayed. Fast evaluation duration 125 ms application for constantly changing noises (traffic noises, neighbourhood noise, industrial noise). Slow evaluation duration 1 s application for constant noise (monotonous hum). Pulse evaluation duration 35 ms application for boom noises (shotguns, hammer hits).

For the frequency evaluation, several options are available for most sound meters / noise meters.

Frequency evaluation A: measured value dBA. Since a human ear does not perceive all frequencies with the same volume, this rating is adapted to the human ear. Sounds with a frequency between 1000 and 4000 Hz are perceived by people louder than the sounds which are below or above this range. Most measurements are performed with this rating (filter). Frequency rating C: measured value dBC. This rating is applied at high sound pressure. DBC is an almost linear frequency evaluation.

The memory function can often be presented differently in the sound meter / noise meter. Very simple sound meter / noise meter memories store only the Min and Max values of the time period measured with the noise meter. On the next memory level, the values displayed on the noise meter, with the set evaluation, are stored in preset time intervals (sound level logger). A further storage option stores the raw data measured with the noise meter which, with the help of the noise meter and PC software for noise meters, can later be displayed with various frequency evaluations. Logging of the values is also possible in the sound meter devices. Since this storage with subsequent evaluation also requires a complex noise meter device or PC software, these memories can be found only in higher-priced sound meter devices.

Application of a sound meter

Simple sporadic measurement in e.g. school or private area can be carried out with a sound meter without or with Min / Max memory function. These sound meters are easy to operate and do not require any special knowledge.

For measurements in areas such as neighbourhood or traffic noise, private users can apply sound level loggers. These are noise meters that log the sound level with the set rating. For this sector, also sound level loggers without display in the form of USB sticks are available. In the noise meter devices with display, the current sound level can also be viewed during recording.

For sound level measurements in the field of occupational medicine, health and safety, class 2 sound meters can be used. These noise meter / sound meter devices should record the raw data as described above in order to evaluate the data with different ratings (filters) even after the measurement with the noise meter. Furthermore, these noise meters can calculate time-weighted average values (Leq) with consideration of the specific logarithm. The LEQ value is used throughout Europe as a measure for occupational health and safety regulations in the field of noise. For this measurement, special sound dosimeters can also be used, which, as well as the microphone, can be carried by the operator on the shoulder. To determine effective noise protection measures, sound meters that divide the frequency range into octave bands (11 sub-ranges) or thirds (33 sub-ranges), should be used. This presents the measured volume per frequency range.

Sound meters of class 1 are used for sound level measurements in the field of environment, laboratory and expert activity as well as in official measurements. These noise meter / sound meter devices also indicate the storage possibility as described in the field of occupational health and safety. A subdivision of the frequency spectrum in octaves and third bands is also available in class 1 sound meters.

Calibration of a sound meter:

To calibrate a sound meter, handy calibrators are available for class 2 and class 1. For calibration, the microphone of the sound meter is inserted into a measuring chamber, which is provided with an opening suitable for the size of the microphone of the sound meter. Then, in case of the calibrators of class 2, a sound pressure of 94 dB 1000 Hz is generated in the measuring chamber of the sound meter calibrator.

The sound meter can then be calibrated based on this value. With class 1 calibrators, the sound meters can be calibrated on two volume levels of 94 dB and 114 dB. Very often, sound calibrators are offered with a sound meter in set.

A laboratory calibration as ISO or DAkkS calibration is possible for almost all noise meters.

Since the dawn of measurement, there has been a need to verify accuracy or calibrate. Calibration is the control of measurement magnitudes correction for a measuring instrument without intervention in the measurement system, or the determination of the systematic display deviation in relation to the real value of the measuring magnitude. A calibration certificate is a document that confirms that the technical properties of test instruments like a sound meter comply with a recognized standard of measurement.

The introduction of International Standards Organization (ISO) 9000 brought about new requirements for quality control. As a result, many companies and laboratories must obtain ISO calibration certificates for their sound meters, noise meters and other measuring instruments. An ISO calibration certificate is a controlled document that confirms that the specific noise meters, sound meters or other test instruments meet the minimum requirements of the appropriate ISO standard. An ISO calibration certificate can be acquired for most sound meters / noise meters offered by PCE Instruments.

A sound meter used for highly accurate measurements, such as reference standards, is referred to as a class I sound meter. Class II sound meters are primarily used for testing and inspection to ensure compliance with workplace safety regulations.

