The metre

A musical wind instrument transforms a constant pressure input from the player's mouth into a fluctuating pressure output in the form of a radiating sound wave. In reed woodwind and brass instruments, this transformation is achieved through a nonlinear coupling between two vibrating systems: the flow control valve formed by the mechanical reed or the lips of the player, and the air column contained by the pipe. Although the basic physics of reed wind instruments was developed by Helmholtz in the nineteenth century, the application of ideas from the modern theory of nonlinear dynamics has led to recent advances in our understanding of some musically important features of wind instrument behaviour. As a first step, the nonlinear aspects of the musical oscillator can be considered to be concentrated in the flow control valve; the air column can be treated as a linear vibrating system, with a set of natural modes of vibration corresponding to the standing waves in the pipe. Recent models based on these assumptions have had reasonable success in predicting the threshold blowing pressure and sounding frequency of a clarinet, as well as explaining at least qualitatively the way in which the timbre of the sound varies with blowing pressure. The situation is more complicated for brass instruments, in which the player's lips provide the flow valve. Experiments using artificial lips have been important in permitting systematic studies of the coupling between lips and air column; the detailed nature of this coupling is still not fully understood. In addition, the assumption of linearity in the air column vibratory system sometimes breaks down for brass instruments. Nonlinear effects in the propagation of high amplitude sound waves can lead to the development of shock waves in trumpets and trombones, with important musical consequences.     

Simplified models of flue instruments: influence of mouth geometry on the sound source

Flue instruments such as the recorder flute and the transverse flute have different mouth geometries and acoustical response. The effect of the mouth geometry is studied by considering the aeroacoustical response of a simple whistle. The labium of a transverse flute has a large edge angle (60 degrees) compared to that of a recorder flute (15 degrees). Furthermore, the ratio W/h of the mouth width W to the jet thickness h can be varied in the transverse flute (lips of the musician) while it is fixed to a value W/h approximately 4 in a recorder flute. A systematic experimental study of the steady oscillation behavior has been carried out. Results of acoustical pressure measurements and flow visualization are presented. The sharp edge of the recorder provides a sound source which is rich in harmonics at the cost of stability. The larger angle of the labium of the flute seems to be motivated by a better stability of the oscillations for thick jets but could also be motivated by a reduction of broadband turbulence noise. We propose two simplified sound source models which could be used for sound synthesis: a jet-drive model for W/h>2 and a discrete-vortex model for W/h<2.     

Radiation characteristics of multiple and single sound hole vihuelas and a classical guitar

Two recently built vihuelas, quasi-replicas of the Spanish Renaissance guitar, one with a small body and one sound hole and one with a large body with five sound holes, together with a classical guitar are investigated. Frequency dependent radiation strengths are measured using a 128 microphone array, back-propagating the frequency dependent sound field upon the body surface. All three instruments have a strong sound hole radiation within the low frequency range. Here the five tone holes vihuela has a much wider frequency region of strong sound hole radiation up to about 500 Hz, whereas the single hole instruments only have strong sound hole radiations up to about 300 Hz due to the enlarged radiation area of the sound holes. The strong broadband radiation of the five sound hole vihuela up to about 500 Hz is also caused by the sound hole phases, showing very consistent in-phase relations up to this frequency range. Also the radiation strength of the sound holes placed nearer to the center of the sound box are much stronger than those near the ribs, pointing to a strong position dependency of sound hole to radiation strength. The Helmholtz resonance frequency of the five sound hole vihuela is influenced by this difference in radiation strength but not by the rosettas, which only have a slight effect on the Helmholtz frequency.     

Numerical and experimental studies on the effects of stage risers

The acoustic effects of stage risers, especially on the sound of lower string instruments, are numerically and experimentally analyzed. To discuss the effects of the vibration of riser’s boards due to the mechanical force from an instrument, a structural-acoustical coupling approach is applied based on the mode expansion and the boundary element technique. Measurement results of the mechanical force from real instruments are used in the numerical study. The vibration of the top board of a riser affects the sound field only around the natural frequencies of the board and the cavity of the riser. In contrast, the acoustic diffraction due to the riser affects the sound field in a wide frequency range. The riser’s sideboard that faces to receiving points increases the sound pressure levels because it reflects waves diffracted at the riser’s edge to the front. To verify the numerical results, the effects of acoustic diffraction due to risers are especially investigated in detail through a scale model experiment.     

