312
6 Shocks and Surprises: Refining the Elementary Model
6.6.1 Factors Affecting the Choice of Wall Material
Although many different materials have been used in the manufacture of members
of the labrosone family (see Sect. 1.1), the most common choice by far has been
the alloy of copper and zinc known as brass. One obvious practical reason for this
preference is that the metal allows the construction of a rigid, strong but relatively
lightweight instrument. The durability of metal instruments is also impressive: the
silver and bronze trumpets from the tomb of Tutankhamun were still playable more
than 3000 years after they were made. Whether plastic trumpets will achieve such
longevity remains an open question.
Another important consideration is the workability of the material (see Sect. 8.1).
The properties of the alloy affect the ease with which internal corners can be rounded
and also influence the extent to which a bell formed on a mandrel retains its shape
when removed. Residual internal roughness can influence the damping coefficient
of the air modes, leading to a divergence between the playing experience and the
predictions of theory for a perfectly reflecting and rigid wall (Watkinson et al. 1982).
Other characteristics of the material may affect the sound of the instrument in an
indirect way. Silver, for example, has a very high thermal conductivity and warms
up more quickly than a low thermal conductivity material such as wood or plastic
(see 2.1.7). The breath from the mouth of the player enters the instrument at a
temperature of around 35 ◦ C and includes significant proportions of water vapour
and carbon dioxide as well as oxygen and nitrogen. During the warm-up exercise
usually performed by the player, a temperature gradient is established along the
bore of the instrument (van Walstijn et al. 1997), and changes in the proportions of
the different gases in the tube influence the frequencies of the acoustic modes and
hence the played pitches (Boutin et al. 2013; Campbell 2014a). These effects could
in principle depend on the thermal conductivity of the wall material, but this topic
requires further study.
6.6.2 Experimental Studies of Brass Instrument Wall
Vibrations
The dominant source of sound radiation from a brass instrument is undoubtedly the
transmission of sound energy from the internal air column through the instrument
bell and any other open apertures. The vibrating wall of the instrument also radiates
sound, but it was pointed out in Sect. 2.1.6 that the direct contribution of wall
vibrations to the acoustic output has been estimated to be around 40 dB lower than
the contribution due to the air column. The relative insignificance of wall vibrations
is a property of wind instruments which differentiates them strongly from string
and percussion instruments, in which the dominant sound source is the vibration of
the structure. On the other hand, when the walls vibrate, they may also influence
the sound output by coupling to and modifying the resonances of the internal air
6 Shocks and Surprises: Refining the Elementary Model
6.6.1 Factors Affecting the Choice of Wall Material
Although many different materials have been used in the manufacture of members
of the labrosone family (see Sect. 1.1), the most common choice by far has been
the alloy of copper and zinc known as brass. One obvious practical reason for this
preference is that the metal allows the construction of a rigid, strong but relatively
lightweight instrument. The durability of metal instruments is also impressive: the
silver and bronze trumpets from the tomb of Tutankhamun were still playable more
than 3000 years after they were made. Whether plastic trumpets will achieve such
longevity remains an open question.
Another important consideration is the workability of the material (see Sect. 8.1).
The properties of the alloy affect the ease with which internal corners can be rounded
and also influence the extent to which a bell formed on a mandrel retains its shape
when removed. Residual internal roughness can influence the damping coefficient
of the air modes, leading to a divergence between the playing experience and the
predictions of theory for a perfectly reflecting and rigid wall (Watkinson et al. 1982).
Other characteristics of the material may affect the sound of the instrument in an
indirect way. Silver, for example, has a very high thermal conductivity and warms
up more quickly than a low thermal conductivity material such as wood or plastic
(see 2.1.7). The breath from the mouth of the player enters the instrument at a
temperature of around 35 ◦ C and includes significant proportions of water vapour
and carbon dioxide as well as oxygen and nitrogen. During the warm-up exercise
usually performed by the player, a temperature gradient is established along the
bore of the instrument (van Walstijn et al. 1997), and changes in the proportions of
the different gases in the tube influence the frequencies of the acoustic modes and
hence the played pitches (Boutin et al. 2013; Campbell 2014a). These effects could
in principle depend on the thermal conductivity of the wall material, but this topic
requires further study.
6.6.2 Experimental Studies of Brass Instrument Wall
Vibrations
The dominant source of sound radiation from a brass instrument is undoubtedly the
transmission of sound energy from the internal air column through the instrument
bell and any other open apertures. The vibrating wall of the instrument also radiates
sound, but it was pointed out in Sect. 2.1.6 that the direct contribution of wall
vibrations to the acoustic output has been estimated to be around 40 dB lower than
the contribution due to the air column. The relative insignificance of wall vibrations
is a property of wind instruments which differentiates them strongly from string
and percussion instruments, in which the dominant sound source is the vibration of
the structure. On the other hand, when the walls vibrate, they may also influence
the sound output by coupling to and modifying the resonances of the internal air