What Is Sound — A Primer?

As air conditioning season draws near, the discussion of “how noisy” is the air conditioner needs to be looked at and what the SRN (sound rating number) means.  In order to do that, we first need to understand what is sound.

Sound can be defined as any pressure variation (could be in air, water, or any other medium) that the human ear can detect. The most familiar instrument for measuring pressure variations in the air is the barometer. However, the pressure variations which occur with changing weather conditions is much too slow for the human ear to detect and, as a result, cannot be defined as sound. If weather related pressure variations were to occur more rapidly, at least 20 times faster, they could be heard and, therefore, called sound.

The number of pressure variation per second is called the frequency of the sound and it is measured in HERTZ (Hz). The frequency of a sound produces its distinctive tone. This is why the rumble of distant thunder has a low frequency while a whistle has a high frequency.  The normal range of hearing for a healthy young person [who has not subjected themselves to loud music :>) ] extends from approximately 20 Hz all the way up to 20,000 Hz while the range of a piano, from the lowest note to the highest note is approximately 27.5 Hz to 4186 Hz.

The second main quality used to describe sound is the SIZE or amplitude of the pressure variations. The smallest or weakest sound a healthy human ear can detect has an amplitude of 1/20 millionth of a Pascal    (20 uPa), some 5 BILLION times less than normal atmospheric pressure. The ear can tolerate sound pressure more than a million times higher than this weakest detectable sound.  If we tried to measure sound in pascals (Pa), we would wind up with some quite large, unmanageable number. To avoid this, another scale is used — the DECIBEL or “dB” scale.

Let’s see if this makes it simpler:


I could go into a lot more information here on decibels but this is the basic concept of sound as we need to know it.  From a practical stand point, the importance of the decibel is that it relates the magnitude of the pressure pulsation to how loud the sound seems to our hearing. [and for the trivia people, it was named for Alexander Graham Bell (bel)].

Air conditioner “noise” is receiving more and more attention. In my next post, I’ll talk about the SRN number and what it means.  Then I’ll go into how we can keep sound levels down both from what manufacturers are doing  but more so, from the INSTALLATION stand point. There are a lot of basic thing to do and avoid to help make that air conditioner sitting outside the home quiet!


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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2 Responses to What Is Sound — A Primer?

  1. roger daigle says:

    Hey Mike, a question ( diversion ). I always recommend an X-13 or ECM motor for multi-story homes. However, when ESP is greater than 0.5 WC. (which is almost always) and homeowners do NOT want to spend money on reducing ESP, which type of motor do you recommend? An X-13 is constant torque, but can it overcome high ESP? I frequently have this dilema, when TRYING to explain high ESP to homeowners their eyes roll back into their heads and $$$$ signs flash. It’s a tough sell even though an X-13 motor is less than a $100 upgrade! Most people say the same things, My heat/a/c has worked so far why are you trying to add cost? In multi-story applications I tell homeowners that newer technology and running the fan constantly will save money and create a better comfort level. Please help!!!

    • Roger
      an X-13 motor is just like any other ECM motor or constant torque motor. It can handle up to .8 IWC static. Once you start getting above that, the motor will start “hunting” to try to compensate for the higher static. So, compared to a PSC motor, the X-13 (or any ECM) motor is much better at the higher static applications

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