Acoustical Resonator, Hi Gain Microphone Amplifier Circuit
Engineered By Pete Zenner , Sioux Falls, SD.
This was my first try at making a resonator. No 35MM film canisters were available, but after some thought I noticed that I had some parts sitting in
old pill containers which are just slightly larger than the plastic film cans. Coffees cans are normally about 5-1/2" to 6" long and 4" wide so I grabbed the nearest tin I could scrounge
from my recycling. Some measuring, drilling, dremeling and hot glue got it together. I think it'll be better once I cover the outside of the cans with something to deaden sound from the
The "coffee" can was a Hersey's chocolate gift tin. I can't find metal coffee cans anymore, but the size of a 1 lb can used to be 6" x 4". Since you
mentioned that you had used a 35mm film cannister, I found a pill bottle of approximately the same size. I traced the pill bottle opening on the bottom of the large can. I used a Dremel
tool to cut the circular hole. it requires a special technique. Use a regular round disc cutter and hold it at a 45 degree angle and just inside the line you scribed, start cutting a series
of small "smiles". Lift the cutter off and move it over, making a series of cuts instead of one single cut.
Once the inside piece falls away, use a half-round file to make the circle smooth and test fit the pill bottle. Once the pill bottle was snugly in place, I hot glue it
together. The microphone element is hot glued into a hole drilled into the base of the pill bottle. I think I mentioned that my test unit would "ring" quite a bit and was not very
directional. I knocked down both problems by finding some tacky 1/8" rubber sheet and putting it out on the outside of both the can and bottle. I attached a picture of 2 other resonators I
made one from hard plastic and a second from an identical metal can and pill bottle, except painted "covert black". Pete
Zenner has taken the 35 MM film Cup mentioned in the parts list for the DAA to another "High Gain" level - by
adding a coffee can to his film cup. Please read the text below his photos - to understand how acoustical resonators work.
(C) 2017 By: Bugplan.com
Last But Not Least - Regarding Self Resonance
Two Excellent Videos, And An Article, I Found On The Net, Among Many That Will Give You Food For Thought
How To Tune and/Or Determine The Resonant Frequency Of Your Cavity Device
One of the problems that I had - back some 21 years ago when I built a similar resonator was that I needed to tune the cavity within a wide bandspread in the audio range
and to keep the audio frequencies that I was trying to self resonant in an envelope so I didn't lose them at the 3db down level. I am a stickler for perfection. I created the Mockup shown
in the photo below.
Using the audio generator ( Unit A ) in combination with a speaker attached, it allowed me to sweep the audio frequency range from 1 Hz to above the hearing range in an
audible signal that I could hear. By Inserting the Electret Element into the cavity ( Unit B ) and then sweeping thru the audio frequency range, I could watch the scope and determine when
the signal peaked at the resonant frequency of the cavity, tube, can, pipe or whatever I had placed my test element into.
I made a chart on graph paper at the peak point at which the particular cavity was resonant at, its audio bandwidth, then the frequency that it fell off at its 3db point
falloff. From there experimentation with various objects from everything from cotton to newspapers stuffed up into the cavity allowed me to achieve a level of broadbanding my desired
frequencies to adjusting its resonant frequencies.I have included a simulation of how it would appear on a Scope below, when the audio signal peaks at the resonant
frequency of the cavity ( Figure 3 Bottom ), falls off ( Figure 4 Bottom ), etc. as shown on the scope when you are sweeping the audio generator thru its range.
I made it a point of using old test equiptment, Scope, Audio Generator etc. in the photo below. You don't need a $2000 Scope nor a $1500.00 audio generator to build this
mockup. Look up your nearest "Hamfest" in the state you are living. If you don't know what a hamfest is, it is a large flea market where radio, ham radio operators, hobbyist, and electronic
experimenters sell their wares and this equipment can be purchased at an inexpensive price. Check out your local hamfest, and they have tons of old test equipment, dirt cheap.
Again, I hope the Info helps. John
Everything Has A Resonant Frequency
Take A Look at the photo of the two glasses below. The small glass would represent Petes small pillbox and the large glass his coffee or larger can. Take two similar size
glasses in your kitchen Pictured Below - and set them out on the counter. Now, take a knife or spoon and tap the larger glass, then tap the smaller glass. You will note that the
larger glass rings at a "Low Tone" and the smaller glass rings at a "Higher Tone."
This is the principle of Audio Resonance, and the principle that Pete has designed his resonator, pictured above, around. These two objects ( Glasses ) will self-resonate
or "Amplify" the frequencys that they are resonant at - without any electronics whatsoever, catered to their respective dimensions. The Smaller glass self amplifies the upper frequencies and
the larger glass self amplifies the lower frequencies. In Electronic Surveillance we try to cater our audio devices from around 300 to 3000 Hz. suppressing anything below 300 and anything
above 3000 cycles.
When you take the combination of the pillbox and the can - what you have achieved here are several factors. You have achieved a static bandpass filter, that would normally
require a 2nd or 3rd order filter that would normally require eight to 10 electronic components and a power source to drive those circuits and an input / output impedance matching circuit.
You have also achieved amplification that would normally require a dual stage amplifier stage, which would require additional components, not to mention - made the unit directional, which
would otherwise require a parabola.
You have seen the commercial where an Opera singer reaches a pitch and a glass shatters ( Self Amplification). You have been around someone who has played a Plain Guitar,
Organ Pipes, Wind Chimes, Speaker Boxs and you name it. They all have one thing in common. They are cut, built or tuned to self-resonate and self amplify based on math formula and/or
expermintation. Have you ever put a sea shell up to your ear and heard the sound of the ocean? Think about it. You were picking up the amplified resonance of the shell cavity itself. Ever
put a glass up to a wall to hear whats going on, on the other side? A wall without sound proofing is one loud resonator ( Self Amplifier ), the glass another - same as Petes can and his
film cup. Finally - The electret element placed into the rear of the small cup Pete has added picks up that amplified resonance of both the can in combination with the small cup, combining
the best of both worlds and pushes the electret element to its maximum specs to feed the intial stage of the amplifier you are feeding the resonator into.
What Pete has created with his "Atomic Amp" is based on solid Physics in acoustic resonance. I have used similar methods over the years, my first resonant device built
over 50 years ago. The principal works! Now, start filling your glasses with different levels of water and then Tap Them Again. Hope the extra info helps. John S. Wilson Jr. Read On Below on how to tune your resonators and/or determine its resonant frequency