If you wish to reproduce the noise spectrum, or use the animated version on your website or elsewhere, you are welcome to do this. However, use must be accompanied by a copyright statement: © Seiche Ltd. 2006 and, where apppropriate, by a text reference: Coates, R., The Advanced SONAR Course, Seiche (2002) ISBN 1-904055-01-X.
The animation and usage instructions may be obtained - free of charge - as a Java 1.5 Application suitable for desktop use and as a Java Applet, suitable for use on a web-site, by contacting Seiche Ltd.
The noise spectrum is easy to use in a "casual" manner. However, there are subtleties in its application which demand careful explanation. The user also needs a degree of competence in handling issues such as the interplay between source spectral width, receiver bandwidth and temporal and spatial resolution. These matters are central to the teaching philosophy employed in the three Seiche Underwater Acoustics courses.
The image is a spectral density. Its horizontal axis is frequency. The frequency scale is logarithmic, in order to accomodate spectral detail over many decades. The units in which the horizontal axis is measured are Hertz [Hz] or "cycles per second".
The vertical axis is a measure of acoustic power-flux density, or "power per square metre, per Hertz of bandwidth". Power-flux is measured in watts per square metre and is usually referred to as the acoustic intensity of a wave.
The vertical axis is also plotted on a logarithmic (deciBel) scale, again to encompass a very wide span. Use of the deciBel scale implies that acoustic intensity is normalised by comparison with a reference wave. The reference wave is a plane wave of rms pressure 1 microPascal (10-6 Pa). Such a wave has an acoustic intensity (the reference intensity) of 0.67 x 10-18 watts per square metre.
The lower region of the image depicts ambient noise. Ambient noise is "all embracing". It emanates from myriad individual sources, both near and far from the observer. If the observer shifts location, the noise remains constant. Ambient noise, for this reason, is not range-dependant.
The upper region depicts Localised Noise Sources. Perception of noise level of such sources does depend on range. With the right-hand slider scale set to 1 m range, the noise levels of the Localised Sources are as depicted. To find the strength of a Localised Noise Source at any other range, simply adjust the slider downwards.
Not only does the applet allow range compensation, it also builds in three spreading models. The preset model is a "practical" one, partway between high-loss free-field or spherical spreading and low-loss cylindrical spreading which occurs, for example, in shallow water with good reflection at the surface and the sea-floor.
Finally, the applet also builds in range-dependant attenuation loss, which increase dramatically as frequency rises. This is the reason why, as range is increased, Localised Noise Sources at higher frequencies disappear below the ambient noise floor more rapidly than low-frequency Localised Noise Sources.