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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