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EYESPYCRAFT SPY DIY Eye Spy presents three DIY projects using a tape recorder that will allow you to make recordings only you can hear; devise a cheap through-wall listening device and make a parabolic microphone for hearing long distance sounds

PROJECT ONE: LISTEN IMPOSSIBLE MAKE RECORDINGS ONLY YOU CAN HEAR aking private messages that stay private is not so easy in today’s digital world. Many people are now migrating from using standard cassette tape recorders to newer digital format recorders such as MP3 and MP4. CDs have also lessened the popularity of the humble tape cassette - so much so that it’s feasible they will one day disappear altogether.


However, the recorded or uploaded content in its raw state is accessible to anyone. Making audio notes on any digital recorder is open to abuse - and we don’t think there is any way of properly coding such information until it is removed from the device. This might be problematic if the recording contains compromising information, notes on a business transaction, or was made covertly using a hidden device.

Digital recorders are easy to use. They can record for hundreds of In this DIY feature, we find uses for hours, and enable operators to make business notes, download and both these types of recorders. However, if you want to make a play music, or listen to the radio.

What you will need: tape recorder, cassette, mini screwdriver

recording only you can hear, then a standard tape recorder is, perhaps, the better option. For this project you will need a cassette recording machine (any cassette player - including the popular ‘Walkman’-size), a small jeweller’s-type screwdriver - those found in the common eyeglass fix-it kit, and a blank cassette tape. When a recording is made on a standard tape-recorder, the actual cassette tape is locked into position so that the thin recording tape moves past the Record/Play tape head at a constant 90 degree angle. This is called the tape head’s “azimuth angle”. The tape-head position can be adjusted by turning a small screw usually located a few millimetres away from this device and usually accessible. If this is done, the quality of any prerecorded tape, music, voice, story book etc. will suffer if played on this machine.



☺   EASY



However, if a blank cassette tape is recorded on this machine, the quality will be fine - but only if it is played back on that particular recorder. Played back on a different machine, the sound will be garbled and incomprehensible. You can take advantage of this effect - called ‘azimuth loss’ - by recording messages on your recorder with the tape head position

Switched in play mode the tape head assembly jumps forward revealing the screw





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changed and then re-adjust the recorder in the normal fashion. If someone plays the tape in another recorder, the signal will be virtually inaudible. When needed, simply readjust the screw until the sound is back to normal.

screw in either direction from its starting point. Leave the screw in the position that produces the lowest audio and quality level.

Next, place a blank tape in the recorder and, with some music playing in the background (to make HOW-TO it more difficult for an eavesdropper to understand what is being said), Since the screw that allows press the record button and speak adjustment of the tape-head position into the microphone. When you play is very small (sometimes crossback the tape, you’ll hear your voice shaped), you will need a tiny clearly: but play the tape in another screwdriver. These can be found in machine and you won’t. most pharmacy/drug stores in an eyeglass repair kit, for example. This is an old trick - probably Cheap sets found on any market originating from the early days of also include a couple of these the Cold War. Those charged with screwdrivers. acquiring audio intelligence, or carrying messages by cassette to Look carefully at the tiny hole or slot colleagues used similar tricks. You on the case near the tape head. can also conceal the “garbled There is a small screw that allows message” in the middle of a preyou to adjust the head position. recorded tape, an album for Turning the screw in either direction example, for extra security. alters the azimuth angle. If you are Recording tags on the upper left and not sure which screw to turn right side of the cassette have been check the user manual, ask an removed so you can’t accidentally electrician, or just test them one by tape over the music. A simple strip one. However, you should be able to of clear tape positioned over these identify the screw in a few seconds. holes allows you to record over the It’s important to note the original music. Fast forward a few minutes position of the screw. and then change the azimuth angle and record your message. Anyone Place a pre-recorded tape cassette listening to the tape will think it has in the tape recorder and press the been damaged at that point - but a PLAY button. The sound will already few moments later it will be be effected. The sound quality “normal” and the music will return. decreases further as you turn the




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rofessional ‘listen-throughwall’ machines are fairly pricey. A descent unit capable of penetrating a few inches can cost anywhere between £200-£400 ($400-$800). More expensive LTW machines are available but can cost six or seven times that amount. These are exceptional, trade standard and often come with a multitude of features and in-built recording and transmitting devices. There is a very cheap solution that can replicate the results of low-end LTW machines.

CONCEPT AND HISTORY As youngsters, one great trick performed with friends was to listen through a wall or door by placing a glass against its side. The results were always dependent on how thick the wall was, incidental background noise (on both sides of the wall), and the size and shape of the glass. At least two of these factors are important even when using quality LTW devices. However, by using an MP3 player or an ordinary

The trick works with an ordinary tape recorder or an MP3 or 4 player



Switched in play mode the tape head assembly jumps forward revealing the screw What you will need: tape recorder, cassette, mini screwdriver...

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