All:
A parabol starts amplifying at a the same wavelengt as it's diameter. There
is no question about this. A telinga will start amplifying at some 650 Hz.
(340m / 0,53m) Then it gives a further gain by 6db/octave.
However, then some other tricky acoustical phenomena occurs. I will not
comment on this.
(In a week or so, I hope to publish the most extensive scientifical paper
made on this topic, by Sten wahlstr=F6m in Audio Engineering for many years=
ago.)
But: Do not forget that a sound also consists of overtones! If a bird has
it's main singing on frequencies below 650 Hz, the dish will still amplify
it's overtones! This is why a parabola "recreates nearness" as distance
tends to attenuate overtones by 6db / octave. Roughly....
At 10 kHz, the focus is a "ball" with a diameter of 34 mm. It does not
matter where you place the (omni) microphone, as long as it is inside this
ball.
Subjectivelly, the parabola seem to cause "amplification" also at very low
frequencies. First of all this may happen when a pidgeon (for example) has
overtones higher than the Telinga 650 Hz. Then there is a "shield off"
effect. The parabol becomes a shield, in the way for side and background
sounds. This gives an impression that the parabola amplifies.
Finally you can sometimes hear an effect cause by the ground. Acoustical
phenomena, like a 6 - 10 db boost by the ground reflection, interact in
strange ways with the dish and microphone.
When I want to compare two different mic's in a parabol, I use pink noise o=
r
a sine tone sweep, outside, at a distance of 30 meters. It doesn't matter
how careful I am, the measurment will always be some 5 - 10 db different
from oneanother. This means that I have to make the printer draw 10-15
measuments on the response of a microphone, then change to another
microphone. When having 20 - 30 such figures on paper, I can compare them
and see if there are any significat pattern of differences.
Talking about MONO and OMNIS - when comparing two modern electrets in such =
a
way, it is very rare that the printer drawings show any relevant differenci=
es.
Klas.
At 21:59 2003-02-21 -0500, you wrote:
>Ranft, Richard wrote:
>
>> I don't think your 20" parabola reflects sounds down to 50Hz; they mostl=
y
>> diffract around it. However, the microphone in the parabola will still =
pick
>> up such sounds - and from all directions in the case of the omnidirectio=
nal
>> Telinga mike. There may also be some directionality at 50Hz due to the
>> 'shadow' effect of the parabola, but not much. Parabolas of 20" are rath=
er
>> poor for recording low frequency sounds.
>
>> This is because the mike also receives sounds directly from the source a=
t
>> all frequencies. The parabola on its own still has a low cut off, and =
it
>> sounds unnatural on some low-pitched sounds. I use a MKH 816 gun mike t=
o
>> record a large owl rather than my Telinga or other parabola. Sure, the =
owl
>> sounds come over in the parabola recording, but not as well as with the
>> Sennheiser. As you say, you have to get closer with a gun mike, but for =
me
>> that's all part of the challenge of 'hunting' such sounds.
>
>Most of the owls I've recorded have been incidental to recording frogs
>at night. Before you volunteer to get close, maybe you should consider
>the conditions. My "typical" recording site is from a road shoulder,
>over the barbed wire fence and past the No Tresspassing sign. To go past
>that sign risks gunshot, arrest, etc. I'm legal on the road edge.
>Second, I'm recording frogs, typically from a wetlands, swamp, etc. The
>owls are typically out somewhere the other side of the swamp. A
>interesting trip in the dark. Here in Georgia slogging through the swamp
>in the dark is for the truly fanatic. Ranks right up there with catfish
>tickling.
>
>I own a pair of MKH-816 shotgun mics. I like them too. Though, since
>I've not worked out the stereo suspension and windscreening for the pair
>I'm not using them a lot. I try to avoid mono.
>
>Now, back to the parabola. You make it clear the parabolic reflector is
>doing nothing. If that's the case, then the mic is doing it all. There
>is only the mic and reflector. Now what can it do?
>
>Well, first off most of what I record starts at 200hz or so, though
>quite a bit of interfering noise goes all the way down. At a distance of
>a mile, a barking treefrog's call is reduced to just the low frequency
>components. All well below the 660hz cutoff that physics predicts. I
>can, on numerous occasions, have picked up barking treefrogs at this
>distance. Once in a while I even record them at this distance, but, they
>are so faint they cannot be heard by ear at all, and on recording are
>still a faint call. So, usually I use the mic to triangulate their
>location. I can get the vector within a few degrees easily off that low
>frequency, faint call. By moving and triangulating I can work out the
>exact direction and estimate of distance. Then I can compare that with
>my GPS map and my paper maps and what I can see to work out if there is
>a road path to take me closer. Again, I stop periodically to verify
>direction. Sometimes that mile can take several miles of driving if it's
>possible at all. If I'm lucky there will be a road right past the pond
>where a few of these frogs are calling. Note I'm picking up frequencies
>below 660hz well, and I'm able to get a precise direction.
>
>If the dish is not doing this, then the mic has to be. That makes it
>some mic. Far more directional than a MKH-816, and super, super
>sensitive. And, since it's frequency response is reasonably flat, and
>there is no change in direction finding as frequencies go up, and no
>breaks in response it would appear that this mic can do this for all
>frequencies. If that's the case, I can throw away the dish, my mic will
>be much more compact. And it will still do everything attributed to a
>parabolic dish.
>
>That is the logical extension of what you are saying. Note it does not
>have to be barking treefrogs; bullfrogs, gopher frogs, etc. will do too.
>Or a 50hz noise source.
>
>Now, myself, I happen to know that the bare Telinga DAT Stereo mic
>cannot pick up barking treefrogs from a mile, and that it has no
>directional ability to speak of. Why, because I tried it, I don't rely
>on the pronouncements of physics. Which is the same way I know for
>certain what a Telinga Pro V with DAT Stereo element can really do. I
>really have used this for many years, and it does what physics says it
>does not.
>
>For some 50 odd years since the first nature recordist put a mic in
>front of a parabolic there has been this problem that physics predicts
>it won't work. (In fact I believe that originally, when consulted
>physics said it would not work for any audio frequencies) Meanwhile a
>somewhat bemused nature recordist keeps right on recording. As will this
>somewhat amused nature recordist. I'll trust what ends up in the
>recording over any theory. Someone else can agonize over why.
>
>And note, once you account for the change of the sound with distance,
>the Telinga does a good job of recording those low sounds in the
>distance. Where you can hear them by ear, what you record will sound the
>same, or as close as mics can do.
>
>Walt
>
>
>
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Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01
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