Ishmael 1.0 user's guide: Ishmael: integrated system for holistic multi-channel acoustic exploration and localization

Original publisher: Seattle, WA : Pacific Marine Environmental Laboratory, [2002] OCLC Number: (OCoLC)317716903 Subject: Sound -- Research -- Computer programs. Excerpt: ... 11 Ishmael 1.0 User's Guide delay determines the beam angle - the angle to which Ishmael preferentially " listens. " When an animal call arrives from this angle, the sound signals from the multiple phones are added constructively. The resulting sum is stronger, and the call shows up in Ishmael relatively well. When a sound arrives from another angle, the delayed signals from the various phones add destructively - with positive and negative parts of the sound waves cancel-ing out to some degree - and the sum is not as loud as an equivalent sound arriving from the beam angle. The amount of gain you get from beamforming depends on several fac-tors, most importantly on the number of phones. You get some improvement with only a handful of phones, but it really takes more than five or six phones to get much gain, and eight is recommended. Many commercial applications of beamforming use 20, 50, or more phones. Another important factor is co-herence. Coherence is a complex topic, but its impact is that beamforming performance can be degraded significantly if your phones are too far apart. As a very rough rule, keep microphones ( in air ) within a few meters to tens of meters, and hydrophones ( in water ) within tens of meters to perhaps 2 - 300 meters. More variable or turbulent environments require closer placement of phones, and more homogeneous environments permit wider spacing. Beam-forming performance also depends on the frequency of the signal: higher frequencies result in narrower beams, but they are subject to sidelobes, an-gles other than the intended one that have strong gain. Lower frequencies, conversely, have broad beams and fewer sidelobes. The overall response of a beamformer is usually expressed as a beam pattern, a circular plot showing the amount of gain for each angle from which s...