Date: Tue, 09 May 2000 11:10:14 -0500
From: "Maddog 'n' Miracles" 
Subject: More Resonant Speaker Stuff

Hi Ed:

Thanks for your feedback on my WE6W resonant speaker prototype.  I've had a
chance to look more closely (with better equipment) at the behavior I'm
getting from my speaker.

With the short-tube/disc combo at the halfway point, the most prominent
peak is at 780 Hz.  I attempted to "null" the small plunger at 900 Hz, but
no null could be found.  So I set it about 1 inch recessed into the plunger
(away from the sphere).  With the sliders at these positions, I had a
prominent peak (not quite as strong as the fundamental peak, maybe 3-5 dB
down) at 2-f and 3-f, and then also multiple weaker peaks at lots of higher
frequencies (maybe 6-10 dB down).  Below 1-f, and from 1-f to 2-f, the
speaker is very quiet, but I do not have any measurements to put on this.
I didn't actually measure the bandwidth at 780, but it's definitely less
than 75 Hz.

I did these tests in a quiet room with small-to-moderate drive, so as not
to come even close to overdriving the speaker.

I wanted my speaker to resonate at about 650 Hz, so I slid the
short-inner-tube/disc outward until the outer end of it was flush with the
outer end of the 8.5"-long 2"-diameter tube.  At this point the resonance
was at 670 Hz.  I was able to get a very weak null at 800 Hz (I think)
using the small plunger at about 3/4" recessed (away from the sphere).
Again, I had significant peaks at 2-f and 3-f, and lots of weaker peaks at
higher frequencies.

I've attached a drawing of my construction for your review.  Maybe you can
see what's causing the peaks at even harmonics.  It's in Word 97 format,
and if you can't read it, let me know what might work better for you.  I
could certainly mail it to you also, if nothing else.  (I'm not quite ready
to put the drawing on the web page yet).

You'll notice a few deviations from your original description, aside from
materials.  The speaker may be physically larger (although the cone
dimensions are very close at 1-1/8").  The short inner tube is a larger
diameter (1-3/8" as opposed to 1-1/8" I think you suggested).  The speaker
wires are brought out close to the speaker.  Perhaps one of these is the
culprit.  I am particularly suspicious of the speaker, which may have some
seriously non-linear properties.  It seems to have plenty of smooth cone
travel, however, and I don't THINK I'm overdriving it.  Maybe also, I have
put the short-tube/disc combo together wrong... for example, in my version,
the single wood ring is epoxied midway along the outside of the short tube,
and the whole assembly slides in and out of the long 2" diameter tube...
since I've never seen a picture, maybe I interpreted the construction wrong
to begin with.

Anyway, thanks for all the attention, Ed!

monty  N5FC