Page 18 - Spring 2009

Fig. 4. Planar view of tow geometry for calibration of a low frequency hydrophone group. In this example the ship and array proceed in a constant speed and heading and consequently a constant array depth. The interference pattern is determined by the closest point of approach and is characteristic for a constant source receiver depth.
Fig. 5. Measurements of the Lloyd Mirror effect for Beaufort Sea State 3 with an array composed of hydrophone groups of 20 series-parallel connected hydrophones. The array was towed at a nominal depth of 175 m past a moored acoustic source at a depth of 300 m radiating a tone at 175 Hz. The closest point of approach, CPA, was 2500 m.
This image interference effect may also be important in determining the response of near surface marine mammals to shipping noise. Is this effect the reason dolphins approach ships from an angle to ride the bow wave impervious to the radiated noise? Is this the reason that near surface whales can not hear an approaching tanker? What role does this effect have on the increased noise levels and, even though tonnage and power is increasing, does the change in propeller effi- ciency and depth offset these increases? These questions are of current interest and certainly the image interference effect will be an important factor even though sophisticated range and frequency dependence are used to describe the environ- mental impact of industrial noise.
Thus the image interference effect—the Lloyd’s Mirror Effect—has continually been an important factor in under- water acoustics since World War II. All texts on underwater and marine acoustics, in my library cite this effect, but few if any provide a reference to Lloyd. Who was Lloyd and why is he so widely cited in optics and acoustics but rarely refer- enced? Perhaps this was the result of a common practice of only citing previous texts and not primary references.
Humphrey Bartholomew Lloyd—the scientist and mathematician
Initially the thought was that Lloyd’s identity would be a simple question to answer—using library resourses and rum- maging in the stacks. However there are no longer card indexes or stacks. Simply put, there are only digital searches and warehouses. Initial searches using the library search
ing, and a balanced array in a stable tow configuration. The corresponding results for this experiment conducted at con- stant ship rpm for a speed of 2.9 knots with the array at 175 m depth, a moored sound source radiating sound at 175 Hz at 300 m depth with the closest point of approach of 2500 m, are shown in Fig. 5. This particular array had 105 hydrophones and the remarkably consistent results are attributable to stable oceanographic and sea state condi- tions. An interesting feature of this calibration is the use of the last interference peak to calibrate the beam forming in the end fire direction. This technique was used prior to sev- eral major sea tests and the results were successful.
Lloyd’s Mirror 17