[CW] Headphones or Loudspeaker?

[Richard Knoppow]

    The acoustical resonator goes back a long time. Long ago I 
found an article from someone at M-G-M, I think, about a 
resonator to fit on the Western Electric condenser microphones 
they were using in the late 1920s. It had a tube and baffle to 
reduce the cavity resonance of the mic (nearly 10db at about 
3Khz). This was before the method of design of electrical filters 
was generally known. I have no idea of how well this worked but 
suspect they had problems with the directional properties of the 
combination.  The idea for the speaker is to put a simple 
Helmholtz resonator in front of it. The resonance is controlled 
by the volume of the chamber in front of the speaker and the 
length of the tube. All good physics and should work. It seems to 
me that T.R. MacElroy sold a code practice oscillator with some 
sort of resonator on the speaker. Not sure how he did that.
     There were a couple of brands of headphones in the 1920s 
that had adjustable resonance. One used adjustable spacing of the 
diaphragm. Never seen one in the flesh.

On 6/15/2018 11:02 AM, D.J.J. Ring, Jr. wrote:
> Steve N4LQ has a wonderful video about resonant speakers.
> 
> He has a SKYTEC speaker and he took it apart and made a practical 
> copy.
> https://www.youtube.com/watch?v=MF8bk6958Hg
> 
> *Magical Audio Filter
> *
> MAGICAL AUDIO FILTER USING MC1458 &
> ​ ​
> MAX295 IC1s BY JIM PEPPER
> ​  BOARD FOR $6.50  available from faircircuits​.net 
> <http://faircircuits​.net>
> 
> 
> 73 Nov 96 
> <https://archive.org/stream/fea_The_Magical_Audio_Filter/The%20Magical%20Audio%20Filter#page/n0/mode/2up> 
> https://archive.org/stream/fea_The_Magical_Audio_Filter/The%20Magical%20Audio%20Filter#page/n0/mode/2up
> 
> DESCRIPTION: This circuit uses two MC1458, one LM386, one MAX295, 
> 78L05, and 78L12.
> 
> Note: Maxim has obsoleted the Max295 IC's; Make sure you can get 
> the IC's before ordering the board!
> (The MAX 295 is available on eBay for two for $13.00.)
> 
> ​Milestone Technologies sells a CW filter, too.
> https://www.mtechnologies.com/misc/filter.htm
> 
> 73
> 
> DR​
>

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL