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Re: what is "shrill"

Subject: Re: what is "shrill"
From: Walter Knapp <>
Date: Sat, 28 Feb 2004 19:27:42 -0500
From: "Rich Peet" <>

> 
> With the dish only do I find it to hot in the highs. I didn't play or 
> view the sound at all.  Once again the high part of the chorus only 
> with the dish do I hear as being shrill.

Which means dish rather than mic. Though I did think of something else 
last night, for which you get to endure a story.

Back in the 70's I earned my way doing environmental impact measurements 
and studies. One of the projects I was involved in was to evaluate the 
impact of a large construction project enlarging the power generation 
facilities in the North Cascades. One of the impacts we had to evaluate 
was the sound impact on the North Cascades Highway of the huge number of 
truck trips involved. For this we had to calibrate our noise model, 
involving taking measurements along the highway to determine it's 
current noise levels.

We got to discussing the effect of vegetation on the measurements. This 
lead to doing some testing of sound attenuation of vegetation at various 
heights. We did this along the North Cascades Highway at sites ranging 
from clearcut stubble to thick forest, at distances from 10 yards to 100 
yards from the pavement. We took 15 minute averages using our big B&K 
sound meter. All of this just preliminaries for taking the actual 
measurements we needed.

The result of all that was we found that below about 5' the sound levels 
began to be attenuated by vegetation. 50 yards of vegetation was enough 
to drop the 5' sound level for between 4 to 15 dB at 2'. Even clearcut 
stubble showed a drop. Armed with that we did all our main measurements 
with the mic of the sound meter at 6'.

I'm still using what we found. Part of why I got into experimenting with 
the high tripod. And I control the Telinga's pickup by deliberately 
varying it's height even when hand held.

How does this apply to your case? Well maybe nothing at all, but I note 
your photo with the large parabolic that it's sitting relatively close 
to the ground. And if you use the Telinga like I do, near head height, 
then you may have the sort of differences we had. We did note when we 
did our stuff that the attenuation was causing a shift toward low 
frequencies, but did not measure the extent of the shift. We were only 
interested in overall sound levels. You might see if you like the sound 
of the Telinga better if it's held at the same height as your big dish.

Something to mull over...

Walt




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