| Vol 31 No 2 | CONTENTS |
August 2003 |
Observations and Explanation of Low Frequency Clicks in Blue Whale Calls
A.D. Jones, R.D. McCauley and D.H. Cato
Australian Aboriginal Musical Instruments: the Didjeridu, the Bullroarer and the Gumleaf
N.H. Fletcher
Why do Bell Plates Ring?
D. Lavan, S. Hogg and J. Wolfe
An Explanation for the Apparent Poor Performance of Some Hearing Protectors
W. Williams
ACOUSTICS FORUM: Top Ten Issues for Acoustics in Australia
M. Burgess and J. Lai
INTERVIEW: Fergus Fricke
H. Pollard
ACOUSTICS FORUM: Solving the Mystery of the Cat’s Purr using the World’s Smallest Accelerometer
E. von Muggenthaler and B. Wright
AAS Council
Future Meetings
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Book Reviews
Diary
Obituary
Letters
Acoustics Australia Information
Australian Acoustical Society Information
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Vol. 31, No. 2 pp 45-50 (2003)
ABSTRACT: A brief study has been conducted on the low frequency "clicks" and tones observed in calls recorded in recent years from Blue Whale aggregations at locations off the Rottnest trench. As has been suggested previously, it is concluded that the 20 Hz clicks most likely are the self-excitation of the bubble resonance of the gas within the lungs by the whale. This is probable, as the click waveform has the appearance of the decay of a resonance, and as there are no other conceivable mechanisms which might give rise to a resonance within a
whale body at 20 Hz. This is explained with recourse to bubble physics, which are extended to include a treatment of both spherical and elongated bubbles, and to considerations of the effects of various depths at which a whale might call. The amplitude of the bubble oscillations required to generate the observed signal levels is shown to be surprisingly large, and appears to be close to the theoretical maximum for a resonant bubble driven by any means.
Vol. 31, No. 2 pp 51-54 (2003)
ABSTRACT: The Australian Aboriginal people developed three musical instruments – the didjeridu, the bullroarer, and the gum-leaf. Most well known is the didjeridu, a simple wooden tube blown with the lips like a trumpet, which gains its sonic flexibility from controllable resonances of the player’s vocal tract. The bull-roarer is a simple wooden slat whirled in a circle on the end of a cord so that it rotates about its axis and produces a pulsating low-pitched roar. The gum-leaf, as the name suggests, is a tree leaf, held against the lips and blown so as to act as a vibrating valve with "blown-open" configuration. Originally intended to imitate bird-calls, the gum-leaf can also be used to play tunes.
Vol. 31, No. 2 pp 55-58 (2003)
Abstract: Bell plates are polygonal plates which, when held in the hand and struck with a beater, produce an initial transient followed by a sustained, pure tone. The presence of the sustained tone depends sensitively on the shape. This paper addresses the question: why does a particular shape ring so well, while slightly different shapes do not? We show that, in the standard ringing shape, the nodal lines of one of the lowest modes of vibration fuse in the handle to produce a region that exerts neither vibrational force nor torque on the hand, and therefore does not transfer vibrational energy to the hand.
Vol. 31, No. 2 pp 59-62 (2003)
Abstract: Hearing protectors do not always perform as well as manufacturers and distributors would wish. Sometimes the attenuation ratings fall well below what was expected. On close examination of the test data it can sometimes be seen that the results are spread over a very wide range thus producing a lower than anticipated mean value and a large standard deviation. In Australia and New Zealand the rating of a hearing protector depends on the value of the mean attenuation minus one standard deviation at seven octave band centre frequencies of onethird octave wide filtered pink noise. A low mean and large standard deviation can reduce the hearing protector rating significantly. Recent work indicates that present methods of analysing data may not always be satisfactory. Perhaps bimodal or other analysis techniques are more appropriate.
Vol. 31, No. 2 pp 63-66 (2003)
Vol. 31, No. 2 pp 67-68 (2003)
Vol. 31, No. 2 pg 69 (2003)