This device is intended for the selective listening of sounds and voices coming from a particular area or direction. These selected sounds by virtue of the geometry of the device are focused to a mike element and fed to a low noise, high gain amplifier powering headsets, recorders, transmitters, etc.
|These are paper plans, requiring parts and assembly (also available as downloadable plans for roughly half the price).|
Sound like light due to phase incoherence has a tendency not to focus at the same point for a given band of frequencies. This is overcome to some extent by the use of a large mike opening located at the focal point of the unit. Even though the exact focal point is not the same for all frequencies, little difference in performance is gained by changing it for selected frequencies.
The advantages to a system of this type is the ability to capture a large cross section of sonic energy approx equal to that intercepted by the area of the reflector. This energy is focused onto a smaller area (mike element) chere its displacement or amplitude value is a function of the quotient of the captured area divided by the area focused down to.
Therefore, an acoustical gain of; GAIN db = 20 Log Area Parabola (this is an ideal case).
The acoustical gain of the device is theoretically about 70 db. (The parameters based on a narrow band of frequencies and will be less for normal sounds due to interference and their lower inherent frequency). Actual gain of voices and other sounds is around 4-8 db.
A disadvantage of a system of this type is the moderate rejection of back and side noise. Hence, the full capabilities of the system as far as long distance listening is often masked by the inability to run the amplifier at high gain. However, on a still winter night, the unit can be run at almost full gain with amazing results, hearing voices and sounds from unbelievable distances. The unit does provide good positive gain over normal hearing in any situation if the amplifier is set not to overload. Optional acoustical shielding of the reflector rear will help minimize unwanted pickup.
Several applications are shown this and other related equipment.