Communications – electrical: acoustic wave systems and devices – Echo systems – Distance or direction finding
Patent
1997-06-18
1998-06-09
Eldred, J. Woodrow
Communications, electrical: acoustic wave systems and devices
Echo systems
Distance or direction finding
367138, 367157, H04R 1700
Patent
active
057645957
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
This invention relates to transducers, and concerns in particular acoustic transducers with controlled directivity.
BACKGROUND TO THE INVENTION
A transducer is a device that converts energy in one form into energy in another form. Sound is a longitudinal waveform comprising pressure waves travelling through a compressible medium. The waves may be at a frequency which matches that of the human hearing capabilities--roughly from about 30 Hz up to about 20 kHz--or they may be above or below this range (respectively ultrasonic and subsonic; dogs and bats can hear ultrasonics up to about 40 kHz, whilst whales appear responsive to subsonics at around 10 to 15 Hz). The medium through which the sound waves travel may be a gas such as air, a liquid such as water, or a solid such as the earth or a metal rod. An acoustic transducer is a device that can be used to convert energy between a sound form (for radiation through such a medium) and another form, usually that of electrical energy.
Most acoustic transducers exhibit the property of reciprocity--that is, they can effect conversion between sound and electricity in both directions. Thus, such an acoustic transducer may convert electrical energy into sound or it may convert sound energy into electricity. A typical example of such a transducer that converts electrical energy into sound energy is a conventional domestic loudspeaker, as in a Hi-Fi system, which is fed with energy in the form of an electrical signal defining some sort of sound--music, perhaps, or speech--and then changes that electrical energy into sound energy by using the former to move some kind of air-encompassed active element such as a diaphragm back and forth in an appropriately-corresponding manner so as to produce matching pressure waves in the air itself, these waves constituting the required sound. Another example of an acoustic transducer is the loudspeaker-like device, known as a projector, employed in a SONAR system to convert an electrical signal into a sound signal travelling through water. A third example is that of those transducers that generate sound to be radiated into the earth; these are employed in the oil industry to send sound into the ground to determine from the received echoes whether the underlying strata are of the type that might be oil-bearing.
A typical example of an acoustic transducer that effects the opposite conversion--sound energy into electrical energy--is a microphone, as used conventionally to receive speech or music. A microphone that receives sound travelling underwater is a hydrophone, while one that receives sound travelling through the earth is a geophone.
All transducers suffer from imperfections in the accuracy with which they convert waveforms in one energy form into waveforms in another, but they can suffer from what at first sight seems to be a rather strange problem; the way they behave, and particularly their directional properties, depends on their physical size and shape. With reference to a conventional loudspeaker, this can be illustrated and explained as follows.
A typical domestic loudspeaker has within its box two, or even three, actual transducer diaphragms involved in the conversion process. One, the "woofer", deals with low frequencies (long wavelengths), and is large; a second, the "tweeter", deals with high frequencies (short wavelengths), and is small; and if there is a third, a "mid-range" unit, then it deals with the intermediate frequencies (and wavelengths), and is of a correspondingly intermediate size. One major reason--there are others--for this use of diaphragms of different sizes being provided to deal with sound of different frequencies (and, of course, different wavelengths) is because as the frequency increases, and the wavelength of the sound decreases to become comparable with the physical size of the transducer's diaphragm, so the way the transducer behaves, particularly in respect of its directionality, changes, not always beneficially. For example, a conventional domestic loudspeaker is generally
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