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Figure 188: Concept of Cylindrical Spreading
12.3.2 Absorption
Absorption is frequency dependent and refers to the conversion of acoustic energy to heat when it
strikes chemically distinct molecules in the water column. Magnesium Sulphate MgSO
4
predominates, with Boric Acid B(OH)
3
playing a major part at lower frequencies. Temperature is also
an influence on absorption. Absorption is one of the key factors in the attenuation of the acoustic
energy based on frequency; the higher the frequency, the greater the absorption. The higher the
sonar operating frequency, the more rapid the vibration (or excitement) of the particles in the
water, and this leads to the greater transference of acoustic energy; thus, the attenuation of the
acoustic wave. This is the reason why lower frequencies are used to obtain deeper data. At 400 kHz,
the normal seawater absorption is approximately 100 dB/km, whereas, at 200kHz, the absorption is
approximately 50 dB/km. These are values for normal seawater (with a salinity of 35 ppt).
Freshwater has little, if any salinity (<0.5ppt), so the absorption is considerably less.
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