**4. Ambient noise level**

Wenz curves are very helpful in the prediction of the level of ambient noise under given constraints. Wenz curves as shown in **Figure 1** represent the average ambient noise for different shipping traffic levels and wind speeds.

Ambient noise level variation w.r.t frequency can be well understood from Wenz curves, and this can be summarized as follows:

**157**

*Underwater Ambient Noise*

*DOI: http://dx.doi.org/10.5772/intechopen.93057*

60–90 dB with less-frequency dependence.

marine mammals or rain is present.

sources are relatively closer to the surface."

**5. Ambient noise in shallow water**

total ocean area.

the sea surface and the seafloor.

• 10–100 Hz—In this band noise level depends mostly on shipping density and activities related to industries. In such cases noise levels are in the range of

• 1–100 kHz—Sea surface agitiation is the dominant factor unless and otherwise

• >100 kHz—Noise in this range is dominated by electronic and thermal noise.

Shallow water ambient noise is highly random due to the waveguide nature of the environment and bottom reflection [12]. There are two definitions of shallow water: hypsometric and acoustic. The hypsometric definition is based on the fact that most continents have continental shelves bordered by the 200 m bathymetric contour, beyond which the bottom generally falls off rapidly into deep water. Therefore, shallow water is often taken to mean continental shelf waters shallower than 200 m. Using this definition, shallow water represents about 7.5 percent of the

Acoustically, shallow water conditions exist whenever the propagation is characterized by numerous encounters with both the sea surface and the seafloor. By this definition, some hypsometrically shallow water areas are acoustically deep. Alternatively, the deep ocean may be considered shallow when low-frequency and long-range propagation conditions are achieved through repeated interactions with

Shallow water regions are distinguished from deep water regions by the relatively greater role played in shallow water by the reflecting and scattering boundaries. Also differences from one shallow water region to another are primarily driven by differences in the structure and composition of the seafloor. Apart from water depth, the seafloor is perhaps the most important part of the marine environment that distinguishes shallow water propagation from deep water propagation. The most common shallow water bottom sediments are sand, silt, and mud, with

In shallow water, in the absence of local shipping and biological noise, wind noise dominates the noise of distant shipping over the entire frequency range [1]. The reason for this is that the deep favorable propagation paths traveled by distant

Shallow water is known for its variability. Waters close to shore and in busy harbors are dynamic locations, where rapid noise changes take place. Even though shallow water has the variability as a notable feature due to variable background of biological and shipping activities, at high frequencies and even at high wind speeds, this level is significantly constant from location to location at the same wind speed. Although noise field characterization is complex in shallow waters, it has to be car-

compressional sound speeds greater than that of the overlying water.

shipping noise in deep water are absent in shallow water.

ried out because of its greater applications in naval operations.

The main inference can be drawn out of Wenz curves as "noise level (NL) decreases as depth increases and also w.r.t change in frequency, because most noise

• 100–1000 Hz—Here shipping is the primary source of noise. Sea surface agitation is also a contributing factor to noise with this frequency range.

**Figure 1.** *Wenz curve [11].*

*Noise and Environment*

**4. Ambient noise level**

the absence of ships and tumid marine life.

curves, and this can be summarized as follows:

discovered that it correlated very well with wind speed [7].

ambient noise for different shipping traffic levels and wind speeds.

to 50 KHz. In this range of frequency, i.e., in KHz, hydrophone is used as an anemometer [1]. Wind speed is considered as the major constituent of ambient noise in

Wind noise can be treated as a typical case of random noise. Wind noise in the frequency range of 500 Hz to 20 KHz is called as Knudsen noise, because Knudsen

Wenz curves are very helpful in the prediction of the level of ambient noise under given constraints. Wenz curves as shown in **Figure 1** represent the average

Ambient noise level variation w.r.t frequency can be well understood from Wenz

**156**

**Figure 1.** *Wenz curve [11].*


The main inference can be drawn out of Wenz curves as "noise level (NL) decreases as depth increases and also w.r.t change in frequency, because most noise sources are relatively closer to the surface."
