Sound Level Meter - Noise Level Meter

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.

It may be quite difficult to fairly evaluate the level of noise in some area, district, at the factory, inside the workshop, at different times of the day etc., because it is rarely continuous and stable. The level of noise may change not only within a day, but within hours and even minutes: it may be very low (almost silence) and then it may be just occasionally interrupted by loud voices, moving vehicles, like motorbikes or cars, or the level of noise may vary depending on the daytime and business of the street, or activity of the workers using noisy equipment, like drilling machines, trucks, jackhammers …

In order to find out the average value, particularly there where the noise emission varies a lot, the Leq measurement is applied. Due to the fact that the device is able to register the measurements, in the best versions, 16 times per second, in the long run, the Leq measurement turns out to be quite accurate and precise and allows evaluating the noise emission over a certain period of time. Such calculations may be very helpful for employers which engage people for carrying out challenging tasks in a noisy environment.

Each country has its own numbers as for “healthy” norm, and this helps to regulate working process and keep the environment under control (regarding noise). If the value meets the requirements stated in the regulations, it relates normally to 8-hour working day. If the value is below the “normal level”, the number of working hours may be extended, and correspondingly, if it exceeds it – the number of hours of exposure should be reduced.

When it goes about sound / noise level measurement, frequency measurement is mentioned. There are mainly three types of it: A, C and Z. In general, the A Frequency Weighting is applied when it goes about assessment of any possible damage the noise may cause to the ear / hearing, and how the ear react to that noise. Then the values may look like dBA. Evaluation comprising A weightings shows how the ear of a person reacts to the noise usually at low levels.

Since, we can never speak about continuous sound level, and the letter fluctuates tremendously, for carrying our accurate measurements, sound level is measures at all the stages the sound may reach. Thus, C weighting is taken when the Peak Sound level is measured (LCeq / LCPeak). C-weightings concern the reaction of the ear particularly to high noise level.

Another weighting applied nowadays is Z (stands for “0”) which is only “no frequency” weighting (LZeq). It is used sometimes as an expanded C, since the latter has a very small difference between its limit values, since it’s peak measurement. Z / “flat” weighting allows getting a better description of the peak sound and it has a linear character, if depicted on the graph.

So, in fact, the main point of the frequency weightings to find out the reaction of the hearing system of a human being to a particular sound (particular sound level).

The standard IEC 61672 (IEC 61672-1:2013, IEC 61672-2:2013, IEC 61672-3:2013 and etc.) covers a wide range of norms and regulations regarding sound level measurements. In its three parts, there is a precise specification of required tests that should be carried out to ensure the conformity of the sound level meters to the existing rules, so that not depending on the specifications and class of each particular device, to exclude generation of false results.

To understand what the sound level actually is, it should be made clear first that when the sound appears, it creates a certain pressure, which sometimes lasts only for a very short time and is never permanent/stable. The sound levels change so quickly that it is impossible to register and display all of them in real time. Time weightings have been created to make the process of monitoring and reading the changes happening with the sound level changes, possible and “smooth”.

Nowadays, there are mostly two types used: Fast (F) and Slow (S). Impulse (I) time weighing used in the past, had its place to be, but since the difference between the real-time sound level change and the Impulse measurement was very big (the decay after the real-time change was much longer that the reaction to the sound) and it was very asymmetric, the two, F and S, have been selected as preferable.

The difference between F and S is in the “reaction” to the sound. The response of the Fast is much quicker than the reaction of S to an unexpected sound. Differently they react also when the sound disappears. In the case of F, the level falls down much quicker and a much higher rate (dB/s) than in the case of S.

It has been already mentioned that the instrument should meet strict requirements and norms, not depending on their design, technical specifications, frequencies they are measuring at etc. The point is that the norms also change with the time and though the basic requirements remain the same, the new sets of rules replace the old ones. Sound Level Meter, as any other device, is presented by a big number of different models, which differ in their “possibilities” and correspondingly, in price. In the past, 4 types of SLM were differentiated. Type 0 was considered to be the most accurate and met the strictest requirements. Type 4 could be applied there where extreme accuracy was not the main priority and the measurements could be carried out over and over again.

