New iMacs as quiet as Jobs claims

Updated Oct 12, 1999

FROM THE RESPONSE I've received to my earlier article below, it appears there is a clear distinction between perceived loudness and sound intensity (or power), which I had assumed to mean the same thing. That being the case, Steve Jobs was absolutely right in saying that 7dB equated to a doubling of "perceived loudness" (the figure ranges anywhere between 6dB and 10dB).

The real crux appears to be that perceived loudness is the more relevant measure of sound level as it is based on the sensitivity of human hearing, although technically, sound intensity (power) does double with each increase of 3dB. Thank you all who wrote in and contributed your knowledge on this subject matter. Here are some responses I received:

Now it's been a while since my last physics class, but if you consider decibels on a natural logarithmic scale, then Jobs's assertion was correct (10 ln 2 = 6.9, or 7 to the layman). If you consider on a common logarithmic scale, then your assertion is correct. And I forget which scale is used.

John Johnson

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While you are technically correctly that a 3 dB increase (or decrease) represents a real power increase (or decrease, respectively) of power output, it does not equivocate to *perceived* sound pressure levels. Empirical testing has shown that it takes an increase of between 7 and 10 dB to represent a perceived doubling of sound levels (and conversely the same amount of decrease to create the perception of halving the sound level).

From an engineering standpoint - you are correct. From a psychoacoustic standpoint, Jobs was correct.

P.S. I took a psychoacoustic course in college, was an electronic technician for many years, and am now a software engineer. I continue my interest in psychoacoustic phenomena.

Best regards,

Greg Meece
Sr. Software Quality Eng.
Pervasive Software, Inc.
Austin, TX USA

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Jobs was correct in his statements. You have to double the sound intensity for the average human ear to perceive a slight change in volume. The most sensitive ear in the proper environment will hear a difference with a 1dB change (say a 3 sigma person, I know there are records of people able to detect a fraction of a dB but they are rare). The sound level in the average office (if memory serves) is 38 dB making the iMac far quieter than that environment. In the average house - I have no clue. I think for my own house, it is 24 dB or so when the HVAC is off and maybe 40 dB when on.

I don't remember the dB increase for a perceived doubling of sound by the human ear. It is either 6 dB or 10 dB (4 to ten times physical change), my reference books for this are at home.

If Jobs had used physical intensity differences, he would have been raked over the coals for obfuscation over how quiet the new iMac really sounds to us.

Chuck White

* * *

I read your article on iMac quietness. It's true that +3dB is an doubling of power (intensity), however the ear is a non-linear device, and Job's 7dB is more accurate in that the "perceived" sound level is doubled.

Peter Richard
Software Engineer
Lambda Technologies

* * *

While it is true that a 3 dB increase results in a doubling of intensity, it requires more of an increase to achieve a doubling of perceived volume. Many people claim it requires 10 dB to achieve a doubling of perceived volume (vice 7 dB), or, to put it another way, -10 dB for half the perceived volume.

Dan Seely

* * *

Just read your discussion about the sound level in the new iMacs, and I think you might be confusing power with perceived loudness.

In fact, a 10 dB increase is perceived as twice as loud, despite requiring ten times as much power.

A good overview of this is at:

http://www.gcaudio.com/volatgeloudness.htm

Brian Hutchison

* * *

And here is the original article:

New iMacs quieter than Jobs claims?

Oct 10, 1999

STEVE JOBS mentioned during the Oct 5 Event that it took an increase of 7dB to double the perceived loudness of any sound, and added that at about 30dB while active, the new iMac is twice as quiet as its competitors, while at 14dB during drive sleep, it is 8 times as quiet and almost below the audible level.

I am wondering if he was probably trying not to make too much noise about how quiet the new iMacs really are, because actually 3dB is all it takes to double sound intensity. So if the new iMac when active is really 7dB less than its competitors, that makes it roughly 5 times as quiet as the competition.

The difference in sound level (B) between any two sound sources could be described thus:

If the sound of the first is B times as intense as the second, then the difference in sound level is expressed as 10 log B. (Time now to take out your calculators.)

So a sound would have to be roughly 5 times as loud as another to register an increase of 7dB, ie 10 log 5 = 7dB (approx). And at 14dB during sleep, the new iMac would 40 times as quiet as when it is active (at 30dB). Do the math.

Decibel Note: The decibel scale is logarithmic, not arithmetic. This means that a doubling of sound intensity is not represented as a doubling of the decibel level. Hence, a 0dB change represents no relative difference in sound intensity, +10dB means a tenfold increase in sound level, while +20dB means a hundredfold increase. Start up the Graphing Calculator that comes with your MacOS and key in 10logx and the graphic representation of this relationship will be clear.

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