David Alan Bies

Flow resistance information for acoustical design

Abstract The measurement and use of flow resistance information for the calculation of the acoustical properties of porous materials has been examined. It is demonstrated that flow resistance information provides a description of a porous material which is sufficient to characterise its acoustical performance for all common applications. Design charts are presented which facilitate the use of flow resistance information for the three most common applications: (1) the control of the reverberant sound field in an enclosure, (2) the improvement of the transmission loss through pipe wrappings and enclosure walls and (3) the attenuation of sound propagating in ducts. Measured values of flow resistance for fibrous and foamed products available in Australia are presented. Information is also presented for estimating the flow resistance of fibrous products of generally uniform fibre diameter. Alternatively, a means for the measurement of flow resistance is described.

The effect of fluid–structural coupling on sound waves in an enclosure—Theoretical part

This paper presents a theoretical investigation into the effect of the interaction between a sound field and its boundaries upon the characteristics of the sound field in an enclosure. Recent experimental measurements have shown that the boundaries of the sound field in a reverberation room cannot be correctly described in terms of a locally reactive normal acoustical impedance. Fluid–structural coupling must be taken into account if the mechanism of sound decay in the reverberation room is to be understood. A solution based on modal coupling analysis is obtained for the decay of sound waves in a panel–cavity system. The vibration of the system is resolved into a number of acoustical modes (including fluid and structural vibrations). The reverberation times and the resonance frequencies of different modes are calculated as a function of the modal parameters of the uncoupled panel and cavity. The effect of the panel characteristics on the decay behavior of the cavity sound field is investigated. The intere...

The effect of fluid–structural coupling on sound waves in an enclosure—Experimental part

The acoustic properties of individual normal modes in a rectangular panel–cavity system were investigated experimentally. The system consisted of a heavy five‐sided concrete box closed on the sixth side with a test panel. Experimental evidence in this paper reveals the dependence of the modal decay time of sound waves in the enclosure upon the modal characteristics of the test panel, such as the modal coupling factors, resonance frequencies, and modal decay times. The results of experimental measurements are used to verify some previous theoretical predictions. The agreement between measured and calculated enclosure modal decay times shows that modal coupling theory can be used to evaluate the sound wave behavior in those enclosures in which the coupling between fluid‐borne and structure‐borne acoustical waves is modal and where the classical model based upon the locally reactive boundary assumption does not apply. The phenomenon of maximum sound energy absorption by the panel was also observed in the exp...

Engineering Noise Control : Theory and Practice, Fourth Edition

Fundamentals and Basic Terminology Introduction Noise-Control Strategies Acoustic Field Variables Wave Equations Mean Square Quantities Energy Density Sound Density Sound Power Units Spectra Combining Sound Pressures Impedance Flow Resistance The Human Ear Brief Description of the Ear Mechanical Properties of the Central Partition Noise Induced Hearing Loss Subjective Response to Sound Pressure Level Instrumentation for Noise Measurement and Analysis Microphones Weighting Networks Sound Level Meters Classes of Sound Level Meter Sound Level Meter Calibration Noise Measurements Using Sound Level Meters Time-Varying Sound Noise Level Measurement Data Loggers Personal Sound Exposure Meter Recording of Noise Spectrum Analysers Intensity Meter Energy Density Sensors Sound Source Localization Criteria Introduction Hearing Loss Hearing Damage Risk Hearing Damage Risk Criteria Implementing a Hearing Conservation Program Speech Interference Criteria Psychological Effects of Noise Ambient Noise Level Specification Environmental Noise Level Criteria Environmental Noise Level Surveys Sound Sources and Outdoor Sound Propagation Introduction Simple Source Dipole Source Quadruple Source (Far-Field Approximation) Line Source Piston in an Infinite Baffle Incoherent Plane Radiator Directivity Reflection Effects Reflection and Transmission at a Plane/Two Media Interface Sound Propagation Outdoors, General Concepts Sound Power, its Use and Measurement Introduction Radiation Impedance Relation between Sound Power and Sound Pressure Radiation Field of a Sound Source Determination of Sound Power Using Intensity Measurements Determination of Sound Power Using Surface Vibration Measurements Some Uses of Sound Power Information Sound in Enclosed Spaces Introduction Low Frequencies Bound between Low-Frequency and High-Frequency Behavior High Frequencies, Statistical Analysis Transit Response Porous Sound Absorbers Panel Sound Absorbers Flat and Long Rooms Applications of Sound Absorption Auditorium Design Partitions, Enclosures and Barriers Introduction Sound Transmission through Partitions Noise Reduction vs Transmission Loss Enclosures Barriers Pipe Lagging Muffling Devices Introduction Measures of Performance Diffusers as Muffling Devices Classification of Muffling Devices Acoustic Impedance Lumped Element Devices Reactive Devices Lined Ducts Duct Bends or Elbows Unlined Ducts Effect of Duct End Reflections Duct Break-Out Noise Line Plenum Attenuator Water Injection Directivity of Exhaust Duct Vibration Control Introduction Vibration Isolation Types of Isolators Vibration Absorbers Vibration Neutralizers Vibration Measurement Damping of Vibrating Surfaces Measurement of Damping Sound Power and Sound Pressure Level Estimation Procedures Introduction Fan Noise Air Compressors Compressors for Chillers and Refrigeration Units Cooling Towers Pumps Jets Control Valves Pipe Flow Boilers Turbines Diesel and Gas-Driven Engines Furnace Noise Electric Motors Generators Transformers Gears Transportation Noise Practical Numerical Acoustics Introduction Low-Frequency Region High-Frequency Region: Statistical Energy ANalysis

