Background info about directional microphones origin and usage

After more than 20 years of experience in designing and providing surveillance devices one of the key points that the AMPFLAB team faces has been gathering information from distance.

First reaction when we are in such situation is to use hidden gadgets for eavesdropping. It is easy and convenient - market supplies almost all types of audio monitoring devices as quality and price vary according to customer specific needs and budget. Using such devices, however, has one major flaw. In many cases the conversation in concern that has to be heard is not had indoors, and clients do not have access to the premises, or the persons who talk are each time outdoors, and speak in varied places.

Not only police officers are facing such complicated situations but many private licensed investigating authorities also. A few examples are: investigation of insurance frauds, investigating frauds in gaming halls or casinos - the list is endless. Usually violators take all precautions like no phone calls, no emails, or they use strong encryption, remove all electronic devices around them, they have conversations in public, sometimes while moving. Companies in this area of business conclude that they need a device to pickup conversations from afar where acoustic waves are inaccessible for the human ear, that is they need a long range directional microphone.

Using traditional long range directional microphone in real situation is quite a difficult task or it seems like a ridiculous absurd. Imagine the following: a private investigator or other investigating body has to gather information from two or more people who talk to each other. They talk for example in a park, coffee shop, restaurant, lounge or while walking in the street. It will be very strange to sit close to your target or to walk after them with a device with dimensions of a satellite dish of at least 60 cm. Correct, 60 cm is the minimum diameter which is necessary to have the acoustic amplification to hear what targets talk about.

No matter how thorough review of the existing options you do, you will find the following types of long range directional microphones:

- gradient directional microphone

Gradient microphones are different from phased receivers, where same phase signals are added to get more sensitivity. Gradient microphones are based on calculation by direction. But this method is limited by sensitivity of discrete microphones.

- directional microphone with running waves

Microphones with running wave or so called pipe microphones receive sound not perpendicularly but along wave direction. Pipe length usually is from 25 – 230mm to 1m. The longer waveguide is, the bigger sensitivity of microphone.

- parabolic type

Parabolic microphones have parabolic shape that reflects incoming sound waves to one focal point where microphone is located. This is one of most known type of directional microphones. Working principal is simple: sound waves incoming along direction axis are reflected towards microphone in focal point. The bigger diameter of reflected surface - the bigger amplification will be reached. Other sounds coming from different angles aren’t amplified much by this effect because of different phases of each reflections in focal point.

- directional microphone using reflected laser beam - laser listening device

Sound waves generated by a conversation indoors causes the glass in a window to vibrate. When a laser beam is bounced off the window, its reflection will be modulated by the vibrations. Then only a demodulating device is needed to receive the audio from the reflected laser beam. Laser beam microphones resemble the so called photophone invented long time ago by Alexander Graham Bell: a device which modulates a beam of sunlight. It had a mouthpiece that concentrated sound energy on a reflecting diaphragm, which, in turn, modulated a beam of sunlight that was aimed at the diaphragm. When a remote receiver consisting of a photovoltaic cell and a sensitive earphone was set in the beam, speech could be heard clearly from the receiver. The problems with positioning caused by the movement of the sun, and the interruptions due to clouds and night, probably prevented the commercial exploitation of the device.

- multitube "shotgun" directional microphone

Shotgun microphones are designed for outdoor recordings because they have a very narrow angle of acceptance or pickup pattern. This means they can pick up the particular sound they are directly towards and reject all other surrounding noise to produce a very clear recording. Shotgun directional microphones only reduce the receiving range in which the microphone is pointed and they do not provide any gain.

- horn type directional microphone

Type of directional microphone which is lesser known. The horn antenna type is normally used in microwave applications. The horn microphone is designed using analogous principles which could, incidentally, also be applied with equal validity to the design of a loudspeaker.

If you consider using shotgun type directional microphone with which you want to pickup a conversation from long distance, think twice. This type of microphones are not designed for surveillance but for isolation of the ambient noise such as taking interview outdoors from a short distance, usually in the range of 5 meters maximum.

Quite often our customers imagine that the process of focusing and amplifying sound waves is the same as this of focusing video images. Unfortunately, it is not so simple. Most important difference is the wavelength. The wavelength of light is millionths of a centimeter. The wavelength of sound is measured in centimeters and meters. For a zoom lens to be effective, its diameter must be hundreds times larger than the wavelength.

Defining the task

Analyzing clients requirements, and thus creating the technical tasks, we have pinned down the basic parameters that a long range directional microphone for covert eavesdropping has to meet:

- not to be spotted by passersby around you

- have acoustic amplification surpassing several times the human ear

- the possibility of focused collection of sound

- suppression of ambient noise outside the target area

- acceptable dimensions and weight

- easy to carry and to position to the target area

- to pickup talk while being carried along

For an electronic device not to be spotted is enough to be integrated in the body of the device with an appearance that is common to the human eye. The situation, however, is complicated when we have the above mentioned parameters.

