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Posted Aug 26, 2002

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- Building a Long Range Directional Listening Device -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- Written by -Q- ==============

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Building a Long Range Directional
Listening Device
Written by -Q-

This brief article will deal with constructing a "DIRECTIONAL MICROPHONE".
The author will also deal with the anceillary equipment related to the
use of the microphone such as amplifiers, filters, sound-cancelling units,

The construction of the devices contained herein are to be considered
"homemade" devices and the reader should understand that their is much
more to this topic than will be discussed. However, just because the
devices discussed here are considered 'homemade' or amateur in nature
one should never understimate the usefullness and effectiveness of such

The author personally builds most of his own equipment, after having
spent years researching the topic, and going through many prototypes
and designs, till I feel my designs rival or far exceed anything
available commercially. Creating ones own equipment allows one to
tailor a unit to the users specific needs. Homemade equipment may
not look as "pretty" as commerically made gear (ie: you may have to
make it out of plastic tubing rather than precision cut Alluminum), etc..
but in the end, its not how the device looks, but rather performs that
is all important.

For a more detailed discussion on directional microphones and high-gain
audio, the reader should attempt to get ahold of the authors book
"Industrial Security Manual: High-Gain Audio Techniques". This 150 page
book discusses everything that one needs to know on the subject including
much more elaborate construction techniques than dealt with in this article.



Shotgun Microphones- A shotgun microphone consists of a long
tube either of metal or plastic with a length of 12-48 inches.
One end of the tube is open while the other end consists of a
super-sensitive microphone. The microphone is surrounded by a
dampner to eliminate vibrations of the tube from being
picked-up. The microphone is connected to a powerfull handheld
amplifier that ususally contains a low pass audio filter to cut
out low frequency sounds such as wind, and vibrations. The
shotgun microphone is extremely directional. A small amateur grade
model can pick up voices at several hundred feet, while a
top-of-the-line model can pick up voices from 1/2 of a mile
away. (It should be noted, that as always, their is usually a good
reason for the difference in performance between amateur grade
stuff, and professional equipment. 'You Get What You Pay For' is
definately a proverb to keep in mind when shopping for surveillance
gear. A top of the line directional microphone [which is usually
2.5 - 4 feet in length] can set you back $500 - $2,000.



The use of shotgun directional microphones has been demonstrated many
times in various films and television shows. Needless to say, the
realistic effectiveness of these devices have been highly exagerated to
perpetuate the "Hollywood magic" and to make the movie more interesting.

THE CONVERSATION are just a few of the films I can think of which feature
this device and overstate its effectiveness.]

These high-gain audio devices, be it shotgun or parabolic microphones,
etc.. are not magical devices. They are tools, which must comply to the
laws of physics. The laws of physics governing the uses of these devices
are many. That includes the microphone itself as well as the amplifiers
which are subject to noise and have a ceartain "noise figure".

Due to the nature of sound acoustics, refelection, absorption and wave
propagation, resonation, etc, etc, etc. directional microphones are NOT
truly capable of picking up sound from a single direction (meaning less
than 5 degrees of angle.) In fact, no device even comes close to such
a pick-up angle, not even the most sophisticated devices manufactured
by intelligence agencies. It is possible to eliminate (or more realistically
"limit" background noise, it can never be totally eliminated, and
extemperanous noise will always be something that must be contended with.
This is where homemade devices have the disadvatage in that their not
capable of filtering (or better yet "cancelling" background noise.)

A typical directional microphone has a true pick-up angular figure
of 100 degrees (50 degrees on each side of the parallel). The sound from
the 100 degree angles is fairly minute, and in reality the shotgun mics
main pick up range is 80 degrees. However, with proper construction
techniques the device can have a pick up range of only 60 degrees
(even less than that when sound-cancelling and filtering is utilized.)

Estimated Figures

E -76 ²
N -70 ± ² ±
S -60 ° ± ² ± °
I d -55 ° ° ± ² ± ° °
T B -50 ° ° ± ² ± ° °
I m -40 ° ° ° ± ² ± ° ° °
V -30 ° ° ° ° ± ² ± ° ° ° °
I -20 ° ° ° ° ± ² ± ° ° ° °
T ------------------------------------
Y 40 50 60 70 80 90 80 70 60 50 40
Angular Figure

The above diagram, although greatly estimated and slightly innacurate,
serves more as an example which demonstrates the relationship between
angle and sensitivity of the microphone at any given incident angle.



