Re: [Phys-L] Suggestions for audio speakers

["John Clement" <clement@hal-pc.org>]

But it all depends on the application and the size of the room.
A home 7.1 system may not have enough volume if you are in a large lecture
hall.

A 7.1 system will not be suitable for demonstrating interference patterns.
For that you need 2 dedicated speakers or 2 stacked arrays of R/L with no
sub-woofer.  Any stacked speakers would have to be identical to prevent
phasing problems.

A 7.1 system works by sensing the phase of the 2 channels and then channels
the sound appropriately to the specified speaker.  7.1 messes up any
interference or phase demos.  Also the signals are added together at the low
frequencies for the sub-woofer so you can not demo superposition at low
frequencies.

So in the end the simplest is to have 2 speakers that fit into a small
duffel for portability.  And of course you will not achieve usable response
anywhere near to 20Hz with such a system.  There are any number of speakers
which claim to be monitors that would satisfy this.  But they will not have
very wide dispersion so the high frequency response will be severly
attenuated at off angles.  You will pay dearly if you want good dispersion,
and broad frequency response.  And adding response below 50Hz at decent
volume will also generally cost a lot as well as adding mass.

When setting up a speaker system it is desirable to have the speakers either
above or below the audience.  You need to aim them at the back row, with the
front row off axis so as to prevent the front from from getting very loud
sound.  The people in the front row will have incresed volume due to
distance, but decreased due to being off axis.  While monitors should be
placed aimed at the listener for a single person or a small group, this
should not be done for a larger group.  Of course if you keep the speakers
at a modest volume level the only problem will be that the back row will not
hear them as well.  But if you are demonstrating how the sound falls off
with distance this may be desirable.

7.1 systems may be very pleasing for reproducing audio designed to take
advantage of them, but they do have some severe limitations, and are
probably not optimal for physics demos.  The subwoofer will probably not
output anything if the 2 channels are out of phase.  I don't know exactly
how the active circuits handle the phasing.  Sometimes this is done
deliberately for special effects.  Newer recordings are probably designed
with the bass in phase so as to prevent subwoofer problems.  Older
recordings or poorly mastered recordings may not have proper phasing.  At
one time recordings were all auditioned to sound good both in mono and
stereo, but I don't know if this is still done.

In the end the best option is to try various speakers until a satisfactory
one is found.  Since the actual purpose of the demo was not specified it is
very difficult to recommend specific equipment.  If it is a one shot demo,
then expensive equipment would not be desirable.

John M. Clement
Houston, TX

> Within the 7.1 category, it would be ideal IMHO to have 7 
> identical main speakers and one subwoofer.  Usually the best 
> you can do is 6 identical main speakers, plus one 
> differently-shaped "front center" speaker, plus the 
> subwoofer.  That should be good enough for the application.