Monitoring plant water status via static uniaxial compression of the leaf lamina.

Turgor pressure is an essential, but difficult to measure indicator of plant water status. Turgor has been quantified by localized compression of cells or tissues, but a simple method to perform these measurements is lacking. We hypothesized that changes in leaf turgidity can be monitored by uniaxially compressing the leaf lamina and measuring the mechanical stress under a constrained thickness (stress relaxation) and that changes in leaf water content can be monitored by measuring the leaf thickness under constant mechanical stress. Using a simple, custom-built leaf squeeze-flow rheometer, we performed different compression tests on leaves from 13 plant species. The mechanical stress measured during stress relaxation was correlated with leaf bulk turgor pressure (R2 > 0.95) and thus with balancing pressure (R2 > 0.94); the leaf thickness measured under constant mechanical stress was correlated with relative water content (R2 > 0.74). The coefficients of these relationships were related to the leaf bulk osmotic pressure at the turgor-loss point. An idealized average-cell model suggests that, under isothermal conditions, the stationary bulk modulus during compression is largely determined by the bulk osmotic pressure. Our study presents an inexpensive, accessible and automatable method to monitor plant water status noninvasively.

Lip hyper-articulation in loud voice: Effect on resonance-harmonic proximity.

Men and women speakers were recorded while producing sustained vowels at comfortable and loud levels. Following comfortable speech, loud levels were produced in three different conditions: first without specific instruction (UL); then maintaining the same pitch as the comfortable level (PL); and finally, keeping both pitch and lip articulation constant (PAL). The sound pressure level, the fundamental frequency ( fo), the first two vocal tract resonances (R1 and R2), the lip geometry, and the larynx height were measured. For women, a closer proximity of R1 to its nearest harmonic, nfo, was observed in UL. However, no such increased proximity was found in PL, when speakers could, and did, hyper-articulate. Also, no increased proximity was observed in PAL, when lip articulation was constrained. No significant increase in R1: nfo proximity was observed in men in any of the three loud conditions. Finally, R2 was not observed significantly closer to a voice harmonic in loud speech, for neither men nor women.

'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.

How the acoustic resonances of the subglottal tract affect the impedance spectrum measured through the lips.

Experimental determinations of the acoustic properties of the subglottal airway, from the trachea below the larynx to the lungs, may provide useful information for detecting airway pathologies and aid in the understanding of vocal fold auto-oscillation. Here, minimally invasive, high precision impedance measurements are made through the lips (7 men, 3 women) over the range 14-4200 Hz during inspiration, expiration, and with a closed glottis. Closed glottis measurements show the expected resonances and anti-resonances of the supraglottal vocal tract. As the glottis is gradually opened, and the glottal inertance decreases, maxima in the subglottal impedance increasingly affect the measured impedance spectrum, producing additional pairs of maxima and minima. The pairs with the lowest frequency appear first. Measurements during a cycle of respiration show the disappearance and reappearance of these extrema. For a wide glottal opening during inspiration, and for the frequency range 14-4200 Hz, the impedance spectrum semi-quantitatively resembles that of a single, longer duct, open at the remote end, and whose total effective length is 37 ± 4 cm for men and 34 ± 3 cm for women. Fitting to simple models of the subglottal tract yields mean effective acoustic lengths of 19.5 cm for the men and 16.0 cm for the women in this study.

Plumbing the depths: extracellular water storage in specialized leaf structures and its functional expression in a three-domain pressure -volume relationship.

A three-domain pressure-volume relationship (PV curve) was studied in relation to leaf anatomical structure during dehydration in the grey mangrove, Avicennia marina. In domain 1, relative water content (RWC) declined 13% with 0.85 MPa decrease in leaf water potential, reflecting a decrease in extracellular water stored primarily in trichomes and petiolar cisternae. In domain 2, RWC decreased by another 12% with a further reduction in leaf water potential to -5.1 MPa, the turgor loss point. Given the osmotic potential at full turgor (-4.2 MPa) and the effective modulus of elasticity (~40 MPa), domain 2 emphasized the role of cell wall elasticity in conserving cellular hydration during leaf water loss. Domain 3 was dominated by osmotic effects and characterized by plasmolysis in most tissues and cell types without cell wall collapse. Extracellular and cellular water storage could support an evaporation rate of 1 mmol m-2 s-1 for up to 54 and 50 min, respectively, before turgor loss was reached. This study emphasized the importance of leaf anatomy for the interpretation of PV curves, and identified extracellular water storage sites that enable transient water use without substantive turgor loss when other factors, such as high soil salinity, constrain rates of water transport.

The role of vocal tract and subglottal resonances in producing vocal instabilities.

