Impulse duration: A new instrument for its measurement

An instrument is described which, when used with a ‘peak hold’ reading sound level meter, will measure the durations of acoustic impulses in accordance with the Atherley and Martin and CHABA criteria for hearing damage risk to impulsive noise. The instrument is small, lightweight, can be battery powered and is designed for field use.Comparison tests show that the impulse duration meter gives more accurate and repeatable results than the oscilloscope trace photograph method or the digital waveform recorder method of impulse duration assessment.By using the instrument with a suitable impulse source reverberation time, measurements may be conducted.     

Evaluation of sound absorbing coefficients in a reverberant room by computer-ray simulation

Standardized methods for the measurement of the sound absorption coefficient, α, of materials in a reverberant room use well known relations between α and the reverberation time of the room. Actually, these relations are correct only in rooms with high diffusion at low α values and for uniform distribution of the absorption on the surfaces. The introduction of acoustic materials with high α values concentrated on only one surface, as recommended by standards, causes a derandomization of the sound field, which may overcome the ‘randomization strength’ of the enclosure and therefore make the use of traditional formulae incorrect. This paper proposes a computer-ray tracing model, which enables these phenomena to be described, and operatively defines the ‘randomization strength’ of room design. Lastly, an alternative approach is suggested for evaluating the sound absorption coefficient, based on the characterization of the reverberant room by its own ‘calibration curve’.     

A method of obtaining the oblique incident sound absorption coefficient through an on-the-spot measurement

A new method (tentatively called the ‘cancellation method’) of determining the sound absorption coefficient is presented. This method makes it possible to obtain the absorption coefficient of a wall material by means of an on-the-spot field measurement which has, until now, been considered very difficult. This paper shows that the reflection from a sample can be obtained by combining the outputs from two non-directional microphones through a phase inverter, and how, by comparing it with the direct sound, measured separately, the sound absorption coefficient can be estimated. Absorption coefficients of several kinds of samples obtained by this method are compared with those calculated theoretically for the locally reactive material.     

Sound attenuation by multiple barriers

A method (tentatively called ‘the method of equivalent sound source’) of predicting the sound attenuation achieved by multiple barriers with knife edges and/or right-angled wedges is proposed. This paper shows that the sound pressure level in the shadow region behind the multiple barriers can be obtained by successively setting imaginary sound sources for respective edges and/or wedges. The locations of the imaginary sound sources are determined by means of the traditional solution for sound attenuation by a single edge. The calculated results for several kinds of small-scale model, obtained by ‘the method of equivalent sound source’, are compared with experimental results.     

Sound transmission through finite lightweight multilayered structures with thin air layers

The sound transmission loss (STL) of finite lightweight multilayered structures with thin air layers is studied in this paper. Two types of models are used to describe the vibro-acoustic behavior of these structures. Standard transfer matrix method assumes infinite layers and represents the plane wave propagation in the layers. A wave based model describes the direct sound transmission through a rectangular structure placed between two reverberant rooms. Full vibro-acoustic coupling between rooms, plates, and air cavities is taken into account. Comparison with double glazing measurements shows that this effect of vibro-acoustic coupling is important in lightweight double walls. For infinite structures, structural damping has no significant influence on STL below the coincidence frequency. In this frequency region, the non-resonant transmission or so-called mass-law behavior dominates sound transmission. Modal simulations suggest a large influence of structural damping on STL. This is confirmed by experiments with double fiberboard partitions and sandwich structures. The results show that for thin air layers, the damping induced by friction and viscous effects at the air gap surfaces can largely influence and improve the sound transmission characteristics.     

Approximate method for obtaining source quantities for calculation of structure-borne sound transmission into lightweight buildings

An approximate approach is described, for obtaining the source quantities required for the calculation of structure-borne sound power from machines into supporting lightweight building elements. The approach is in two stages, which are based on existing international Standards for measurement. The first stage involves direct measurement of the source free velocity at each contact, to give the sum of the square velocities. The second stage is based on the reception plate method and yields the single equivalent blocked force, which approximates the sum of the square blocked forces. The applicability of the source data obtained has been investigated in a case study of a fan unit on a timber joist floor. The approach contains several significant simplifying assumptions and the uncertainties associated with them are considered. For the case considered, the power transmitted into the floor is estimated by the approximate method to within 5 dB of the true value, on average.     

Airborne sound insulation referred to the ‘A’ weighting curve

The ‘A’ weighting curve, drawn from the 56 dB datum, forms a very good compromise between the British House Party Wall Grade curve, and the International Standard Organisation R 717 reference curve. Since the ‘A’ weighting curve is already internationally standardised for noise measurements, its adoption as a reference curve for sound insulation measurements would make for a unification and rationalisation of acoustic criteria.The adoption of the ‘A’ weighting curve would also provide a theoretical basis for a quick test for sound transmission using a sound level meter to measure the overall levels on each side of a wall, of a white noise source. When the received level is measured on the ‘A’ weighting network, any serious adverse deviation of the partition below the reference curve would be shown by the higher overall received level.Suggestions for the revision of ISO R 717 to incorporate this proposal are included.Good correlation is shown between the sound level difference, as measured by a meter, and the revised airborne sound insulation index measured by the ISO R 140 procedure, on one sample test.     

