Raimund Specht wrote:
> Walt,
>
> please do not misunderstand my postings. I really appreciate your
> broad and impressive knowledge on recording techniques and your long
> experience in sound recording.
I do not consider this personal at all if that's what you mean. This is
a difference of opinion.
> However, I'm sure, that you are completely wrong at these points:
>
>
>>Essential or not, there are reflections internally in a microphone
>
> at
>
>>ALL frequencies. For reflections to occur from any of these
>
> surfaces
>
>>which are tiny compared to the wavelengths, proves you absolutely
>
> wrong
>
>>that the vastly larger surfaces of a parabolic reflector will not
>>reflect things longer wavelength than their size.
>
>
>>Note that reflection in a microphone, which you do admit occurs, is
>>exactly the same thing as reflection off a parabolic surface. If it
>>occurs in a microphone, it definitely occurs off the much larger
>
> surface
>
>>of a parabolic.
>
>
> I guess, that you got your impression on the directionality of the
> parabola at all frequencies from the fact, that our brain is able to
> restore information that got lost during the travel through air.
> For instance, we can determine the pitch of a sound, that contains
> harmonics (e.g. human speech) even if the signal has been high-pass
> filtered with a cutoff-frequency higher than the fundamental
> frequency of that sound. This effect can be heard in a telephone
> speech. The transmitted bandwidth ranges from 300 to 3500 Hz only.
> Male speech may have a pitch lower than 300 Hz and we can still
> understand what one says (and have teh impression of a low-pitched
> voice). Another example would be piano music broadcasted over a
> cheap radio. Thanks to the harmonics in a piano note, we can still
> determine the pitch of the note, even if the fundamental frequency
> of that note is not transmitted.
>
> I guess, that the same happens when you listen to your parabola
> recordings. Because the parabola is very directional for the high-
> frequency components of a sound, you may have the impression, that
> the parabola is also very directional at lower frequencies (which is
> not true).
>
> As I have seen from your website, you have been recording frogs and
> toads, that may have strong harmonics and can be very noisy (broad
> frequency spectrum). This would explain your impressions.
That's a rumor started by the bird folks ;-)
Frogs have quite precise calls, and while they can be very noisy in
groups, that does not change the nature of the individual calls unless
they deliberately change them to new precise calls. It certainly does
not make for a broader frequency spectrum.
And if you think I'll go this far just because I've casually listened in
a non-critical way, you are wrong. Every recording I make is gone over
in great detail by sonogram. I know the exact structure of each species
call. I know the relative intensity of each band in each call. A frog's
call is pretty much hard wired genetics, we think, though there is some
indication that at least some learn or invent their calls.
For instance, I might record a bullfrog. At 10', or at nearly a mile, or
anywhere in between. I'll compare that sonogram to those of other
recordings, including recordings made by others, often with mics other
than parabolics. I have mics that are not parabolics. I use full color
sonograms that give me pretty accurate scaling by dB level. I compare
the relative banding at each frequency and do the same for recordings
that are not recorded with a reflector of any kind. If the parabolic was
loosing gain level as the frequency went down, this is sensitive enough
to find it even if it's just a few dB. I don't find that, I find the
bullfrog calls all have the same relative banding no matter if recorded
with a shotgun, a omni, a parabolic. The bottom band on their call is
just above 200 hz, the top is at about 2500 dB. Frogs often use a
multifrequency system that's keyed off two main frequencies working
against each other, it's not all just passive harmonics, but
constructed. I think, by the structure of the calls that a bullfrog may
be one of those.
Now, what was I doing a couple weeks ago just after I'd found the
Brimley's. I went to a pond known to contain gopher frogs hoping to make
some recordings and get some photos, it's one I don't have a photo of,
the one on the webpage is one of John Jensen's. Unfortunately the pond
was full of egg masses and only two gopher frogs were making a few calls
or so a hour. The pond can be waded in hip boots throughout. I spent a
couple hours with the Telinga triangulating exactly which grass clump
each gopher frog was at. I then spent another two hours in a fruitless
long stalk. The gopher frogs were exactly where I'd located them with
the Telinga. Unfortunately these very wary frogs got away before I could
net them for photos. I clearly saw each of them.
I've done these same sorts of things with quite a variety of wildlife,
many of which are giving low calls. The Telinga is very reliable for
triangulating. Even when the entire sound is below this critical point
based on wavelength.
I think I'll take my delusion to your reality any day.
> Please also note, that I use parabolas too (just purchased one from
> Telinga). Nobody says, that parabolas were bad. But I would not use
> them for recording signals with wavelengths smaller than the dish
> (because of the distortion in the frequency response curve).
Try it, you might be surprised. Then your parabola will be even more
useful, as it is for me. I learned what my Telinga could do by trying it
and looking in detail at the results. I did not limit myself to theory.
I also don't keep doing things that don't work.
> Perhaps I will conduct some measurements using a sine sweep
> generator, a speaker and a MKH 800 microphone both with and without
> the dish. I will then post the results.
Do this. Set up at a decent distance for a parabola, mid range, say
200'. You want it definitely far enough to make the bare mic barely able
to pick up. Use individual frequency bands, not swept stuff, it will
take too long to set up each frequency. And for each frequency use a
calibrated sound meter to standardize the speaker output to a fixed dB
level. You don't want your source to be what's causing the results.
You should probably also use a pretty flat field and have both source
and mic well elevated. Say, 15-20'. Since ground effects are frequency
dependent too. That won't totally eliminate them, but will minimize them.
It goes without saying you measure the output the same way at all
frequencies.
I'd have already done this, I have a good sound meter, but I don't have
any portable speakers I'd trust. Takes some fairly large stuff to do the
low end right.
Walt
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