Third grade science report: Physics of Sound

[Leigh L. Klotz Jr WA5ZNU]

I am presenting a 2 hour adjunct to the "Physics of Sound" science unit
for the 3rd grade at my local elementary school here in Palo Alto, CA.  
I have recently finished the unit with one classroom, and it went quite
well.  Below is the report of the first class, with which I did one hour
units each on "Time and Frequency Domain Analysis" and "Communicating
with Sound (but without speech)" presented one week apart.

I have a copy of the syllabus I developed (sketchy though it is) at
http://wa5znu.org/2006/11/sound and it includes a link to the take-home
sheet I made showing where to get the PC software for home experiments.

The first hour was on time and frequency domain, and I appeared at the
appointed hour in a lab coat with the name label "Joey" sewn on.  
(Thanks Joey!) I brought a half-yardstick and marker, an HP audio
oscillator, speaker, microphone, laptop with free PC oscilloscope
program, a stringed instrument, a microphone, a toy piano, and an AM
radio.

As I set up the laptop and projector, the teacher talked about pitch and
loudness, a subject previously discussed.  Once set up, I asked the
class to demonstrate 3 of the 4 attributes (high, low, soft) and asked
for ideas about why I wouldn't ask a 3rd grade class to demonstrate
"loud."

I explained cycles and seconds and connected Hz to other units they had
studied (degrees, centimeters, inches) through questions to the class,
and asked for the range of hearing, and eventually arrived at the
standard answer.  I then demonstrated a 1KHz tone and we wrote down a
few guesses, but I didn't disclose the answer.  I showed the oscillator
and explained a brief version of the HP founding story (it was started
and is still here in Palo Alto: only one kid's parent worked at HP; I
had expected more).

I got a few volunteers help make a mechanical oscilloscope...we taped a
marker to the end of a yardstick (I used an 18" one) and set it to
oscillate while another kid pulled a sheet of flip-chart paper, and the
class timed it.  We all observed the sine waves (a bit distorted) and
many kids gasped as they saw the same shape they'd seen in the diagrams
they'd been shown before in class.  We then counted the cycles (9) and
divided by the time in seconds (6) to ger 1.5Hz.  About 5 kids could do
this division, and I had previously related unit division to
miles-per-hour.  One kid suggested if we shortened the stick it would go
faster.

Then I showed the PC scope and the oscillator with the 1KHz tone, and
adjusted both the frequency and amplitude controls up and down, showing
this all in the time domain.  With an older class, I might have done the
division at this point, but in order to make the concepts more
accessible, I told them we could use the computer to do the frequency
calculation for us, and switched to a power spectrum (FFT) display,
which showed a nice sharp peak at 1000 Hz.  We then compared their
answers, which had been in the 2000-6000Hz range.  We discussed the
range of hearing of children, adults, people, dogs, and bats.

I told them that volume, although we wouldn't be exploring it, was
measured in deciBels, and the teacher wrote it on the board and
underlined the metric prefix, to tie it into a previous unit on
measurement.

We tried the various instruments, and multiple sounds to see multiple
peaks.  I pointed out the 1KHz peak among the noise of 20 kids talking,
and showed them a chart of the cochlea and likened its spiral to the
power spectrum X axis, and said that roughly speaking that is why they
can hear both the tone and the noise at the same time.  I asked if
anyone ever had sudden ringing in the ears and said that some doctors
think it is caused by a single hair cell in the ear.  I pointed out the
height of tone above the noise, implicitly demonstrating s/n ratio.

We then tried pitch matching and saw that the voice contains multiple
frequencies, and that the stringed instrument had strong 3rd harmonics,
whose values they calculated.  This led to a discussion of octaves, as
about a third of the class took piano.  (I wanted to get into other
intervals and ratios but there wasn't time.)

We did some matching of the oscillator to the tones of the instruments
and voices, and closed with a review of pitch and volume with the
oscillator on both the time and frequency displays.  There was a lot of
demand for higher and higher frequencies (mostly from the boys, for some
reason), so next time I'll try to find a higher fidelity speaker.  But I
did demonstrate leaving the audio range, and went up the 1 MHz, where I
showed the electrical waves now being called radio waves, and picked up
the scratch signal on an AM broadcast band radio at 1MHz.

By the end of the hour, the kids could answer questions about
measurement of frequency, knew the range of hearing and could estimate
the frequency of an audio tone,  knew what Hz was, and had a few ideas
for experiments to do at home (no PC required).

