My daughter's science fair project is testing the frequency range of
various animals hearing.  What should I look for in a device that can
generate a sound in various frequencies.  If possible I would like it
to accept a short recording.  A kit would be OK.  (Remember, "science
fair" =
inexpensive).

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Request for Question Clarification by haversian-ga on 11 Nov 2002 12:59 PST

Hello dvh-ga,

What exactly are you looking for?  There are tons of different ways to
approach this question, and while you mention a kit, that's only one
option.  So, we need to narrow down what your requirements are and
what sorts of solutions you can handle (vis. soldering / bringing a
computer in / etc.).

Do you need to know the frequency of the tone, or just change it?

Do you want a circuit diagram?  A kit?  A pre-assembled device?

Do you have a computer and speakers so a software package would
suffice?

Does the volume need to be adjustable?  How loud should it be?

Answering some or all of these questions will help us give you a
better answer, rather than a long list of options that may or may not
suit you.  Thanks.

-Haversian

Sorry about that, I clicked the wrong button, but I'll do my best now.

What you are looking for is the PIC 16F84 based tone generator,
purchaseable from http://members.itol.com/~wolson/chipordr.htm. 
However, this requires you to wire the entire circuit yourself, using
schematics downloadable here:
http://members.itol.com/~wolson/projects.htm
Another such buildable frequency generator is:
http://www.imagineeringezine.com/PDF-FILES/noisegen.pdf

Using a laptop+speakers, there are a large variety of programs out
there that can do exactly what you are looking for, for much less
money and less hassle.
Some of them are even free:
Frequency Generator (Simple, free, works quite well.  I recommend
using this one):
http://www.pdacentral.com/epoc/preview/48073.html
NCH Audio Tone Generator:
http://educate-yourself.org/nch/

Lastly, sound generators, albeit less accurate then the ones listed
above, are also available in kit form that are simple and easy to put
together:
http://www.qkits.com/serv/qkits/velleman/pages/MK134.asp
http://www.bullnet.co.uk/shops/live/electron.htm (ULTRASONIC DOG
WHISTLE KIT for higher frequencies)
http://www.electronics123.com/amazon/catalogue/c3-3-1.htm (The siren
product comes with a variable resistor, which can alter the frequency
of the output)

Your best bet would be to go with the software solution.  If that is
not possible, the next is to use the sound generating kits.  You can
test the frequency of the sound produced by using a tuner.  It is also
possible to calculate the frequency produced, depending the resistor
used.  The last option would be to build the circuit from scratch,
though that would be quite time consuming and laborious for a science
project.  Professional sound frequency generators with high accuracy
are in the $100+ range and I'm guess that's out of the question.

Hope that helps.

-Tox-ga

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Request for Answer Clarification by dvh-ga on 12 Nov 2002 07:00 PST

In regards to your questions:

All this is for is a 7th grade Science Fair Project.  My daughter
wants to be able to set the frequency and volume and test at what
levels our dogs, cats and birds can hear.

I just thought it would be better at getting the animals attention if
she could do this using their names rather than just a tone.

I have downloaded the NCH software and it will should work, but using
a PC would not be as portable as a standalone device.

Thanks very much.

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Clarification of Answer by tox-ga on 12 Nov 2002 12:23 PST

dvh-ga,
Tones are the most accurate for specific frequency generation.  This
is because their wave form is completely sinuodial.  If you record and
alter the frequency of the recording, you get a large range of
frequencies because human speech registers very eratic patterns when
you graph its wave form. You can use programs like Cool Edit
(http://www.syntrillium.com/cooledit/) to bump up the frequency of a
recorded sound but like I said, it will be quite inaccurate.  And
also, the links I supplied also contain build it yourself kits, though
a voice recording one with altered frequency would most likely be too
complex for a grade 7 science fair.  You best bet would probably be to
output it directly from the speakers of your computer for optimal
control.  If you do not mind sacrificing quality, then recording it
onto a cassette tape and then playing it would also work.

Hope that helps.

-Tox-ga

Keep in mind that with the software option, the computer speakers may
not be able to accommodate higher and lower frequencies.

I have used NCH tone generator before, and with Harmon Kardon
speakers, it was having problems above the 17 KHz range. Also, in the
lower frequencies, I'll often just get a pop and static.

Depending on what frequencies you are trying to produce, the speakers
used to produce the sounds are probably more important than what you
are using to generate them with.

Hello dvh,

I use a the Radio Shack Super tweeter, (Catalog number 40-1310) to
test the hearing of cats and mice in the laboratory.
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F002%5F006%5F005%5F007&product%5Fid=40%2D1310
This speaker is capable of presenting sounds from 2,000 to 40,000
Hertz.  It retails for $30.  I have been told that you can modify it
to present low frequency sounds using a wire snips and soldering
equipment.  However, ordinary computer speakers are capable of
presenting low frequency sounds.

A major obstacle to testing hearing is determining the loudness of the
sounds that you present.  I calibrate my speaker each time that I use
it, because the loudness (dB SPL) can be changed by factors such as
the temperature and humidity of the room.  Accurate microphones are
expensive.

Good luck with your project.  I will try to comment on some of your
other questions as time allows.  What are your time constraints?

--neurogeek