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AW: bose pa speaker thingie



Matthias and also Tim already said some things regarding this topic, so
I'll only add what hasn't been said before:

The directional characteristics are a property of the source (or sink).
This property of the source or sink is usually dependant on the
frequency, but the directional characteristics are not a property of the
sound wave or its frequency.

A directional source (like a dynamic speaker) tends to become
omnidirectional if the wavelength of the waves reproduced considerably
exceeds the dimensions of the source (rule of thumb here). The
wavelength is the phase velocity (the velocity with which the wave
distributes, here "the speed of sound") divided by the frequency. That
means, with a phase velocity of ~300m/s and our speaker a diameter of
0.3m, we get f=1000Hz or a frequency of around 100Hz where the
wavelength exceeds the speaker dimensions considerably. Yet, if we
reproduced this same 100Hz wave with a speaker a diameter of thirty
meters, this speaker would be directional all right...

The same is true for the sink (in this case: your head): if the
wavelengths considerably exceed the dimension of your head, then it
becomes harder to make out the direction a sound comes from from the
difference in sound pressure level received by both ears. If a tone of a
sufficiently high frequency comes directly from the right side of your
head, then the perceived sound level in your right ear is much higher
than in your left ear because your head shields your left ear from the
directed sound wave. Note that there are other effects you can use to
make out from which direction sound comes: decrease in intensity  with
distance travelled and delay from distance travelled. Actually, two
standard stereo miking techniques make use of either the level
difference by the directionality of your ears (X/Y, coincident miking)
or of the delay (A/B, spaced pair) exclusively and in an exaggerated
way.

Still, you can perceive the direction sound comes from even for
extremely low frequencies, although this only works for very high sound
pressure levels. During thunderstorms, you sometimes feel the very low
frequency components with your body and are able to make out a
direction.


There has been some confusion going on about this topic in recent years,
caused by companies who try to tell you that you can get "cinema" sound
with a sub and a set of miniature sattelites with xover frequencies of
130Hz, 180Hz or even higher. If you do however look up the specs for
Dolby Digital, you'll find that while the LFE (which stands for "low
frequency effects" and is to be used for the "big bangs" of planes
exploding or atomic bombs going off exclusively) channel has a frequency
range of 3-120 Hz, the remaining five (or more for the EX format) front,
center and surround channels have a specified frequency range of 3-20000
Hz.

--- Travis Hartnett <travishartnett@gmail.com> wrote:

> Maybe I'm just weird, but I've never found bass to omnidirectional.
> Less directional than higher frequencies, okay, but
> it takes a lot
> less than a gun to my head to figure out where the
> bass is coming from
> when I see a band play.

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