For the second week, I brought two laptops loaded with the PC version of
PocketDigi (a very uncluttered digimode program available at
http://pocketdigi.sourceforge.net) an external mic, and a CW practice
set, composed of the ARRL No-Solder oscillator and the IOWA QRP key.  
(These last two are appropriate level science fair projects for
elementary kids.)

I asked for some ways that we could communicate with sound but without
talking, and we discussed speech acquisition by babies (or is it cry
recognition acquisition by parents ;) and a few other ideas.  One kid
suggested morse code, so I brought out the key and showed them the
PocketDigi waterfall display (note to self: ask Vojtech for a
right-to-left scroll option), but we didn't get into CW at that point.

I briefly showed a PSK31 idle signal in both time and frequency domain ,
and quickly explained that when computers communicate with each other,
whether it is the Internet at home or over a telephone dialup, or
through air like we were doing, they always do it with waves, and that
these waves were one way of doing that communications.  I then typed on
one keyboard and they saw the signal on the waterfall and the text
appear, and we tested how far we could get before the signal faded into
the noise and the recognition stopped.  They had fun reading the text
messages I typed.

Then we segued to Morse code; one kid had studied it at a cub scout
troop (bless them) but didn't remember it.  Rather than doing a long
explanation of morse code, I said that I would now demonstrate how to
communicate by sound without talking, just by using my ears (and
cochlea) instead of the computer program.  Fortunately, I had a
confederate planted in the class who could send CW reliably.(Sorry if
this part is like like those recipes in the newspaper that say "Now
ladle on the sauce, which you should have prepared in early spring...")

I treated this demonstration like a magic trick, and with just a hint of
explanation, turned my back, covered my eyes, and asked my trusty
assistant to pick kids who had raised their hand (all of them did, of
course) and send their names, and I would figure out who each was.  My
pronouncements of RENEE and EMMA and ANDREW were greeted with as much
excitement as if I had pulled a live guinea pig out of my hat.  JOLLY
though produced quite a bit of laughter, though, and we ended the
demonstration and switched back to discussion with an explanation of how
someone could confuse H and J.

By this time, the class was ready for some information and discussion,
and I wrote down (though cautioned that it's not he way to learn the
sounds) E I S H A N T M and asked for opinions along the way about why
the letters were different length.  I mentioned a bit of the Alfred Vail
controversy to get them to cheer for the underdog assistant, and asked
for ideas about how to determine letter frequency.  After a bit of
discussion on this topic, I suggested that they all try making a letter
frequency chart of a page or two of their favorite book and see how it
worked out.  Intermixed was a bit more demonstration of sending the
high-point Scrabble letters Q, X, J, and Z, and a note that all numbers
are 5 elements and punctuation 6 (shown with period).  A few other free
discussion topics came up as well, in response to questions, and I
mentioned the Jay Leno (K7JA/K6CTW) results.

I picked a three-letter word and asked the class to call out the letters
as I spoke them (di-di-dit dit dit), and right afterwards was quite
surprised when a girl who spoke halting English asked to come up to the
board.  She wrote down something in dits and dahs and asked the class to
decipher it; she had written EINSTEIN.

We went back to discussing frequency and I wrote down the metric
prefixes going through KHz, MHz, GHz, and THz.  (I made a mistake on the
numerical value of THz and akid corrected me!).  We discussed familar
examples of each (sound, radio, computer speeds) and then for THz, I
told them about the frequency of light.  I then switched to showing
sound waves modulating light, using a fiber optic experiment kit (about
$20).  I went around the class showing the light going through the 1m
optical fiber, and used my finger to start and stop it in a tacit
demonstration of CW modulation.   Back at the demo table, I turned on
the tone generator in the experiment kit and showed sending CW over
light; several kids were sure it was a wire, so I pulled it out and
aimed the IR parts at each other; we saw how the sound transmitted over
light through air, and I explained that the frequency of the light was
something on the order of 300 THz.

It was then time for recess, and though I had brought my KX1 with the
intent of letting them listen to people "communicating with sound" over
the radio, there was no time.  So I left the teacher with 21 copies of
the handout sheet to be sent home.  On the way out the door, the girl
who wrote EINSTEIN stopped by and asked me please for a Morse code
chart.

The next week, the class had a test on sound and frequency, and reports
were that the two hours had really made the ideas "come alive" for the
students, and the classroom newsletter reported that the Morse code
demonstration was the big hit of the week.

73,
Leigh/WA5ZNU
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