Oceans of noise - Whale and Dolphin Conservation Society

2.2. Types of sound sourceSounds can be transient (short duration sounds with obvious start and finish points) such as thoseproduced by explosive blasts or percussion piling, or continuous (such as drilling). Transient soundsare usually described by their peak level (the maximum amplitude measured in dB) with anaccompanying indication of how this varies in time. If a transient sound is impulsive (with a durationof less than 0.5 seconds) it is best described by its energy level. Continuous sounds and long durationtransient sounds are conventionally described by their mean square pressure.2.3. The propagation of underwater noiseSource path receiver model of soundWhen describing the propagation of underwater noise it is useful to apply a simplistic model to thisprocess. These models are based on the sonar equation and perhaps the simplest of these is the SourcePath Receiver Model. The basic parameters of this model are:(1) Source: the noise source, e.g. ship, sonar etc. Parameter of interest = source level (SL)(2) Path or medium: the water column. Parameters of interest include transmission loss (TL), andambient noise level (NL)(3) Receiver: e.g. whale, hydrophone etc. Parameters of interest include signal to noise ratio (SNR),received sound intensity level (RL) and detection threshold (DT).A simple model of sound propagation is:RL=SL − TLWhere RL is the received level, SL is the source level and TL is the transmission loss.Transmission Loss (TL)Transmission loss is the decrease in intensity of a sound as it propagates through a medium, and is theresult of spreading, absorption, scattering, reflection and rarefaction. Transmission loss can also beestimated by adding the effects of geometrical spreading (TL sp ), absorption (TL a ) and the transmissionloss anomaly (A). The transmission loss anomaly includes scattering loss and losses due to reflectionand rarefaction at boundary interfaces.TL= TLspreading + TLabsorption+AFor simplicity we'll only deal with spreading (TL sp ) and absorption loss (TL a ):TL = TL g+ TL a(1) TL sp - Spreading lossSpreading loss is a major component of transmission loss and is range (distance) dependent. Twoforms of spreading loss are common underwater (1) Spherical or Geometrical spreading loss (TL g ),and (2) Cylindrical spreading loss (TL cy ).22