Re: [Phys-L] Suggestions for audio speakers

[John Denker <jsd@av8n.com>]

Let me address a couple of points that are relevant to the body of
the original message.  Some of this is relevant to the subject line,
and some not.

On 03/11/2013 07:20 AM, Forinash III, Kyle wrote:

> I will be teaching a physics of sound class in the fall

OK.

> I have a CD that plays a sine wave starting at about 20Hz and sweeps
> up to 20,000 Hz

For those who don't already have the CD and/or want to do things
that aren't provided by that CD ... you can make simple audio files
from scratch using the "mkwav" utility.  The C++ source is here:
  http://www.av8n.com/sound/

It will make sine waves or frequency sweeps.
It can do mono if you like.
 ++ It can also do stereo, with the right channel offset from the
  left channel by a fixed musical interval (i.e. frequency ratio).
 ++ It can also do N channels if you like, for any N.
You can specify the frequencies.
You can specify the amplitude.
You can specify the duration of the recording.
You can specify the sample rate.

In the same directory you can find pre-fabricated frequency sweeps
(as .wav files) sweeping from 20 Hz to 20 kHz at the rate of 
0.5 octaves per second.

> I ask students to raise their hands when they can't hear it anymore

You'll get better psychophysics data if you play a short beep-beep-beep
sound at a selected frequency ... and selected amplitude ... and ask them 
to raise their hands if/when they hear something.

====

There are "virtual piano keyboard" apps that run on your computer, so
you can play any sound you want, whenever you want.  There are midi
patches that put out a sine wave.

Midi codes span more than 10 octaves, which would be more than you need
for the given application, *except* that the midi low-end and high-end
are both lower than is ideal for this application.  You can fix this by 
finding a "transposing instrument" that plays all midi codes an octave
higher than written.  I assume a transposing sine-wave patch exists or
could easily be constructed, although I can't point to one at the moment.
See e.g. http://freepats.zenvoid.org/

============

>  I would like to get a good set of speakers/monitors and amp system (has to be portable). 

To serve the narrow purpose of the raise-your-hands demo, and a lot 
of other purposes besides, your best option might be a consumer-grade
"home theater" system, perhaps a 7.1 system, perhaps something like 
this:
  http://www.pyleaudio.com/sku/PT798SBA
I have no experience with that particular item;  I cite it only as an
existence proof, not a brand-specific or model-specific recommendation.

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.

Having a large number of smallish speakers works waaaay better than having
a couple of larger speakers ... especially in a room as big as an ordinary 
classroom.  The smallish speakers are definitely portable.  You can port them
one at a time ... or you can pile them into a box and port them all at once,
and they will /still/ weigh less than two big speakers with comparable overall
output quality.

Also, having a multi-channel system allows flexibility to do lots of other
stuff in the future, including sending different signals to each of the N
channels.

Note that for a few bucks you can get a dongle that takes USB in and puts
7.1 S/Pdif out.  It looks to the operating system like a sound card.  I
mention this so that you don't need to worry about being limited by the
capabilities of your existing sound card.

Along the same lines, there are good reasons why you do not want an analog
connection from your computer to your fancy audio stuff.  S/Pdif is a fine
way to make this problem go away once and for all.