Our task was to create an extremely sensitive device, and for this purpose we had to use acoustic summer and amplifier. The task for acoustic amplification automatically fails every attempt for traditional minimization, unfortunately, no one can escape the laws of physics. Our technicians applied a different approach. The first question which we asked was: what is this thing which can be taken as a commonly used item that can be carried, and to be with fairly big surface for receiving acoustic signals, and how an acoustic concentrator can be installed into it. One of the obvious answers is a suitcase or a laptop case.

Carrying a laptop case in public can not raise any suspicion, and at the same time the surface of the face and back of the device is big enough to receive clearly the incoming acoustic wave . After all, the bigger the surface which receives distant and weak signals is, the bigger amplification of the signals in target we have. Next step was design of a device that has the size of a laptop case and in the same time to be able to amplify acoustically and electronically sound wave with low intensity. For this purpose our technicians developed flat acoustic summer which works in the way all contemporary radars do. The acoustic summer has the size of a laptop, it has 64 independent acoustic channels thus providing the acoustic amplification requirement, diagram of directionality (angle of targeting) and size that allows covert usage in real life surveillance situations.

The need of acoustic summer with large surface comes from the following simple setting: you need to amplify a signal that is impossible to be heard because signal is very low, and human ear does not have area and sensitivity to perceive it. This refers to the microphone also - due to its small acoustic area and physical limitations the signal still can not be received when electronically amplified. Smaller receiving area means smaller amplification, and this is the main issue which shotgun and gradient type directional microphone have. This issue can be solved by using acoustic amplification of a device which has area big enough to span and amplify maximum area of the sound wave arriving low.

The same principle is used also in parabolic type directional microphone, with the difference that the flat grid acoustic type antenna for long range directional microphone produced by us, is mobile and covertly targeted.

How far away will directional microphone pickup, or calculating the necessary distance from the object under investigation. Range of interception with directional microphone.

First of all, there is no such thing as constant distance when you use a directional microphone in urban area. We can provide average result which varies depending on the environment and on speech volume. These are the primary factors which have to be considered when asking how far away the directional microphone will pick up. We all want to be able to hear conversations in 500 m or more, without being spotted, but in urban conditions this is impossible because of the high ambient noise and the need for the device to be inconspicuous.

Far more important for the customer is operating in real situation. Our practice and the requirements of customer show that a directional microphone with a range of 100 and more meters range is what is necessary.

The most important point for the client is to be as close as posible to the target, at a distance where no suspicion can occur, with a long range directional microphone that is not visible. Our directional microphone with flat acoustic grid X64 ACS performs excellently in such circumstances providing one very interesting opportunity. Carrying the laptop case on your shoulder you can hear talk of target objects while they speak behind your back, which additionally puts using our device out of suspicion.

Technical requirements concerning directional microphones

Suppressing unwanted sounds

May be the first requirement which comes to your mind when purchasing long range directional microphones is the maximum distance from which it can clearly pick up conversations. Reality, however, is different. Just having high sensitive directional microphone is not the answer to all your needs.

Using directional microphones in real situation means dealing with a very complicated acoustic scene. It is a complex scene because besides the amplification all ambient noises must be suppressed to the level where they do not hinder the clear reception of sound from the tapped area. Every directional microphone must provide comfortable listening in sudden onset of severe irritating noises - a typical example is noise of a vehicle engine. Without meeting the requirements mentioned above, you will hear only normally or very slightly the target sounds, and with a sudden noise of a car engine or just an increase of the ambient noise you run the risk of hearing loss. Our tests show that even a noise above the limit of human ear automatically reduces your sensitivity to perceive sounds for a period from 10 seconds to 3 minutes, and the interval depends on the time for which your ears have been exposed to high ambient sound.

This complex technical problem is resolved by using the combination Automatic Gain Control (AGC) and bandpass filter. This combination has shown in our practice that using fixed and swift adapting AGC system for directional microphones for medium or long range has a negative result. Why is there a negative result because the received in the directional microphone signals do not come from one single source but from many sources. After all, you will surely not be in a public library.

The complex dynamics of the acoustic scenes makes using the standard AGC system unacceptable. ACG system, which is based on a standard engineering decisions can not work in combination with directional microphone which has acoustic amplifier. What matters is the signal, or the frequency range of human speech, to be amplified and picked up. This is a very difficult task and that is why producing directional microphones is done by using electronic human voice bandpass filter. Once filtered, and with a high level of suppression of the ambient specter, the entering signals are processed by smart AGC. Thus the ambient audio signals are 1.eliminated and 2. the AGC activation is limited to the frequency range of the human voice.

Facts about using long range directional microphones for eavesdropping

What to have in mind when using X64ACS directional microphone for surveillance

- situation in which ambient noise is high

Logical conclusion of this high level of ambient noise is that the area where the target conversation is had is populated and your presence within 20 to 40 meters cannot seem strange. Using a highly sensitive long range directional microphone, which is well covered, can solve the problem.

- situation in which ambient noise is low

Logically, again, there will be lower presence of people and machines. Examples include parks and areas in the open. In this case the recommended distance is 60 meters so that you stay out of suspicion. Our long range directional microphone with flat acoustic grid X64ACS performs excellently in both situations without raising suspicion.

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