Porous metal absorbers for underwater sound

Rubber has traditionally been used for underwater sound absorption. Porous metal is a relatively lightweight material and also has higher strength than rubber. However, exactly how porous metals can be used as effective underwater sound absorbers remains unclear. This paper shows how to use porous metal absorbers so that they work well under water, even under fairly constrained conditions. A method of nondimensional analysis is proposed that allows identification of vital characteristics. This means that such characteristics can be varied and the absorbers themselves filled with different types of viscous fluids. Such analysis suggests that the sound absorption coefficient of porous metals does not always increase when there are either increases in porosity or decreases in average pore size. The same method of analysis can show how, by choice of the right characteristics to choose a suitable viscous fluid, a porous metal absorber can be built that takes up little space but still effectively absorbs underwater sounds at low frequencies.     

A small-scale multi-purpose reverberation room

A one-fourth scale model of the large (425 m³) National Bureau of Standards reverberation room is described. This facility was constructed to carry out acoustical research at relatively low cost in a frequency range two octaves higher than that used in the larger facility. Initial experimental measurements carried out in this facility concern sound power emitted by small sources. The pure tone qualification procedure specified in American National Standard S1.21-1972 ‘Metods for the Determination of Sound Power Levels of Small Sources in Reverberation Rooms’ was carried out using computer control of the experiment. This standard is of particular interest to the international acoustical community since it is technically equivalent to ISO Documents DIS 3741 and DIS 3742. The effect of increased low frequency absorption upon room qualification was tested in the model room. Future research plans to make use of the small-scale reverberation rooms are described.     

Determination of the sound energy level of a gunshot and its applications in room acoustics

In some cases an impulsive noise source such as a gunshot can be a preferred alternative when investigating building acoustics, including sound insulation measurements, when compared to conventional steady state noise sources. A gun equipped with blank cartridges is an impulsive noise source that is lightweight and small enough to be easily transported. The differences in the noise characteristics between individual cartridges for the same gun are usually small, so the impulsive source can be replicated to a high degree. This paper is focused on the practical application of the sound exposure levels produced by a gunshot with a known sound energy level in the rooms under investigation. In this way, the equipment and methods required by the conventional method are simplified significantly. Furthermore, reverberation times need not be measured, since the equivalent absorption area can be directly obtained from the measured sound exposure levels. Using Green’s theorem, the roles of the sound source and measuring microphone were exchanged, which simplified the determination of sound insulation as it was easier to change the position of the gun than the microphone. The results obtained using the impulsive noise source were in good agreement with those obtained using the conventional method. Above 100 Hz, their difference in any frequency band of interest was less than 1 dB.     

An investigation of simple methods for assessing reverberation time

There is considerable interest in the development of a simple test for sound insulation between dwellings. The assessment of reverberation time is the most difficult part of the procedure to simplify. In this paper six alternative methods are described and evaluated. The first three require no electronic apparatus but are not accurate enough for general use. The fourth involves using sufficient absorbing material in the receiving room to effectively determine the RT. This approach appears worth further development. The fifth approach requires a source of known sound power and the final method employs a simple meter giving a direct reading of the decay time. The last two methods appear to be accurate enough for inclusion in a simplified test method but the simple meter seems to have some advantage.     

Non-resonant sound transmission through double walls using statistical energy analysis

Although SEA is a suitable framework for predicting sound transmission through double walls it has been found that the standard method of computing the non- resonant coupling loss factor from a room to cavity underestimates the coupling. A revised model for computing this coupling loss factor is presented which gives much better agreement with measured data. This allows better predictions to be made of sound transmission through lightweight double walls.     

多层多孔吸声材料结构参数优化设计*

高开孔率的发泡材料(如三聚氰胺、聚氨酯发泡材料)具有优良的吸声、隔热防火、防腐及环保性能,可以作为吸声、阻尼等材料应用于建筑、航空、交通工具等领域。该文基于Biot理论和多层介质声波传播理论(传递矩阵法),建立多层多孔吸声结构背衬刚性壁的理论模型,利用遗传算法优化多层结构厚度和质量。将理论模型计算结果与阻抗管测试结果进行对比,验证了理论模型的准确性。结果表明:优化后的多层多孔吸声结构在整体厚度降低18 mm的基础上,吸声能力并未降低,且部分结构低频吸声增强,结构的整体重量也有所降低,达到轻薄化的目标,具有较大的应用价值。     

Experiment on investigating the voice cords functioning by the speech signal

A practical application of linear prediction methods for calculating the pulse function that models the functioning of vocal cords is described. Some characteristics of the pulses of this function enable one to draw some conclusions about the speaker’s individual features (and, possibly, about the quality of sound). The first part of the paper is devoted to the theoretical background of the described method. The second part presents a detailed algorithm of the program realization of the method in the MATLAB 5.2 environment and analyzes the results of the experiment made on Russian vowels.     