The directional microphone system consists of two main components.
[a] The shotgun microphone [directional mic]
[b] The electronics package [amplifier, filters, DSP's, Equalizers,recorder]

The following is my simple "recipe" which I concocted long-long ago
for creating a simple (yet quite high quality) directional microphone.
My simplified unit, although obviously not as good as the $2,000 models
does serve a usefull purpose and is much better than the than the
$200 - $500 "cheapie" models you buy from various 'spy' catalogs.

It should be noted that this design is very simple and is indeed quite
primitive.. I have made some wonderfull enhancements on the design
over the years, but I cannot discuss those here for a variety of reasons.
The main reason, is that this is supposed to be a 'home-made' device
which uses readily available materials, and more importantly requires
a very small layout of money (less than $15 in goods) [excluding all of
the electronics and microphone element].
You can enhance your shotgun microphone if you choose by using all sorts of
advanced ceramics, vibrational dampening mechanisms, custom machining of
metal parts, as well as using advanced physics calculations to create the
precise tube length and the precise resonance properties for the tube...
but then again thats the topic of a more advanced article..

(1) Microphone. Either [a] Elecret [b] Dynamic [c] Field Effect Transistor.
It is preferable that you use an electret microphone as they are
usually the best all-around. They are cheap, yet have the highest
sensitivity and frequency response. FET microphones actually have
more sensitivity than any other type which makes it the BEST
microphone you could possibly use, only problem is that FET mics
usually retail for well over $120. While Dynamic and Electret mics
sell for less than $60 on average. It is preferable that your
microphone have a sensitivity rating of at least -65dBm -75dBm and an
average frequency response rating of 40 - 15,000kHz.
Dynamic microphones do have 1 advantage over all others, and that is
that they provide the greatest overall "clarity" in sound pick-up.
This clarity aspect can be and is very important, however, when using
various through-wall or long-range listening devices, the clarity is
usually severly distorted anyhow through the use of an amplifier so
the slightly greater clarity that the Dynamic mic offres really does
not make much of a difference, and it is better to go with higher
sensitivity rather than clarity or frequency response.
BTW: A "Dynamic" microphone is the type most people think of when they
think of microphones. It is the type often used on "the stage" by
performers. You can buy several kinds at Radio Shack.
ALSO NOTE: That it is best to use a "directional" or "Cardioid"
microphone as opposed to an "omni-directional" mic, as a cardiod
mic picks up the sound mostly from the front of the mic (which is
what you want) while an omni-directional mic picks up sound from
all 360 degrees which means alot of background noises may be
present in the audio, plus it may make your microphone more sensitive
to vibrations (which is not a good thing). The only problem with
directional (or cardioid) mics is that they usually are not as
sensitive as omni-directional mics (which is also a bad thing since
you want the highest sensitivity microphone that you can afford).
So the key is to try to find a cardiod mic that has a
high sensitivity rating.

You could also purchase "stand-alone" condensor microphones from
most any electronics suppliers, (including Radio Shack) however,
the reader should be advise that these $2 - $5 are not very good
at all, and DO NOT compare in sensitivity to $60++ microphones.

(2) The reader has two options for purchasing an amplifier unit.
A small low-power hand-held amplifier could be utilized with a
power output in the range of 250mW - 2 Watts, however the results
of the audio will be most unsatisfactory and in fact the user will
realize almost no gain whatsoever (in otherwords it will appear that
a person could hear better without even using the unit). However,
a trained ear will be able to discern the difference. Although the
amplification power is miniscule and in fact you truly can hear better
with just your ears, such a set-up will still allow you to pick up
sounds from a specific direction while cutting out background sounds.

If a low power amplifier is chosen it is very important to have an
extremely low "noise figure" for the unit. Unfortunately no such
units exist that cost less than $250. [See the authors review on
general purpose amplifiers]. The user could start experimenting with
a $12 Radio Shack 1/4 Watt amplifier (available at all Radio Shack
outlets), but this is only for experimental purposes to get used to
listening through a shotgun mic... but after experimentation is over,
your going to need to step up to a better amplifier unit. Such small
amps are really only usefull for small handheld shotgun or parabolic
mics (ie: shotgun mics smaller than 12 inches (which really arent
designed for listening over long ranges).

The next step up is to use an amplifier in the range of 25 - 300 Watts.
The author personally uses two seperate set-ups on his long range
microphone systems (both parabolic and shotgun mics). I use a
35Watt Radio Shack P.A. (Public Address) amplifier for doing
"low level" non-critical work.. (ie: just playing around or testing
new directional mic designs).. And when I need to do serious professional
work, I am forced to use either my 100 Watt high-fidelity amplifier
or on occasion my 300Watt unit. (the latter 2 amplifiers cost me
$1,500 and $4,200 respectively) while the 35 Watt Radio Shack amplifier
only cost me approx $150 (actually I bought it for $99 on sale).

