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<< <i>By the way, 20dBa is considered to be inaudibe >>

Not really. Zero on the dBA scale is considered silent or completely inaudible. Anything higher than is a noise that can be heard by human beings—as long as you get close enough 20 dBA fans like the Panaflo, NMB, Papst, etc are plainly audible to just about anyone from a meter away.

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dBa

Let’s get all the references to dBa straight for everyone.

The problem of how loud a sound is, or is perceived to be, or how annoying it is, can be discussed from the points of view of physics, or of physiology, or of psychology. I am going to take each of these in turn. Of these, believe it or not, it is physics that is easiest!

In physics it is possible to specify and measure a sound level with great precision, and the decibel scale (dB) has its basis in good physics. Sound is a form of energy, and energy is expressed in a very precise unit called the joule. The sound intensity arriving at your dwelling can be very precisely measured in terms of the rate of arrival of energy across unit area, and is expressed in joules per second per square metre. There is nothing at all subjective about it. Nor does it depend on the frequency of the sound wave.

The decibel scale is used to express the ratio of a particular sound intensity to some standard, usually taken to be an intensity of 1.E-12 joules per second per square metre. (Sorry for the technicality there, but physics demands precise terms.) The scale is such that if one sound has TEN times (NOT twice!) the intensity of another, the difference in sound levels is said ten decibels, or 10 dB. Those familiar with logarithms will understand (and those unfamiliar will be mystified!) that a factor of two in sound intensity corresponds to a difference of THREE (NOT ten!) decibels.

In summary: 53 dB is twice the sound intensity of 50 dB, 60 dB is ten times the sound intensity of 50 dB.

And do not let anyone tell you otherwise!!!!

We now move on to a science a little less precise than physics, namely physiology. The ear is not equally sensitive to all frequencies, and entirely insensitive to very high and very low frequencies. The frequency response of the ear varies from individual to individual, and especially it varies with age, and it also varies with the intensity of the sound. The relative sensitivity of the ear to different frequencies can be measured (somewhat subjectively) by asking an individual to compare two sounds of different frequencies, and by varying the intensity of one sound until the subject judges them to be equally loud. Thus we can measure something that might be called “perceived loudness”, which is not the same thing as sound intensity; it does depend on the sound intensity, but it also depends on the frequency spectrum.

In order to allow for the properties of the human ear, a “dBA” scale, or “decibels on the A scale” has been set up, in which a decibel meter is set up to imitate as closely as possible the frequency sensitivity of the human ear. This varies from human to human; however, the dBA scale is set up in reference to a “standard” human ear, whose frequency sensitivity is in fact precisely defined, even if it may not coincide exactly with your own ear. Thus the dBA scale, even though intended to imitate a sort of average human ear, is quite precisely defined in the sense that the sound intensity on the dBA scale is not a matter of opinion but it is capable of almost as precise definition and measurement as a frequency-independent scale.

I have seen no justification in any scientific literature of the common statement that “perceived loudness” doubles for every increase of ten on the dBA scale. I have seen this incorrect statement accompanied by an explanation that perceived loudness is proportional to the logarithm on the sound intensity.  I have even made the statement myself , but it is not true.

In fact one of the reasons that the decibel scale was first set up was to accommodate a physiological “law” known as the Weber-Fechner law, in which it was supposed that perceived loudness was proportional to the logarithm of the intensity; or, put another way, if the sound intensity increases geometrically, the perceived loudness increases arithmetically. According to this law, the perceived loudness would be linearly proportional to the decibel scale. The Weber-Fechner law is, however, only a rather approximate rule of thumb rather than a physical law, although it is fairly good over a moderate range of intensities. No very simple mathematical expression exists, for accurately describing perceived loudness over a wide range of frequencies and intensities, and there is no basis at all for the “doubling for every 10 dB”. What cannot be denied, however, is that sound intensity, if not perceived loudness, increases tenfold for every 10 dB.

