Hi John--
I realized this morning that I should have written, "I start by
aligning the center axis of the array with the direction of the
longest decay,.." Previously, I described this step as pointing it at
the "loudest reflector" which is less accurate.
Good point that the pitch can make a difference-- especially with an
array that employs cardioid mics such as NT1-A's or other mics where
the polar pattern is quite treble-centric. The rear array on my
portable surround rig has two NT1-A's between a baffle and I've have
them independently adjustable to accommodate the kind of fine tuning
you suggest. I can hear the high frequencies increase in volume when
I tilt my head down about 20 degrees with live insect sounds coming
from my speakers. I haven't noticed pronounced tonal changes with
small mic angle or pitch changes in the field, but they might exist.
I'll try the head tipping experiment you suggest. The NT2000's in
omni mode are unusual in that their treble "bumps" are 90 degrees
off-axis.
My intuitions about where the animal and other dynamic subjects are
going to be are not strong enough to divert me from trying to
_build-in_ as much overall balance to the stereo image as I can. If
it turns out that a greater percentage of the intermittent sounds
occur on the left side for example, balanced reverberation still
establishes a right side to the image. If the reverberation produced
a strong decay on the left side with the calls, the image would feel
more lop-sided. Directional arrays are a larger challenge to set-up
for unattended recording to be sure!
As for the pitch adjustment, you can see in my revision that I
create this angle when I align the center axis of the array with the
direction of the longest decay. The angle turns out to be close to
parallel to the ground in open space, pitched-up slightly when the
reflectors are tall and pitched-down slightly when the reflections
are in a space below. Vertical imaging in speakers is pretty minimal
so it might be clearer to place the horizon line straight ahead where
most listeners expect it (assuming speakers are at ear level). With
speaker playback in mind and a stereo array that is well-suited for
vertical imaging, positioning the array head-on should create enough
contrast to work with. Experienced recordists might notice when the
orientation is radically distorted or exaggerated, everyone else
probably takes the spatiality in uncritically.
You make another good point about the use of live monitoring when
placing an array. If it an array I have little experience with, I'd
hook-up headphones and walk around listening to impulses and
sustained sounds. I'd probably end-up positioning the array using the
same, basic steps and naked ears, but it is necessary to have a good
sense of the array's "blind spots" and strengths first. One
significant array performance factor I immediately think of is rear
imaging. Some arrays have poor pick-up to the rear (or _good_ rear
rejection depending on one's goals), other arrays have good pick-up
to the rear (and superimpose these sounds onto those sounds towards
the front of the array) and some have some extended side "wrap" but
still with some rejection at full rear. Most of my effort goes
towards avoiding imbalance which some recordists would feel more
comfortable assessing with headphones. Rob D.
At 12:20 AM +0000 8/9/09, John Hartog wrote:
>Hi Rob,
>Thanks for the detailed descriptions of your mic placement methods.
>
>Another aspect of mic placement is the vertical angle of the mics.
>The difference though slight adjustments up or down can be huge.
>This effect can usually also be heard by tipping ones head up and
>down. After picking a spot and determining mic height, I usually aim
>the mics in the general directions of the subject and then monitor
>while first turning left and right and next tipping the mics up and
>down - listening for a desired tonal balance.
>
>This fine tuning can be very difficult in louder ambient settings
>because leakage from outside interacts with monitored sounds in the
>headphones thus rendering a pretty much useless perspective. Any
>tips for determining best vertical angle without monitoring?
>
>John Hartog
>
>> Thanks for the stimulations John and Mike.
>>
>> I am not very good at it, but the most important factor in
>> positioning a stereo mic array might be knowledge of the habitat and
>> animal behaviors-- that is-- what places animals are likely to visit,
>> likely paths they will take, spots they could pause and rest and the
>> associated animal, plant and weather- related sounds that are most
>> likely to occur. I go on the assumption that places that provide
>> food, water, protection, transition zones between habitats and
>> greater purview often present higher degrees of activity.
>>
>> For recordings that attempt to capture communicative or "ambient"
>> space, the subject to mic distances can be quite large. Positioning
>> the array where the sonic events and the local acoustics are not at
>> cross-purposes is one way to imagine the task more comprehensively.
>>
>> I'm most fascinated by sound imagery that portrays as much space as
>> possible while maintaining the unique nuances of the contained
>> sounds. I agree with others that adjectives like, "full," "open,"
>> "spacious," "clear," "transparent," and "immersive" hint at qualities
>> behind this listening experience. Add to this the selection of
>> passages when the combination of sound elements and patterns take on
>> characteristics of language, music, narrative, (create expectation
>> and engagement) -- the experience of the reproduction can become
>> quite enveloping.
>>
>> Here are most of the acoustic factors I usually consider with steps I
>> tend to follow:
>>
>> (1) I usually avoid setting-up near any sound source that creates
>> long duration or sustained sounds including obvious /air traffic
>> intrusions and natural subtleties like insects, water, leaves in
>> wind, etc. I try to place sustained elements on opposing sides of the
>> stereo field when their rhythmic natures are compatible and in the
>> front and rear of the field when they are not .
>>
>> (2) I avoid exploring positions close to sound reflecting surfaces,
>> including the ground at least initially. Close reflectors can be
>> incorporated into the overall reverberation scheme but I find it
>> easier to start studying reflections in the open. My portable stands
>> extend to 7-8 feet but I sometimes set-up as high as 10 feet by
>> tying-off to trees. Positions near ledges, of course, can increase
>> the effective height on one side dramatically.
>>
>> (3) In order to get a sense of the local acoustics and ways to
>> incorporate them, I stand in a mic location selected on the basis of
>> biological clues and use hand claps or a clicker to determine where
>> the primary sound reflectors relative to this point are located (
>> e.g. the directions from which the claps echo back to me). I imagine
>> the direction towards the loudest reflector as a potential center
>> axis of my stereo array, then I try to judge whether reflections to
>> the sides are balanced (whether similar or different in terms volume,
>> mostly). If one side is louder, I move away from that side clapping
>> about every 10 steps until the side reflections start to blend with
>> more interesting character.
>>
>> (4) After I find a spot where the reflections from the sides blend
>> together well, I experiment moving towards and away from the primary
>> reflector noting changes in the overall character of the
>> reverberation as the reflections mix. Sometimes small distances
>> create profound changes, other times, the changes are slow and
>> consistent. The reverberation formed when reflections decay together
>> smoothly is often preferable to distinct reflections and
>> significantly delayed echoes. Differing decay times can often be
>> adjusted by changing the array position to feel more related in
>> proportion. When the reverberation is very long and pronounced, I
>> then consider orienting the array so the direction of the movement of
>> the reverberation is aligned with its center line. I try to avoid
>> positions between two parallel reflectors with chances of flutter
>> echoes. When I find that the movement of the reverberation sounds
>> best when it moves from one side of the stereo field to the other, I
> > try to find ways to balance-out this inherent asymmetry. Reflections
>> from above are often less localized but if they are localized, they
>> can blend better if placed more towards the center axis.
>>
>> (5) I try to get to the site early so I can go through these
>> preparations, set-up, start the gear recording and be gone hours
>> before the expected "prime time." Rob D.
>>
>>
>> --
>>
>
>
>
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