Investigation of phase coupling phenomena in sustained portion of musical instruments sound

This work investigates aperiodicities that occur in the sustained portion of a sound of musical instrument played by a human player, due to synchronous versus asynchronous deviations of the partial phases. By using an additive sinusoidal analysis, phases of individual partials are precisely extracted and their correlation statistics and coupling effects are analyzed. It is shown that various musical instruments exhibit different phase coupling characteristics. The effect of phase coupling is compared to analysis by means of higher order statistics and it is shown that both methods are closely mathematically related. Following a detailed analysis of phase coupling for various musical instruments it is suggested that phase coupling is an important characteristic of a sustained portion of sound of individual musical instruments, and possibly even of instrumental families. Interesting differences in phase deviations where found for the flute, trumpet and cello. For the cello, the effect of vibrato is examined by comparing the analysis of a closed string sound played with a natural vibrato to analysis of an open string sound that contains no vibrato. Following, a possible model for phase deviations in the cello is presented and a simulation of phase fluctuations for this model is performed.     

中国筝的声功率级测试

筝是中国古老的弹弦乐器。但迄今为止,对其声功率级一直未进行科学的测定。本文在一混响室内根据ISO及GB标准,对二十一弦筝的声功率级进行了首次测定。两位资深乐师在混响室内分别演奏各自的乐器,通过围绕乐师和乐器布置的四通道测试设备,对筝所辐射的声功率级和动态范围进行测定。测试结果表明,中国筝在以不同力度演奏单音、音阶和乐曲时所辐射的声功率级及其频率特性均有所不同。考虑到乐器演奏音阶时所辐射的声功率级及其动态范围与演奏乐曲时的声功率级接近,并且,音乐的空间感也大都在乐器以f力度演奏乐曲的强音标志乐段时最为显著,故此我们建议中国筝所辐射的声功率级用其以f力度演奏音阶时的平均声功率级表示。本文测试的两架中国筝以f力度演奏音阶时的平均声功率级为85.9dB。文中不仅首次公布了中国筝声功率级的测试结果,并且所介绍的测试方法对其它乐器声功率级测试也具有借鉴意义。民族乐器所辐射的声音性能的确定是民族音乐厅堂音质研究的基础。     

Vortex sound in the presence of a low Mach number flow across a drum-like silencer

The sound generated by a vortex propagating across a two-dimensional duct section with flexible walls (membranes) in an infinitely long rigid duct conveying a flow is investigated numerically using the matched asymptotic expansion technique and the potential theory. The effects of the initial vortex position, the mechanical properties of the flexible walls, and the mean flow on the sound generation are examined in detail. Results show that the presence of a vortex inside a uniform mean flow can strengthen or attenuate the sound generation, depending on the phase of the membrane vibration when the vortex starts vigorous interaction with the membranes and the strength of the mean flow. The results tend to imply that there is a higher chance of sound amplification when a vortex stream is moving closer to the lighter membrane under a relatively strong mean flow or when the mean flow is weak. The chances of sound amplification or attenuation are equal otherwise.     

椰胡声功率级的半消声室测试

椰胡是具有地方特色的拉弦乐器之一,多用于演奏广东音乐和广东戏曲、曲艺的伴奏。但对其发声强度一直未进行过科学的测试。本文介绍在华南理工大学半消声室内参照ISO(GB)标准对椰胡声功率级的测量工作。由两位资深乐师分别用两把椰胡在pp,mp,f和ff力度下演奏单音、音阶和乐曲,对每一把椰胡的每一测试内容,由十通道测试设备同步测试中心频率为100～10000 Hz的1/3倍频带声压级谱,通过计算获得每把椰胡在演奏每项内容时的声功率级和动态范围。通过对两把椰胡的测试结果进行平均,获得该乐器在演奏上述内容时的典型声功率级数值及频谱。文中并将半消声室内的测试结果与混响室内的测试结果相对比,探讨测试环境对测试结果的影响。民族乐器发声强度及其频谱特性的获得是开展民族音乐厅堂音质研究的基础。     

Acoustical properties of petung bamboo for the top plate of guitars

In this paper, the use of petung bamboo (Dendrocalamus asper) as an alternative material for the top plate of acoustic guitars was evaluated. In the first research stage, the analysis was carried out on petung bamboo splits, which were treated with three different preservation conditions, namely unpreserved, preserved by boiling in water and preserved by boiling in a solution of 5% borax and boric acid. The vibration damping ratios and the sound radiation coefficients of these three types of splits were measured. It was found that their vibration damping ratios were not significantly different, whereas the sound radiation coefficient of the petung bamboo splits preserved in borax and boric acid was 40–60% higher than the other two variants. Based on this result, in the second research stage, three acoustic guitars with top plates from the borax–boric acid preserved petung bamboo, spruce, and pine were constructed. The top plate frequency response function of the three guitars was evaluated. Here, the spruce guitar was used as a reference for the generally preferred guitar sound characteristics, whereas the pine guitar was included as a sample of guitar made from local Indonesian wood. The results showed that the frequency response function of the petung bamboo guitar were generally five times lower than that of the spruce guitar, but two times higher than that of the pine guitar. The response amplitudes of the bamboo guitar were significantly higher than those of the spruce and pine guitars for frequencies between 200 and 400 Hz. Based on the results, it is concluded that petung bamboo has the potential to be applied as an alternative material for guitar top plates. However, since the bamboo guitars exhibits different resonances, the produced sound will have distinctive characteristics compared to the sound from guitars with spruce top plates.     

