**3.6.3 Intermittent sources of noise**

14 Will-be-set-by-IN-TECH

the swarm, irrespective of the operating depth. As will be discussed in the next section on Self Noise, the expectation is that this additional 'ambient noise' which relates to the 'Shipping Noise' component of ambient noise will have limited affect on the acoustic communication

Self noise is defined as the noise generated by the vehicle itself as the platform for receiving signals. This noise can reach the hydrophone mounted on the AUV either through the mechanical structure or through the water passing over the hydrophone. The degree to which turbulent flows cause transducer self noise depends on the location (mounting) of the transducer and its directivity characteristics (Sullivan & Taroudakis, 2008). Self Noise can also be seen as an equivalent isotropic noise spectrum as presented by Urick from work done during WWII on submarines. In general, as with ambient noise, there is decreasing levels of self noise with increases in frequency however self noise is also significantly affected by speed with decreasing noise spectra when the vessels are travelling at slower speeds or are

Kinsler (1982) notes that at low frequencies (<1kHz) and slow speeds machinery noise dominates and at very slow speeds self noise is usually less important than ambient noise. Whereas at higher frequencies (10kHz) propeller and flow noise begins to dominate and as speed increases the hydrodynamic noise around the hydrophone increases strongly and becomes more significant than the machinery noise. This is due to the cavitation from the propeller due to the entrainment of air bubbles under or on the blade tip of the propeller. At higher speeds, self noise can be much more significant than ambient noise and can become

The self noise of different size and types of vehicles are as varied as there are vehicle designs and there is little recent published values. Each vehicle itself produces large variations in self noise with speed and operating conditions (Eckart, 1952). Self noise can be controlled by selection of motor type, configuration, mounting and motor drivers. The trend for most AUV's will be the use of small brush-less DC electric motors which have been used on the development of the SeaVision vehicle (Mare, 2010). Preliminary testing of self noise on these vehicles shows an increase in noise due to increases in speed, as has been predicted, but there was no way to distinguish between machinery and hydrodynamic affects. Higher frequency components (up to 20kHz) were present as the speed increased due to the increased work from the thrusters. When the SeaVision vehicle hovered in a stationary position the frequency

Holmes (Holmes et al., 2005) at WHOI recently investigated the self noise of REMUS, their torpedo shape AUV, used as a towed array. At the maximum RPM of the AUV, the 1/3rd octave noise level, when converted to source level by the calibrated transmission factor, was 130 dB re 1*μPa* at 1m directly aft of the vehicle for a centre frequency of 1000 Hz. This would represent the radiated noise source level for a vehicle moving at 3 knots (1.5m/s). Vehicles typically radiate less noise in free operating conditions than in tethered conditions, so the second trail on the REMUS was measuring the radiated noise of the vehicle to examine the power spectral density of the noise as recorded on the hydrophone array as it was towed behind the vehicle. The results showed the RPM dependent radiated noise in the aft direction at a distance of 14.6 m behind the vehicle looking at frequency range up to 2500Hz which is

which generally uses frequencies above 10kHz.

stationary (Eckart, 1952; Kinsler et al., 1982; Urick, 1967).

of the noise psd centred around 2kHz, which is out of band noise.

**3.6.2 Self noise**

the limiting factor.

again out of band noise.

The sources of intermittent noise can become very significant in locations or times that they occur close to operating AUV swarms. The two major areas where research has been undertaken are in the marine bio-acoustic fields and also the effect of rain and water bubbles created by raindrops.

Major contributors to underwater bio-acoustic noise include;


Rain creates different noise spectrum to wind and needs to be dealt with separately as it is not a constant source of noise. Urick (1967) showed examples of increases of almost 30dB in the 5 to 10kHz portion of spectrum in heavy rain, with steady rain increasing noise by 10dB or sea state equivalent increase from 2 to 6. Eckart (1952) presented average value of rain at the surface from 100Hz to 10kHz of -17 to 9dB.

These main contributors to intermittent sources predominate in the lower frequency ranges up to 20kHz. Thus, interference in the operating frequencies of communication data signals is considered low.
