Curious - microphone 'distance control' focussing?

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bleyrad

New member
Out of curiousity, I'm interested in knowing if it's possible for a mic to 'focus' on sounds created within a certain distance range from the capsule... like 0-3 inches for cellphone mics, 50-75 feet for shotgun micing an orchestra but rejecting ambience and crowd noise (but shooting right THROUGH the crowd)... you get the idea.

is it physically possible to design a mic like this? the only way i could imagine it's possible is by somehow calculating the amount of time the sound is taking to arrive at the mic, by frequency-dependant phase variations or something (don't low frequency waves behave differently over time and space than high-frequency ones?) i don't really know... any thoughts?
 
No, there would be no way to systematically remove sound from a signal without destroying it's integrity as 'music'. Keep in mind of course, that the audience is no where near as loud as the rock band, so it's not a big deal once the song starts... and it's pretty acceptable to have audience sounds between songs on a live tape. Regardless, the audience is a very ambient sound... It will get picked up to some degree no matter where you put the mic assuming that it's a pretty big audience. The only way to avoid it, would be to have the microphone pointing away (i.e. on stage with the band) from them, and even still, you'll probably get some of the sound leaking in...
 
thanks, though i'm not concerned with the best way to do something like that, i already realize that there are easy alternatives for many practical applications. just thinking theory here.
 
This is just off the top of my head - I'm no expert.

If sound is anything like light, in order to focus it by reflection or refraction, the instrument to do this has to be orders of magnitude larger than the wavelength. Since sound has a wavelength of about a foot at 1khz, and about 50 feet at 20 hz, this doesn't seem practical unless you are building a concert hall or stadium.

There may be ways to use diffraction to create nulls and nodes, but again this would be frequency dependent, not "hi fi", and, I would guess, not practical in the real world.
 
the sort of focussing you're talking about (if i understand you correctly) is basically a "beam" to a specific xz coordinate. Like a satellite dish in reverse. There's something like that already in development - beaming sound like a laser, so no one outside a 3-foot radius (or whatever) of the beam can hear it. kind of like a super extreme cardiod SPEAKER.

but i'm talking about the y, or depth axis. distance.

upon further thought, it's probably impossible (or highly impractical). but i'm still curious as to what would be needed.
 
Here's another way of looking at it from wikipedia.org.

"Parabolic microphones are generally not used for standard recording applications, because they tend to have poor low-frequency response as a side effect of their design. This is a direct result of the physical laws that govern sound waves. Sound entering the parabolic dish from the front axis is reflected into a sphere around the "focus" of the dish, where the microphone element is located. The higher the frequency of the sound, the smaller the size of this sphere. Thus, high frequency sounds tend to be more concentrated onto the pickup element, while low-frequency sounds are "diluted" across a larger volume of space."
I suppose you could design some heavy compensating low freq boost or high freq cut eq into the mic preamp, like the RIAA phonograph compensation.

Here's some more info.

http://www.bahnhof.se/~telinga/attach.htm
 
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