Time-domain simulation of sound production of the sho

A physical model based on the sound production mechanism of the sho is proposed with intention of applying it to sound synthesis. Time-domain simulation was done using this model, and effects of the tube length and blowing pressure on the sounding frequency and sounds spectra were investigated. The reed vibration, pressure variation inside the tube, and threshold blowing pressure for oscillation were measured by artificially blowing air into the sho. The experimental results are in acceptable agreement with simulation results in terms of the relationships between tube length and threshold pressure and between tube length and the sounding frequency. In addition, recorded sound waveforms and simulated ones have a common feature in the sense that high-frequency components of their spectra increase with increasing blowing pressure. Further, it is concluded that a sho reed acts as an "outward-striking valve."     

Effects of nonlinear sound propagation on the characteristic timbres of brass instruments

The capacity of a brass instrument to generate sounds with strong high-frequency components is dependent on the extent to which its bore profile supports nonlinear sound propagation. At high dynamic levels some instruments are readily sounded in a "cuivre?" (brassy) manner: this phenomenon is due to the nonlinear propagation of sound in ducts of the proportions typical of labrosones (lip-reed aerophones). The effect is also evident at lower dynamic levels and contributes to the overall tonal character of the various kinds of brass instrument. This paper defines a brassiness potential parameter derived from the bore geometries of brass instruments. The correlation of the brassiness potential parameter with spectral enrichment as measured by the spectral centroid of the radiated sound is examined in playing tests using musicians, experiments using sine-wave excitation of instruments, and simulations using a computational tool. The complementary effects of absolute bore size on spectral enrichment are investigated using sine-wave excitation of cylindrical tubes and of instruments, establishing the existence of a trade-off between bore size and brassiness potential. The utility of the brassiness potential parameter in characterizing labrosones is established, and the graphical presentation of results in a 2D space defined by bore size and brassiness potential demonstrated.     

Infrasonic sound pressure in dwellings at the Helmholtz resonance actuated by environmental noise and vibration

The Helmholtz resonator effect of a room with an open window or ventilation duct has been studied theoretically and experimentally. The effect results in a sound pressure buildup at infrasonic frequencies. For comparison, the frequencies of the standing-wave room resonances are above the infrasonic range for residential dwellings. The relations between the sound pressure inside a room and outside (environmental) sound pressure or vibration acceleration have been calculated for the third-octave frequency band incorporating the Helmholtz resonance frequency. The experiment on a small-scale model illustrated the Helmholtz resonator effect caused by environmental vibration.     

Acoustic characteristics inside cylindrical structure with end plates excited at different frequencies

In order to study coupling between vibration of a structure and a sound field in contact with the structure, a cavity surrounded by a rigid cylinder having thin elastic plates at both ends is adopted as an analytical model. When excitation forces of different frequencies are applied to the respective plates, the plate vibrations and the sound field inside the cavity become aperiodic, because of the coupling between the systems. In the present investigation, distribution of the sound pressure level inside the cavity is studied in detail in order to clarify the coupling behavior. The results show that when the respective plates vibrate at the same circumferential order, the vibration modes of the plates, which effect the coupling of the plate vibrations and the sound field, cause the aperiodic nature of each system to develop. In this case, since the dominant mode exists in formation of the sound field, it significantly influences the aperiodic nature of the coupling systems. In the case of vibration modes where the plates vibrate at different circumferential orders, the behaviors of the three systems, whose coupling has been restrained, approximate a steady state. Consequently, the dominant mode does not appear in the sound field.     

声速测量及声波干涉演示仪器的研制

本介绍了声波干涉、反射的两种物理演示仪器的研制,一种是通过调节波导管中液面的高度形成驻波,实现对声速的测量;另一种是由一个信号发生器驱动两个扬声器,这两个相干的声源在自由空间中形成强弱变化的声波干涉。本分别介绍了这两种装置的原理、制作及使用效果。     

Particle motion measured at an operational wind turbine in relation to hearing sensitivity in fish

The effect of sound pressure on the hearing of fish has been extensively investigated in laboratory studies as well as in field trials in contrast to particle motion where few studies have been carried out. To improve this dearth of knowledge, an instrument for measuring particle motion was developed and used in a field trial. The particle motion is measured using a neutrally buoyant sphere, which co-oscillates with the fluid motion. The unit was deployed in close vicinity to a wind turbine foundation at Utgrunden wind farm in the Baltic Sea. Measurements of particle motion were undertaken at different distances from the turbine as well as at varying wind speeds. Levels of particle motion were compared to audiograms for cod (Gadus morhua L.) and plaice (Pleuronectes platessa L.).     

