SELECTED PUBLICATIONS RELATED TO MEASUREMENT AND PROCESSING OF MUSICAL INSTRUMENTS SIGNALS
Kemp, J., van Walstijn, M., Campbell, D., Chick, J. & Smith, R. (2010), "Time domain wave separation using multiple microphones", Journal of the Acoustical Society of America. Vol. 128(1), pp. 195-205.
Abstract: Methods of measuring the acoustic behavior of tubular systems can be broadly characterized as steady state measurements, where the measured signals are analyzed in terms of infinite duration sinusoids, and reflectometry measurements which exploit causality to separate the forward and backward going waves in a duct. This paper sets out a new multiple microphone reflectometry technique which performs wave separation by using time domain convolution to track the forward and backward going waves in a cylindrical source tube. The current work uses two calibration runs (one for forward going waves and one for backward going waves) to measure the time domain transfer functions for each pair of microphones. These time domain transfer functions encode the time delay, frequency dependent losses and microphone gain ratios for travel between microphones. This approach is applied to the measurement of wave separation, bore profile and input impedance. The work differs from existing frequency domain methods in that it combines the information of multiple microphones within a time-domain algorithm, and differs from existing time domain methods in its inclusion of the effect of losses and gain ratios in inter-microphone transfer functions.
BibTeX:
@article{jasa10,
  author = {Jonathan Kemp and Maarten van Walstijn and D.M. Campbell and John Chick, and Richard Smith},
  title = {Time domain wave separation using multiple microphones},
  journal = {Journal of the Acoustical Society of America},
  year = {2010},
  volume = {128},
  number = {1},
  pages = {195-205},
  doi = {http://dx.doi.org/10.1121/1.3392441}
}
de Sanctis, G. & van Walstijn, M. (2009), "A Frequency Domain Algorithm for Wave Separation", In 12th Int. Conference on Digital Audio Effects (DAFx-09), Como, Italy., pp. 498-505.
Abstract: We propose a frequency domain adaptive algorithm for wave separation in wind instruments. Forward and backward travelling waves are obtained from the signals acquired by two microphones placed along the tube, while the separation filter is adapted from the information given by a third microphone. Working in the frequency domain has a series of advantages, among which are the ease of design of the propagation filter and its differentiation with respect to its parameters. Although the adaptive algorithm was developed as a first step for the estimation of playing parameters in wind instruments it can also be used, without any modifications, for other applications such as in-air direction of arrival (DOA) estimation. Preliminary results on these applications will also be presented.
BibTeX:
@inproceedings{dafx09g,
  author = {Giovanni de Sanctis and Maarten van Walstijn},
  title = {A Frequency Domain Algorithm for Wave Separation},
  booktitle = {12th Int. Conference on Digital Audio Effects (DAFx-09), Como, Italy},
  year = {2009},
  pages = {498--505},
  url = {http:\sarc.qub.ac.ukvanwalstijnpubspapers9g.pdf}
}
Chatchiioannou, V. & van Walstijn, M. (2009), "Inverting the Clarinet", In 12th Int. Conference on Digital Audio Effects (DAFx-09), Como, Italy., pp. 133-138.
Abstract: Physical-modelling based sound resynthesis is considered by estimating physical model parameters for a clarinet-like system. Having as a starting point the pressure and flow signals in the mouthpiece, a two-stage optimisation routine is employed, in order to estimate a set of physical model parameters that can be used to resynthesise the original sound. Tested on numerically generated signals, the presented inverse-modelling method can almost entirely resynthesise the input sound. For signals measured under real playing conditions, captured by three microphones embedded in the instrument bore, the pressure can be successfully reproduced, while uncertainties in the fluid dynamical behaviour reveal that further model refinement is needed to reproduce the flow in the mouthpiece.
BibTeX:
@inproceedings{dafx09v,
  author = {Vasileios Chatchiioannou and Maarten van Walstijn},
  title = {Inverting the Clarinet},
  booktitle = {12th Int. Conference on Digital Audio Effects (DAFx-09), Como, Italy},
  year = {2009},
  pages = {133-138},
  url = {http:\sarc.qub.ac.ukvanwalstijnpubspapers9v.pdf}
}
van Walstijn, M., Campbell, D., Kemp, J. & Sharp, D. (2005), "Wideband Measurement of the Acoustic Impedance of Tubular Objects", Acta Acustica united with Acustica. Vol. 91, pp. 590-604.
Abstract: A method is discussed for measuring the acoustic impedance of tubular objects that gives accurate results for a wide range of frequencies. The apparatus that is employed is similar to that used in many previously developed methods; it consists of a cylindrical measurement duct fitted with several microphones, of which two are active in each measurement session, and a driver at one of its ends.

