Posted by: "Steve Pelikan"
>
> Walt et. al.:
>
> This is a question I've worried about a fair bit.
>
> Walt wrote: (about where to mount microphones)
>
>> > The higher you get the less cluttered the signal. Near the ground you
>> > are recording a mix of lots of reflections as well as the direct signa=
l.
>
> Is this because, by putting the mic well above the ground you're making
> the sound reflected off the ground follow such a longer path that
> (inverse square law) it has much less energy than the direct path?
>
> This I understand, but the then same or better effect would be attained
> by putting the microphone directly at ground level, since then there'd
> be no reflections off the ground?
>
> Or are you referring to some aspect of sound propagation through the
> vegetation near ground level, in which case how does the optimal height
> depend on the habitat? (I know and agree with you that the best answer
> is "listen and you'll know" but could you offer some guidelines as
> starting points?)
It's really all of the above and some more.
Back when I did EIS work for a living we were studying a area for a
campground on the North Cascades Highway. The question came up just what
height we should use to measure the sounds from the highway to the
campground area. This was a forested area with undergrowth, typical NW
forest. We did some measurements from ground up to 10' in height.
Depending on the exact location there was considerable attenuation of
the highway sounds at ground level up to about 6' - 8' or so. Above that
height fairly consistent sound levels were found. Note we were only
interested in attenuation and did not get into any frequency balance
checks or so on. We ended up reporting for that EIS the standing and
sitting heights if I remember right. So, you see, placing a mic at
ground level will not work all that well unless it's a flat ground, like
a indoor stage. Any irregularity in the ground will attenuate sound.
Yes, attenuation will be site dependent. You really do have to listen
and think about where you put that mic.
Yes, reflections do attenuate with distance. Also if a mic is
directional and high then the local reflections may not be picked up at
all, or greatly attenuated. Many of the local reflections are quite
weak, so won't make it up to a high mic much, this is probably the
greatest removal of clutter.
The polar pattern of your mic is given on spec sheets as 2D, but it's
really a 3D pattern you work with. You need to be aware of the
soundfield in 3D.
In my case with frogs where I often have both immediate and distant
callers the height means the immediate callers are not right at the mic.
This makes it easier to include the distant callers.
If you are trying to record what a bird in a tree might hear, then your
mic should really be up there, as the bird will hear something quite
different from what you hear at ground, or even standing level.
Note I don't say you must go high, I record a lot handheld, and that's
not all that high. I can hear the difference between holding the mic at
chest height and holding it at arm's length above my head, however. I
just suggest it's worth experimenting. It will add a new aspect to your
recording to have the height option.
One of the places I record has a pond with tall large trees to the right
and pretty open to the left and straight ahead. Recording with the SASS
at ground level you cannot really read that arrangement very well. But
put the SASS up on the high tripod and you can practically point at each
large tree trunk to the right listening to the incoming signal with
headphones. The open space ahead is evident as a silent hole, and so on.
That's the sort of thing I mean by improved clarity at height.
And, back to the subject of mic stands, the same tripods I have for 17'
up will also hold the mic at 4' up, or any height in between. Something
to think about, a stand that can go high adds more choices. It will
necessarily be heavier than one that can only do low.
Walt
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