According to the modern regulations (part of IEC 61672-1), Types classification is not valid anymore and instead there are two Classes instead of four types. Class I and Class II sound level meters, in spite of pretty much similar structure and composition, differ from each other in tolerances. No matter, which frequencies it goes about, the tolerances for Class I devices are always much tighter than for Class II. At higher frequencies, the tolerance for the Class I devices gets wider than for the same device at the lowest frequencies, but at each frequency level, the difference between Class I and Class II is evident. The choice of the device should be determined by the application task, and if, for example, it goes about workers health and safety of people, it absolutely makes sense to select devices meeting the Class I requirements and compliant with the latest norms of the International Electrotechnical Commission.

When making assessment about the noise in a certain environment, there may be different parameters, like sound pressure, intensity of the sound, level of the intensity or frequency. Not all sounds can be heard by the human ear, but it does not mean that the unheard sounds have no influence on the person. The frequency between 20 – 20.000 Hz is the range that can be perceived, as against to the ranges below 20 Hz and above 20.000 Hz (infra- and ultrasound correspondingly)
Not only how intense the noise is determines the level of its harmfulness, but also the frequency of the oscillation of the sound pressure per a unit of time. Noise in the workplace is often characterized by the range of sound waves of different frequencies (divided into octave bands). To correctly arrange, for example, the working process, without extreme exposure to noise, it is necessary to know all the noise characteristics, in particular spectral analysis. The whole spectrum for convenience is divided into the octave bands, the noise pressure in which is measured by the noise analyzer. One octave band is when the upper limit value (f2) is a double of the lower (f1). Depending on where and what type of noise is to be analyzed, the octave band is sometimes split into the bands of ½ or 1/3 octave. The latter (f2 / f1 =2 1/3) is more appropriate for noise analysis at the enterprise or where it often goes about technical noises (equipment, working machines). As a rule, the octave and 1/3 octave bands are determined by the geometrical mean value ((16, 31.5) 63, 125 … 8000 HZ). The sound pressure in the octave bands with these geometrical mean values is one of the parameters of the continuous noise that must be carefully controlled with the help of the sound measuring equipment. For each particular case (hospital, school, hotel, restaurant, factory), the standard sound pressure value is different and it should meet the requirements of the health organization.

Impact sound is a special form of structure-borne sound that is generated when people walk on floors. The lower the impact sound level of a construction, the less footstep noise enters into the adjacent rooms. The impact sound level is measured according to DIN EN ISO 140-7. In the transmitting room, the separating ceiling to be measured is stimulated with a standard hammer mechanism. The sound level meter in the receiving room records the incoming signal. In the case of hard floor coverings, the noise level is also recorded in the transmitting room and an airborne sound measurement is carried out. Building services equipment, such as lifts, heating and water installations and electrically operated shutters, also generate structure-borne noise that is radiated into the room via the room surfaces. These noises are recorded with the sound level meter according to the specifications of DIN EN ISO 10052. If the noise is too high due to the impact sound or building services equipment, improvements can be achieved through structural measures for decoupling.

The TA Lärm (TI Noise) (Technical Instructions on Noise Protection), as an administrative regulation to the Federal Immission Control Act, regulates noise immissions that may pose dangers, significant disadvantages or significant nuisances to the general public or the neighbourhood. The TI Noise specifies immission guide values for immission locations outside and inside buildings. The requirements for protection against external noise increase with the need for protection of the neighbourhood. For sports and leisure facilities, separate criteria are determined in the Sports Facilities Noise Protection Ordinance (18th BImSchV) and the Recreational Noise Guideline. Traffic noise on roads, railways and waterways is measured according to DIN 45642 and aircraft noise according to DIN 45643. Construction noise is to be recorded according to the AVV Baulärm (Construction Noise).

The sound level meter can be used to check whether noise from one's own installations has an impermissible impact on the neighbourhood or whether the noise from the surroundings exceeds the permissible limits. Even if values or assumptions from the sound immission prognosis are doubted, recording one's own comparative measurements can be useful before commissioning an expert. As an operator of facilities or as an organiser of concerts or festivals, one is responsible for ensuring that the noise level does not exceed the guideline values. If the sound level meters with analogue output or switching output are used, warnings can be triggered when preset limit values are reached, or noisy installations can be throttled or switched off.

The sound effect of noises is strongly dependent on room geometry and room furnishings. They have a significant influence on the reverberation time. In addition to the suitable sound level meter, a signal generator is needed for the test signals. The sound level meter records how long the test signal reverberates. The measurement procedure is described in DIN EN ISO 3382. DIN 18041 specifies the values for the reverberation times of rooms depending on their size. Group A includes rooms in which speech must be clearly audible over long distances, such as conference or training rooms, while group B includes rooms in which speech must only be clearly understood over short distances.