We have seen, then, that from the point of view of physics, the decibel scale (dB) is perfectly well defined. From the point of view of physiology, the “dBA” scale has been set up to approximate the response of the human ear. The third word in I used was psychology, and this deals with how annoying or disturbing a sound actually is. Psychology is the least amenable to quantification of the three sceinces in the title. It is barely possible to set up a mathematical scale to determine how annoying a particular noise is. For example, it is sometimes held that normal conversation is about 58 dB and a noise at this level is therefore nothing to complain about. Indeed our local Municipality on this basis allows 58 decibels continuously from 9 a.m. to 10 p.m. on the basis that 58 dB is “only” normal conversation and is not harmful and it is therefore unreasonable to complain about. But try yourself to hold a conversation with someone, or to read a book or to watch television, or to study or to go to sleep when someone or your computer is unceasingly making a noise at 58 dB! Such an unwanted intrusion is utterly intolerable, and it is no answer at all to say it is “only” 58 dB and is not “harmful”.

Basic common sense as well as physics and physiology says that any level of constant noise is additive to the sound environment that we have to deal with.  Although our brain will filter out anything below 20dBa and become inaudiable according to the physiologists of the world, it is still there.  Each additive sound source must be dealt with and filtered and at some point we reach a saturation point.  At this point, the inner ear transmission points begin to fail and we become “hard of hearing”.  Speaking as someone who wears hearing aids in both ears from exposure to high levels of sound, I know what I am speaking about.  That is why I believe in Quiet Powerful Computers, not “the noise is OK if I get a high benchmark number”.

Thanks for that learned post, Renod. I think most of what you wrote is well-substantiated in the literature I have read as well. However, you suggest that the relationship between “perceived loudness” by people and measured decibels on any type of scale is so vague as to defy clear description:

<< No very simple mathematical expression exists, for accurately describing perceived loudness over a wide range of frequencies and intensities, and there is no basis at all for the “doubling for every 10 dB”. What cannot be denied, however, is that sound intensity, if not perceived loudness, increases tenfold for every 10 dB. >>

From the POV of a listening or hearing human being, I don’t give a hoot about sound intensity increasing 10X for 10 dB. That information unfortunately does not advance our understanding of what a decibel or 10 decibels means to you and me, in terms of what we hear.

I can’t say that I have seen clear evidence either—for the generally bandied rule of thumb “sounds twice as loud for every 10 dB”; you may be right that this is just another myth. So what’s our recourse? Just say the lower the quieter, never mind how much because you hear differently than I do?

In the hope of promoting clarity, here is what I personally can hear & perceive with broadband noise:

1 dBA—just barely perceptible, either as an increase or a decrease, depending on background noise
2 dBA—usually always perceivable but not generally a big deal
3 dBA—significant; can be enough to bring a noise from acceptable to annoying, or vice versa, depending on where the original level is
4 dBA—if jumping 4 dB in a quick instantaneous step, very obviously audible
6 dBA—a lot louder or quieter
10 dBA—difficult to say whether it’s twice as loud (or quiet) but close (perhaps more, perhaps less), and very dramatic, again, depending somewhat on the starting point. Going from 85 to 95 dBA, for example, does sound at least twice as loud to me; going from 50 to 60, however, doesn’t for some reason. Probably my own hearing’s dynamic range non-linearity showing up.

The above comments are based on sitting ~5-6 feet in front of a very good hi-fi loudspeaker system in a quiet living room with an SLM meter in my lap, playing pink noise from a test CD, and a wireless remote for the amplifier volume control (has presets that can be programmed so specific levels can be dialed in instantly). 

Mikec,

I think we are in complete agreement (although someone reading our threads may not think so ).  What you measured and confirmed is that a 3dBA difference is noticeable and significant.  That every increase in dBA is to be avoided unless absolutely necessary and that we should make decisions on our fans and equipment based upon dBA and not just CFM.

Boy, give us a big enough lever and WE WILL move the world (or at least document it and be able to explain it).