Science

Publication Search Results

Now showing 1 - 10 of 184
  • (2006) Tarnopolsky, Alex Z.; Fletcher, Neville H.; Hollenberg, Lloyd C. L.; Lange, Benjamin D.; Smith, John; Wolfe, Joseph
    Journal Article
    The didjeridu, or yidaki, is a simple tube about 1.5 m long, played with the lips, as in a tuba, but mostly producing just a tonal, rhythmic drone sound. The acoustic impedance spectra of performers' vocal tracts were measured while they played and compared with the radiated sound spectra. When the tongue is close to the hard palate, the vocal tract impedance has several maxima in the range 1-3 kHz. These maxima, if sufficiently large, produce minima in the spectral envelope of the sound because the corresponding frequency components of acoustic current in the flow entering the instrument are small. In the ranges between the impedance maxima, the lower impedance of the tract allows relatively large acoustic current components that correspond to strong formants in the radiated sound. Broad, weak formants can also be observed when groups of even or odd harmonics coincide with bore resonances. Schlieren photographs of the jet entering the instrument and high speed video images of the player's lips show that the lips are closed for about half of each cycle, thus generating high levels of upper harmonics of the lip frequency. Examples of the spectra of 'circular breathing' and combined playing and vocalization are shown.

  • (2005) Smith, J.R.; Wolfe, Joseph; Bavu, E.
    Journal Article
    Bowing a string with a non-zero radius exerts a torque, which excites torsional waves. In general, torsional standing waves have higher fundamental frequencies than do transverse standing waves, and there is generally no harmonic relationship between them. Although torsional waves have little direct acoustic effect, the motion of the bow-string contact depends on the sum of the transverse speed v of the string plus the radius times the angular velocity (rw). Consequently, in some bowing regimes, torsional waves could introduce non-periodicity or jitter to the transverse wave. The ear is sensitive to jitter so, while quite small amounts of jitter are important in the sounds of (real) bowed strings, modest amounts of jitter can be perceived as unpleasant or unmusical. It follows that, for a well bowed string, aperiodicities produced in the transverse motion by torsional waves (and other effects) must be small. Is this because the torsional waves are of small amplitude or because of strong coupling between the torsional and transverse waves? We measure the torsional and transverse motion for a string bowed by an experienced player over a range of tunings. The torsional wave spectrum shows a series of harmonics of the translational fundamental, with strong formants near the natural frequencies for torsion. The peaks in rw, which occur near the start and end of the 'stick' phase in which the bow and string move together, are only several times smaller than v during this phase. We present sound files of the transverse velocity and the rotational velocity due to the torsional wave. Because the torsional waves occur at exact harmonics of the translational fundamental and because of similarities in the temporal envelope, the sound of the torsional signal alone clearly suggests the sound of a bowed string with the pitch of the translational fundamental. However, the harmonics that fall near the torsional resonances are so strong that they may be heard as distinct notes.

  • (2006) Inta, R.A.; Smith, J.R.; Wolfe, Joseph
    Journal Article
    This is a report on the first three years of a long-term experiment designed to measure how two very similar violins change with time. After being constructed 'in parallel,' one is stored under controlled conditions in a museum and is played infrequently, while the other is played regularly by a professional musician. Vibro-acoustic measurements were performed on the instruments and parts thereof during and after construction. Playing and listening tests by a panel of experienced violinists were conducted at completion, after three years with no adjustment, and then after minor adjustments were made to the played violin only. Panels of players and listeners rated the two violins at all stages, and all results are consistent with the null hypothesis: at present there is no significant preference for either instrument over a range of categories.

  • (2005) Smith, J.R.; Tarnopolsky, A.; Wolfe, Joseph; Fletcher, N
    Journal Article
    The effect of evanescent mode generation, due to geometrical mismatch, in acoustic impedance measurements is investigated. The particular geometry considered is that of a impedance probe with an annular flow port and a central microphone, but the techniques are applicable to other geometries. It is found that the imaginary part of the measured impedance error is proportional to frequency, and that the sign of the error is positive for measurements made on tubes with diameter much larger than that of the inlet port,,but negative for tubes with diameter close to that of the inlet. The result is a distortion of the measured frequencies of the impedance minima of the duct while the maxima are largely unaffected. There is, in addition, a real resistive component to the error that varies approximately as the square root of the frequency. Experiment confirms the results of the analysis and calculations, and a calibration procedure is proposed that allows impedance probes that have been calibrated on a semi-infinite tube of one diameter to be employed for measurements on components with an inlet duct of some very different diameter. © 2005 Acoustical Society of America.

