Page 20 - Summer 2021

MUTES FOR MUSICAL HORNS
Figure 10. Input impedance curves for a tenor trombone with a straight mute removed (blue) and inserted (red). A: slide in first position. B: slide in seventh position. Adapted from Campbell et al., 2021, with permission of Springer Nature.
the lips to the second acoustic mode at 116.5 Hz; because the parasitic resonance is not close to this frequency, it does not disturb the playing of the note. When the slide is extended to the seventh position, however, the pitch of the second natural note drops to E2, at a frequency of 82.4 Hz. This is now dangerously close to the para- sitic resonance, and the input impedance curves shown in Figure 10B reveal that the insertion of the mute has split the second peak into two smaller peaks. Lacking the support that comes from coupling to a single strong acoustic resonance, the player’s lips will struggle to sound a stable E2 with the mute inserted.
Some other common alternatives to the straight mute are illustrated in Figure 1. The “Tuxedo Plunger” (Figure 1A) is a commercial variant of the simple drain plunger previ- ously discussed. The cup mute (Figure 1D) is a straight mute surrounded by an annular bowl that almost covers the bell. It can thus be considered as a combination of a straight mute and a plunger (Sluchin and Caussé, 1991). The increased trapping of high-frequency sound energy by the bowl gives this design of a mute a warmer sound than a straight mute. The space between the bowl, the bell of the instrument, and the external surface of the straight
mute has its own internal resonance, at a frequency that depends critically on the gap between the bowl rim and the bell. For a trombone cup mute in its normal position, Sluchin and Caussé found an attenuation of 20 dB in a fairly narrow frequency band around 1,000 Hz, which they ascribed to this additional cavity resonance.
The harmon mute shown in Figure 1E has a cork strip surrounding the inner neck, closing off the annular gap through which sound radiates in a straight or cup mute. The radiating aperture in a harmon mute is a circular open- ing in the outer face of the mute. A short cylindrical tube, known as the “stem,” can be inserted into this aperture, and manipulation, removal, or partial covering of the stem by the fingers allows for a wide variety of timbral effects to be achieved. A harmon mute without the stem was used by Miles Davis to create the “cool jazz” trumpet sound that became his hallmark (available at bit.ly/3u34HMK).
Finale: The Active Mute
An interesting recent development in the design of mutes for brass instruments has been the application of the active control technique (Nelson and Elliott, 1991) to cure the problem caused by parasitic resonances in straight mutes (Meurisse et al., 2015). The principle of the method is illustrated in Figure 11. Because the Helm- holtz resonance of the mute cavity plays no useful role in the sound production of the muted instrument, it can be suppressed without unwanted side effects. A microphone inside the cavity senses the internal acoustic pressure, providing an input signal to the control electronics. The amplified and phase-shifted signal drives a loudspeaker embedded in the mute, with the aim of canceling the pressure changes due to the Helmholtz resonance.
An experimental test of the method was carried out using the modified trombone mute shown in Figure 11. When the mute was used in a trombone with the active control switched off, the pedal note B♭1 was very difficult to play because its frequency (58 Hz) was very close to the para- sitic peak. Switching on the active control system, with gain G = 2 and a phase shift of Φ = π radians, effectively canceled the pressure signal arising from the Helm- holtz resonance. The parasitic peak disappeared from the input impedance of the muted trombone, and the playability of the note B♭1 was restored. It seems likely that future developments in electronic enhancement and active control will not only help to correct faults in the
20 Acoustics Today • Summer 2021