101 lines
8.4 KiB
Plaintext
101 lines
8.4 KiB
Plaintext
Phile 1 of 4 Audio Surveillance
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-----====| SANctuary |=====-----
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Audio is the most common surveilance method in use. Most listening devices depend on some form of electronics, and it is important to understand
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the usual steps to audio electronic surveillance. It is basically a 5 step process.
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1) Input- usually a microphone
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2) Preamplifier- used to boost the nominal signal of a mic to usable levels
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3) Processing- eliminates excess noise and unwanted sounds from the output
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4) Output- headphones, recorder, transmitter, etc.
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5) Post-processing (sometimes)
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This phile will deal with microphones. Other files will deal with each of the other steps. Microphones are judged by frequency response,
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sensitivity, signal-to-noise ratio (S/N), durability, and size.
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Frequency response is the range of sound that will give usable output from the mic. Human hearing is roughly 20 Hz to 20,000 Hz, but, in surveillance
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work, we only need to hear the frequencies that deal with human speech.
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Sensitivity is the amount of electrical output we get for a given sound level. surveillance mics need to be very sensitive to pick up the whisper
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or speech from a distant room, so we look for the most sensitive mic that performs well in the other areas.
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Signal-to-noise ratio is the number of decibels (dB) louder than the mic's noise the input signal is. All mics introduce hissing, cracking electrical
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noise into the output. A good compact disc player can have a S/N ratio of 90 dB, totally inaudible to humans. Records give a S/N ratio of 50-60 dB,
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which gives some noise during quiet passages, but a good record on a good player will have very little audible noise except during quiet parts. 50 dB
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is usually considered VERY good for surveillance gear. Condenser mics give a less-than-extraordinary 35-43 dB S/N ratio. All electrical equipment
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add noise to the signal. Each stage introduces more noise, so, while the noise introduced by the mic might be almost unnoticable, when added to the
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inevitable noise of the other components, it can become quite annoying.
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Impedance is the opposition to alternate current. This is only important because a transformer is needed to couple a mic and amp if they have
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different impedences. Mics are classified either high impedance or low impedance. High impedance mics tend to lose some of their high frequency
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response in long runs of cable. Low impedance mics are usually between 50 and 600 ohms. High impedance mics are in the 5000 to 20,000 ohm range.
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Some mics come with built in transformers that are switchable to make them high or low impedance, but these add bulk and noise to the mic, and a
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better transformer can be built into the preamplifier. It is imperative in surveillance that we match the mic impedance with interfacing machinery,
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or a loss of signal and lower S/N ratio may occur.
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Durability is the mic's ability to stand up to changes in humidity and temperature, as well as it's ability to withstand shock. Dynamic and
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electret mics are generally the most durable.
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Size is very important in surveillance work. As a rule of thumb, a small mic is always preferable because it can go unnoticed more easily than a
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large one, but sometimes a large mic can be incorporated well into the environment (A large dynamic mic can be installed in a stereo speaker system
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and blend perfectly with the speakers inside.
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There are several types of mics, but only a few are suitable for surveillance work. The most common are crystal, condenser, dynamic, and electret.
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Crystal mics are microphones that use a crystal of Rochelle salt as it's piezoelectric element. Piezoelectricity is the property of acquiring
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oppositeelectrical charges on opposing faces of assymetrical crystals when they are subjected to pressure. It is closely related to the ceramic mic,
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which uses barium titanate instead of Rochelle salt. The ceramic mic is more weather resistant and has slightly lower impedance. Condenser mics have
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replaced crystal mics in most applications, but their high output and high impedance and low cost still find use in some applications. they find use
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in surveillance mainly in contact mics (such as spike mics) where a probe is linked directly to the crystal.
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Condenser mics are one of the favorites for clandestine work. They are very small, offfer wide, smooth frequency response, and are fairly
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inexpensive. Condenser mics have to membranes, and the change in distance (which causes a change in capacitance) between them causes the electrical
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output. One or both of the charged membranes is flimsy, and sound alters the distance between them. They have built a built in ampifier which
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changes the variable capacitance to variable voltage or current, and it also drops the impedance from millions of ohms to 500-2000 ohms. It requires
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a power supply, usually either an internal battery or, more commonly, the mic draws power from it's output leads (often called phantom power).
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Frequency response is very good. For most surveillance work, it is too good, because it reaches down below the range of human voice. The high end
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extends above the normal voice levels (some sopranos can reach the high end, though.)
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Dynamic mics are basically speakers designed to work in reverse-instead of changing electrical signals into sound, they change sound into
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electrical signals. They are durable, low impedance, and very large when compared to electret mics that are a fraction of the size of a dime. They
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often pick up a 60 Hz AC hum unless shielded. These perform poorly in surveillance work.
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Electret mics are without a doubt the best all-around surveillance mics. They work similarly to condenser mics, but require less power because
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they have a permanent charge across their membranes. Condenser mics use their input voltage to create a charge across the membranes.
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There are other mics which just aren't cut out for surveillance work except in most unusual circumstances. The large ribbon mics used in recording
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studios are too expensive and fragile for surveillance work, along with giving much to wide a frequency response. Carbon mics used to be used in
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telephone mouthpieces, but that is fairly unusual now. They are large and give mediocre resaponse. If you ever watched mission impossible or any old
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spy films, you may have seen the hero unscrew the mouthpiece of a phone and take out the mic and drop in his special transmitter. It was called the
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drop in transmitter, and could be inserted in any "standard" phone and transmit the conversations over short distances. Pressure zone mics are
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perhaps the best of the uncommon mics. They are not really a mic, but a design, because they can have an electret, condenser, dynamic, etc. element
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in them. Pressure zone mics have a boundry about 1/32" in front of the mic. This results in the arrival of direct and reflected sound in a way that
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cancels echos. It enhances intelligability, but is very large. The smallest of them will fit into a shirt pocket, andthey are very expensive and
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fragile. Still, there are situations where they fit the bill better than any other mic.
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That's it for this phile. Watch for more philes on surveillance by,
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The Gaurdian
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-=|=-=|=-=|=-=|=-=|=-=|=-=|=-=|=-=|=-=|=-=|=-=|=-=|=-=|=-=|=-=|=-=|=-
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+++++++++++++>>>>>>>>>>>| SANctuary |<<<<<<<<<<<+++++++++++++++++++++
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<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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Oi! Red here! Thanks Gaurdian for the First of the SANBug
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series! Hopefully It'll be as big as the SANlock series!
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Guardian has already written the first 4 Philes in this series, so
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look for all four on a Hard Drive near you!
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Dont forget to call the SANctuary WORLD HEADQUATERS!
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-=-=-=-==HELLFIRE BBS!!! (908) 495-3926
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If you would like to be a SANctuary site, A SANctuary ubiter
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(Courrier) an ANSi/VGA Artist, or a SANctuary writer, E-mail myself
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or Havok Halcyon at HELLFIRE! OR write to us at our SANctuary Mail bag
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SANctuary!
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PO BOX 286
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Keansburg,NJ
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07734
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We also need programmers to help us w/ our New SANMAG which we
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hope to release before christmas.
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----===Red===----
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Greet's go out to:
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Pieman: What's going on w/ PITS???
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Humble Slave #4: GIve me a call....
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C.A.S. : Nice work!
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ICE : We could use a few good ad's!!!
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EAT THE RICH!
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