Doubting The Decibel Scale

From: Ron Pellegrino
Subject: decibel scale
Date: 7/9/98

In the world of physics the decibel is a unit of power ratio used to express the intensity of a sound wave - the ratio of pressure produced by a sound wave relative to a reference pressure. The decibel scale derives its meaning in the sound world from the responses of human test subjects. Its supposed "objectivity" is arrived at by averaging the responses of a group of subjects who rate the test sounds from "softer than barely audible" to "louder than intolerable" and all the loudness levels between the two extremes. Some people believe the decibel scale is an objective scale because they see it printed in books and on the face of a machine when in fact it´s intersubjective and lacking in precise meaning. At this point it´s extremely crude in its unweighted and weighted forms and commonly misused to make a point on either side of the noise issue. It fails to account for age, history, location, individual sensitivity, preference, and a host of other psychoacoustic factors that mean a lot to people.

As a sound researcher one of my favorite games is to walk throughout a soundspace - any soundspace - observing the radical sound intensity changes at various frequency levels as I change my location relative to the source - shadowing first one ear with my head then the other ear, standing in front of and behind the source, close to it and further away, to the right and the left of it, above and below it, facing and turning away from it, and all the intervening locations. Today's decibel meter correlates poorly with those hearing tests. It´s not inconceivable that future decibel meters will include many additional weightings that better reflect the hearing of a broader range of human subjects with more conditions. Until then I prefer Muriel Strand´s position that"our very own ears (and brains) are the best. if the purpose of dBA measurements is to evaluate noise effects on people, why not just listen to the people? cheaper than a meter, and more to the point."

Ron Pellegrino

The exchange that follows is a continuation of the discussion.

The dBA Issue and Sonic Sensitives

From: Ron Pellegrino
Subject: Federico Miyara´s explanation of the dBA issue
Date: 7/14/98

>=== Quiet-List message from Federico Miyara ===
>Let´s understand that A-weighting sound level is the worst noise measure
>available--but there is no better one! :)

The worst aspect of decibel meters is that they lead unsuspecting users to think they know a whole lot more about noise than they actually do. In my 10 years of teaching an upper level university course in the Physics of Music the chief value I found for the decibel meter was to use it as a foil. As a tool for measuring loudness in any meaningful way it´s an utter failure but it´s a great vehicle for demonstrating both the complexity of human sound perception and the current conceptual limits of quantifying what relatively little is known about our perception of sound.

»Even at low levels A weighting (or B weighting at medium levels) is
»probably inappropriate. If you listen to a good sound system and vary
»the level you should hear substantial changes in tonal balance if the
»equal loudness contours (which are the basis for claims for the need
»for weighting) had anything to do with reality. But I haven´t observed
»such changes in tonal balance (and it seems neither have most audio-

Whether or not one can hear changes in the timbre (tonal balance) of spectrally rich tones that are changing in pressure levels (loudness) depends upon the evolution of one´s hearing; human responses to acoustic energy run the gamut from "tin ears" to sonic sensitives. As a composer I´ve spent a lifetime developing my own hearing acuity and for decades I´ve taught others to do the same. Electronic music laboratories that include oscilloscopes, spectrum analyzers, and computer displays make that task easier and faster. My experience with audiophiles is that, with few exceptions, they tend to be collectors of expensive audio equipment rather than sonic sensitives.

>That´s because the brain fools the ear. You have a tendency to adapt
>yourself to a continuously changing context. This does not mean that
>the psychoacoustic principle does not hold. It means that, much in the
>same as you can speak in a noisy environment by filtering noise out, you
>filter out the tonal disbalance in order to be able to recognize the
>source as one and the same regardless of if it is loud or low.

When it comes to perceiving sound, the brain is not separate from the ear and neither fools the other. Both ears (they don't have the same response to sound) and the brain are parts of a complex system that responds to sound; included in that system are flesh, bones, body cavities, organs, and circulatory systems. The decibel meter is deaf to the rest of that complex system. It also doesn´t measure vibrations that have frequencies below and above the response of the ears yet have an impact on other sensors in the system.

One of the many flaws in conventional western academic science is that it is built on a divide and simplify approach which produces an army of specialists. Specialists tend to act like the universe is made of separate machines that barely communicate with each other (brain/ear problem). The interdisciplinary study of the nature of complex systems like the human response to sound is in its infancy with specialists doing their utmost to slow the infant´s growth. What the specialists give us is the mass distribution of tools like the decibel meter, a tool representing a conclusion based on a badly flawed premise so, of course, the conclusion (the decibel meter) is also badly flawed. In an earlier post I said "I cringe every time I see someone raise the decibel flag" because the discourse inevitably leads into such murky waters. On the decibel issue I promise not to cringe any more on the quiet-list.

Ron Pellegrino

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