Page 50 - Summer 2006

Scanning the Journals
􏰀“Virtual Acoustic Prototypes: Listening to Machines that Don’t Exist” is the title of a paper in the December issue of Acoustics Australia. A virtual acoustic prototype (VAP) is a computer representation of a machine such that it can be heard without having to exist as a physical assem- bly. Whereas visualization tools are well developed in the field of visual design, the analogous tools for auralization are still in their infancy. In order to construct a VAP, a method is needed to represent the excitation and trans- mission mechanisms. Airborne sound, fluid-borne sound, and structure-borne sound must be considered. This paper was adapted from the keynote lecture at Acoustics 2005.
􏰀Physicists have developed a mathematical model to explain the breathing patterns of canaries when they sing, according to a paper in the 10 February issue of Physical Review Letters. By treating both a bird’s vocal organ and neurons as nonlinear systems, researchers have found that complex songs, involving notes of many frequencies and lengths, might be produced by surprisingly simple neuro- logical structures and processes. The new model shows that birdsong is produced from the interplay between the air sac and the neural system in contrast to the long-held view in which a nervous system sends instructions to a passive body. This suggests that subharmonic behavior can play an important role in providing a complex variety of responses with minimal neural substrate.
􏰀“Sound ideas” is the title of a feature article on phononic crystals in the December issue of Physics World. When a wave passes through a periodic structure, interference leads to the formation of “band gaps” that prevent waves with certain frequencies from traveling through the struc- ture. Band gaps are observed for electron waves in semi- conductors, electromagnetic waves in photonic crystals, and sound waves in phononic crystals. The periodic varia- tion in the density and speed of sound that is needed to make a phononic crystal can be achieved by making air holes in an otherwise solid structure. Negative refraction in phononic crystals is possible due to multiple scattering of sound waves at the solid-air interfaces. Phononic crys- tals could provide researchers in acoustics and ultrasonics with new components that offer the same level of control over sound that mirrors and lenses provide over light. [Phononic crystals were reported in Phys. Rev. Letts. 93, 024301 (July 9, 2004); see Fall 2004 issue of ECHOES].
􏰀 The spiral shape of the cochlea increases sensitivity to low frequency sound, according to a paper in the March 3 issue of Physical Review Letters. Although calculations show that curvature has little effect on the average vibra- tional energy traveling along the tube, energy increasingly accumulates near the outside edge of the spiral rather than remaining evenly spread across it. Low frequencies travel the furthest into the spiral, so the effect is strongest for them. Concentration of sound intensity translates into
higher sensitivity. The researchers liken the sound propa- gation to the “whispering gallery modes” found in domes such as London’s St. Paul’s Cathedral.
􏰀 The complex songs of humpback whales have their own syntax or grammar, according to an article in the 23 March issue of New Scientist. Male humpback whales produce songs that last anywhere from six to thirty minutes, and these vocalizations vary across the seasons. During breed- ing periods they are thought to help attract female part- ners. Now computer programs have been used to analyze complete songs and to demonstrate their hierarchical syn- tax. Shorter whale songs appear more complex than longer ones. The investigators admit that we are still a long way from understanding the meaning of whale songs, however. Some of the whale songs can be heard at www.newscientist.com/channel/life/dn8886.html.
􏰀 The acoustics of the singing voice is the topic of a review article in the April issue of Physics World. Scientists are now able to record spectra of the human voice using relatively simple equipment, and this is having a major impact on the way singing is learned, performed, and recorded. Displaying the spectral signature of the voice in real time on a computer screen, for example, provides an effective teaching aid. It is possible to detect piracy in com- mercial recordings and even to synthesize the human voice to recreate lost sounds. Almost all songs recorded nowa- days have undergone some degree of pitch shifting, the article claims, to disguise the fact that many pop stars can- not sing very well.
􏰀 The use of ultrasonic communication by the concave- eared torrent frog is reported in the 16 March issue of Nature, Males of this species emit birdlike melodic calls with pronounced frequency modulations that often con- tain spectral energy in the ultrasonic range. This extraor- dinary upward extension into the ultrasonic range is likely to have evolved in response to the low-frequency ambient noise near streams.
􏰀 Regular didgeridoo playing has been found to be an effective treatment for patients with obstructive sleep apnea, according to a report in the 23 December issue of the British Medicine Journal. Participants practiced an average of 5.9 days a week for 25.3 minutes.
􏰀 New digital video technology can reveal shock waves as never before according to an article in the January- February issue of American Scientist. Shock waves, like sound waves, are usually as invisible as the air through which they travel. However, schlieren and shadowgraph techniques have been used for flow visualization for at least 100 years. Now high-speed digital cameras with retroreflective screens can record shock position over time and use this information to determine post-shock fluid properties.
􏰀 “Drowning in Sound” is the title of an article in the April issue of IEEE Spectrum that discusses the sonar vs. whales
Thomas D. Rossing
48 Acoustics Today, July 2006