>John Hartog wrote:
> > As has become clear from these discussions so far, there are many
>factors to consider when >determining the best height for a
>recording:
> > - sound horizons
>> - primary ground reflections
>> - phase issues
>> - shadows from ground contours and obstacles
>> - vertical angle of mic axis
>> - shelter from wind
>> What else?
>>
At 10:56 PM +0000 8/6/09, picnet2 added:
>Temperature and wind ?
>
>- ground material and coverage e.g snow is an excellent absorber.
>- resonance / recording in a cave?
>- reflection, or flutter echo if in an enclosed cavern as the sound bounces
>from one wall to the next. - Mind you that would be natural, where
>the mic is placed
>to capture the effect may be challenging.
>- diffraction effects, sound over walls, bushes/trees/houses, road
>noise barriers (specifically for low frequencies) - apertures / gaps
>between trees etc. someone's head in the way, standing too close to
>the rig.
>- refraction - time of day, temperature and wind effects
>a sound source in uniform conditions will reach the mic directly (or
>rather the wavefronts will) a source propagating where the
>temperature change is from cool to warm, causes the source to
>propagate down. - opposite is where the change is from cool to warm,
>the source bends up. Depending on the temperature gradient. - This
>may be in effect near lakes / sea - or in general temperature
>boundaries.
>- wind gradients, a source will be heard downwind, yet will be in an
>acoustic shadow if upwind.
>
>some of the above is paraphrased form the Master Handbook of
>Acoustics 5th Edition.
>
>Quite an eye opener.
>
>BR,
>Mike.
>
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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