The object under study is fitted at the other end. The impedance of the object is determined from the microphone signals obtained during excitation of the air inside the duct by the driver, and from three coefficients that are pre-determined using four calibration measurements with closed cylindrical tubes. The calibration procedure is based on the simple mathematical relationships between the impedances of the calibration tubes, and does not require knowledge of the propagation constant.

Measurements with a cylindrcal tube yield an estimate of the attenuation constant for plane waves, which is found to differ from the theoretical prediction by less than 1:4% in the frequency range 1 kHz - 20 kHz. Impedance measurements of objects with abrupt changes in diameter are found to be in good agreement with multimodal theory.

BibTeX:
@article{aca05,
  author = {Maarten van Walstijn and D.M. Campbell and Jonathan Kemp and David Sharp},
  title = {Wideband Measurement of the Acoustic Impedance of Tubular Objects},
  journal = {Acta Acustica united with Acustica},
  year = {2005},
  volume = {91},
  pages = {590-604},
  url = {http://www.sarc.qub.ac.uk/~mvanwalstijn/pubs/papers/aca05.pdf}
}
van Walstijn, M. & Campbell, D. (2002), "Large Bandwidth Measurement of Acoustic Response of Tubular Objects", In Forum Acusticum Meeting, Sevilla, Spain.
BibTeX:
@inproceedings{forum02imp,
  author = {Maarten van Walstijn and D.M. Campbell},
  title = {Large Bandwidth Measurement of Acoustic Response of Tubular Objects},
  booktitle = {Forum Acusticum Meeting, Sevilla, Spain},
  year = {2002},
  url = {http:\sarc.qub.ac.ukvanwalstijnpubspapers2imp.pdf}
}
van Walstijn, M., Campbell, D.M. & Sharp., D.B. (2002), "Measurement of Input Impedance of an Acoustic Bore with Application to Bore Reconstruction", In Institute of Acoustics Meeting, Salford, England.
Abstract: This paper reports on the development of a method for experimentally determining the input impedance of an acoustic bore. The method and the experimental setup are designed having in mind the requirements associated with the possibility of applying high-resolution bore reconstruction algorithms to the measurement data. As such, the measurement bandwidth at which the method needs to produce accurate data is much larger than that in previously developed experimental methods for determination of input impedance. In the paper, we discuss the details of the design of the experimental setup and the initial results obtained, as well as the particular problem that occurs when measuring an acoustic bore whose input diameter is different from the exit diameter of the measurement head.
BibTeX:
@inproceedings{ioa02,
  author = {Maarten van Walstijn and D. M. Campbell and D. B. Sharp.},
  title = {Measurement of Input Impedance of an Acoustic Bore with Application to Bore Reconstruction},
  booktitle = {Institute of Acoustics Meeting, Salford, England},
  year = {2002},
  url = {http:\sarc.qub.ac.ukvanwalstijnpubspapers2.pdf}
}
Buick, J.M., Kemp, J., Sharp, D.B., van Walstijn, M., Campbell, D.M. & Smith, R.A. (2002), "Distinguishing between similar tubular objects using pulse reflectometry: a study of trumpet and cornet leadpipes", Measurement Science and Technology. Vol. 13(5), pp. 750.
Abstract: This paper considers the measurement of the internal radius of a number of similar, short, tubular leadpipes using pulse reflectometry. Pulse reflectometry is an acoustical technique for measuring the internal bore of a tubular object by analysing the reflections which occur when an acoustical pulse is directed into the object. The leadpipes are designed to form the initial, or lead, part of a trumpet or cornet and their internal radii differ by less than 0.1 mm between similar pipes. The ability of the reflectometer to detect these small differences, which are considered by players to produce a noticeable difference in the sound of an instrument, are investigated. It is seen that the pulse reflectometer is able to distinguish between leadpipes with different nominal radii varying by as little as 0.03 mm, demonstrating its potential in the study of musical instruments and showing that it can be used as a diagnostic tool by the instrument manufacturer to detect defects which are significant enough to acoustically alter performance. The absolute accuracy of the radius measurements is also considered at the end of the leadpipe, where the uncertainty is ±0.05 mm.
BibTeX:
@article{iop02,
  author = {J M Buick and J Kemp and D B Sharp and M van Walstijn and D M Campbell and R A Smith},
  title = {Distinguishing between similar tubular objects using pulse reflectometry: a study of trumpet and cornet leadpipes},
  journal = {Measurement Science and Technology},
  year = {2002},
  volume = {13},
  number = {5},
  pages = {750},
  url = {http://stacks.iop.org/0957-0233/13/i=5/a=313}
}

Created on 30/03/2011.