  • (2005) Tarnopolsky, A.; Fletcher, N.; Hollenberg, L.; Lange, B.; Smith, J.; Wolfe, Joseph
    Journal Article
    The Australian didgeridoo (or yidaki in the Yolngu language of northern Australia) is a simple musical instrument that, at the lips of an experienced player, is capable of a spectacular variety of timbres - considerably greater than those that can be coaxed from orchestral instruments, for example. To understand this phenomenon, we simultaneously measured the sound produced by the didgeridoo and the acoustic impedance of the player's vocal tract. We find that the maxima in the envelope of the sound spectrum are associated with minima in the impedance of the vocal tract, as measured just inside the lips. This acoustic effect is similar to the production of vowel sounds made during human speech or singing, although the mechanism is different, and leads to the surprising conclusion that experienced players are subconsciously using their glottis to accentuate the instrument's tonal variation.

  • (2007) Longmore, Steven; Burton, Michael; Barnes, P; Wong, Tony; Purcell, Cormac; Ott, J
    Journal Article
    We present observations of the (1,1), (2,2), (4,4) and (5,5) inversion transitions of para-ammonia (NH3) and 24-GHz continuum, taken with the Australia Telescope Compact Array towards 21 southern Galactic hot molecular cores traced by 6.7-GHz methanol maser emission. We detect NH3(1,1) emission towards all 21 regions and 24-GHz continuum emission towards 12 of the regions, including six with no reported 8-GHz continuum counterparts. In total, we find the 21 regions contain 41 NH3(1,1) cores but around half of the regions only contain a single core. We extract characteristic spectra for every core at each of the NH3 transitions and present both integrated intensity maps and channel maps for each region. NH3(2,2) emission was detected towards all NH3(1,1) cores. NH3(4,4) emission was detected in 13 of the NH3(1,1) cores with NH3(5,5) emission coincident with 11 of these. The NH3(4,4) and (5,5) emission is always unresolved and found at the methanol maser position. An analysis of the NH3(1,1) and (2,2) line ratios suggests that the cores with NH3(4,4) and (5,5) emission are warmer than the remaining cores rather than simply containing more ammonia. The coincidence of the maser emission with the higher spatial resolution NH3(4,4) and (5,5) emission indicates that the methanol masers are found at the warmest part of the core. In all cores detected at NH3(4,4) (with the exception of G12.68-0.18 core 4), the measured linewidth increases with transition energy. The NH3(1,1) spectra of several cores show an emission and absorption component slightly offset in velocity but it is unclear whether or not this is due to systematic motion of the gas. We observe large asymmetries in the NH3(1,1) hyperfine line profiles and conclude that this is due to non-local thermodynamic equilibrium conditions arising from a number of dense, small clumps within the beam, rather than systematic motions of gas in the cores. Assuming that the 24-GHz continuum emission is optically-thin bremsstrahl

  • (2006) Maercker, M; Burton, Michael
    Journal Article
    L-band data of 30 Doradus at 3.5 mu m taken with SPIREX (South Pole Infrared Explorer) is presented. The photometry was combined with 2MASS JHK data at 1.25- 2.2 mu m. Colour-colour and colour-magnitude diagrams are constructed and used to determine the sources with infrared excess. These are interpreted as circumstellar disks, and enable the fraction of sources with disks (the cluster disk fraction or CDF) to be determined. We find that similar to 42% of the sources detected at L-band in 30 Doradus have an IR-excess.

  • (2005) Burton, Michael; Minier, Vincent; Hill, Tracey; Purcell, Cormac; Walsh, Andrew; Longmore, Steven; Pestalozzi, M; Garay, Guido
    Journal Article
    We present a multiwavelength study of five methanol maser sites which are not directly associated with a strong (>100 mJy) radio continuum source: G31.28+0.06, G59.78+0.06, G173.49+2.42 (S231, S233IR), G188.95+0.89 (S252, AFGL5180) and G192.60-0.05 (S255IR). These radio-quiet methanol maser sites are often interpreted as precursors of ultracompact H regions or massive protostar sites. In this work, the environment of methanol masers is probed from mid-IR to millimetre wavelengths at angular resolutions of 8−34. Spectral energy distribution (SED) diagrams for each site are presented, together with mass and luminosity estimates. Each radio-quiet maser site is always associated with a massive (>50 M), deeply embedded (Av > 40 mag) and very luminous (>104 L) molecular clump, with Ltotal ∝ M0.75 gas . These physical properties characterise massive star-forming clumps in earlier evolutionary phases than H regions. In addition, colder gas clumps seen only at mm-wavelengths are also found near the methanol maser sites. These colder clumps may represent an even earlier phase of massive star formation. These results suggest an evolutionary sequence for massive star formation from a cold clump, seen only at mm wavelengths, evolving to a hot molecular core with a two-component SED with peaks at far-IR and mid-IR wavelengths, to an (ultra-compact) H region. Alternatively, the cold clumps might be clusters of low-mass YSOs, in formation near the massive star-forming clusters. Finally, the values of the dust grain emissivity index (β) range between 1.6 and 1.9.

