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The source level SL of an acoustic source is defined as the sound pressure level in dB relative to 1 uPa at a reference distance of r9=1 m. The source level of a sound source with A common transducer for high-frequency application is the piezoelectric disk with an epoxy layer in the front, towards the water, and with a backing of a porous material with acoustic properties like air, Fig. 6. The resonance frequency is the frequency where the thickness of disk equals half the acoustic wavelength of the ceramic.

Figure 6 The source level SL of an acoustic source is defined as the sound pressure level in dB relative to 1 uPa at a reference distance of r9=1 m. The source level of a sound source with A common transducer for high-frequency application is the piezoelectric disk with an epoxy layer in the front, towards the water, and with a backing of a porous material with acoustic properties like air, Fig. 6. The resonance frequency is the frequency where the thickness of disk equals half the acoustic wavelength of the ceramic.

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The absorption of sound in sea water depends on many factors; the most important being the temperature and the salinity. Figure 1 shows the absorption coefficient, expressed in dB per meter as function of frequency, calculated for a temperature of 15 degrees for fresh water and for salt water with a salinity of 3.5%. The absorption increases very rapidly with frequency and this means that transmission over long ranges requires relative low fre-
The absorption of sound in sea water depends on many factors; the most important being the temperature and the salinity. Figure 1 shows the absorption coefficient, expressed in dB per meter as function of frequency, calculated for a temperature of 15 degrees for fresh water and for salt water with a salinity of 3.5%. The absorption increases very rapidly with frequency and this means that transmission over long ranges requires relative low fre-
The solution for the particle velocity is, from Eq. 3 It is assumed in the expressions above that the amplitudes are given as the effective values.
The solution for the particle velocity is, from Eq. 3 It is assumed in the expressions above that the amplitudes are given as the effective values.
Fig. 3 Examples of transducer elements: Jeff, a disk and right, a cylindrical tube
Fig. 3 Examples of transducer elements: Jeff, a disk and right, a cylindrical tube
Fig. 4 Typical receiver sensitivity of a ring hydrophone using a tube element with diameter 10 mm and height 20 mm With c=1,500 m/s and p=1,000 kg/m’, the numerical value of the equation’s first term is approximately 50.8, so that
Fig. 4 Typical receiver sensitivity of a ring hydrophone using a tube element with diameter 10 mm and height 20 mm With c=1,500 m/s and p=1,000 kg/m’, the numerical value of the equation’s first term is approximately 50.8, so that
Fig. 5 Circuit diagram equivalent to the ring hydrophone, for frequencies far below the resonance frequency Figure 2 shows ray paths in the Norwegian Sea during typical summer and winter conditions, respectively. The sound speed profiles are shown at the left sides of the figures; the source depth is 5 m. For the summer profile, we notice that the direct sound’s range is very limited because it is bent downward. In the winter the propagation conditions are quite different. For shallow source depths, the sound rays have a marked upward curve, with possibilities of good sonar coverage for shallow targets.
Fig. 5 Circuit diagram equivalent to the ring hydrophone, for frequencies far below the resonance frequency Figure 2 shows ray paths in the Norwegian Sea during typical summer and winter conditions, respectively. The sound speed profiles are shown at the left sides of the figures; the source depth is 5 m. For the summer profile, we notice that the direct sound’s range is very limited because it is bent downward. In the winter the propagation conditions are quite different. For shallow source depths, the sound rays have a marked upward curve, with possibilities of good sonar coverage for shallow targets.
Fig. 6 Half-wavelength transducer in 33-mode
Fig. 6 Half-wavelength transducer in 33-mode
Fig. 8 The directivity factor is defined by comparing the maximum intensity (@=0) of a directional antenna with the intensity of a point source with intensity Ler
Fig. 8 The directivity factor is defined by comparing the maximum intensity (@=0) of a directional antenna with the intensity of a point source with intensity Ler
Fig. 9 Searching for an object with a hull-mounted sonar in an environment having both noise and reverberation In order to distinguish between two targets having a small velocity difference Av, the receiver must be able to carry
Fig. 9 Searching for an object with a hull-mounted sonar in an environment having both noise and reverberation In order to distinguish between two targets having a small velocity difference Av, the receiver must be able to carry
Fig. 10 Transmission loss as function of range for the frequencies of 0, 25, 50 100 and 200 kHz. The transmission loss includes spherical spreading and absorption according to Eq. 27
Fig. 10 Transmission loss as function of range for the frequencies of 0, 25, 50 100 and 200 kHz. The transmission loss includes spherical spreading and absorption according to Eq. 27
Fig. 11 Target strength of cod as a function of the fish’s length The transmission loss is determined by the sound speed variation in depth and range, which again is determined by oceanographic conditions: the water’s temperature, salt content, and density. If sound propagation involves inter- action with the sea surface or the seabed, the properties of these boundaries also may have a significant impact on transmission loss. In order not to complicate the discussion too much, in this section we will apply a very simple transmission model. For transmissions in relatively deep
Fig. 11 Target strength of cod as a function of the fish’s length The transmission loss is determined by the sound speed variation in depth and range, which again is determined by oceanographic conditions: the water’s temperature, salt content, and density. If sound propagation involves inter- action with the sea surface or the seabed, the properties of these boundaries also may have a significant impact on transmission loss. In order not to complicate the discussion too much, in this section we will apply a very simple transmission model. For transmissions in relatively deep
Table 2. Nominal range and transducer size for typical sonar systems.
Table 2. Nominal range and transducer size for typical sonar systems.
Table 1 Parameters for a hypothetical sonar. 4.5 Noise water and at short distances, we can often assume spherical sound propagation, which along with absorption gives the transmission loss
Table 1 Parameters for a hypothetical sonar. 4.5 Noise water and at short distances, we can often assume spherical sound propagation, which along with absorption gives the transmission loss
Fig. 12 Echo level EL, reverberation level RL, and noise level NL as functions of the distance to a target for which TS=—30 dB The strength of the echo from a target is characterized by its target strength (TS) defined by the reflected intensity divided by the incident intensity as
Fig. 12 Echo level EL, reverberation level RL, and noise level NL as functions of the distance to a target for which TS=—30 dB The strength of the echo from a target is characterized by its target strength (TS) defined by the reflected intensity divided by the incident intensity as
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Materials EngineeringUnderwater AcousticsAcoustic Waves
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