During speech and singing, the vibrating vocal folds are acoustically loaded by resonant ducts upstream (the trachea) and downstream (the vocal tract). Some models suggest that the vocal fold vibration (at frequency fo) is more stable at frequencies below that of a vocal tract resonance, so that the downstream load is inertive (mass-like). If so, vocal fold vibration might become unstable when fo and resonance frequencies "cross over" and the load varies rapidly in phase and magnitude. In one experiment, singers produced a slow diphthong at constant pitch, thus shifting the first tract resonance R1 across fixed fo. In another, pitch glides took fo across the tract and subglottal resonances. Few instabilities occurred when singers could change lip geometry and thus alter R1. This suggests that avoiding resonance crossings can aid vibrational stability. In experiments in which R1 was constrained using a mouth ring, instabilities occurred at frequencies above R1. When subjects sang into an acoustically infinite pipe, which provided a purely resistive load at the lips, R1 was eliminated. Here, instabilities were reduced and concentrated near the lower limit of the head voice.

Easy Come, Easy Go: Capillary Forces Enable Rapid Refilling of Embolized Primary Xylem Vessels.

Protoxylem plays an important role in the hydraulic function of vascular systems of both herbaceous and woody plants, but relatively little is known about the processes underlying the maintenance of protoxylem function in long-lived tissues. In this study, embolism repair was investigated in relation to xylem structure in two cushion plant species, Azorella macquariensis and Colobanthus muscoides, in which vascular water transport depends on protoxylem. Their protoxylem vessels consisted of a primary wall with helical thickenings that effectively formed a pit channel, with the primary wall being the pit channel membrane. Stem protoxylem was organized such that the pit channel membranes connected vessels with paratracheal parenchyma or other protoxylem vessels and were not exposed directly to air spaces. Embolism was experimentally induced in excised vascular tissue and detached shoots by exposing them briefly to air. When water was resupplied, embolized vessels refilled within tens of seconds (excised tissue) to a few minutes (detached shoots) with water sourced from either adjacent parenchyma or water-filled vessels. Refilling occurred in two phases: (1) water refilled xylem pit channels, simplifying bubble shape to a rod with two menisci; and (2) the bubble contracted as the resorption front advanced, dissolving air along the way. Physical properties of the protoxylem vessels (namely pit channel membrane porosity, hydrophilic walls, vessel dimensions, and helical thickenings) promoted rapid refilling of embolized conduits independent of root pressure. These results have implications for the maintenance of vascular function in both herbaceous and woody species, because protoxylem plays a major role in the hydraulic systems of leaves, elongating stems, and roots.

Frequencies, bandwidths and magnitudes of vocal tract and surrounding tissue resonances, measured through the lips during phonation.

The frequencies, magnitudes, and bandwidths of vocal tract resonances are all important in understanding and synthesizing speech. High precision acoustic impedance spectra of the vocal tracts of 10 subjects were measured from 10 Hz to 4.2 kHz by injecting a broadband acoustic signal through the lips. Between 300 Hz and 4 kHz the acoustic resonances R (impedance minima measured through the lips) and anti-resonances R¯ (impedance maxima) associated with the first three voice formants, have bandwidths of ∼50 to 90 Hz for men and ∼70 to 90 Hz for women. These acoustic resonances approximate those of a smooth, dry, rigid cylinder of similar dimensions, except that their bandwidths indicate higher losses in the vocal tract. The lossy, inertive load and airflow caused by opening the glottis further increase the bandwidths observed during phonation. The vocal tract walls are not rigid and measurements show an acousto-mechanical resonance R0 ∼ 20 Hz and anti-resonance R¯0∼200 Hz. These give an estimate of wall inertance consistent with an effective thickness of 1-2 cm and a wall stiffness of 2-4 kN m(-1). The non-rigidity of the tract imposes a lower limit of the frequency of the first acoustic resonance fR1 and the first formant F1.

The effect of blowing pressure, lip force and tonguing on transients: A study using a clarinet-playing machine.

Wind instrument players control the initial and final transients of notes using breath, lips, and tonguing. This paper uses a clarinet-playing machine and high-speed camera to investigate how blowing pressure, lip force, and tonguing parameters affect transients. After tongue release, the reed quickly comes to rest, losing its mechanical energy. However, the changing aperture past the reed rapidly changes the airflow. For pressure above the oscillation threshold, successive interactions between reflections of this pulse of airflow and the reed produce an exponential increase in the sound. The rates r of exponential increase in the fundamental of the sound range from several tens to several hundreds of dB s(-1), as functions of blowing pressure and lip force. Because the reed's initial mechanical energy is lost, tongue force and acceleration have little effect on r. However, larger tongue force and acceleration produce more rapid changes in flow, which start notes sooner after tongue release. Further, large tongue force increases the third harmonic during the transient. There is a hysteresis region on the (pressure, lip force) plane where regenerative oscillation is not produced spontaneously by increasing blowing pressure only. Here, tongue action can initiate sustained notes at low pressure.

How clarinettists articulate: The effect of blowing pressure and tonguing on initial and final transients.

