From: "Rich Peet" <>
>
> I guess the only other dimension you really want is it is about a
> 8.5" focal length. Would need to measure it again to be sure.
There is one more measurement. Plunk a straightedge across the dish and
measure from that to the vertex, the dish depth. It's the ratio of that
divided by focal length that's the critical measure in Sten's analysis.
We can try and line your dish up with his graphs.
> I could only find one comparitive recording in my archive of the Greg
> Clark next to the Telinga. It was the same target, same location,
> same recorder, about 2 mins apart. Target about 200' and 6 lane
> expressway at 90 degrees off in the distance. Stereo Telinga and I
> don't know if it had a low filter on or not.
>
> To be fair about my comment of shrill, use a FFT filter on the sound
> and cut everything below 1,000 cycles.
Making it the characteristics of the high end that's being compared.
Actually, my question is why your dish is not shrill under such
conditions. While it's risky as the gain is probably not evenly matched,
I did another sonogram:
http://naturerecordist.home.mindspring.com/Clark-Telinga.jpg
I'm assuming the right hand part is the Telinga and left is your dish.
If so, you had the Telinga set with it's low cut on by the look of it.
And you did not have a low cut running on the Clark.
Note the sharper definition of the Telinga of the frogcalls, and that it
got more of the higher frequency components? This could be a gain
difference, but if it's real that sharper definition may be all it takes
to sound more shrill. Particularly with a 1khz high pass.
Also note the dropoff in background sound in the Clark above 1.34kHz
while the Telinga continues the background sound higher. The same sort
of dropout was in the other Clark sonogram, but having it here too makes
it a bit more likely it's the Clark and not the environment or sound source.
Also note the band of sound down at the 330 - 664Hz range in the
Telinga, that looks typical for what I get for traffic, it's mostly tire
sounds. Since your dish has poor gain in this range it acted as a
traffic filter in this instance, the greater sound levels in this band
mask it's gain dropout we saw in the other sono.
The other thought I've had on shrill is that it might be possible the
Telinga is resonating a bit with intense high frequencies, more than the
Clark as it's much lighter material. Though this sono seems to say the
opposite.
> A download to choke a horse, 5 megs.
> http://home.comcast.net/~richpeet/gcvte.wav
>
Indeed, my modem was not at all happy about this load. Took some time
and two tries.
Walt
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>From Tue Mar 8 18:27:02 2005
Message: 14
Date: Thu, 26 Feb 2004 12:40:51 -0500
From: Walter Knapp <>
Subject: Re: the nature of parabolic reflectors
From: Bret <>
> I may be wrong, but I think the size of the globe must be frequency
> dependent. I believe this is true especially if you have an ideal
> parabolic reflector.
>
> As you move a given distance from the absolute point of focus, this
> distance is a larger phase difference for shorter wavelengths than it
> is for longer wavelengths. This phase difference at the off focus
> position is one mechanism that would give rise to interference or
> cancelation rather than the addition of pressure as occurs at the
> absolute focus (because the arrival at focus is in phase from all dish
> reflecting points).
> bret
In real parabolic reflectors it may be unwise to assume that the focus
contains everything in phase. Or rely too heavily on phase differences
to explain parabolics.
You also have the problem that the air molecules that move to carry the
sound occupy space as well as having mass. They cannot all arrive at a
point, which is what the focus would be in a perfect world. The pressure
created would be way too high even before they got to a point. So, there
is a pressure limit on the focus spot. Dispersal of molecular movement
on out past the focus is going to be somewhat influenced by this
interference in the soundpath as well.
My only point is that the 'focus globe' idea may be somewhat misleading,
too simplistic. It's a concept those of us using parabolics have used
for some time, but may not stand up to better measurement. The idea was
that there was a zone where the sound amplitude was even that was
relatively large, I think better measurement will show this zone is very
small, much smaller than was previously assumed. We will have to pay
more attention to diaphragms being exposed to varying sound pressures
over their area and not rely on a 'focus globe' to avoid that.
And in real life that 'focus globe' will be less meaningful as the
soundpaths are not all on axis but quite mixed. Even for something
that's out there right along the axis line.
Walt
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>From Tue Mar 8 18:27:02 2005
Message: 15
Date: Thu, 26 Feb 2004 12:54:48 -0500
From: Walter Knapp <>
Subject: Re: the nature of parabolic reflectors
From: Dan Dugan <>
>
> Walt, you wrote,
>
>
>>>To say nothing of natural sound not being some kind of even distribution
>>>of all frequencies, or a single frequency as are the popular methods of
>>>testing.
>
>
> Neither is music, but that doesn't make tones and pink noise any less
> useful for testing musical equipment.
Music is much better behaved and recorded in much simpler environments
than nature recording. This makes it closer to the test sounds in simple
environments. So such test sounds are much more predictive for recording
music than they are for predicting the outcome from the much more
complex natural environment.
You can get some info from such tests, but the distance it gets you down
the road to that perfect recording in nature recording is much less.
It's more a case of what proportion of your time and thought should be
spent on testing as opposed to actually recording your subjects in order
to learn. There are certainly some, when asked if a piece of equipment
will work for a particular nature recording task that rely entirely on
testing. Even if they are out where they can do actual recording. I
don't do that, I'll take a look at testing, but I know to find out what
will really happen I'll have to spend considerable time out doing that
task. I will allot most of my time in making a determination to real
field recording. I believe I will know far better about the suitability
of the equipment and the details of using it than those that base their
judgement on testing only.
Walt
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