42
Henry C. Bennet-Clark
of the maximum velocity, the muscle produces its maximum mechanical power
(the product of its force and shortening velocity). Power outputs of up to 250
Wkg-1 have been recorded.
Contraction frequencies of over 40 Hz for the flight muscles of humming birds
(Weis-Fogh 1972) and of between 100 and 500Hz from insect sound-producing
muscles (Josephson and Young 1985) have been recorded. At a frequency of
contraction of about 100 Hz, the tymbal muscle of the cicada Cyclochila
austra/asiae produces forces of 22·1 o3 Nm-2 (Bennet-Clark and Daws 1999). One
atmosphere is 1 00·1 03 Nm-2 , so these tymbal muscles are producing a working
pressure of 0.2 atm. An acoustic change of ± 1 atm corresponds to a sound
pressure of 194 dB (!). However, the highest sound pressures recorded inside the
abdominal air sac of C. austra/asiae are 155 to 159 dB, or ± 0.01 to 0.02 atm
(Young and Bennet-Clark 1995). There is a 20-fold stage of reduction of pressure
between the muscle and the inside of the insect; this must be a stage of impedance
matching because the transduction efficiency between the insect's muscles and the
sound that it radiates is at least 18 %. and possibly as high as 46 %. (Bennet-Clark
and Daws 1999).
Muscle pressures are appropriate for locomotion or feeding but are far larger
than those in acoustic communication; loud music in a concert hall, 94 dB, is 1 o-5
atmospheres and many animals can hear sound pressures of below 10-3 Nm-2 or
10- 8 atm.
Frequency multiplication usully occurs between the muscle and the first
mechanical stage. The muscle contraction rate in C. austra/asiae is about 120 Hz
but the song frequency is 4.3 kHz (Josephson and Young 1981). Each muscle
contraction causes the tymbal to buckle and produce a series of clicks at close to
the song frequency (Young and Bennet-Clark 1995; Bennet-Clark 1997). The
tymbal acts as a frequency multiplier (see Sect. 3.3).
In summary, muscle produces pressures that vastly exceed those encountered
in everyday acoustics in air at frequencies that are far lower than those typical of
acoustic communication.
3.3 The Role of Oscillators and Other Vibrating Structures
Typically, a single muscle contraction powers one or a series of sound pulses.
Where continuous sounds are produced, they are produced by the overlapping
alternating contraction of contralateral muscles as in the whistling song of the
periodical cicada Magicicada septendecim (Young and Josephson 1983). The
ubiquity. of frequency multiplier mechanisms that convert a long slow muscle
contraction into a higher frequency has been highlighted by Michelsen (1983).
Frequency multipliers may be driven by a forced air current, by impacts, by
escapements etc. (for a comprehensive early review, see Dumortier 1963a) but,
although such structures may set the frequency of the sound, they do not always
act as the effective sound source.
Henry C. Bennet-Clark
of the maximum velocity, the muscle produces its maximum mechanical power
(the product of its force and shortening velocity). Power outputs of up to 250
Wkg-1 have been recorded.
Contraction frequencies of over 40 Hz for the flight muscles of humming birds
(Weis-Fogh 1972) and of between 100 and 500Hz from insect sound-producing
muscles (Josephson and Young 1985) have been recorded. At a frequency of
contraction of about 100 Hz, the tymbal muscle of the cicada Cyclochila
austra/asiae produces forces of 22·1 o3 Nm-2 (Bennet-Clark and Daws 1999). One
atmosphere is 1 00·1 03 Nm-2 , so these tymbal muscles are producing a working
pressure of 0.2 atm. An acoustic change of ± 1 atm corresponds to a sound
pressure of 194 dB (!). However, the highest sound pressures recorded inside the
abdominal air sac of C. austra/asiae are 155 to 159 dB, or ± 0.01 to 0.02 atm
(Young and Bennet-Clark 1995). There is a 20-fold stage of reduction of pressure
between the muscle and the inside of the insect; this must be a stage of impedance
matching because the transduction efficiency between the insect's muscles and the
sound that it radiates is at least 18 %. and possibly as high as 46 %. (Bennet-Clark
and Daws 1999).
Muscle pressures are appropriate for locomotion or feeding but are far larger
than those in acoustic communication; loud music in a concert hall, 94 dB, is 1 o-5
atmospheres and many animals can hear sound pressures of below 10-3 Nm-2 or
10- 8 atm.
Frequency multiplication usully occurs between the muscle and the first
mechanical stage. The muscle contraction rate in C. austra/asiae is about 120 Hz
but the song frequency is 4.3 kHz (Josephson and Young 1981). Each muscle
contraction causes the tymbal to buckle and produce a series of clicks at close to
the song frequency (Young and Bennet-Clark 1995; Bennet-Clark 1997). The
tymbal acts as a frequency multiplier (see Sect. 3.3).
In summary, muscle produces pressures that vastly exceed those encountered
in everyday acoustics in air at frequencies that are far lower than those typical of
acoustic communication.
3.3 The Role of Oscillators and Other Vibrating Structures
Typically, a single muscle contraction powers one or a series of sound pulses.
Where continuous sounds are produced, they are produced by the overlapping
alternating contraction of contralateral muscles as in the whistling song of the
periodical cicada Magicicada septendecim (Young and Josephson 1983). The
ubiquity. of frequency multiplier mechanisms that convert a long slow muscle
contraction into a higher frequency has been highlighted by Michelsen (1983).
Frequency multipliers may be driven by a forced air current, by impacts, by
escapements etc. (for a comprehensive early review, see Dumortier 1963a) but,
although such structures may set the frequency of the sound, they do not always
act as the effective sound source.