Projector
Frequency Properties
Projectors can be designed to work at high or low frequencies, depending on how the projector will be used. One consideration in selecting a frequency is the distance that the sound will travel. Low frequency signals are absorbed less rapidly in the ocean than high frequency signals. They can therefore travel longer distances and still be detected. (See Sound Absorption). Echo sounders used to measure water depth in the deep ocean, which is 4000-5000 m deep, typically use frequencies of about 12 kHz. Echo sounders used in shallower water typically use much higher frequencies, often 30 kHz or more. Signals designed to travel thousands of kilometers have to use very low frequencies. The Acoustic Thermometry of Ocean Climate (ATOC) project transmitted signals at 75 Hz, for example.
 |
| ATOC HX-554 acoustic source (75 Hz) on the DSVSS Laney Chouest ready for deployment off Kauai during July, 1997. The acoustic source is a large ceramic cylinder mounted on a frame. Electrical current passed through the cylinder will cause it to expand and contract, creating the 75Hz sound waves for the ATOC project. Photo Credit: Scripps Institution of Oceanography. |
Another consideration in selecting the frequency is the size of the target. High frequency signals give higher resolution and can generate detailed images of the target better than low frequency signals. Fish finders, for example, typically operate at frequencies of 20-200 kHz. Side scan sonars use frequencies as high as 500 kHz to generate detailed images of objects on the seafloor, but the sonar then needs to be towed near the seafloor very close to the object.
The frequency at which a sound source transmits has a large effect on its design. In order to transmit sound efficiently the size of the source needs to be roughly comparable to the wavelength of the sound being transmitted. Low-frequency sounds have long wavelengths, and low-frequency sources therefore need to be large. The 75 Hz sound transmitted for the ATOC project has a wavelength of about 20 meters, for example, and the sources are roughly 2.1 m (6.9 ft) high by 0.9 m (3.0 ft) in diameter (comparable in size to a large water heater) and weigh 2268 kilogram (kg) (5000 pounds). High-frequency sounds have much shorter wavelengths. At 75 kHz (75,000 Hz), for example, sound has a wavelength that is 1000 times smaller than at 75 Hz, 0.02 meters or 2 centimeters, and a 75 kHz sound source can therefore be 1000 times smaller than a 75 Hz sound source.
|