Articulation, including initial and final note transients, is important to tasteful music performance. Clarinettists' tongue-reed contact, the time variation of the blowing pressure P¯mouth, the mouthpiece pressure, the pressure in the instrument bore, and the radiated sound were measured for normal articulation, accents, sforzando, staccato, and for minimal attack, i.e., notes started very softly. All attacks include a phase when the amplitude of the fundamental increases exponentially, with rates r ∼1000 dB s(-1) controlled by varying both the rate of increase in P¯mouth and the timing of tongue release during this increase. Accented and sforzando notes have shorter attacks (r∼1300 dB s(-1)) than normal notes. P¯mouth reaches a higher peak value for accented and sforzando notes, followed by a steady decrease for accented notes or a rapid fall to a lower, nearly steady value for sforzando notes. Staccato notes are usually terminated by tongue contact, producing an exponential decrease in sound pressure with rates similar to those calculated from the bandwidths of the bore resonances: ∼400 dB s(-1). In all other cases, notes are stopped by decreasing P¯mouth. Notes played with different dynamics are qualitatively similar, but louder notes have larger P¯mouth and larger r.

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.

The effect of nearby timpani strokes on horn playing.

Horn players have observed that timpani strokes can interfere disruptively with their playing, especially when they are seated close to the timpani. Measuring the horn's transfer function in the bell-to-mouthpiece direction reveals that the horn behaves as an acoustic impedance matching device, capable of transmitting waves with pressure gains of at least 20 dB near horn playing resonances. During moderate to loud timpani strokes, the horn transmits an overall impulse gain response of at least 16 dB from the bell to the mouthpiece, while evidence of non-linear bore propagation can be observed for louder strokes. If the timpani is tuned near a horn resonance, as is usually the case, further bore resonance interactions may be observed leading to gains of ∼26 dB from bell to mouthpiece. Finally, measurements of horn playing made under conditions approximating playing reveal that timpani strokes sounding near the horn bell are capable of disrupting horn playing by affecting the amplitude, periodicity, and frequency of the pressure signal generated at the horn player's lips.

Vocal tract resonances in singing: variation with laryngeal mechanism for male operatic singers in chest and falsetto registers.

Seven male operatic singers sang the same notes and vowels in their chest and their falsetto registers, covering the overlap frequency range where two main laryngeal mechanisms can be identified by means of electroglottography: M1 in chest register and M2 in falsetto register. Glottal contact quotients determined using electroglottography were typically lower by 0.27 in M2 than in M1. Vocal tract resonance frequencies were measured by using broadband excitation at the lips and found to be typically lower in M2 than in M1 sung at the same pitch and vowel; R1 typically by 65 Hz and R2 by 90 Hz. These shifts in tract resonances were only weakly correlated with the changes in the contact quotient or laryngeal height that were measured simultaneously. There was considerable variability in the resonance tuning strategies used by the singers, and no evidence of a uniform systematic tuning strategy used by all singers. A simple model estimates that the shifts in resonance frequencies are consistent with the effective glottal area in falsetto register (M2) being 60%-70% of its value in chest register (M1).

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.

The clarinet: how blowing pressure, lip force, lip position and reed "hardness" affect pitch, sound level, and spectrum.

Using an automated clarinet playing system, the frequency f, sound level L, and spectral characteristics are measured as functions of blowing pressure P and the force F applied by the mechanical lip at different places on the reed. The playing regime on the (P,F) plane lies below an extinction line F(P) with a negative slope of a few square centimeters and above a pressure threshold with a more negative slope. Lower values of F and P can produce squeaks. Over much of the playing regime, lines of equal frequency have negative slope. This is qualitatively consistent with passive reed behavior: Increasing F or P gradually closes the reed, reducing its equivalent acoustic compliance, which increases the frequency of the peaks of the parallel impedance of bore and reed. High P and low F produce the highest sound levels and stronger higher harmonics. At low P, sound level can be increased at constant frequency by increasing P while simultaneously decreasing F. At high P, where lines of equal f and of equal L are nearly parallel, this compensation is less effective. Applying F further from the mouthpiece tip moves the playing regime to higher F and P, as does a stiffer reed.

Experimental evaluation of inverse filtering using physical systems with known glottal flow and tract characteristics.

The technique presented here uses an impedance head to measure the input impedance spectrum of a physical model of a vocal tract, and then to inject a known glottal flow waveform into the tract. The sound measured outside the mouth is used to evaluate inverse filtering techniques by comparison with the known glottal flow and measured acoustical properties of the tract. The normalized least square errors in the glottal flow were typically a percent or less in the time domain and several percent in the frequency domain. Accurate determination of resonance frequencies and bandwidths required a suitable order of inverse filter.

Do trumpet players tune resonances of the vocal tract?

The acoustic impedance spectrum was measured in the mouths of seven trumpeters while they played normal notes and while they practiced "bending" the pitch below or above the normal value. The peaks in vocal tract impedance usually had magnitudes rather smaller than those of the bore of the trumpet. Over the range measured, none of the trumpeters showed systematic tuning of the resonances of the vocal tract. However, all players commented that the presence of the impedance head in the mouth prevented them from playing the very highest notes of which they were normally capable. It is therefore possible that these players might use either resonance tuning or perhaps very high impedance magnitudes for some notes beyond the measured range. The observed lack of tuning contrasts with measurements for the saxophone which, like the trumpet, has weak resonances in the third and fourth octaves. Saxophonists are only able to play the highest range by tuning resonances of the vocal tract, so that the series impedance has a very strong peak at a frequency near that of the desired note. This difference is explained by the greater control that the trumpet player has over the natural frequency of the vibrating valve.