What Might the Trombone Teach Us About the Singing Voice?-A Tutorial Review.

The trombone and the male voice cover similar frequency ranges and, at a physical level, the basic anatomies of the voice and the trombone show some qualitative similarity: both have two vibrating flaps of muscular tissue (the vocal folds and the trombonist's lips, respectively), and in each case, these are loaded acoustically by resonant ducts both upstream and downstream. There are also large differences. For example, the downstream ducts differ in length. The trombone usually operates with an oscillation frequency close to that of one of the downstream resonances; this is only occasionally true of the voice. Because the lips of a trombone player are much more readily accessible for experiments, they have yielded more detailed measurements of longitudinal and transverse motion, AC and DC pressures, and flow under varying acoustic loads. In normal operation, the downstream motion of the lips or vocal folds leads the lateral opening motion, resulting in a sweeping flow that leads the flow through the aperture. The relative timing of these flow components is related to the phases of the pressure across the tissues and the downstream acoustic load. Further, the work done on trombonists' lips due to their sweeping motion makes an important contribution to maintaining oscillation with both inertive and compliant acoustic loads. This probably explains why trombonists can "lip" the pitch smoothly from above to below a downstream resonance. Similar calculations on measurements of vocal fold motion show a similar work contribution and suggest that this sweeping motion is significant in powering this component of laryngeal motion. Comparing and contrasting the operation of the two "instruments" gives new perspectives on the basic science of the voice, with practical applications including the use of resonances. This could be helpful to voice scientists but also useful background knowledge for singers and singing teachers.

'Warming up' a wind instrument: The time-dependent effects of exhaled air on the resonances of a trombone.

To study the effect of 'warming up' a wind instrument, the acoustic impedance spectrum at the mouthpiece of a trombone was measured after different durations of playing. When an instrument filled with ambient air is played in a room at 26-27 °C, the resonance frequencies initially fall. This is attributed to CO2 in the breath initially increasing the density of air in the bore and more than compensating for increased temperature and humidity. Soon after, the resonance frequencies rise to near or slightly above the ambient value as the effects of temperature and humidity compensate for that of increased CO2. The magnitudes and quality factors of impedance maxima decrease with increased playing time whereas the minima increase. Using the measured change in resonance frequency, it proved possible to separate the changes in impedance due to changes in density and changes in acoustic losses due to water condensing in the bore. When the room and instrument temperature exceed 37 °C, condensation is not expected and, experimentally, smaller decreases in magnitudes and quality factors of impedance maxima are observed. The substantial compensation of the pitch fall due to CO2 by the rise due to temperature and humidity is advantageous to wind players.

Trombone lip mechanics with inertive and compliant loads ("lipping up and down").

Trombonists normally play at a frequency slightly above a bore resonance. However, they can "lip up and down" to frequencies further above the resonance (more compliant load) and below (inertive load). This was studied by determining the pressures, flows, and acoustic impedance upstream and downstream and by analyzing high-speed video of the lips. The range of lipping up and down is roughly symmetrical about the peak in bore impedance rather than about the normal playing frequency. The acoustic flow into the instrument bore has two components: the flow through the lip aperture and the sweeping flow caused by the moving lips. Variations in the phases of each of these two components with respect to the mouthpiece pressure allow playing regimes loaded by bore impedances varying from compliant to inertive. In a simple model, this sweeping motion also allows the pressure difference across the lips to do work on the lips around a cycle. Its magnitude is typically about 20 times smaller than the work input to the instrument but of the same order as the maximum kinetic energy of the lips. In some cases, this sweeping work may, therefore, contribute most or all of the energy required for auto-oscillation.

Acoustic dissipation in wooden pipes of different species used in wind instrument making: An experimental study.

In this study, the acoustic dissipation is investigated experimentally in wooden pipes of different species commonly used in woodwind instrument making: maple (Acer pseudoplatanus), pear wood (Pyrus communis L.), boxwood (Buxus sempervirens), and African Blackwood (Dalbergia melanoxylon). The pipes are parallel to the grain, except one which forms an angle of 60° with the fiber direction. An experimental method, involving input impedance measurements with several lengths of air column, is introduced to estimate the characteristic impedance and the attenuation factor in the pipes. Their comparison reveals significant differences of acoustic dissipation among the species considered. The attenuation factors are ranked in the following order from largest to smallest: maple, boxwood, pear wood, and African Blackwood. This order is the same before and after polishing the bore, which is an essential step in the making process of wind instrument. For maple, changing the pipe direction of 60° considerably increases the attenuation factor, compared to those of the other pipes, parallel to the grain. Further, polishing tends to reduce the acoustic dissipation in the wooden pipes, especially for the most porous species. As a result, the influence of polishing in the making procedure depends on the selected wood species.

Laryngeal flow due to longitudinal sweeping motion of the vocal folds and its contribution to auto-oscillation.

Analysis of published depth-kymography data [George, de Mul, Qiu, Rakhorst, and Schutte (2008). Phys. Med. Biol. 53, 2667-2675] shows that, for the subject studied, the flow due to the longitudinal sweeping motion of the vocal folds contributes several percent of a typical acoustic flow at the larynx. This sweeping flow is a maximum when the glottis is closed. This observation suggests that assumption of zero laryngeal flow during the closed phase as a criterion when determining parameters in inverse filtering should be used with caution. Further, these data suggest that the swinging motion contributes work to overcome mechanical losses and thus to assist auto-oscillation.

Relationships between pressure, flow, lip motion, and upstream and downstream impedances for the trombone.

This experimental study investigates ten subjects playing the trombone in the lower and mid-high range of the instrument, B♭2 to F4. Several techniques are combined to show the pressures and the impedance spectra upstream and downstream of the lips, the acoustic and total flows into the instrument, the component of the acoustic flow due to the sweeping motion of the lips, and high speed video images of the lip motion and aperture. The waveforms confirm that the inertance of the air in the channel between the lips is usually negligible. For lower notes, the flow caused by the sweeping motion of the lips contributes substantially to the total flow into the mouthpiece. The phase relations among the waveforms are qualitatively similar across the range studied, with no discontinuous behavior. The players normally played at frequencies about 1.1% above that of the impedance peak of the bore, but could play below as well as above this frequency and bend from above to below without discontinuity. The observed lip motion is consistent with two-degree-of-freedom models having varying effective lengths. These provide insight into why lips can auto-oscillate with an inertive or compliant load, or without a downstream resonator.

Producing undistorted acoustic sine waves.

A simple digital method is described that can produce an undistorted acoustic sine wave using an amplifier and loudspeaker having considerable intrinsic distortion, a common situation at low frequencies and high power. The method involves, first, using a pure sine wave as the input and measuring the distortion products. An iterative procedure then progressively adds harmonics with appropriate amplitude and phase to cancel any distortion products. The method is illustrated by producing a pure 52 Hz sine wave at 107 dB sound pressure level with harmonic distortion reduced over the audible range to >65 dB below the fundamental.