Acoustic Impedance of a Helmholtz Resonator at Very High Amplitude

The acoustic impedance of a Helmholtz resonator terminating a ten‐inch diameter tube has been investigated for sound pressure levels in the resonator of from 100 db to 170 db and for a range of particle velocities in the neck of from 1.3 cm/sec to 1.2×104 cm/sec (rms). Two different mounting orientations showed the same general rise in acoustic resistance and rise in resonant frequency with increasing sound pressure level but gave quite different results in detail.

Propagation of sound in a curved bend containing a curved axial partition

Sound transmission in a 180° bend containing a curved partition positioned on the axial center line is investigated theoretically and experimentally using equations for sound propagation in straight and curved ducts. Good agreement is obtained and small discrepancies are discussed. The partition is found to significantly alter the sound propagation through the bend and reasons for the different acoustic behavior are given.

Optical holography for the study of sound radiation from vibrating surfaces

Time‐averaged holography has been used to quantitatively investigate sound radiation from an edge‐clamped circular flat plate mounted in an infinite rigid baffle. For a particular mode of vibration, the plate response is measured using holography and the sound power radiated is measured in a reverberant room with the plate mounted in one of the room walls. From these measurements a radiation efficiency is determined. The theoretical plate responce is calculated using both classical and Mindlin–Timoshneko plate theory and is shown to agree well with experimental measurements. Radiated sound power is calculated for each mode of interest by solving the wave equation in oblate spheroidal coordinates at the plate surface. These calculations are verified by direct evaluation of the Rayleigh integral in the farfield. Good agreement is obtained between experimentally measured radiation efficiencies and theoretical predictions. Small discrepancies between theory and experiment are discussed.Subject Classification:...

On acoustic radiation by a rigid object in a fluid flow

Abstract A problem of sound radiation by an absolutely rigid object, moving with respect to the surrounding fluid, is considered on the basis of the Lighthills equation for aerodynamic sound. An integral representation of the radiated acoustic field is utilized, where the field is characterized as the sum of three fields, generated by a volume distribution of monopoles and by distributions of monopoles and dipoles on the surface of the rigid object. It is shown that, due to a discontinuity of Lighthills stress tensor on the rigid boundary, a layer of surface divergence of hydrodynamic stresses on the boundary must be taken into account when evaluating the volume integral over Lighthills quadrupole sources. When the contribution of the surface divergence is included in the solution of Lighthills equation, amplitudes of the monopole and dipole sound radiated by the rigid object are shown to depend on the potential components of the normal velocity and the pressure on the rigid surface. The obtained solution is compared with Curles solution for this problem, which establishes that the sound radiation by a rigid object is determined by the force exerted by the object upon the fluid. Both solutions are applied to two known problems of sound scattering and radiation by a rigid sphere in variable pressure and velocity fields. It is shown that predictions based on the obtained solution are equivalent to the results known from literature, whereas Curles solution gives predictions contradicting the known results. It is also shown that the Ffowcs Williams and Hawkings equation, which coincides with Curles equation for an immoveable rigid object, does not lead to the correct predictions as well.

The effect of fluid–structural coupling on acoustical decays in a reverberation room in the high‐frequency range

The effect of the fluid–structural coupling between the modes of a sound field and the modes of test panels placed in the sound field upon their decay behavior is investigated in the high‐frequency range. Quasitransient solutions of the average energies in the room and the panels are used to estimate the reverberation times in the room and to calculate the decay curves of a test panel. Experiments are conducted that verify these solutions. The estimated and measured results are in good agreement, which indicates that the average decay behavior of the room and panels can be analyzed in terms of the average modal parameters. The average modal parameters of the test panels and the reverberation room can be related to the Sabine absorption coefficient αSab of the panels. This result suggests an interpretation for the Sabine absorption coefficient of a modally reactive surface and provides a way to estimate this quantity using the statistical energy analysis (SEA) method. The dependence of the αSab upon the ro...

An experimental investigation into the interaction between a sound field and its boundaries

The traditional description of the sound field in an enclosure begins with the assumption that the walls are locally reactive and that they may be characterized by local normal acoustic impedance. This assumption allows the introduction of sound absorption coefficients and a formalism that is supposed to adequately predict the acoustic response of an enclosure. The assumption of locally reactive walls and the concepts that follow from it have been applied to architectural acoustics, apparently without question, for several decades. However, it is known that this method is useless when applied to enclosures such as the interiors of aircraft and motor vehicles. Experimental work carried out in a reverberation room indicates that the walls are not locally reactive and the coupling between wall structural modes and room acoustic modes affects the reverberation time. This article will review the experimental evidence in support of these conclusions.