Jittering wave-packet models for subsonic jet noise

Three simplified wave-packet models of the coherent structures in subsonic jets are presented. The models comprise convected wave-packets with time-dependent amplitudes and spatial extents. The dependence of the radiated sound on the temporal variations of the amplitude and spatial extent of the modulations are studied separately in the first two model problems, being considered together in the third. Analytical expressions for the radiated sound pressure are obtained for the first and third models.Results show that temporally localised changes in the wave-packet can lead to radiation patterns which are directional and which comprise high-amplitude bursts; such intermittency is observed in subsonic jets at the end of the potential core, and so the models may help explain the higher noise levels and intermittent character of the sound radiated to low emission angles for subsonic jets. By means of an efficiency metric, relating the radiated acoustic power to the fluctuation energy of the source, we show that the source becomes more powerful as its temporal localisation is increased. This result extends that of Sandham et al. (Journal of Sound and Vibration 294(1) (2006) 355–361) who found similar behaviour for an infinitely extended wavy-wall.The pertinence of the model is assessed using two sets of data for a Mach 0.9 jet. One corresponds to a direct numerical simulation (DNS) of a Reynolds number 3600 turbulent jet and the other to a large eddy simulation (LES) of a Reynolds number 4×10⁵ jet. Both time-averaged and time-dependent amplitudes and spatial extents are extracted from the velocity field of the numerical data. Computing the sound field generated by the wave-packet models we find for both simulations that while the wave-packet with a time-averaged envelope shows discrepancies of more than an order of magnitude with the sound field, when the wave-packet ‘jitters’ in a way similar to the intermittency displayed by the simulations, we obtain agreement to within 1.5 dB at low axial angles. This shows that the ‘jitter’ of the wave-packet is a salient source feature, and one which should be modelled explicitly.     

船用超声液位传感器的分析与设计

介绍了一种船用某型超声液位传感器的基本结构和工作原理。在对脉冲式超声探头的轴向声压分布特点及其声透射强度分析研究的基础上,给出了该传感器的超声信号发射端与接收端之间距离d和压电晶片保护膜的壁厚δ值的确定方法。最后设计了一套超声液位传感器性能测试实验平台,对上述理论研究结论进行了实验验证。     

Acoustical design of the multi-purpose ‘Hjertnes’ hall in Sandefjord

‘Hjertnes’ is a typical multi-purpose hall primarily intended for concerts. A three-dimensional ray tracing computer program was used to provide a picture of the sound energy distribution on the audience area of the hall, various room shapes being investigated. This resulted in tilted side walls and a convex ceiling. Variable acoustics have been obtained through the use of movable ceiling absorbing and diffusing elements. In addition, a demountable orchestra enclosure has been constructed within the stagehouse. Control measurements of pulse response and reverberation time are reported.     

A ray model of sound focusing with a balloon lens: an experiment for high school students

A weather balloon filled with carbon dioxide gas is used as a positive spherical acoustic lens. High frequency but audible sound from a circular loudspeaker ensonifies the balloon and produces increased sound pressure levels in a region along the principal axis according to a ray acoustics model. This enhancement was measured experimentally and was found to agree with theory. The possibility that interference from reflected sound off walls or the floor could mask or mimic the expected focusing was countered by calculating and measuring within a "shadow zone" in which only direct rays or rays refracted by the balloon exist by the method of Fresnel volumes. The experiment described in this paper would be a suitable learning experience for junior high and high school students showing how rays and Snell's law apply to sound as well as light and giving them a measurable predicted focal region for enhanced sound pressure levels.     

Potential solutions to improved sound performance of volume based lightweight multi-storey timber buildings

Lightweight building systems in general suffer from poor sound insulation, especially in the low frequency region. Since no reliable mathematical models that can predict the impact sound pressure level exists, the lightweight building design is to a high extent based upon previous experience and upon measurements. A special difficulty is related to experimental measurements since the variation among identical units must not be neglected. A modern volume based lightweight wooden building concept has here been tested by numerous well controlled measurements, in laboratory as well as in more field like conditions. The volume construction technique offers new possibilities and challenges to improve sound insulation in light weight timber construction. The main purpose was to investigate how different constructional solutions in the floor, like plaster board, mineral wool, elastic glue, dividing board, floating floor etc., affect the sound insulation. Many of the tested modifications resulted in only marginally changed impact sound pressure level but parameters that substantially can improve the sound insulation were found in using elastic glue to mount the floor boards, to install extra board layers and to use floating floors.