Needless to say, their is truth in the age old statement, that
"YOU GET WHAT YOU PAY FOR".... I can assure every reader, without
question. The most significant factor in the quality of the directional
amplification system is NOT the microphone design itself but the
quality of the amplifier. P.A. Amplifiers are very low quality and
they are designed as such. PA Amps are not serious audio amps, they
are general purpose devices for use in DJ'ing, and addressing audiences
where sound quality is not at all critical. The noise figure for
PA systems is very very very high. And it is a great hinderance if you
plan to do serious audio interception work. You need a real professional
high fidelity low noise amplifier. Get it if you can afford it.

Just to elaborate a bit on why amps must produce little noise, lets
take as an example a typical P.A. Amplifier. The units produce such
an amazing amount of distortion which creates so much background
noise, that it 'drowns out' the audio your trying to hear.
The distortion, basically just sounds like 'static' and 'humming'
and 'buzzing' and well just .. 'noise'.

You could filter it out, but that requires yet another very expensive
piece of equipment known as an audio notch filter (more advanced filtering
methods exist [DSP, etc..] but for the purposes of this article they will
not even be mentioned.. but dont even think of buying one, unless your
intent on building the ultimate surveillance system. Its a hell of alot
simpler just to go out and buy an amplifier that doesnt produce so much

if you do plan on using a P.A. Amplifier because you cant afford a
professional audio engineers amplifier ($1,000 +++ dollars) then the
proper way to use a P.A. is as follows:

The key is to keep the volume LOW to MEDIUM on the P.A. Amplifier.
And at the same time, you will need a second small mini amplifier which
has the volume at high gain inputted into the P.A. AMP.
(I could get technical and tell you that your not really supposed to have
1 Amp on high and another on low or medium as that causes distortion by a
phenomenon known as "mismatched gain staging" but nevermind that technical
bullshit because while it looks good in theory on paper it doesnt really
equate to real world use.) Technically your supposed to have 2 seperate
amplifiers set at close to the same gain level within a
10 - 20 percent (+/-) tolerance.

You can buy a mini-amplifier from RADIO SHACK, it sells for $11 and has an
output gain of 250mW (milliwatts). It has an input for a mic and an output
for an earphone which you then plug into the P.A. Amplifier. The reason
you set the Mini-Amp on High and not the P.A. AMP is because the
mini-Amp has far less distortion than the P.A. Amp which gets very
distorted at high power levels.

Another option, if you dont have, or canot afford a 35Watt P.A. AMP
is simply to use 2 or 3 mini AMPS chained together. (2 will
usually suffice, 3 is overkill and causes technical problems
which I cant discuss for the sake of brevity in this article).
To connect 2 RADIO SHACK (or any brand for that matter) Mini-Amps
together is by a Patch Cord.. You plug the microphone into the IN Jack
of AMP 1. Then connect the EARPHONE jack of Amp 1 into the Input jack of
Amp 2, then you connect your earphones to the EARPHONE(OUTPUT) Jack
of AMP 2. Sounds complicated, but it only requires a $3.50
"patch cable", readily available from Radio Shack....

Of course, as usual, nothing is as simple as it seems.. I fooled you
a bit.. What you need is not actually a "regular" patch cord,
you need what is known as an "ATTENUATING PATCH CORD" (4-20dB
attenuation) which will lower the gain a bit and allow you to connect
low-impedance earphone output to the high impedance microphone input.
If you try to use a regular patch cord, you will get a tremendous sqeel
in the unit, or various 'humming' or other strange noises from impedance
mismatch and/or overpowering the audio circuit which causes audio feedback
through oscillation or saturation.

However, using an attentuationg patch-cord with the small 250mW
Radio Shack amplifier is unwise (use a regular patch cord) because
the attenuation will end up eliminating most all (50percent) of the
gain that you just created with the amp.

You could of course always invest in a "REAL" audio amplifier.
This is not a cheap task. A quality audio amp with high gain
(100dBm) and very low noise [High (S/N ratio)] can run you
between $250 to $500 for a handheld unit, and $1,000 to $5,000
for a very high power, extremely low noise power amplifier
which is often used by professiona musicians and audio engineers.
An example of a high quality handheld amplifier is the Martin L.
Keiser model 1059 PreAmplifier which is a personal favorite of
the author, and I take it every where I go (really!) (or even better
the model M.L.K. 2040 power amp.) [the 1059 has a bit more noise
than the 2040].

