320
6 Shocks and Surprises: Refining the Elementary Model
Fig. 6.30 (a) Reproduction of the Tintignac carnyx (see Sect. 9.1.2). (b) Measured input
impedance of the reproduction of carnyx: just below the fifth acoustic resonance at 402 Hz, there
is a small perturbation at 396 Hz corresponding to a structural resonance
from the perfectly rigid case can be found. In this case of frequency coincidence, the
spectral content of sounds can be slightly different from that of the perfectly rigid
case if the coincidence is close to a harmonic of the played note. In Nief et al. (2008),
the tube was constructed with an unusually thin wall to make the mode coupling
effects easily measurable. The experiments of Smith (1986) suggested that this type
of frequency-localised coupling of acoustic and structural modes is unlikely to be
of musical importance in realistic brass instruments.
Nevertheless, sometimes it is possible to see small perturbations on impedance
measurements of real brass instruments (see, e.g. Fig. 6.30, around the fifth resonance of a carnyx input impedance). Such small-magnitude secondary peaks, which
arise from coupling of structural resonances with the internal acoustic field, have
been measured on a trumpet by Macaluso and Dalmont (2011). It was found that
when the structural resonances of the bell were damped by wrapping it in a light
plastic film, the main acoustic resonances were not significantly altered but the small
secondary peaks were suppressed.
Such coincidence effects are influenced by details of the mechanical boundary
conditions. The effect of clamping the brace of a trombone is illustrated in Fig. 6.31.
The additional damping caused by the clamp suppresses a mechanical mode which
coincided with an acoustic mode, and as a consequence, a small perturbation near
780 Hz disappears from the measured input impedance.
Experiments involving the heavy damping of brass instrument bells to investigate
the influence of wall vibrations on the radiated sound were described in Sect. 6.6.2.
The effects of structural resonances on the transfer function and input impedance
of a B trumpet have also been studied using this method by Kausel et al. (2010).
In the transfer function experiments, the instrument was excited sinusoidally by an
acoustic driver; the input pressure was measured by a microphone in the delivery
tube between the driver and the mouthpiece and the transmitted pressure by a second
6 Shocks and Surprises: Refining the Elementary Model
Fig. 6.30 (a) Reproduction of the Tintignac carnyx (see Sect. 9.1.2). (b) Measured input
impedance of the reproduction of carnyx: just below the fifth acoustic resonance at 402 Hz, there
is a small perturbation at 396 Hz corresponding to a structural resonance
from the perfectly rigid case can be found. In this case of frequency coincidence, the
spectral content of sounds can be slightly different from that of the perfectly rigid
case if the coincidence is close to a harmonic of the played note. In Nief et al. (2008),
the tube was constructed with an unusually thin wall to make the mode coupling
effects easily measurable. The experiments of Smith (1986) suggested that this type
of frequency-localised coupling of acoustic and structural modes is unlikely to be
of musical importance in realistic brass instruments.
Nevertheless, sometimes it is possible to see small perturbations on impedance
measurements of real brass instruments (see, e.g. Fig. 6.30, around the fifth resonance of a carnyx input impedance). Such small-magnitude secondary peaks, which
arise from coupling of structural resonances with the internal acoustic field, have
been measured on a trumpet by Macaluso and Dalmont (2011). It was found that
when the structural resonances of the bell were damped by wrapping it in a light
plastic film, the main acoustic resonances were not significantly altered but the small
secondary peaks were suppressed.
Such coincidence effects are influenced by details of the mechanical boundary
conditions. The effect of clamping the brace of a trombone is illustrated in Fig. 6.31.
The additional damping caused by the clamp suppresses a mechanical mode which
coincided with an acoustic mode, and as a consequence, a small perturbation near
780 Hz disappears from the measured input impedance.
Experiments involving the heavy damping of brass instrument bells to investigate
the influence of wall vibrations on the radiated sound were described in Sect. 6.6.2.
The effects of structural resonances on the transfer function and input impedance
of a B trumpet have also been studied using this method by Kausel et al. (2010).
In the transfer function experiments, the instrument was excited sinusoidally by an
acoustic driver; the input pressure was measured by a microphone in the delivery
tube between the driver and the mouthpiece and the transmitted pressure by a second