Date: Mon, 16 May 2005 17:11:56 +0000
From: "Nicholas Drozdoff" <email@example.com>
Subject: [TPIN] Some
fun trumpet demos, and a neat day with my students a bit off...
I want to share some neat things that have been going on with my
physics classes and remind you of a couple of neat demos.
My classes are in the final stages of my favorite unit - the physics
of music. I had some excellent presentations already. A couple of
students built PVC flutes and played them beautifully - excellent sound
great pitch. On top of that they prepared a power point worthy of a
collge class. Two other groups made hollow metal chimes and discussed
the physics beautifully. Two gilrs beautiful balaika and discussed the
acoustics very nicely. We had one fantastic presetation on piano
construction. All in all we are off to a great start.
In the process of observing, I was reminded of two neat demos that
shed light on how a trumpet works. I going to be doing them both
today. I got them from the ITG Journal. You can search the journals and
find the articles. I think one was written by Thomas Moore. The other
was a scholarly article submitted about trombone. I've adapted it to
One: trumpet versus the bubble
I take a tupper-ware container just big enough to fit the bell of my
trumpet into it and then fill it with diswashing soap and water
solution. I mix in a bit of glycerine. The object is to be able to make
really nice bubbles. Then I dip the bell of my trumpet into the soap
solution. This will leave a nice bubble film in the bell of the horn.
It will recede into the throat of the bell opening a bit, but you'll
see it there. Now, once you've got a good bubble film in there, you'll
need to work fast. In this relaxed state the bubble film will succumb
quickly to gravity. Play a note and play it loud. It is amazing to see
how slowly the bubble fills even at FFFF! Many students will assume
that because sound moves at 340 m/s that the bubble should fill quickly
and burst in an instant. This speed isa WAVE (AC) speed and the waves
will pass harmlessly through the bubble. The actual air-speed (DC) at
the bell is remarkably slow. It is interesting to estimate how many
liters per second one is expelling while playing trumet. Check it out!
It's a lot less than you think!
Two: the stopped trumpet
This is an odd ball. I take a plastic bag and seal off the end of my
bell by duct-taping the bag over the bell as tightly as possible
without breaking the bag. Thus I can't get any air through the trumpet.
Of course, when I try to play, I get a very short burst of sound as the
bag stretches tight and then nothing as the air-flow is stopped. Then I
take a prepared special mouthpiece and put it in, and play just fine
(ableit with a slightly muted sound). The kids take a minute to figure
it out. Usually somebody notices about 2.5 inches of coffee stirrer
straw sticking out of the side of my mouthpiece. This demonstrates the
difference wbtween AC and CD. The AC part goes right through the bag
(as it did with the bubble). The DC part, or flow was required only for
one thing: to cause my lips to vibrate. I just drilled a small hole
through the side of the mouthpiece bowl into the cup so that air will
vent to the side instead of through the horn. The air doesn't even have
to go through the horn in order to make it play! This certainly raises
questions about bore sizes and air, doesn't it?
Here's another twist on the second demo. If I don't use the straw, but
only just let the hole vent the DC part, the horn doesn't resonante
well at all - no slots. The RESISTANCE is TOO low! I need to insert
some straw to add resistance even to a side flow like this. As the
straw gets longer, I will hit a maximum resonance point. As it gets TOO
long, the resisitance gets too high and it gets harder to blow. Again,
the resonance starts to dwindle.
Fun stuff and easy to do (as long as you have a mouthpiece that you
don't mind messing up). Also, the bubble dem is a nice prelude to
cleaning your horn. Plusses all the way.
Thanks for your patience with me, recently! I'll be OK. ;-)