LAST NOTE: Their are a few other technical details which you need to
be aware of which pertain to audio distortion. Make note
that you should ALWAYS ground your P.A. Amplifier unit
to the radiator. This will eliminate a minute (or significant)
amount of humming, and/or other distortion. ALSO, make
note that you should not operate audio equipment in the same
room as flourescent lights if you can help it. Particularly
a flourescent light with a defective "ballast" (which is
easily identified by a light which flickers ON and OFF
or BRIGHT and DIM constantly. Defective ballasts cause
serious RFI (Radio Frequency Interference) problems which
causes a static and crackling sound. Also note that you should
always ("should") have an RFI protector on your PA-AMP which
plugs into the wall outlet (available from Radio Shack for
only $12.00) which will cut down Carrier Current & RFI such as
that caused by electric motors on refrigerators (particularly
old ones), and blenders, and hair dryers. Lastly, you should
be aware of the phenomenon of Inductive RFI coupling which
is when your audio patch cords pick up RFI from 110VAC wires.
This will often happen when you have your patch cords rolled
up into a circle causing a transformer type effect which
transfers the A.C. "hum" onto your audio cables through induction.
This causes serious audio background noise, most distinctly
a "humming" type noise. To combat this, always fully unravel
all microphone and audio patch cords, as well as laying
all patch cords and microphone cables at 90 DEGREE angles to
any 100VAC wires which may lead to various pieces of audio
equipment such as your P.A. Amp or Equalizer.

(3) 10 band (per channel, 20 total) Graphic Equalizer... This is
not an absolute necessity.... But if you have it laying around as
part of your home stereo,... then use it. It makes the resultant
intercepted audio all that much better by enhancing the audio and
filtering out unwanted frequencies caused by various background

(4) Headphones (preferably a "headset" with a 1/4 inch jack, not the flimsy
headphones with the 1/8th inch jack which cannot handle as much power)
his is an ABSOLUTE NECESSITY! You cannot output the audio to a speaker
system. Their are complex reasons which I wont get into. The first is...
That you need headphones to produce the "stereo/hall effect" which makes
it easier to decipher the audio when it is being played into both ears.
If you play it from a speaker, you will never be able to understand what
is being said, because usually the audio is quite distorted. This is
especially important in advanced surveillance applications where multiple
microphone are being utilized (some for sound pick-up, others for
sound-cancelling, etc..)

In addition, speakers have serious "feedback" problems, as well as
"power overload" problems that headphones do not encounter. You can
crank the power output on the AMP up full blast and your earphones
can easily take it. But if you so much as turn the AMP up more than
30 percent and send it into the speaker, then that speaker is
going to give you a loud 'squeel' like you never have heard before
due to "feedback". Feedback is where the outputted sounds (from your
directional microphone) throught the speaker, get fed back into the
directional microphone and that perpetuates a continuous loop which
causes a saturation of the amplifier due to resonation.

(5) Sound Absorbant Material...This is the tricky part of the whole
operation. This is a major factor in what seperates the amateur grade
directional mic versus the professional grade shotgun microphone.
The sound absorbant material in the unit has multiple uses, and the
tricky part is to find a sound absorbant material that is suitable
for use to combat the various problems which the directional
microphone encounters. Unfortunately, one cannot simply use the
material which provides the highest coefficient of sound absortion.
If only it were that simple!

The Sound Absorbant material is used for the following reasons:
[a] To prevent the microphone from picking up audio from
within the listening post where the user is operating the
unit. Without sound absorbant materials and dampeners
The shotgun microphone would pick up every sound you (the
user of the unit make, including every breath [thats how sensitive
the microphone is; especially when highly amplified].
[b] To prevent unwanted extemporaneous background noises caused
by vibrations when the directional mic tube is shaken. This
shaking can happen for a variety of reasons, including the
operator adjusting the position of the tube, or a passing truck
or an overhead flying aeroplane. All of which can cause a deafening
roar in the users headphones if the unit were not protected
against vibrations. In addition, flowing air (which is actually
flowing around us quite vigourously, us humans just do not notice
it. But the flowing air vibrates the PVC pipe on both the
inside and the out, and this causes unwanted noise in the
intercepted audio. Without proper insulation and dampening, one
is likely to hear lots of 'noise' in the image, which as you
might of guseed sounds like rushing air, kinda like when you stick
your ear into a seashell.
[c] Flowing Viscous Air. This is the most complex of all to
discuss, and I cannot even discuss it fully without getting into
some advanced physics. This has alot to do with flow dynamics,
pressure dynamics, resonation and standing waves, etc.. etc..
This is a really annoying problem which plagues shotgun (but not
parabolic mics). Their are ways to reduce flowing air sounds but
I cant discuss it without visual demonstration and diagrams.

