Perception of Loudness

To: quiet-list@igc.org
From: Ron Pellegrino <ronpell@microweb.com>
Subject: Re: How is noise different?
Date: 4/20/98

>=== Quiet-List message from jamdancer1@juno.com (Jam
>Dancer) ===

>Can someone show me, in the form of a bar graph of some kind,
>the two scales side by side: temperature in centigrade
>degrees, and noise in decibels?

RP - This is another one of those apples and oranges comparisons. Far better to focus on sound and avoid the inevitable comparison confusions. Also, temperature is a quantitative term and noise is a qualitative term. If by noise you mean sound then check out the graph included in the essay on my site titled Why are Audio Engineers the Enemies of Our Ears. It´s not a bar graph because a bar graph doesn´t show enough dimensions. In a nutshell, what the equal loudness curves graph shows is that the human ear is a nonlinear transducer - the acoustic energy (intensity or decibels) at the input (the ear/organisim) is not mapped in a one-to-one loudness correspondence over the entire range of frequency sensitivity of the hearing mechanism (ear/neuro system including brain/state of the organism). This classic diagram shows the connections among sound pressure level (intensity), frequency, and perceived loudness. Humans are most sensitive to sound in the middle of our frequency sensitivity range (where we talk and sing) and least sensitive at the bottom and top of our range (but those frequencies are still important important information carriers).

The diagram is great as far as it goes; anyone interested in sound should meditate on it often. However there´s been plenty of new research since that diagram came onto the acoustics scene. Today we know that the complex of factors that relate to the human perception of loudness not only includes frequency level and sound pressure level but also includes spectral complexity, dynamic complexity, textural complexity, an individual´s variable threshold levels, personal preferences, and state of the organism. When assessing excessive sound too many people, including some on this list, focus on decibels (sound pressure level) as the only variable. In fact, the notion of threshold level is just as important though it seldom surfaces in discussions. Threshold levels figure into the notions of just barely noticeable, very soft, soft, moderate, loud, very loud, irritating, painful, destructive, and murderous. And those threshold levels aren´t only based on intensity; they also relate to iteration, complexity, and an individual´s idiosyncrasies.

Ron Pellegrino


To: quiet-list@igc.org
From: Ron Pellegrino <ronpell@microweb.com>
Subject: Re: How is noise different?
Date: 4/23/98

>=== Quiet-List message from "Muriel Strand"
><mstrand@arb.ca.gov> ===

>i took a look at ron´s equal loudness contours, which are quite
>similar to other versions i have seen. upon reflection i began to
>wonder about exactly what these narrow lines represent; are
>they averages (i assume) and if so, what is the statistical
>distribution?

RP - As I mentioned in my original post the graph of "equal loudness curves" is a classic diagram to be found in any good book on acoustics. It's also called the Fletcher-Munson diagram after the two scientists who did the research and testing. If you want the nitty-gritty on the diagram you can always go back to the original source: Fletcher and Munson, Jour. Acoust. Soc. Amer., Vol. 5, No. 2, p. 82, 1933. A good explanation is also to be found in MUSIC, PHYSICS, AND ENGINEERING by Harry F. Olson, Dover Publications.

As a test tone they used a pure tone (a sine wave) with 1000 Hz as a reference. The narrow lines do represent average responses. The bottom line is the threshold of audibility and the top line the threshold of pain; the other lines represent musically useful loudness levels between the extremes. Again what´s important to notice is that the ear is a nonlinear transducer - its loudness response to a signal of constant energy changes according to the frequency of the test signal with the greatest sensitivity to be found in the middle of the human hearing frequency range.

>taking an analogy from air quality standards, which are health
>based and designed to protect the health of the most sensitive
>members of the population, what would noise standards be if
>the criteria was protection of the most sensitive?

RP - Wouldn´t it be great if what you´re saying were true. Asthma has reached epidemic levels precisely because air quality standards are set not for the most sensitive member of the population but for the average or normal member. The general consciousness level for air quality is very low. The regulation of smoking is only the first step. Our homes, schools, workplaces, and transportation systems are filled with noxious fumes from outgassing furnishings and building materials and inadequate ventilation; yet few people seem to be able to figure out why we have so many respiratory ailments. Our society is quickly and stylishly going deaf because the general consciousness level of noise and sound pollution is off the bottom of the scale; the quiet-list is an example of the birth pangs of audio consciousness but it´ll take a while before it´s mainstream. Best assume responsibility for your own hearing system and those close to you; bureaucratic support systems may be a while.

>the catch is how to separate the physical and psychological
>effects of sound. personally, i have a hard time believing that
>ice cream trucks are really a problem; they are frequent in my
>neighborhood and i have always found them basically charming.
>even if the music is rather repetitive, they only stand still when
>making a quick sale.

>muriel

RP - The problem of what to accept, even find charming, in the soundscape is a tough one to solve because it boils down to personal and subcultural preferences and shifting threshold levels of tolerablity. It´s clear that some people have had it with sound and would prefer absolute silence. Though it´s not likely to happen. Even if you lived in an anechoic chamber you´d have to contend with the pounding of your beating heart, the buzzing of blood coursing through your veins, and the gurgling of your digestive system plus any other strange sounds your body might feel compelled to make.

Ron Pellegrino


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