**3.3 Human activities**

*Noise and Environment*

tional waters [3].

as threshold [6].

of ocean.

**3.1 Marine life**

190 dB at 10–25 Hz frequencies [6].

underwater dB at 1 m.

**2. Ambient noise: frequency dependency**

**3. Ambient noise due to intermittent sources**

The classification of primary sources of noise can be done by taking frequency as a parameter. For example, ambient noise because of distant ship traffic exists between 20 and 500 Hz. In the absence of noise due to ships that are nearer to the receiver, also noise can be identified, which is obviously due to distant ships. Noise is higher in high ship traffic locations. For example, in the southern hemisphere, where there exist fewer ships, naturally low-frequency ambient noise levels are recorded. With the increased ship traffic, the noise produced is also high in interna-

Ambient noise generated due to spray and bubbles related to breaking waves fall in the range of 500–100,000 Hz [4, 5]. With the increase in wind speed, ambient noise also increases. After 100,000 Hz, thermal noise dominates. Thermal noise is the noise due to random motion of water molecules, which is sometimes considered

Knudsen et al. [7] studied ambient noise in harbors and in coastal waters. This study reveals the fact that the noise level raises with the increase in speed of the wind and wave height. At a specified wind speed, noise level decreases along with the incremental values of frequency. It has been observed that in the frequency range of 500 Hz to 50 KHz, ambient noise is a function of wind speed. This encourages researchers to use this as a means to measure wind speed over the ocean. Apart from the sources of ambient noise that have been listed out, several intermittent sources exist in the ocean, which include marine life, man-made, and natural processes. These intermittent sources are limited to particular regions

The influence of intermittent sources on ambient noise is significant, which

A distinct and innumerous variety of sounds are produced by marine life. Humpback whales, dolphins, and other marine mammals generate sounds of a broad range of frequencies, and these sounds can be treated as infrasonic or ultrasonic. Specific categories of fish and marine invertebrates are also sources of sound. For example, special species of fish includes toadfish and croaker, and some of the marine invertebrates are snapping shrimp. Information pertained to surroundings is obtained by marine animals using sound. This dependency on sound extends in communicating, navigating, and even feeding. For example, dolphin depends on sound to search food and smell enemies. This is done by transmitting sound pulses for short intervals, and then with the echo received, dolphins detect objects in sea. Most of the time, this kind of communication contributes to raise in ambient noise levels for a maximum value of 25 dB. In the case of whales, noise levels are up to

Sound produced by snapping shrimp inhabiting in shallow waters contributes to background noise. In general, prevalence of this can be seen in semitropical and shallow tropical waters which contain seabed with rock and shell, as they provide the necessary concealment. Colonies of snapping shrimp produce sounds at frequencies of 2–15 KHz. Individually these have peak-to-peak source levels of 189

includes various factors such as marine life, rain noise, shipping noise, etc.

**154**

Human activity is one of the important sources of background noise. Sounds in underwater are used for many purposes like communication, navigation, and fishing, and the point of observation is that one act is source for the other activity. Several by-products can also be seen like noise generation by offshore activities such as oil drilling, manufacturing, and other industrial acoustic pollutions. Incidentally and evidently, all of these factors, more or less, have a human hand in it. So, it is very clear that human activities also contribute to ambient noise of the underwater acoustics.
