In my last post, we talked about what is sound . I included a chart showing dB and uPa comparisons so you can better understand “noise”. In this post, I’ll try to explain what the SRN (sound rating number) is in relation to air conditioner noise. In part 3 on this subject, I’ll discuss what you can do to keep that unit outside as quiet as possible.
So what is an SRN? The Air Conditioning and Refrigeration Institute (ARI) created a standard, ARI 270, to rate the sound level of air conditioners. The unit of measure they use is the “BEL”. The BEL is simply 10 times a decibel. In the ARI standard, they use the BEL to delineate between “A weighted” decibel (dBA), which is what we use in the field for measuring the sound level. The ARI Standard is used for calculating a Sound Rating Number in a laboratory environment and cannot be used in the field. So, the Sound Rating Numbers are calculated numbers based on this laboratory test to a “standard”. This then produces the SRN. The reason the Standard cannot be used in the field is because it requires a reverberant room to perform the testing. Without going into everything done in this testing, suffice it to say that the purpose of the SRN is to establish a minimum acceptance level for the purpose of comparison and contract bidding. Basically, the lower the number, the quieter the unit.
Here is a typical chart provided by a manufacturer for the SRN. Note the final number is the dBA (decibels “A” weighted”) rating for field use:
When you are using a decibel meter to measure the “sound” of an air conditioner, you are using the “A weighted” scale on your meter. The reason for using the dBA scale is the A scale is more in tune with the normal healthy ear.
Sound Level vs. Distance: sound propagation in air can be compared to ripples on a pond. the ripples spread out uniformly in all directions, decreasing in amplitude as they move further from the source. for sound in air, when the distance doubles, the amplitude drops by half, which is a drop of 6dB. Now, if you are at a position one meter from the source and move 1 meter further away from that source, the sound will drop 6 dB. If you move 4 meters it will drop by 12 dB, 8 meters by 18 dB, and so on. However, this is true only when there are no reflecting or blocking objects in the sound path. Such ideal conditions are called FIELD FREE conditions.
In our real world, we don’t have this luxury. Sound will bounce off the wall. It will reverberate if the unit is between buildings, or any other factor that can make a unit seem “noisy”.
In my next and final post on this subject, I’ll try to present what manufacturers are doing to keep the noise down on their equipment and what YOU can do to keep noise down.
(information in this post was obtained from ARI Standard, and from information presented by Don Branch, retired York UPG Service Manager, at a seminar on Sound in 1996)