Sound Level Meter / Noise Level Meter

Among a sound level meter there are various models suitable for different measuring tasks. For sure, it is impossible that all the devices have the same performance and provide the same measuring results. In spite of the differences in design they all are divided into certain categories, and thus, Class 1 and Class 2. These division is based on the accuracy of the sound level meter, and of course, compliance of the sound level meter with the standards. Since the latter have not been permanent and unchangeable, the measuring devices have gone through certain changes as well. To comply with all the modern standards, the measuring devices must meet all the current requirements.
One of the most recent standards IEC 61672-1:2002 specifies those limits for the tolerances that should be preferably met by the devices of Class 1 and Class 2. For the Class 1 devices the requirements and limits for the permissible tolerances are just much stricter, which does not mean that devices of Class 2 are bad and unsuitable for measurements. In some applications, Class 2, or what the users could see before, Type 2 (which relates to the old standards) is a minimum requirement and they may be sufficient for a number of measurements. For serious measuring tasks, when it goes, for example, about safety in the workplace, environmental studies, construction of new buildings, Class 1 device is a “must-have” instrument. It has a higher accuracy performance and is considered to be more professional.

The first thought might be that, of course, the other name for the maximum sound level is peak. It is not quite correct. In case of sound level measurement, Peak or Lpeak has nothing to do with Maximum (Lmax). Depending on the frequency weighting (for example, most common A-weighting) and time constant (slow or fast), there may be LASmax and LAFmax or LASmin and LAFmin. They are all different from each other, but speaking in general, they show the maximum and minimum sound level registered by the sound level meter during certain time period.

By using these values, it is possible to create an image or graph of how the noise changes over a certain time. Peak relates more to sound pressure and it cannot be calculated using RMS. It is an instantaneous sound and often it exceeds the maximum value. The measurement of peak is not always necessary and to the point. Nevertheless, in some spheres it is a requirement, for example, for noise level measurement in the workplace.

As well as for many other occupations and activities, there is also a limit for noise exposure over a certain time period. It must not be the same every day, but it is desirable that the 100% limit of the noise dose is not exceeded. The noise exposure may vary during the day, but the daily dose, for example, is calculated then by summing up all the time periods with different noise levels. In has been internationally accepted to take an 8-hour working day as standard and to calculate the daily maximum noise exposure dose in relation to 8-hour duration. In general, the 100% value varies between 85 and 90 dBA. Especially for people exposed to noise at work a lot, it is important to adjust the working schedule in such a way that the noise exposure does not exceed the permissible level and, of course, to take all the required measures of precaution to minimize the risks. The sound level pressure may be higher than 85, then it is important to remember that the 100% noise dose will be reached much quicker than after 8 hours. It is a mistake, to take a slight increase of the sound level by only 6 dB as harmless and to keep the noise exposure duration without any changes (8 hours) Here, the 3-dB exchange rate rule applies, which means that the slightest change of the sound level pressure causes extreme tension and the time of exposure should be significantly reduced. And at the same time, if the sound level is below the 85-dBA mark, then the exposure time may be prolonged without any risks and damage to health.

Noise may really cause pressure and tension. Human ear is a perfect detector for that and when the sound gets too loud, it causes inconvenience and even pain. Taking into account the ability of the human ear, the quietest sound has been defined as 20 micro Pascal (sound pressure is measured in Pa) or p0, and it has become a reference for calculating the sound pressure level Lp (the latter is logarithmic measure and shows the intensity of the effective sound pressure related to the threshold of the human ear). Any sound creates a certain vibration, which intensity depends on how loud the sound is. This vibration “goes through” the air, of course creating a certain pressure, a pressure within the sound wave. It is different from the air pressure, of course. Each person who hears the sounds receives a certain portion of vibration coming from the sound wave, in other words, each of us is affected by the loudness of the sound. The level of the pressure which human ear undergoes, is measured in dB.

Since noise may be of different origin and come from different instruments and sources, a certain system was needed to be able to describe the character of each particular sound / noise and determine its compliance with the official regulations. To develop the methods of reducing the noise is impossible without knowing its characteristics. The frequency at which the noise sounds appear is not always the same, and to exactly determine the intensity of the noise, octave bands have been created. In fact, it is a subdivision of all the frequency range into octave bands, where the upper limit frequency value is always a double of the lowest (the third one is the middle value). A generally accepted range of the middle or centre frequencies goes from 31 Hz to 16kHz (63Hz, 125Hz, 250Hz, 500Hz, 1000Hz, 2000Hz, 4000Hz, 8000Hz). Thus, each of octave bands in the frequency range is characterized by the middle value, from which the upper and lower values may be easily calculated. Depending on the source of noise and location the limit value for the sound pressure level within the octave band of a certain frequency rangy (mostly, from 63Hz to 8000Hz) will be different and it is calculated for each particular case individually.

