Improving the stability of a hybrid wind instrument using two microphones

Buys, Kurijn; Sharp, David and Laney, Robin (2016). Improving the stability of a hybrid wind instrument using two microphones. In: Proceedings of the International Symposium on Musical and Room Acoustics (Miyara, Federico ed.), Asociación de Acústicos Argentinos, AdAA, Buenos Aires.



A hybrid wind instrument is constructed by putting a theoretical excitation model (such as a real-time computed physical model of a clarinet embouchure) in interaction with a real wind instrument resonator.

In previous work, the successful construction of a hybrid wind instrument has been demonstrated, with the interaction facilitated by a loudspeaker and a single microphone placed at the entrance of a clarinet-like tube. The prototype was evaluated using physical models of a single-reed, a lip-reed and a bow-string interaction. Musically relevant results were obtained when the negative gradient of the nonlinear excitation function was limited to a certain threshold. When surpassed, erroneous noises appeared.

In the present paper, a study of the open-loop system (the input-to-output response excluding the excitation model) reveals that this instability is caused by strong, high-frequency resonance peaks combined with an inverted phase response. The high frequency resonance peaks appear to result from non-planar air vibration modes in the small cavity in front of the loudspeaker. Hence, they are avoided by repositioning the microphone at the centre of the loudspeaker cavity. Meanwhile, the inverted phase state occurs due to various phase lag sources such as the inevitable input-to-output latency of the computing system. This is accounted for by introducing a second microphone a distance c.Δt along the tube (where c is the speed of sound and Δt the latency).

The excitation models are implemented on a new digital real-time audio platform, “Bela”, supporting multiple audio inputs. A better stability is obtained and evaluation with a real clarinet gives musically relevant results.

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