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Acoustical Brass-instrument Tone Quality:
Is it the Player or the Instrument?
Robert Pyle - rpyle@post.harvard.edu
S. E. Shires Co.
11 Holworthy Place
Cambridge, MA 02138, USA
Popular version of paper 4aMU5
Presented Thursday morning, November 18
2nd Pan-American/Iberian Meeting on Acoustics, Cancun, Mexico
Most experienced brass players are convinced that the material from which the instrument is made has a small but important effect on the instrument's tone quality. Players and instrument makers generally agree that a bell made of thin yellow brass (70% copper, 30% zinc) will produce a brighter tone than a bell made of somewhat thicker red brass (90% copper, 10% zinc).
The experiment described here was an attempt to distinguish between the influence of the alloy and the player's ability to control the tone quality, independent of the bell alloy.
Two players were used, player W, who prefers a light-weight yellow brass bell, and player G, who uses a heavy-weight red brass bell. ("Light-weight" and "heavy-weight" are relative terms; the sheet brass from which the bells are fabricated differ in thickness only by about 20%.) In a previous experiment, player W produced essentially the same timbre on both bells, while player G showed consistent differences.
The test instrument was a symphonic tenor trombone made by S. E. Shires. It is constructed in a modular fashion, so that the two bells can be interchanged while leaving the remainder of the instrument completely unchanged. A microphone was mounted on the bell axis 75 cm from the plane of the bell rim. This was supported by an aluminum rod attached to the trombone's hand slide. The trombone was played with the slide fully retracted ("first position"), so that its position relative to the bell was the same throughout the tests.
The ability of the players to control the timbre was severely reduced by making them listen to loud masking noise while playing. Four pitches were recorded by the two players on both bells, with and without the masking noise.
The players were told to play each note in a slow decrescendo from "as loud as possible" to "as soft as possible". The analysis software moved a small window along this decrescendo, small enough that the playing level was essentially constant within the window. At several hundred positions of the window, the spectrum of the sound was calculated.
Each spectrum was reduced to a single number, the "average frequency". If one imagines the sound energy distributed along a "frequency line", the average frequency is the center of gravity of that distribution. Higher values of average frequency correspond to a brighter tone quality. For brass instruments, the average frequency generally increases with increasingly louder playing level.
Both players were asked to use the same mouthpiece, one which neither of them normally plays, but which both thought they would find comfortable. As it turned out, neither was happy with the mouthpiece. They also found the masking noise quite disconcerting.
Figures 1 and 2 plot average frequency against playing level for player W on the pitch B-flat 2 (117 Hz), with and without the masking noise.
Figre 1. Player W with masking noise. The vertical dashed line shows the fundamental frequency of the note played (here, B-flat 2).
With the masking noise, the average frequency with the yellow brass bell is higher than with the red brass bell, apparently confirming the conventional wisdom.
Figure 2. Player W without masking noise.
Without the masking noise, player W does indeed unconsciously succeed in producing very nearly the same timbre on both bells. Player G showed a much greater difference, as expected from the earlier experiment, but he too reduced the difference between the bells when the masking noise was absent.
Based on B-flat 2, everything appears clear cut. The average frequency produced by the yellow brass bell is higher than for the red brass bell, and the introduction of masking noise reveals more clearly the difference between the bells.
When the other notes are analyzed, the picture becomes murkier. Figures 3 and 4 show the results for player G on B-flat 3, an octave higher.
Figure 3. Player G with masking noise, B-flat 3 (233 Hz).
Figure 4. Player G without masking noise, B-flat 3 (233 Hz).
As before, the use of masking noise magnifies the difference between the bells, but at this pitch, it is the red brass bell whose average frequency is greater.
At F 3 (175 Hz), for player G, the red brass bell was also the "brighter" of the two. However, for player W, the reverse was true. Also, unexpectedly, player W showed a greater discrepancy between the bells without the masking noise.
At F4 (350 Hz), both players had very similar results.
The difference between the bells was much less pronounced, with or without the masking noise, and the yellow brass bell had the higher average frequency.
What have we learned? It seems almost certain that skilled players always "correct" the timbre based on what they hear, but not necessarily consciously.
If the players and makers are correct in stating that yellow brass produces a brighter tone than red brass, then why, at some pitches, did the reverse appear to be true? It is widely believed that yellow brass encourages a crisper attack than red brass, so it is possible that the brighter-darker judgement is based on the attack transient as well as on the steady-state tone that was tested here.
It appears that further experiments would be useful.
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