  • (2007) Wright, Christopher; Burton, Michael; Van Dishoeck, Ewine; van Langevelde, Huib-Jan; Wilner, David; Hughes, Annie; Lommen, Dave; Maddison, Sarah; Jorgensen, Jes; Bourke, Tyler
    Journal Article
    Context. Low-mass stars form with disks in which the coagulation of grains may eventually lead to the formation of planets. It is not known when and where grain growth occurs, as models that explain the observations are often degenerate. A way to break this degeneracy is to resolve the sources under study. Aims. Our aim is to find evidence for the existence of grains of millimetre sizes in disks around T Tauri stars, implying grain growth. Methods. The Australia Telescope Compact Array (ATCA) was used to observe 15 southern T Tauri stars, five in the constellation Lupus and ten in Chamaeleon, at 3.3 mm. The five Lupus sources were also observed with the SubMillimeter Array (SMA) at 1.4 mm. Our new data are complemented with data from the literature to determine the slopes of the spectral energy distributions in the millimetre regime. Results. Ten sources were detected at better than 3 sigma with the ATCA, with sigma approximate to 1-2 mJy, and all sources that were observed with the SMA were detected at better than 15 sigma, with sigma approximate to 4 mJy. Six of the sources in our sample are resolved to physical radii of similar to 100 AU. Assuming that the emission from such large disks is predominantly optically thin, the millimetre slope can be related directly to the opacity index. For the other sources, the opacity indices are lower limits. Four out of six resolved sources have opacity indices <= 1, indicating grain growth to millimetre sizes and larger. The masses of the disks range from < 0.01 to 0.08 M-circle dot, which is comparable to the minimum mass solar nebula. A tentative correlation is found between the millimetre slope and the strength and shape of the 10-mu m silicate feature, indicating that grain growth occurs on similar (short) timescales in both the inner and outer disk.

  • (2006) Hill, Tracey; Thompson, M; Burton, Michael; Walsh, Andrew; Cunningham, Maria; Pierce-Price, D; Minier, Vincent
    Journal Article
    We report the results of a submillimetre continuum emission survey targeted towards 78 star formation regions, 72 of which are devoid of methanol maser and UC H II regions, identified in the Swedish ESO Submillimetre Telescope (SEST)/SEST IMaging Bolometer Array (SIMBA) millimetre continuum survey of Hill et al. At least 45 per cent of the latter sources, dubbed `mm-only`, detected in this survey are also devoid of the mid-infrared MSX emission. The 450- and 850-mu m continuum emission was mapped using the Submillimetre Common User Bolometer Array (SCUBA) instrument on the James Clerk Maxwell Telescope (JCMT). Emission is detected towards 97 per cent of the 78 sources targeted as well as towards 28 other SIMBA sources lying in the SCUBA fields. In total, we have identified 212 cores in this submillimetre survey, including 106 previously known from the SIMBA survey. Of the remaining 106 sources, 53 result from resolving a SIMBA source into multiple submillimetre components, whilst the other 53 sources are submillimetre cores, not seen in the SIMBA. Additionally, we have identified two further mm-only sources in the SIMBA images. Of the total 405 sources identified in the SIMBA survey, 255 are only seen at millimetre wavelengths. We concatenate the results from four (sub)millimetre continuum surveys of massive star formation, together with the Galactic plane map of Pierce-Price et al. in order to determine the dust grain emissivity index beta for each of the sources in the SIMBA source list. We examine the value of beta with respect to temperature, as well as for the source classes identified in the SIMBA survey, for variation of this index. Our results indicate that beta is typically 2, which is consistent with previous determinations in the literature, but for a considerably larger sample than previous work.