A cylinder with a closed end is a "closed cylinder" and requires the
formula that uses 2f instead of f.
The "real" formula accounts for lots of variables, including diameter of
the sphere. But a ballpark formula that works just fine for less complex
designs is just Length (in feet or fractions of a foot) equals the speed
of sound in air (in feet per second which is approximately 1100) divided
by either f (for an open end cylinder) or 2f (for a closed end cylinder)
where f is the desired frequency of resonance you want. Much like
making a simple dipole, you can start with the formula but it takes
trimming to get it optimized.
Example: You want a speaker resonant at 600 Hz. It is closed at one
end ( the end in which you mount the speaker). The tube is *about* two
inches in diameter. L=1100/2f -- L=1100/1200 -- L=..916 ft. --
.916 inches x 12 inches = 10.9 inches.
The preceding is not perfect but, like a dipole, it needs final
trimming. That's where an adjustable design (like the PVC versions with
a sleeve) make it easier to trim vs. building a wooden box (like the
KD1JV box design).
73, Stan WB2LQF
On Fri, Feb 17, 2017 at 06:07 PM, Mark Petiford via Elecraft wrote:
I think a pipe with a speaker in the end is considered a closed end
cylinder, but I am not completely sure. If so, resonant lengths are
shorter. Here is the page from one of my favorite websites if you are
interested in experimenting with resonant speakers:
http://hyperphysics.phy-astr.gsu.edu/hbase/Waves/opecol.html#c1
Good educational website from Georgia State University.
Mark,
KE6BB
On Friday, February 17, 2017 2:14 PM, Mike Furrey wrote:
I have not but you certainly can do it. The length of the tube will
determine the peak resonant frequency and you would have to experiment
with the length to tune it to the pitch you like best to listen to.
Remember the days you would blow across a bottle to make a tone? The
more you drank out of the bottle the lower the pitch. I like this
project and I am going to build one and it will be with a straight
tube mounted on a little stand so it aims towards me.
73, Mike WA5POK
On Friday, February 17, 2017 12:58 PM, P.J.Hicks wrote:
Has anyone built the resonator with a straight length of tube?
Perhaps terminated with a 45 deg. cut? Might be an interesting
experiment.
The 45 deg long radius street elbow in 2" is an order only item in my
area so thought I'd substitute a straight length.
PJH, N7PXY
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