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Old 05-01-2005, 05:27 PM   #31
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Originally Posted by voyager_663rd
Any acoustical engineers out there that could whip up some tables, and suggest frequency detecting apparatus?
I sometimes work on sonar. Does that qualify?

A real-time spectrum analyzer would be the tool of choice with a frequency weighting to account for the characteristics of human hearing..

As previously mentioned, low frequency acoustic energy decays less rapidly with distance. Fog horns and sonar systems take advantage of this.

Wooded areas surrounding a field would absorb some acoustic energy.

High frequency sound tends to be more spectral (beam like light) and low frequency sound more omnidirectional. In confined spaces, low frequency sound is more prone to standing waves (becomes modal).

The sound of a model engine is quite tonal (a few select frequencies dominate) and not broadband (like pink or white noise).

The human ear has a very characteristic (and un-flat) frequency sensitivity. The Fletcher-Munson equal loudness contours attempt to quantify this. In order to block out what would normally be a very annoying amount of body noise (beating heart, thumps from walking etc.) human hearing is very insensitive to low intensity low frequency sound.

By nature, human senses are more sensitive to change than steady state. Look at one spot in a picture long enough and it eventually dissappears. This is no less true of sound. The siren of an emergency vehicle with its varying pitch is more effective at attracting our attention.

In my opinion, it is the last three characteristics that make the sound of model airplanes unwanted noise to more people. A small two-stroke IC engine produces most of its acoustic energy in the form of tonals in the frequency range the human ear is most sensitive to. As the throttle setting changes and the loading on the prop changes in flight the RPM and consequently the frequency of sound produced by the engine changes.

In contrast, a lawnmower engine is lower in pitch, relatively constant in pitch and is a socially accepted necessity of suburban living.

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