Page 16 - Summer 2021

MUTES FOR MUSICAL HORNS
The problem with the sock is that it intrudes on the inter- nal acoustic field of the instrument; muting devices that do this are known as internal mutes. Disturbance of the acoustic resonance frequencies and natural note pitches can be minimized if the muting object does not penetrate significantly into the horn mouth. Mutes that satisfy this rule are known as external mutes. Some examples of exter- nal mutes are reviewed in Derby Hats and Drain Plungers, after which we examine various ways in which internal mutes have been adapted to serve musical purposes.
Derby Hats and Drain Plungers
An important characteristic of a brass instrument bell is the cutoff frequency, which depends on the diameter and rate of flare of the bell near its exit. For frequencies well below this cutoff, most of the sound energy in a wave trav- eling down the instrument tube is reflected back into the instrument on reaching the bell, whereas for frequencies well above the cutoff, most of the energy travels outward as a radiated wave. Because the standing waves in the instru- ment arise from the addition of the forward traveling and reflected waves, the peaks marking the acoustic resonances in the input impedance curve diminish rapidly above the cutoff frequency. Inspection of the trumpet impedance curve in Figure 4 shows that the cutoff frequency for this instrument is in the region of 1,200 Hz.
The sound radiated from the instrument can be partially interrupted by placing any solid object in front of the bell. Inventive early jazz musicians discovered that derby hats and the rubber cups used on drain plungers made effec- tive mutes. External mutes are most effective at reducing the amplitudes of the high-frequency components in the sound because these are radiated more strongly along the bell axis. The instruction “in hat” on a big band trum- pet or trombone score instructs the player to play into a derby hat or a specially manufactured mute in this shape (available at bit.ly/3pIj9qh). Because the hat intercepts much of the high-frequency radiation, the sound is both quieter and more mellow than when the instrument is unmuted. A presentation of trumpet mutes by Jon-Erik Kellso (available at bit.ly/3qu6Xum) includes an enter- taining demonstration of the use of an aluminum derby (at around 4 minutes 25 seconds).
The “plunger” mute (Figure 6A) performs a similar func- tion to the hat, but because it can be firmly gripped in the player’s hand, it can be easily manipulated to make
rapid timbral changes. One characteristic effect, popular in swing band arrangements in the 1940s, involves the play- ing of a succession of notes with the plunger alternately close to the bell and swung away from it (Multimedia 2 at acousticstoday.org/campbellmultimedia). Closed and open positions of the plunger are marked “+” and
“o,” respectively, on a musical score. A spectrogram of a performance of this effect on a tenor trombone is shown in Figure 6B. Notes that are played with the plunger in the open position have a rich harmonic spectrum with significant components up to at least 8 kHz. Moving the plunger to the closed position strongly attenuates the high frequencies, with little energy above 4 kHz. The effect is similar to that obtained by singing the vowel “ah” while alternately opening and almost closing the lips, creating a “wah-wah” sound.
Baroque Transposing Mutes
The earliest reference to the use of mutes in brass instruments appears to be in a description of a carnival
Figure 6. A: trombone plunger mute. B: spectrogram of trombone note with plunger closing (+) and opening (o) the bell mouth. Adapted from Campbell et al., 2021, with permission of Springer Nature.
16 Acoustics Today • Summer 2021