Silencing the silent service

It is well known that the first few Collins class submarines suffered from a higher level of noise emissions at the beginning of their service life than was desirable. The good news is that the early issues are being successfully corrected with the use of instruments including a number of sound and vibration analysers.

Both sound and vibration are subject to the same rules of physics and represent energy seen to be in different forms.

A hammer dropped on the deck plate of a submarine creates vibration in the deck which is converted to sound in the adjacent air or in the water in contact with the outside of the hull.

In the same way, all the machinery and operations of the vessel can generate vibration and sound waves in the internal air space and surrounding water. These waves can be sensed over long distances in sea water and are a threat to the security of the vessel.

FFT-based signal analysers in defence use are a challenge for designers of data acquisition and analysis instruments.

The frequency bandwidth for most applications is low - usually less than 2000 Hz and in marine applications often below 1000 Hz - but it is necessary to cover the full audio range up to 20 kHz.

Further, designers have long aimed to meet all requirements in a single platform and while the original FFT analysers built on visible minicomputers or dedicated 'boxes' achieved this, they weren't very portable.

A family of dynamic signal analysis systems from Kingdom uses ethernet to interface to the controlling computer or to multiple chassis for very high channel counts.

[image] 32 channel ABACUS dynamic signal analyser showing two channel 1/3 octave analysis.

This family, under the trade name Data Physics SignalCalc, provides a toolbox of features and includes a wide bandwidth and ultra portability with powerful functionality.

ASC Pty Ltd (formally Australian Submarine Corporation) and Defence Science and Technical Organisation (DSTO) have used three Data Physics analysers to help solve the vibration and acoustic issues in the Collins class boats.

All the analysers were selected for either their ultra portability, such as two ACE 2 channel analysers, or for their portability plus high channel count, such as SignalCalc 620 VXI which DSTO is using with 48 input channels. (An alternate instrument now available is ABACUS DSP Engine, which permits up to 1024 channels.)

In each case the ability to get the instruments into restricted and awkward places in a submarine operating at sea, combined with the very powerful functionality and ease of use decided the selection.

Mike Osborne of ASC said that "Data Physics has put a large amount of work into developing the very logical and excellent user interface of ACE".

The user interface is common across the range of analysers.

Data Physics analysers are not only useful in analysis of mechanical structures.

Among the large list of options available for the SignalCalc family are three choices which are relevant to Occupational Health & Safety requirements.

'Human Vibration' is an option, which answers an emerging trend in OH&S requirements to protect workers from hazards in environmental circumstances. Human Vibration can be used to determine the impact on the human body, either the hand/arm or whole of body, of vibrations, which arise from machinery in contact with humans.

When coupled with a seat pad sensor which includes either a uni-axiel or tri-axiel accelerometer built into a soft seat cushion, multiple measurements can be obtained from the seat, the seat back and the steering wheel or controls to assess the degree of vibration.

Human audio is also very important in OH&S investigations and the Data Physics analysers include two options to assist these investigations.

One provides both synthetic and real-time octave analysis including full octave, third octave, down to 1/24 octave.

The second option addresses the emerging need to relate the loudness of sound more closely to the manner in which the ear perceives sound intensity.

Sound quality/loudness analysis provides this type of measurement. Both of these options can be used in measuring human audio impact in different ways.

The combined effects of both vibration and sound cause problems both for the military and industry. Instruments are available with a suitable range of facilities to measure, analyse and record these phenomena.

The resulting analysis will assist modification of the operating machine to reduce the vibration or noise to allowable levels or, if necessary, enable protective equipment or clothing to be selected for the work force as well as providing solutions to other important issues.