Posted by: "Bruce Wilson"
>
> Having just listened to Jim's "Brown Canyon Storm" recording
> (http://www.wingsofnature.com/downloads/, I listened to it many times on =
a
> row, great stuff there) I realize there is a fundamental difference betwe=
en
> recording with a dish and recording with a studio pair: intimacy. Those d=
ish
> recordings have a feel that the birds are close and the storm is distant,=
> exactly the opposite feel that I get with a similar recording I made with=
an
> ORTF pair of Rode NT2000's, where the wind and rumble feel closer than an=
y
> of the birds, especially the canyon wrens.
On the Telinga site you will find a white paper that goes into this.
> So maybe I'll start a switch over to that technology. Thing is, I like th=
e
> low end of the specturm as much as the highs.
I'd not completely switch. A dish is not a universal mic for all
situations. I do believe it's a better choice than a shotgun for zeroing
in on distant callers.
> How does the diameter of a dish relate to the low-end response?
Sound waves are movements of particles along the direction of travel, so
even a really tiny dish would reflect all frequencies. The larger the
dish, the greater it's gain, at all frequencies. Below a dish about the
size of the Telinga the low end gain suffers a lot relative to the rest.
Dish gain is lowest at the low end, but not zero. It increases with
increasing frequency. That's why you find that your recordings above
gave you a closer feeling for the birds. The increasing gain with
frequency compensated for the fall off with distance, which is the
opposite. Fall off with distance is greatest at high frequencies and
decreases as you go lower. The combined effect gives you a frequency
response like you had the mic physically closer to the bird.
> When the low end falls off, is it completely gone, or just 3 dB down? Doe=
s
> it get phasey?
There are several things entering in here. This has been discussed to
death several times. It generally boils down into two camps. One camp
tries to explain it mathematically, the other just get's out and the
field and determines what it will do by actual recording. The two do not
agree well. A complication is that many don't seem to notice that
electromagnetic waves are quite different from sound waves on a
fundamental level. The math of electromagnetic waves is often
erroneously applied to analyzing parabolics for sound. And I definitely
have no desire for another of those free for alls.
The fall off in gain does not really go to zero, but it gets pretty
close. Note at zero you still have the same gain as the bare mic without
the reflector, so there is no cutoff, no drop below that of a bare mic,
gain does not go negative. And a specific value to the gain would depend
on frequency as well as dish geometry, it's not a single set value.
The position of the focus relative to the dish depth determines how much
interference there may be at the mic from phase differences between the
direct and reflected sound. In general ideal is a focus about 1/4 dish
depth, but such a deep dish has other practical problems. The usual
compromise is to have the focus at the plane of the front of the dish or
just inside. Note that this interference notch can get bad enough with
the wrong geometry that for certain frequencies you may have negative gain.
The best way to find out what your dish will do is take it out in the
field and record with it. Listen carefully to the results. You will then
know more than all the math.
> What difference in the low end is there between using cards (facing the
> dish) and omni's?
>
> And finally, does using a dish narrow the stereo image much over an MS se=
tup
> (which is about 45 to each side, in my experience), and does the low end=
> remain largely omnidirectional while the mid's and hi's become increasing=
ly
> directional?
The angle of the stereo image in M/S is quite variable. It can be varied
even more by choice of polar pattern of the mid. It can be wider than a
90 degree field for sure.
A stereo image from a parabolic is different. In the case of the Telinga
I view it as being a elongated pear shape. Most of the width of the
field is local sound, but as you approach the center it reaches farther
and farther out. This is somewhat similar to what we have after our
brain has filtered our sound pickup when we are listening to a distant
sound. (note this is a different aspect than the variable gain with
frequency)
Angle of pickup for stereo will be limited by the dish as it acts as a
barrier as well as a reflector. The location of focus relative to the
dish will determine the maximum possible angle.
Walt
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