a flush mounted omni in fact works exactly like a pzm
microphone. i cannot offhand refer to them but there
are several papers and articles pointing this out.
think of this: at the boundary, the direct and
reflected sound are essentially in phase, and the omni
microphone is sampling this. in what way can the
direction in which it points really matter?
umashankar
--- Curt Olson <> wrote:
> umashankar responded to my post:
>
> > a microphone on a boundary does not necessarily
> accentuate low
> > frequencies. a small boundary in fact works as a
> very effective low
> > cut filter (i have often used microphones mounted
> on six inch
> > boundaries for dialogue).
> >
> > what happens is actually quite interesting. there
> is a transition
> > frequency (i think it the wavelength in question
> is six times or five
> > times the dimensions of the boundary) where the
> boundary no longer
> > works. the microphone then becomes omnidirection.
> before the
> > transition the pick up is a hemisphere. so just at
> the point where the
> > six db boost due to the boundary effect starts to
> disappear, the
> > microphone is sampling a much larger sound space.
>
> Thank you for this. My physics theory might be way
> off the mark on this
> one. I'm happy to defer and learn more. That's what
> I like most about
> these lists!
>
> Dan Dugan also responded:
>
> > Umshankar explained about small boundaries. That's
> why I asked, too,
> > because your report was puzzling.
> >
> > I wonder if you were hearing a reduction in the
> high end rather than a
> > boost in the bass. Two possibilities for that, 1)
> the 183s are
> > directional at high frequencies (most omnis narrow
> down at the top
> > end), and forward sounds are "off axis" to your
> 110 degree wedge
> > mount. Or 2) a dip in the hf response caused by
> how the 183s are
> > inserted into the board--something that could be
> tuned out by moving
> > the mic in and out to find the sweet spot. Just
> speculating.
>
> Good questions, Dan. You and umashankar are both
> making me think very
> carefully here, and I appreciate that.
>
> From my observations, high-end performance of the
> 183s seems to be the
> same with or without a boundary. I have noticed some
> frequency response
> changes in the lower-mid and very low ranges,
> depending on the size of
> the boundary. I can report confidently, however,
> that going to a
> smaller boundary recently seemed to reduce the
> low-end bump that was
> bothering me. Exactly why that is, I won't try to
> explain.
>
> I understand about the narrowing pattern at the high
> end and the fact
> that both capsules are off axis to sound sources
> directly in front of
> the array. As a practical matter, it's pretty much a
> non-factor -- even
> at the angles I'm using.
>
> Your idea of "tuning" the mic in and out relative to
> the boundary plane
> is interesting. I've wondered about it myself a few
> times, but with the
> way I'm constructing these boundary mounts up to
> now, there hasn't been
> much physical room for that, so I've never pursued
> it.
>
> >> I'll continue beyond the scope of your question.
> The reason for the
> >> boundaries is that in my experience, spaced omnis
> can give terrific
> >> stereo imaging in an enclosed space with a
> specific targeted sound
> >> source, such as a choir or orchestra. But for
> outdoor ambience, phase
> >> interaction between the mics makes imaging a mess
> and destroys mono
> >> compatibility every time. (I'm talking about
> close spacing here --
> >> say 4" - 9" -- that someone can easily carry in
> the field, not wide
> >> spacing.)
> >
> > Same thing happens with an orchestra.
>
> Yup. That's right. Instead of "terrific stereo
> imaging" I should have
> said that spaced omnis can yield an "exciting
> recording." That would
> have been more accurate.
>
> >> But when the mics are incorporated into boundary,
> this phase
> >> interaction at close distances seems to be
> virtually eliminated while
> >> the distinct and desirable omni characteristics
> seem to be preserved
> >> -- with a little low-end boost thrown in.
> >
> > I've experimented more with barriers than with
> boundaries. My
> > shoulder-mounted 183s face directly forward to
> focus the hf response,
> > but they have my neck in between. Since the mics
> aren't mounted right
> > on my neck, it's a barrier rather than a boundary.
> I don't worry about
> > mono compatibility.
>
> I do, mostly because I'm in the habit from 30 years
> in radio
> production. I always have in the back of my mind
> that somewhere down
> the line something I record could end up on the
> radio, and I worry
> about what mono listeners would hear.
>
> Rob Danielson added:
>
> > When I look at the spectral displays of field
> recordings made with a
> > range of different mics, they all reveal that a
> very, very high
> > percentage of the total sonic energy in our
> environments is below 300
> > Hz-- even in remote areas in the middle of the
> night. Very low
> > frequencies are powerful and travel great distance
> and our powerful
> > machines generate tons of it. I can roll-off
> everything above 1000Hz
> > in a field recording and still retain 90% of the
> amplitude. If I use
> > shelf filtering to "roll-off" below 150Hz, I'll be
> left with 10-25% of
> > the sound recorded. Its traditional to "roll off."
> The harmonics
> > produced by these Hz's are lost and the ability to
> define the mid
> > range is adversely affected. Of course, the
> intrigue of recording,
> > "space" has only been considered possible for
> about 15-20 years.
> >
> > In short, "I never met a low Hz I didn't like" --
> that I couldn't find
> > something useful in or about.
>
> I've noticed this too, but... when I'm standing
> there taking the
> headphones on and off, comparing what I hear with
> "naked" ears to what
> I hear through the recording chain, I realize that
> the mics are often
> picking up and amplifying a whole lot more low end
> than my "naked" ears
> ever perceive. Maybe it's just me, but I'm not
> interested in recording
> an over-hyped low end any more than I want to record
> an over-hyped high
> end or midrange.
>
> Rob again:
>
> > I'm not sure your front-facing, flush with
> boundary mics behave
> > similarly, I know that PZM boundary mics need at
> least 4' X 4'
> > collectors for full frequency response. A 12" area
> produces much less
> > low Hz. Crown lit mentions using smaller
> collectors for low Hz
> > attenuation. Likewise, the SASS collector seems
> too small to use as
> > free-standing suspended boundary mic rather than
> on a stage where the
> > floor creates a large collector. Maybe I'm totally
> wrong.
>
> This is the concept I had in mind when I thought to
> try attenuating the
> low end response by going to a smaller boundary, and
> it seemed to work.
> (You're absolutely right, though, to point out that
> a front-facing,
> flush-mounted boundary mic will not necessarily
> behave the same as a
> PZM.) So how would that square with umashankar's and
> Dan's comments
> above? (A serious question here, not a jab.)
>
> Curt Olson
>
>
>
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>
>
> "Microphones are not ears,
> Loudspeakers are not birds,
> A listening room is not nature."
> Klas Strandberg
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>
>
__________________________________
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"Microphones are not ears,
Loudspeakers are not birds,
A listening room is not nature."
Klas Strandberg
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