As for recalibration of a noise meter / sound meter, as a rule, only the results of measurements over time may show when it is appropriate to recalibrate a sound meter. Very important points to consider also are how and how often the noise meter is used. If a sound meter has been used frequently during a short time interval, it may make sense to recalibrate the noise meter / sound meter to make sure that all the functions of the noise meter are correct and the sound meter’s accuracy corresponds. Depending on the environment the sound meter is used in, the calibration time intervals for the sound meter may also be changed. Laboratory conditions can differ greatly from conditions in the field. Humidity, temperature and contaminants can have a negative influence on the device and recalibration of the noise meter may be desirable.

In reality, many modern sound meters and other devices remain in peak calibration condition. In the past, all instruments like noise meters or sound meters required calibration. Nowadays, instruments like sound meters are very precise and stable. Of course, in the process of operation of the noise meter, damage is possible, especially with fragile and vulnerable components like microphones. In this case, calibration of the noise meter is applied to restore or verify the accuracy of a sound meter.

Class I sound meter calibration is the most accurate, with a very small uncertainty (or tolerance). Class II allows a bigger deviation. Depending on the sound meter class, different kinds of calibration can be applied.

For example, for sound:

- Class I should give +/-0.3 dB tolerance
- Class II should give +/- 0.5 dB tolerance

The period of time between noise meter calibrations is called a calibration interval. It is the responsibility of each sound meter user to determine and check the interval between sound meter calibrations. PCE Instruments' friendly and knowledgeable technical experts are available to answer any questions related to the process of establishing a calibration interval fore noise meters / sound meters.

PCE Instruments is a trusted manufacturer and supplier of accurate, reliable sound meter products. Most noise meter test instruments can be calibrated and adjusted according to the highest possible quality standards by PCE Instruments itself, or by a first-class calibration laboratory partner.

Nowadays it is near to impossible to find a place in a town or city where there would be an absolute and total silence. Too much traffic, too many people, factories, plants, railway stations, airports and so on – from the one side, life is unimaginable without all these things, from the other – life may turn into torture if they get out of control and do not meet any requirements. Outdoor noise or sound is one of the stress causing factors and that is why the permissible level of sound emission is under permanent control by a noise meter / sound meter. Any equipment used outside (vehicles, devices, machinery) should meet the requirements of the EU directive concerning noise and outdoor equipment. Especially when it goes about living districts, there are particular time periods when work by means of drills and other noisy equipment can be carried out. Even though these sounds are transient and do not last permanently, their noise level is extremely high for a normal unprotected human ear and that is why this work is strictly regulated and must measured by a noise meter / sound meter. Problems with noise level are also experienced by those who live in the proximity to railway stations, airports, noisy streets or even hospitals and police stations. Excessive and long-term exposure to high-level noise may have dramatic negative impacts on human health (hearing damage, raised nervousness and irritability, unstable psychological condition), and in some areas noise monitoring and control by use of a noise meter / sound meter is an essential task! A noise level of 120 … 150 dB /decibel (airport noise, planes taking off and landing, road drills) is absolutely dangerous to human beings and that is why continuous presence or living very close to the sources of such kinds of noise may have consequences. The perfect level of 30 … 50 decibels, which is quite a favourable atmosphere with the sounds of a more quiet and pleasant character, is sometimes not reachable anymore.

To understand how bad the noise really is just by description of your physical perception is impossible - You have to use a noise meter / sound meter. Different people perceive noise in different ways, and what is perfect for some people, is unbearable for others. Therefore, the limits of noise exposure have been set based not on individual opinions, but on scientific conclusions concerning healthcare. It applies to all people who are exposed to noise, not depending on their personal preferences: workers operating drilling equipment and similar should wear protective clothes, airports should mostly be built outside the living areas, the sirens should be adjusted to an acceptable level and many other similar points.

The possibility of mounting a noise meter / sound meter on a tripod makes the measuring process easy and convenient and there is no necessity to hold the noise meter / sound meter. The readings collected after a certain period of time may be processed and an absolutely clear picture of the noise level in a particular area / region is received.

It is necessary to remember that there are noise limits, for both, its level, as well as for its duration. The measurements received with the help of the noise meter / sound meter show discrepancy between the desirable and real values and thus, any problem connected with the noise emission can be solved in a correct way, so that all the requirements were met and the working process (construction, airport, vehicles with sirens) was not stopped anyways.