Noise is an extremely important topic that many people struggle with. It's no wonder, as there is a certain apparent "mess" in acoustic parameters, which many manufacturers use to define the acoustic parameters of their product. With the help of the KOA module of the IX CHART program, I will try to explain in the simplest possible way the issues related to acoustics. For some, this will be a reminder, but I am sure that a significant number of people will discover America again :) I would also like to add that equipment manufacturers are aware of certain shortcomings of designers in this area and use it with premeditation to "hide" the true nature of their product.
In acoustics, we deal with 2 basic concepts:
Sound power level expressed as Lw [dB] and
Sound pressure level Lp [dB].
Since our "measurement instruments" given to us by nature, called ears, are characterized by a certain "deafness" to certain frequencies and are more sensitive to other frequencies, a characteristic "A" was introduced, and then the sound power and pressure levels are expressed as Lw(A) and Lp(A). But wait a minute - we are talking about two different physical quantities, i.e., acoustic power and acoustic pressure, and both quantities are expressed in decibels [dB], so what is the point? The matter is quite simple - our ears recognize an increase in volume only when the acoustic pressure or power increases tenfold compared to the previous value, so the resolution of our ears is truly impressive.
The lowest value of acoustic pressure that the human ear is able to detect is p0=210^-5 Pa, which is equivalent to 0 dB. Therefore, 1 dB corresponds to 210^-4 Pa, while 100 dB is the noise level with a pressure value that may surprise many: p1=200000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 Pa, which is 2*10^95. It is clear that it was very inconvenient to operate with pressure or power values as absolute values with such long numbers, and someone smart came up with the idea to use the mathematical properties of the decimal logarithm, which elegantly shortens the notation to the ones we know. However, not everyone liked logarithms because their algebra is not identical to the algebra of basic numbers, but it is enough to learn their basic mathematical relationships. Hence the term "sound pressure level" and "sound power level," which refer to the logarithms of these numbers and are expressed in [dB], but as a principle, acoustic pressure is expressed in Pa, and power, like any power in watts.
Now, to better understand the topic, I propose to think about acoustics in analogy to our industrial heating devices. Let the sound power be the equivalent of the boiler power, and the acoustic pressure be the temperature. Looking at a boiler that has a specified power, we are not able to estimate the temperature it can achieve in a room because it depends on many factors such as heat transfer coefficients, ventilation type, but we can calculate it and vice versa - being in a room and experiencing a certain temperature, we are not able to tell what power the boiler maintains to keep this temperature. It is analogous with noise - the sound pressure level is what our ears hear, but what we hear depends not only on the power of the sound source but also on:
the acoustic absorption of the room,
the room's volume,
the direction from which the sound comes,
and of course, the distance from the sound source (the further from the focus, the colder it is).
The CONCLUSION is, therefore, one - the only correct value to pay attention to when comparing different devices from an acoustic point of view is the SOUND POWER LEVEL! If the manufacturer gives the sound pressure level, it only means that he assumed some acoustic absorption of the room, some directional coefficient, and some distance at which the noise is measured. The question is only whether these "some" parameters describe our room, because if not, the noise level will be different!
To illustrate this better, here is an example (without the names of specific manufacturers). I took an air conditioner as an example, but it could be any noise-emitting device. The analysis will be performed, of course, in the IX CHART program in the KOA module. We have everything we need for such calculations. In addition, I assume that the room has dimensions of 5x5x3 and a fixed workstation is located 3m from the air conditioner. The room is finished in an industrial climate, without curtains, only with a small amount of furniture.
Here, the producer provides the noise level as follows (for a 3.5 kW air conditioner):
At a distance of 1 meter from the unit, the maximum sound pressure level is 46 dB(A). The problem is that we don't know what parameters were used to determine this noise level, and in fact, such data should not be considered reliable.
Another producer provides the following information:
Here, we have the sound power level (great) and sound pressure level, but the producer did not indicate at what distance the noise was measured, etc. Therefore, the only measurable parameter is the sound power level.
Let's put this into the IX CHART and see how it will be in our case. It will be difficult to deduce anything for the first example because there is no sound power level. But for the second example, we can do it.
So, below are the results for the second case:
And what did it turn out? The producer declared that the highest noise pressure level is 39 dB(A), but in our case, it turned out to be 50.4 dB(A) - that's a huge difference, right!
Out of curiosity, let's iteratively check for which room conditions it would be possible to achieve a value of 39 dB(A).
I was able to achieve such a result only for a much larger room with dimensions of 15x15x4m and for an absorption coefficient of 0.4, which characterizes rooms that are very well soundproofed.
And in the catalog, it looked so good :) That's why it's extremely important to approach acoustic issues consciously, and the best tool for this is the IX-CHART ;).
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