Timpani-horn interactions: effects on the lips of horn players seated too close to the timpani
Horn players have observed that a timpani sounding nearby can interfere disruptively with their playing. During playing, the bell of the horn increases the radiation of sound out of the horn: it acts a little like the horn on an old gramphone. It can also act like the horn of an ear-trumpet, a device (predating hearing aids) that 'concentrates' sound coming into the large end of the horn. For horn players, this second property can be disturbing.
Technically, the horn behaves as an acoustic impedance matching device in both directions. In this study, we show that it is capable of transmitting sound waves from the bell to the mouthpiece with sound pressure gains of at least 20 dB near horn-playing resonance frequencies, and also transmits an overall impulse gain response of at least 16 dB. Further bore resonance interactions show gain responses of up to 26 dB, if the timpani is tuned near a horn resonance, as is usually the case.
This page is an electronic appendix to the scientific paper: Chen, J.M., Smith J. and Wolfe, J. (2014) "The effect of nearby timpani strokes on horn playing" J. Acoust. Soc. America. 135, 472-478. Copyright (201) Acoustical Society of America. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the Acoustical Society of America. The preceeding article appeared in (JASA134, 2247-2255) and may be found at (J. Acoust. Soc. America).
Here we present examples of sound signals measured at the bell and in the mouthpiece during horn playing when the timpani is tuned (and struck) at the horn playing note, and away from it, showing examples of disruptive interactions occuring during horn playing.
The schematic diagram (not to scale) on the right shows one of the set-ups used to make measurements under extreme conditions, with the horn and timpani unnaturally close. We also report 'ecological' measurements with more realistic geometires. There is a microphone positioned in the centre of the bell of the horn and a microphone coupled to the mouthpiece, and their signals are presented below.
In this example, the horn nominally plays C3+15¢ (concert pitch), mp, while the timpani sounds C3+15¢, mf.
Here we observe destructive intereference in the mouthpiece signal coinciding with the arrival of the first timpani stroke, followed by constructive interference at the arrival of the second stroke.
In both instances, there is a momentary 'roughening' of the horn tone.
By the way, you'll notice that the sound mouthpiece is nothing like the sound that comes out of the bell of the horn!
Pressure signal at bell
Pressure signal at mouthpiece
Microphone pressure signal measured in the bell of the horn (top) and in the mouthpiece (bottom). (Different channel gains used)
When the timpani is tuned: Standing wave in the bore
In this example, the horn nominally plays C3-10¢ (concert pitch), p while the timpani sounds C3-10¢, mf.
Because the player is playing much softly here than the timpani, we observe a small rise (bulge) in the pressure signal measured in the mouthpiece
about 0.1 to 0.3 seconds after the arrival of the timpani stroke, due to the sound energy produced by the vibrating drum skin being stored as a resonant standing wave in the bore of the horn.
Like the previous example, there is a momentary 'roughening' of the horn tone accompanied with slight pitch instability.
Pressure signal at bell
Pressure signal at mouthpiece
Microphone pressure signal measured in the bell of the horn (top) and in the mouthpiece (bottom). (Different channel gains used)
When the timpani is tuned: Pitch instability (horn playing moderately softly)
In this example, the horn nominally plays C3−10¢ (concert pitch), mp, while the timpani sounds C3−10¢, mp.
The moderately softly sustained horn note suffers from some audible pitch instability here which is associated with the timpani strokes, even though the timpani strokes are also moderately soft.
Pressure signal at bell
Pressure signal at mouthpiece
Microphone pressure signal measured in the bell of the horn (top) and in the mouthpiece (bottom). (Different channel gains used)
When the timpani is tuned: Pitch instability (horn playing loudly)
In this example, the horn nominally plays Bb2+25¢ (concert pitch), f, while the timpani sounds Bb2+20¢, mf.
Although less obvious than the previous example (horn playing moderately softly), there is still some audible pitch instability observed in the horn associated with the timpani strokes, even though the horn is now playing loudly.
Pressure signal at bell
Pressure signal at mouthpiece
Microphone pressure signal measured in the bell of the horn (top) and in the mouthpiece (bottom). (Different channel gains used)
When the timpani is mistuned: Sharp
In this example, the horn nominally plays Bb2−50¢ (concert pitch), mp, while the timpani sounds B2+0¢, mf.
Interference beating pattern (~11 Hz) can be seen in the mouthpiece
signal each time after the timpani is struck, resulting in an audible momentary raspy flutter in the horn.
Pressure signal at bell
Pressure signal at mouthpiece
Microphone pressure signal measured in the bell of the horn (top) and in the mouthpiece (bottom). (Different channel gains used)
When the timpani is mistuned: Flat (−20¢)
In this example, the horn nominally plays Bb−20¢ (concert pitch), p, while the timpani sounds Bb2−40¢, mf.
Although the horn and timpani are mildly mistuned, noticeable pitch instability can be observed here as the horn struggles to maintain a steady note with the arrival of each timpani stroke.
Pressure signal at bell
Pressure signal at mouthpiece
Microphone pressure signal measured in the bell of the horn (top) and in the mouthpiece (bottom). (Different channel gains used)
When the timpani is mistuned: Flat (-95¢)
In this example, the horn plays C3+5¢ (concert pitch), mp, while the timpani sounds B2+10¢, mp.
Both the timpani and horn are playing moderately softly, almost a full semitone apart. Like the previous example, general pitch instability can be observed with the timpani strokes. With the second timpani stroke in particular, the horn playing pitch is pulled down some 85¢ (almost a semitone) to
sound B2+20¢ – near the timpani pitchfor almost 1 second.
Pressure signal at bell
Pressure signal at mouthpiece
Microphone pressure signal measured in the bell of the horn (top) and in the mouthpiece (bottom). (Different channel gains used)
What to do about the problem
It has been suggested by hornists that the timpani should be 3 m or more from the horns. The data in this study may be regarded aas support for this suggestion.
Frequency response and acoustic impedance.
Here are plots of the impedance spectrum at the mouthpiecee of a a Bb and F horn, fingering OOO, with and without a hand in the bell. Why two horns? Most horns are double horns: the player has a valve for the thumb that introduces extra tubing to convert between a Bb horn (lowest playable resonance Bb2 and pedal Bb1, like the trombone) and an F horn (lowest playable resonance F2 and pedal F1.) The horn also has three keys, each of which is connected to two valves, one in the F and one in the Bb horn. The keys add appropriate lengths of tubing to lower the pitch by about 2, 1 and 3 semitones respectively.
(If you want to cite these spectra, the reference is: Chen, J.M. (2009) Vocal Tract Interactions in Woodwind Performance, PhD thesis, UNSW, Sydney.)
More information and links
Brass Acoustics: A musician's introduction to the acoustics of brass instruments.
For background on topics in acoustics (waves, frequencies,
resonances etc) see Basics.
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