Examples of directional projectors are echosounders and side scan sonars. Echosounders that are used to measure ocean depth are designed to point down toward the seafloor (see Echosounder). Side scan sonars have narrow beams directed perpendicular to the direction in which the sonar is being towed, to image the seafloor on either side of the ship track (see Side Scan Sonar).
A projector whose physical dimensions are small compared to the wavelength of the sound being generated transmits sound equally in all directions. A projector must be large compared to a wavelength in order to generate directional beams. Small individual projectors can also be used together to create directional beams. These are called projector arrays. In a projector array, all of the individual projectors (also called array elements) transmit at the same time to generate directional beams. The arrays are designed so that the sound waves generated by each of the array elements add together in the direction that the sound is wanted, but do not add together in other directions. Projector arrays are often more practical than large directional projectors.
Another reason to use a projector array is to achieve higher effective source levels in the direction of the beam. The acoustic power transmitted into the water by an array is calculated as the number of elements multiplied by the power transmitted by each element. Because of the way the sound adds together from the individual elements, however, the source level in the direction of the beam is proportional to the square of the number of elements. This means that much less power is needed to obtain a desired source level using a projector array than would be needed using a single source that transmits sound equally in all directions. For example, the SURTASS-LFA sonar system has 18 projectors hanging beneath the source ship (see SURTASS LFA Sonar). The total acoustic power transmitted is 18 times the power transmitted by each of the projectors. The source level in the beam, however, is the square of 18 or 324 times higher than that from a single non-directional projector. Source levels are usual given in decibels (see Advanced Topic: Introduction to Decibels), a logarithmic unit. The source level in the direction of the beam is then higher than the source level of a single projector by
The source level of individual SURTASS-LFA projectors is about 215 underwater dB; therefore, the source level of the overall projector array is theoretically 215 + 25 = 240 underwater dB in the direction of the beam. Although the source level of a projector array is defined at a range of 1 m from the array, the sound level in the ocean is never that high. This is because the sound pressure levels close to the array are a complicated sum of the sound waves transmitted by each of the array elements. The diagram below illustrates how the sound waves from the individual projectors merge at a distance from the array to create the effect of a single stronger source. The equivalent source level of the point source is 227 dB re 1 uPa because the figure below only shows 4 elements in the array rather than the full 18 elements that the SURTASS LFA projector really includes..
The sound field close to the array is irregular and does not fall off smoothly with distance, as it does far from the array. Therefore, the source level of an array must be calculated from measurements at much larger ranges. The equivalent or effective source level at 1 m is then calculated assuming spherical spreading.
Additional Links on DOSITS
- Cylindrical vs. Spherical Spreading
- How is sound used to find objects on the ocean bottom?
- Surveillance Towed Array Sensor System Low Frequency Active (SURTASS LFA) Sonar Technology
- Surveillance Towed Array Sensor System Low Frequency Active (SURTASS LFA) Sonar Sound