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 or speech from a distant room, so we look for the most sensitive mic that performs well in the other areas.
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 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, 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 is usually considered VERY good for surveillance gear. Condenser mics give a less-than-extraordinary 35-43 dB S/N ratio. All electrical equipment 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 inevitable noise of the other components, it can become quite annoying.
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 different impedences. Mics are classified either high impedance or low impedance. High impedance mics tend to lose some of their high frequency 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. 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 better transformer can be built into the preamplifier. It is imperative in surveillance that we match the mic impedance with interfacing machinery, or a loss of signal and lower S/N ratio may occur.
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 electret mics are generally the most durable.
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 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 and blend perfectly with the speakers inside.
There are several types of mics, but only a few are suitable for surveillance work. The most common are crystal, condenser, dynamic, and electret. Crystal mics are microphones that use a crystal of Rochelle salt as it's piezoelectric element. Piezoelectricity is the property of acquiring opposite electrical charges on opposing faces of assymetrical crystals when they are subjected to pressure. It is closely related to the ceramic mic, which uses barium titanate instead of Rochelle salt. The ceramic mic is more weather resistant and has slightly lower impedance. Condenser mics have 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 in surveillance mainly in contact mics (such as spike mics) where a probe is linked directly to the crystal.
Condenser mics are one of the favorites for clandestine work. They are very small, offfer wide, smooth frequency response, and are fairly inexpensive. Condenser mics have to membranes, and the change in distance (which causes a change in capacitance) between them causes the electrical 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 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 a power supply, usually either an internal battery or, more commonly, the mic draws power from it's output leads (often called phantom power). 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 extends above the normal voice levels (some sopranos can reach the high end, though.)
Dynamic mics are basically speakers designed to work in reverse-instead of changing electrical signals into sound, they change sound into 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 often pick up a 60 Hz AC hum unless shielded. These perform poorly in surveillance work. Electret mics are without a doubt the best all-around surveillance mics. They work similarly to condenser mics, but require less power because they have a permanent charge across their membranes. Condenser mics use their input voltage to create a charge across the membranes.
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 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 telephone mouthpieces, but that is fairly unusual now. They are large and give mediocre resaponse. If you ever watched mission impossible or any old 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 drop in transmitter, and could be inserted in any "standard" phone and transmit the conversations over short distances. Pressure zone mics are 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 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 cancels echos. It enhances intelligability, but is very large. The smallest of them will fit into a shirt pocket, andthey are very expensive and fragile. Still, there are situations where they fit the bill better than any other mic.
The main factors of preamplification are gain and noise. Gain is the increase in signal given by the preamplifier. Noise is unwanted sound that the preamp generates. A good preamp can make up for a mediocre mic or a bad signal due to the location of the target with relation to he mic, but nothing can compensate for a bad preamp. A good preamp has high gain and low noise. There is no limit to the amount of gain that can be applied, but noise and electrical breakdown limit the practical application of it. Noise increases with gain, so the limit is where noise overwhelmes the signal. Electrical breakdown occurs when gain is so high that inaudible ultrasonic feedback causd by the location of the components with relation to each other causes the preamp to shut down. Electrical breakdown also occurs when gain is so high that oscillation occurs and a squeal is sent through the mic or speaker. That can sometimes be lessened by shielding or changing the location of the mic.
Operational amplifiers (op Amps) are often used because they are inexpensive, simple to use, easier to handle, and offer higher gain and lower noise than normal-component amplifiers. Op Amps are integrated circuits (ICs) that were originally developed for use in analog computers in the 1940s. They are high performance linear amplifiers with 2 inputs, allowing for inverted and non-inverted output (negative and positive gain). The gain is determined by a resistor that feeds some of the amplified signal back to teh inverting input. The smaller the resistor, the lower the gain. An Op Amp amplifies the difference between the input and ground. This may seem complicated, but it actually makes amplifier design much simpler. Even a novice could design a simple amplifier using only the Op-amp's data sheet. It is important to keep the battery leads short, but most amps avoid that restriction by using a capacitor to keep the input from oscillating. Op Amps are the components that make miniature bugs posible.
The first step to processing is the removal of sounds outside the speech band. This makes the rest of the processing go more smoothly because the sounds that are unwanted anyway aren't dealt with. The speech passband goes from 300-3000 Hz. By eliminating the sounds outside this range, we cut the unwanted noise considerably. Filters that eliminate the sounds above and below are very easy to build (an Op Amp and a few resistors and capacitors can be thrown together to make a passable filter), but, for surveillance, we sometimes make complex filters with high dB/octave slopes.
Slope measures how quickly response drops below nominative level (3 dB below input level). Steepness is expressed in dB/octave, which occurs in multiples of 6. A 36 dB/octave filter eliminates all sound below about 150 hz, and sound above that is practically inaudible up to almost 300 Hz. A 6 dB/octave filter would dampen the sounds, but they would be audible down to around 100 db, and still noticable down to around 50dB. The high end filters work the same way, only response is lower for higher signals instead of lower ones. A 24 dB slope at each end of the passband is a fair negotiation, and, to make design simpler, we could drop it to 18 dB/octave but raise the low end to 500 Hz and drop the high end to 2000 Hz and not miss much. A filter below 18 dB/octave is almost a waste of time because the filter would barely dampen the sounds that need to be removed.
The next step is compression. It would be unessacary if the target would stand in one place and speak in a clear, medium voice. Unfortunately, if you ask someone to do this, they might get a teeny bit suspicious. We all have the tendancy to speak at various levels, from a whisper to a shout, and everyone tends to move around and change the direction that they're facing when they are speaking. In a surveillance recording, we want to hear whispers as if they had been spoken aloud, and we want to hear shouts at the level of a normal voice. That's where a compressor comes into play. It raises the level of a low sound, and lowers that of a loud one. With a compressor made from an IC compander, a -80 dB signal is boosted to -40, and a +20 signal is cut to +10. The chip I use is capable of double compression, which means that a -80 dB signal is boosted to -20 and a +20 signal is cut to +5. It is possible to use 2 compressors together to bring the range within 6.25 dB of each other, but that is really unnecessary and causes the component to be bulkier than it should be. A limiter can be used with or (shudder) instead of a compressor. A limiter suppresses signals above certain levels, so your recorder or ears won't be overloaded. The last step of signal processing is equalization. Equalization is the process of removing sounds within the speech passband that can be as annoying as those outside it. For example, if you are listening with a laser bug, your speech passband will remove 90% of the noise, and the compressor will make all the sounds audible without battering your eardrums, but the mark has a refrigarator next to the window you are useing as a reflector that is obscuring some of the sound. So you need to get rid of the narrow band that the refrigarator is on without obscuring the voices. A parametric equalizer can do the job. This is not the same as a "graphic" equalizer that you can find on a stereo system, although that can substitute if necessary. The graphic equalizer has set center frequencies and bandwidths, usually at octave points. If the sound you want to eliminate is between 2 frequencies, you have to adjust both and sacrafice some of the speech. A parametric lets you set the center frequency and bandwidth. A good parametric should operate from about 200 Hz to 4000 Hz. Anything below or above will be filtered by the passband filters. (from 200-300Hz and 3000-4000Hz will be damped, but not eliminated) A parametric equalizer with 3 bands can run rings around a graphic equalizer in the same range with 30 bands. You can also use a parametric to boost the high frequency sibilants to make speech more clear.