Sounds precepted by the human ear are very different, they occur at different frequencies, last for different time periods, have different intensity. Due to the existence of the digital measuring instruments sound level and different sound parameters may be measured easily and precisely. Since some sounds do not stay the same and may quickly change, the measuring instrument must react immediately to these changes in order to provide a sufficient level of accuracy and repeatability. Time weightings have been introduced so that to make it possible to compare the measurement of a sound level meter from different producers and of different models. In simple words, it stands for how quickly the sound level meter is able to react to any change in the noise or sounds level. The standard three-time weightings are fast, slow and impulse. Depending on the type of noise, different time weightings may be applied, though fast is the most commonly applied one. Slow – just helps to get a clearer overview of the changes (rise and fall) of the noise. Impulse makes sense only when particular sounds come unexpectedly and are very short-lasting. This weighting is much quicker than fast, especially in its rising phase, and is slow in its falling phase.

Sounds may all be very different and how they are precepted depends on a few factors. For example, human ear is capable of hearing sounds at certain frequencies much better than the at the others, but it does not mean that the sound will not exist, it just does not affect the humans. Such a topical issue, as noise exposure, has led to the development of a variety of noise measuring instruments which are capable of detecting the noise exposure accurately and very precisely. To measure the noise exposure, it is necessary to assess the amount of noise that the human hears, is affected or may be hurt by. Thus, frequency weightings allow carrying out the measurements on the best level, without involving unnecessary data. The standard modern frequency weightings are A, C and Z. the A-weighting is the most common, since it is the closest to how we, humans, perceive the sounds, starting from the lowest of 20 Hz to much higher values of 16 and even 20kHz. That is why such important measurements as LEPd or Leq (see the paragraphs below) are measured based on A-weightings. The national standards and regulations clearly define the recommended noise exposure values based on A-weighting measurements.

C-weighting is applied for sounds which occur at lower frequencies. It actually applies for peak-sound measurements. With the C-weightings those sounds are measured, which the ear may hear, but which are much rare, happen unexpectedly and last very shortly. As a rule, they are very loud and spontaneous, and though the frequency range is wide, the response is much flatter than in the case of A-weighting.

Z-weighting stands for zero frequency weighting (flat frequency response).

In order to systematize the noise and sound measurements, a few main values have been introduced and Leq is one of them. It stands for equivalent continuous sound level. Noise almost never remain on the same level over very long time periods, its intensity may fluctuate depending on activities, machinery used or time of the day. By calculating the Leq value, or LAeq, since mostly it is measured by using the A-weighting, it is possible to make judgements about the average noise exposure. It is a bit more complicated than just a traditional average calculation, since there are logarithms behind noise/sound level measurements, but nevertheless, it gives an exact information whether the noise level in a certain area or in the workplace is satisfactory or unacceptable. The slight, as it may seem to be, increase or decrease in the Leq level, for example, by standard 3 or 5 dB, either requires an immediate significant reduction of the noise exposure duration (cut the time in half), or allows prolonging it (double). For generally noisy environments, the Leq measurement is a significant value for noise situation assessment, for working process planning, for taking corresponding measures (like introduction of protective clothing and accessories, informative seminars).

Noise may be a harmful and dangerous health destructing factor and this is one of the points that every employer should care about. Noise exposure is strictly controlled on the governmental level and the working conditions as well as the devices used for the necessary measurements should meet the standards and official requirements.
In private life, noise is a controversial issue and it often depends on the personal perception. At work, noise daily exposure must not exceed the permissible limits which do not depend on the individual estimations. The amount of the total noise exposure per day is defined as LEPd. For calculating the LEPd value it takes LAeq (equivalent continuous sound level) at A frequency weighting (the latter is used in the noise measurements, since it in the best way reflects how the human ear perceives the sound/noise) and of course, the duration of the measurement.
The existing regulation take 8-hour working day as standard. The noise level meter is used for carrying out the measurements in the workplace of the workers exposed to noise continuously. The sound level may vary during the day, that is why continuous measurement is required to give an assessment of the LEPd or LEPw (weekly exposure).
The LEPd calculation is required to adjust the working conditions, for example, reduce the duration of noise exposure, provide hearing protection, make the changes on the equipment etc. In general, the louder the noise and the longer the noise exposure, the more the LEPd value grows. The average 80 – 85 dB is the limit which is recommended to focus on. Significant norm exceeds are forbidden and may have aggravating consequences.
For more accurate noise exposure measurement, it is also required to register the peaks, if such occur. LCPeak values, since they are measured with C-weightings, may influence the total noise exposure value, depending on how frequently they occur and which intensity they achieve.

The official standards require regular calibration of the sound/noise measuring equipment. Not depending on the model, the device should present reliable and accurate measurements.
Depending on the frequency of application and specifics of the measurements (private life or working environment check), the noise meters may have different wear-out. To ensure that the device delivers good repeatable results, it is recommended to carry out professional annual calibration, and pre-check before every new application. Besides, it is necessary to remember about correct storage conditions and do not leave the device in appropriate conditions, like very high temperatures or excessive humidity.