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.
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.
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. 
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.
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.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.
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.
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.
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.
A calibration certificate is a document that confirms that the technical properties of test instruments comply with a recognized standard of measurement.
For the new buildings and reconstructions requiring approval, sound insulation calculations are required in the building permit procedure. For existing buildings and reconstructions not subject to approval, often only the minimum requirements apply without a separate agreement. With a suitable sound level meter, the reduction of extraneous noise achieved due to the components used can be checked according to standardised procedures and then compared with the specifications. In the sound insulation assessment of the buildings, a distinction is made between airborne sound insulation, impact sound level and installation noise of building services systems. Sound level meters can also be used to determine whether existing external noise from traffic or commercial operations exceeds the permissible maximum values.