I have experienced a good example of what Stewart is
talking about here.

Where I work, there was a large all metal building about
35 x60 feet with 25 foot sloping ceilings. It held some electrical
and electronic equipment all around the periphery. There
were about 3 dozen microprocessor monitor panels with
each piece of equipment being monitored. When certain
conditions occurred, an alarm would sound from that panel.
It sounded like maybe 1khz, and a sine wave or close to it.


Well if an alarm sounded it was pretty much impossible in this
highly reflective open space to figure out where it was coming
from. The alarm was the tone only. So you had to start at
one end, pull up the alarm display screen, and work your way
through it until you came across the right one. A terrible
design. A light should also have blinked on the panel or
something. This is also highly disconcerting to move where
it seems to be coming from only to have it sound like some
place else when you get there. You simply couldn't
tract it down via the sound.

Dennis

"Stewart Pinkerton" wrote in message
...
On Wed, 22 Oct 2003 18:24:15 GMT, chung wrote:

Stewart Pinkerton wrote:
On 21 Oct 2003 22:48:38 GMT, chung wrote:

All Ears wrote:
"Bruce Abrams" wrote in message
...
snip

You just can't reproduce sounds from behind you with
speakers in front of you. It's impossible.

This is not entirely true, actually the ears works as a filter, so
that
sounds comming from behind will have a dramatic drop in the 8 kHz
area. This
means that if you cut the 8kHz area from a sound image, it will
appear as
the sound is comming from behind.

KE

I don't think it's that simple. Let's say you play some bass notes on
speakers in front of you. There is no 8KHz in those bass notes. How can
the ear interpret those as coming from the behind?

How can it interpret them as coming from *any* location? Ever tried to
decide which phone in a modern office is ringing?

OK Stewart, I'll bite. How does one know if sound is from the back?

One doesn't, unless one turns one's head. We often have difficulty in
locating sound directly behind us - until we turn our heads slightly.

I have always thought that it is the difference in amplitude and phase
of the sounds arriving at the left and right ears that provide location
information. But obviously, if a sound is coming from behind you in the
middle, there may not be any difference in what each ear perceives.

Quite so.

On the other hand, I don't think it's as simple as a notch filter around
8 KHz.

Not notch, low pass.

A telephone ringtone does not have much 8KHz component, yet
clearly I can tell if a phone is ringing behind me.

A mechanical bell tone most certainly does have significant HF
content, and such a ringtone is *much* easier to locate than modern
tone generator 'rings'. Do you really claim to be able to locate an
electronic ring, such as most mobile phones or office phones?

When one of the many
phones rings, I think most of us will turn our heads to find a spot
where the sound is somewhat front-center of us, and then focus on the
ringing source. Well, most of the time at least. What if a 1KHz tone is
played. Can one tell if the tone comes from the back or front without
turning one's head?

There is absolutely no way to locate a steady sinusoid, except by
walking around to follow amplitude variation. Try it sometime.
--

Stewart Pinkerton | Music is Art - Audio is Engineering