168
4 After the Lips: Acoustic Resonances and Radiation
Fig. 4.62 Measured input impedance of a trombone straight mute
Fig. 4.63 Low-frequency impedance peaks for a trombone with (red curves) and without (blue
curves) a straight mute. (a) Slide in first position. (b) Slide in seventh position (Color figure online)
impedance minima more visible in Fig. 4.62, the curve is plotted with a logarithmic
impedance axis.
The first dip in the mute impedance curve occurs at 166 Hz. This is the Helmholtz
resonance frequency of the mute (see Sect. 4.1.1); the corresponding pitch E3 can be
sounded by blowing across its open neck. The effect of the Helmholtz resonance on
the input impedance of the muted trombone is limited to the low-frequency region
shown in Fig. 4.63. A small additional peak appears when the mute is inserted.
This extra peak is described by Sluchin and Caussé (1991) as ‘parasitic’, since it
plays no useful role in the sound generation process and can in fact disrupt it. The
frequency of the additional peak is lower than the Helmholtz resonance frequency
of the mute by an amount which increases when the trombone slide is extended or
the mute pushed further into the bell.
Figure 4.63a shows that with the trombone slide in first position, the parasitic
peak appears at 77 Hz. The note B 2 is played with this slide position, the player’s
lips coupling to the second main impedance peak at 116.5 Hz. The parasitic peak is
4 After the Lips: Acoustic Resonances and Radiation
Fig. 4.62 Measured input impedance of a trombone straight mute
Fig. 4.63 Low-frequency impedance peaks for a trombone with (red curves) and without (blue
curves) a straight mute. (a) Slide in first position. (b) Slide in seventh position (Color figure online)
impedance minima more visible in Fig. 4.62, the curve is plotted with a logarithmic
impedance axis.
The first dip in the mute impedance curve occurs at 166 Hz. This is the Helmholtz
resonance frequency of the mute (see Sect. 4.1.1); the corresponding pitch E3 can be
sounded by blowing across its open neck. The effect of the Helmholtz resonance on
the input impedance of the muted trombone is limited to the low-frequency region
shown in Fig. 4.63. A small additional peak appears when the mute is inserted.
This extra peak is described by Sluchin and Caussé (1991) as ‘parasitic’, since it
plays no useful role in the sound generation process and can in fact disrupt it. The
frequency of the additional peak is lower than the Helmholtz resonance frequency
of the mute by an amount which increases when the trombone slide is extended or
the mute pushed further into the bell.
Figure 4.63a shows that with the trombone slide in first position, the parasitic
peak appears at 77 Hz. The note B 2 is played with this slide position, the player’s
lips coupling to the second main impedance peak at 116.5 Hz. The parasitic peak is