Decibel Meter

What Noise is and How It Is Measured

As there is no unanimous opinion what “a real beauty” is, there is no exact definition of what “noisy-very noisy”. It depends on the observer / listener and on some personal subjective perception. For sure, speaking about noise, there are certain reasonable limits which define normal and excessive noise levels, and they are based more not on the opinions of the people, but on the affect or damage the noise may cause to the human health. How to measure noise? In fact, to measure noise itself is quite an unfulfillable task, but how intense the sound is, how big the pressure, coming from the source of the sound and directed to the listener, is – is absolutely and easily doable, for example, with the help of the decibel meter. Human ear catches any sound vibration coming from any possible source: musical instrument, voices, autos, equipment. Sometimes the sounds are hardly heard, but sometimes unbearably loud. Decibel meter is a device capable of registering the change in vibration or pressure frequency in the air and presents the measurement result in decibels, which are units for measurement of sound strength.

For all who work with decibel meter it is important to understand that the mathematical linear rule “2+2=4” may not be applied here. Logarithmic principle does not work like we expect: 20 dB is 2 times more than 10 dB, and correspondingly the loudness is two times more intense. In fact, in this case, the loudness is 10 times higher. If the measurement value is 40 dB (which is only 4 times higher than the initial 10 dB), the loudness in fact is 1000 times higher. That is why there are strict limits as for permissible upper sound / noise levels. Human ear is not able to perceive any levels of sounds and that is the damage from very long exposure to loud noise or very loud sounds may cause serious hearing problems.

Variety of decibel meters and their functions

Depending on the purpose and area of application, different kinds of decibel meters may be selected. They may possess data logging function, and thus give a possibility to monitor the situation over a certain time period. Or they may be used for instantaneous measurements, right here-right now. The latter are simple to use devices, but absolutely sufficient for quick noise level detection. Depending on the level and complexity of the device, only highest (Max) and lowest (Min) values may be stored in the memory or all the records made at preset intervals. The meters of a higher price class have got the function of storing and transferring data to the PC for later assessment.

Basically, there are two main types or classes of the decibel meters. Since the permissible levels are all set and described in the standards and regulations, the division into classes is also carried out in compliance with the existing norms. Class 1 is more accurate and more precise and is recommended for laboratory application or studies where the smallest sound peculiarities may be detected and analyzed. Class II is a good choice for measuring noise in the workplace, stadiums, during sports competitions etc.

All sounds have different duration, some are instantaneous, some very resonant and deep, some are very monotonous, some are repeatable or changing their nature, from being very quiet to suddenly reaching their peak and then subsiding again. Decibel meter is capable of carrying out measurement at given time intervals.in addition, it takes ms for the device to detect and evaluate the sound (depending on the character of the noise, it may take from 1/10th ms to 1s).

An ear works in such a way that not all sounds are perceived in the same way. And it pretty much depends on frequencies. Thus, the widest range is 500 Hz – 5 kHz – it is when the ear is the most sensitive. The device used for measurements should provide the best results, and thus, may be adjusted by application of the frequency weightings or filters. During the measurements it is important to select the correct filter / weighting so that to get the most accurate result. “A” filter is the most widely used and it is the closest to how the human ear reacts to the noise. The range is 20 Hz – 2 kHz and is presented as dB(A).

“C” filter is desirable to apply when peak sound / very high level sounds measurements need to be performed (dB(C)).

In the past, “B” filter was used, but there is no need in it anymore. “Zero” filter occurs occasionally and stands for no frequency response.

Different types of Decibel Meter Memory for Different Applications

As mentioned above, the memory in different decibel meters is not the same and in each device it differs in the number of stored data, frequency of performed recordings and the possibility of further processing on the PC.

1-time measurements do not require sophisticated and expensive devices with various functions and filters, and this makes it easy for even inexperienced users to apply the device without auxiliary help.

For continuous measurements of noise in the streets, near the highways, airports and anywhere where the measurement are supposed to be continuous or at regular time intervals, the data logging function is desirable. The models of the device may also vary (USB stick data logger or digital data logger with the screen), depends on each particular application.

Noise in daily life and noise at work – these are two different aspects of one general issue. If noise in daily life may be omitted, avoided or at least reduced, noise exposure at work is sometimes unavoidable. Compliance with the Directives of the Health Organizations is obligatory and noise level monitoring is mandatory. For this application, raw data registration is important, and the device should possess filters for correct data evaluation. Here, the total sound energy (Leq) needs to be calculated with the purpose of correct evaluation of the situation in eth workplace for each worker. Based on the results of the measurement the corresponding changes, like reduction of exposure time, improvement of protective clothes at work, repair of equipment and similar, should be carried out. To evaluate the sound pressure level in acoustical assessment, octaves (the upper frequency is two times higher than the lowest limit) and third octave bands (the spectrum is divided into minimum 33 parts) are usually used.

Sound dose
Human ear detects sounds of different levels and it seems to be normal and natural, since all their life long people are surrounded by the sources of noise and sounds. However, even though there is a big difference between the level and dose of noise, in both cases, when the permissible limit is exceeded, very serious consequences like health damage may follow. Besides a pure medical side, undesirable sounds or sounds that are much louder than the norm, may have consequences for the infrastructure, when people leave or do not want to purchase real estate objects close to the noisy enterprises, or financial, economical negative results for the employers, when pollution of the working environment brings the performance down.

Control of the noise load is required in daily life, for example, near the construction sites, stadiums, in the residential areas near highways, but of course, one of the most important areas is the workplace. Noises may be very different, and during the working shift, the workers may be exposed to the noises of different levels, interrupting, monotonous or with a few peaks, impulse. Sound level meters applied at the enterprises for monitoring and control of the noise load in the workplace perform the auxiliary function in the calculation of the noise dose that the worker is exposed to, for example, during the day or during certain period of time. This allows for stating whether or not the safety and health requirements are met and the conditions of work comply with the ISO regulations. Continuous noise may create the same noise dose as just few short time periods with interrupting and occasional noises. Sound control belongs to the hygiene in the workplace, and is important not only for creating current healthy and comfortable working conditions, but also for evaluation of the possible consequences in the future. Calculation of the noise dose is meant to see the total noise load on the person during a certain period of time, but not just the average noise value.