So What is a Good (Yet Low Cost) Sound Absorbent Material?
[a] CLAY (modelling clay, great for sound absorption, bad for
vibrational dampening)
[b] Water (tricky, but is the good for eliminating sound and is the
best for vibrational absorption.)
[c] Steel Wool (good for sound absorption, bad for vibration
[d] Powders (you can use any kind of "powder" [baby powder, chalk,
flour, corn starch, anything].. It absorbs sound very
good [slightly less than clay, but yet provides better
protection against virational noise)

(10) Vibrational Dampener. You need some kind of vibrational dampener
mechanism to seperate and isolate the microphone from the resonation
tube (which is the 3/4 inch by 3.5 foot PVC pipe). This is necessary
in order to fight background noises which are caused by vibrations
in the resonation tube. You can seperate the microphone with a simple
air gap (which usually ends out working best... Take note of that.)
You could also use various rubber washers, or styrofoam, however be
aware that by using such materials, you may actually make the
background noise worse, since even rubber and styrofoam transfer
sound quite well from the resonation tube to the microphone. I cannot
get into discussing the complexities of the isolation mehcanism
as it is rather complex, and involves alot of trial and error based
on various factors such as your resonance tube length, resonance tube
material, etc.. So my advice is to just use an air gap of 1/4 inch.
Also note, that their is another even better way to do this which
is simpler. You can simply place the microphone into the resonation
tube, this will protect the microphone from resonations on the
outer (1 Inch) PVC tube which are much greater than the resonations
on the inside 1/2 Inch resonator tube).

I'd like to point out, that all of the above comments on dampening
may seem very vague and nonsensical. The fact is, I did not write the
above very clearly and its confusing. The fact is, this is just one
of those things the reader has to work out for themselves by trial and
error and many years of building these units and refining it.
I could not get specific about which is the best method because it is
different for every shotgun microphone design. Their are thousands of
methods to build your directional mic, and although they each perform
to roughly the same standards in the end, you can employ different
methods in your system. So when creating your mic, just do what you
think is the obviously smart thing to do. Dont go overboard with elaborate
dampening schemes because as I said above, in the end... all mics
perform to nearly the same standard. No microphone is going be a 100
times better than another.. twice as good...maybe.. but not more than that.


DESIGN #1: Simple model (fairly effective)

<- - - - - - - - - Overall Length is 3 Feet - - - - - - - - - - ->

mic |°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°
cable °ÛÛÛ--------------------------------------------------------------
--> ==================================================================
| +++++ KEY +++++
1.75 Inch === 1.75 Inch PVC Pipe + Mic cable runs through clay
PVC Pipe --- 0.75 Inch PVC Pipe ² Microphone
Û Vibrational Dampener ° Sound Absorbant Material
(rubber or clay) (modelling CLAY)


DESIGN #2: More complex model (provides slightly better results
than design #1 but alot lighter and
more 'interesting looking' )

This unit is unique in that it can be broken into two seperate pieces
to reduce its overall length so that you can carry it inconspicuously
in a carrying case. Although my design below does not show it because
ASCII diagrams do not allow precision, a little ingenuity will allow
the creator to seperate the front barrel from the unit (create a
threading system so that it screws into the main barrel, etc..) reducing
the unit to only 27 inches.

(All pipe is white PVC pipe)

Total device length 42 inches (3 feet & 6 inches)

Pipe diameter Pipe length Key
1.75 2'3" 27 inches =====
1.00 42 inches +++++
0.50 40 inches -----
Microphone -76dBm sensitivity MIC
Modeling Clay sound absorber *****

[ASCII picture NOT to scale]

^^^^^^Tripod mounting holes
should be situated at the exact center
of gravity on the unit.

|-----------------------------42 inches long total-----------------------|

The user should also tap holes for a scope mount (a 2X - 4X scope should
be sufficient) and should be mounted on the top of the tube and another
hole should be tapped for a tripod mount (on the bottom).


Your finished device should vaguely resemble
something like the above diagram.

Tripods can be purchased cheaply from almost any camera store.
A very convenient place to also purchase the tripods is CALDOR, K-MART,
and/or WAL-MART (all of which have electronic departments [which is where
the cameras usually are]). Interfacing the tripod with the PVC tubes is
very simple. You need not worry about puncturing the PVC tubes or any
such thing. If you drill a few holes in it it will not really affect the
prformance of the microphone. Tripods cost between $30 - 90 for something
that is sufficient for your purposes.. You need not get a $300 tripod
like is used for advanced photography or telescopes.


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