EXPERIMENTAL STUDY OF NOISE PRODUCED BY STEADY FLOW THROUGH A SIMULATED VASCULAR STENOSIS

An in vitro experiment is carried out in order to study the acoustic effects of a vascular constriction (stenosis) in people and provide correlations between these effects and parameters relevant to the hydrodynamic and acoustic processes. For this purpose, we measure the sound produced when water flows through an elastic tube which is either unobstructed or contains a rigid axisymmetric constriction. The sound is measured at the outside of a large annular container filled with water and bounded at the inside by the coaxial elastic tube. The analysis of the acoustic fields shows that a stenosis has two basic acoustic effects. These are a general increase in the sound level and the production of a number of additional distinct peaks (new frequency components) in the acoustic power spectrum. The frequencies of these peaks are close to the characteristic frequencies of vortex formation in the disturbed flow region behind a stenosis and the resonance frequencies of vibration of the post-stenotic segment of the tube. Another important result is that the stenosis generated acoustic power is approximately proportional to the fourth power of the stenosis severity and the same power of the flow Reynolds number.     

Experimental study of AO and T1 modes of the concert harp

String instruments are usually composed of a set of strings, a soundboard, and a soundbox with sound holes, which is generally designed to increase the sound level by using the acoustic resonances of the cavity. In the case of the harp, the soundbox and especially the sound holes are primarily designed to allow access to the strings for their mounting. An experimental modal analysis, associated to measurements of the acoustic velocity in the holes, shows the importance of two particular modes labeled A0 and T1 as it was done for the guitar and the violin. Their mode shapes involve coupled motions of the soundboard's bending and of the oscillations of the air pistons located in the sound holes. The A0 mode is found above the frequency of the lowest acoustically significant structural mode T1. Thus, the instrument does not really take advantage of the soundbox resonance to increase its radiated sound in low frequencies. However, contribution of mode A0 is clearly visible in the response of the instrument, confirming the importance of the coupling between the soundboard and the cavity.     

Feature dependence in the automatic identification of musical woodwind instruments

The automatic identification of musical instruments is a relatively unexplored and potentially very important field for its promise to free humans from time-consuming searches on the Internet and indexing of audio material. Speaker identification techniques have been used in this paper to determine the properties (features) which are most effective in identifying a statistically significant number of sounds representing four classes of musical instruments (oboe, sax, clarinet, flute) excerpted from actual performances. Features examined include cepstral coefficients, constant-Q coefficients, spectral centroid, autocorrelation coefficients, and moments of the time wave. The number of these coefficients was varied, and in the case of cepstral coefficients, ten coefficients were sufficient for identification. Correct identifications of 79%-84% were obtained with cepstral coefficients, bin-to-bin differences of the constant-Q coefficients, and autocorrelation coefficients; the latter have not been used previously in either speaker or instrument identification work. These results depended on the training sounds chosen and the number of clusters used in the calculation. Comparison to a human perception experiment with sounds produced by the same instruments indicates that, under these conditions, computers do as well as humans in identifying woodwind instruments.     

内部体积源作用下的圆柱壳内外声场特性

圆柱壳内各型体积源辐射噪声特性研究是声场建模和声场预报的前提.为了研究具有指向性的大尺度体积源特性对水下航行器结构内外声场的影响,本文结合薄壳理论、等效源和柱腔Green函数构造了体积源激励下的壳体振动耦合方程,研究了体积源表面声散射作用和指向性强弱对圆柱壳内外声场的影响.数值计算结果表明,体积源构造的准确性与其等效源位置有关,等效源配置在体积源几何中心与其结构表面之间0.4—0.6时,可以提高声场计算结果的准确性;大尺度体积源表面的声散射作用会导致壳体内部声场结构发生改变,内声场声腔共振峰发生偏移,并且在部分频段引起较强的声透射现象;此外,体积源指向性变化对壳体内外声场强弱影响较小,其显著作用表现在改变了外辐射声场的远场指向性.该研究结果对噪声预报和控制有一定的参考价值.     

点簧簧片振动特性数值分析

簧片是笙等中国传统簧管类乐器的核心部件之一.通过点簧工艺可调整簧片振动频率,点簧簧片的差异对整体乐器的声音效果具有重要影响.针对点簧簧片的振动特性问题,对其建立非均匀截面并具有质量负载的振动模型,采用有限元方法计算点簧簧片的固有频率.大量算例分析了点簧质量、位置以及簧片的边界条件对振动频率的影响,研究发现:若点簧质量不...