Direct Sound Field Research Articles

Selection of measurement positions for measuring sound reflection of noise barriers

The number of positions for the in-situ measurement of sound reflection under direct sound field conditions according to EN 1793-5 for road traffic noise reducing devices (NRDs, e.g. noise barriers) depends on the surface structure of the device under test. The surface structure and material homogeneity may be categorized as flat and non-flat as well as homogenous and inhomogeneous. Flat and homogenous NRDs are measured with one central measurement position, whereas non-flat and or inhomogeneous devices require additional measurements points to assess the overall sound reflection properties. In the current version of the standard, NRDs are considered flat, if the depth of their surface structure is below 8.5 cm. Also, they are considered homogenous if the lateral extent of each material is at least 8.5 cm. It is discussed to reduce this criterion from 8.5 cm to 1.7 cm, as research showed significant differences on devices considered as flat between measurement positions for the sound reflection index. An extensive analysis of sound reflection measurements shows the influence of the measurement position for noise barriers with structural depths below 8.5 cm. A new strategy for selecting measurement positions is proposed, which also considers the period length of the surface structure.

Directional Sound Field and Spatial Speech Decay: Impact of Sound Absorption and Screen Height in Performance Spaces and Open-Plan Offices

This study investigates the spatial decay rate of speech in performance spaces and open-plan offices, focusing on the effects of sound absorption and screen height. Utilising parameters such as D2, SD_2, SD2, S (decay rate) and Lp, A, S, 4mL_p, A, S, 4mLp, A, S, 4m (sound pressure level at 4 meters), the research evaluates how acoustic environments impact speech transmission. Through detailed experiments on different screen heights (1.2 m, 2.1 m, and 2.9 m) and material absorption, it was found that increasing screen height significantly reduces the spatial speech decay in open-plan offices, but only up to a certain threshold. The results suggest that while acoustic enhancements like screens and absorptive materials improve speech privacy, over-reliance on single- number ratings can lead to misclassification of office acoustics. This research provides insights for optimising room acoustics, ensuring speech privacy, and meeting acoustic comfort standards.

Time-domain model for spherical wave reflection in a flat surface with absorber character – Application to the SOPRA measurement method

This paper presents a developed time-domain model for the reflection of spherical waves in an absorber-like surface. The model is made to enable evaluation of measurement methods for assessing the sound absorptive properties of traffic noise barriers in direct sound field, it is thus called the Direct Field Absorption (DFA) model. In this study, the DFA model is applied to the in-situ SOPRA method for quick sound reflection index measurements on road noise barriers. The DFA model results in an impulse response (IR), based on a theoretically derived impedance of a certain absorber. The DFA IR is entered to the SOPRA formula to calculate the reflection index, RIQ, of the absorber, which is subsequently compared to the measured RIQ of a wall fitted with the absorber in question. If the results are similar, it is reasonable to assume that the SOPRA measurement results are valid. But if there are any significant differences between the DFA and SOPRA sound reflection indices, possible reasons for them should be examined.The first results are encouraging, showing that the DFA model can be a valuable tool to evaluate the results of reflection measurements. Furthermore, the DFA model could be useful for estimating the sound absorptive performance at lower frequencies of a noise barrier limited to its spatial extent in width and height, i.e., when it is physically impossible to measure. Further studies are necessary though, since the conclusions of this paper are based on only one kind of absorber.

A miniaturized sensing system for liquid level and concentration based on piezoelectric micromachined ultrasonic transducers

This paper presents a system utilizing scandium-doped aluminum nitride (ScAlN) piezoelectric micromachined ultrasonic transducers (PMUTs) for simultaneous detection of liquid level and concentration. The PMUTs comprise a grid of 13×13 square units, covering an area of 2.8×2.8 mm2 in total. The PMUTs exhibit a resonant frequency of approximately 1 MHz in water and a beam angle of 30.16°, providing excellent sound field directivity. Additionally, a custom package housing with a cantilever baffle is designed to measure sound velocity in real time without modifying the container's structure. Finally, the fabricated PMUTs transceiver is combined with the TDC1000 ultrasonic sensing analog front-end to produce a high-precision liquid level and concentration detector. Glycerol and NaCl solutions are used as test samples. The system demonstrates measurement errors of ±0.5 % / ±1.1 % and ±0.5 mm / ±0.5 mm for concentration and liquid level, respectively, with resolutions of 0.323 % / 0.914 % and 0.077 mm / 0.070 mm. Compared with the related work results, the system based on PMUTs demonstrates advantages in miniaturization and high precision, making it particularly suitable for small-volume fluid containers, where sensor size and detection precision are crucial.

Direct sound field estimation based on sound source modeling with sparse equivalent sources

The measurement of multi-point room impulse responses (RIRs) is challenging because it requires many microphones or a microphone traverse system. In our previous study, we proposed a spatial extrapolation method to measure early RIRs by the locally-positioned microphones. However, the estimation accuracy was degraded at a far distance from the microphones owing to the directivity of the sound source. In this study, we proposed a modeling method to estimate a direct sound field using a source radiation model based on the sparse equivalent source method. First, the source radiation was modeled using an enclosed microphone array. Then, we modeled the direct sound field using locally positioned microphones. The evaluation experiment was conducted in an anechoic chamber under 2.5D condition.

Application of sound absorption measurements according to EN 1793-5 on gabion walls

The standard EN 1793-5 describes a method to determine the sound reflection index of road traffic noise reducing devices under direct sound field conditions in-situ. The method is commonly used to assess the intrinsic characteristics of conventional, absorptive noise barriers and yields results with a limited measurement uncertainty. Recently, the sound reflection index of a gabion wall was determined by means of the in-situ measurement method leading to unexpected results. An in-depth analysis was made of the results of the in-situ measurements on a conventional noise barrier and on a gabion wall. The main conclusion of this comparison is that application of the in-situ method on a gabion wall has a much higher measurement uncertainty than to be expected.

Characterizing noise barriers: SOPRANOISE final report and outcomes

SOPRANOISE targets a three-step approach for assessing the in-situ intrinsic acoustic performance of road/railway noise barriers in regard to airborne sound insulation and sound reflection under direct sound field conditions. This approach goes from the in-situ inspections method (targeting at the acoustic effect in the far-field) to the standardized measuring methods EN 1793-5 and 6, with "quick and safe" measuring methods in between. Papers about SOPRANOISE have already been presented in 2020 and 2021 as progress reports. The research ended in February 2022 and the final outcomes are now presented. The main part of the pending work has been done within Work Package 4 (WP4). Herein, a new "quick and safe" method was designed and validated both in the lab and in situ. The new measuring equipment is portable, lightweight and uses a linear antenna; the measurement itself is fully automated and the results show a very good correlation with full EN 1793-5/-6 results. WP4 results are particularly successful and, together with the in-situ inspection, those methods are ready to be submitted to the relevant CEN/TC226/WG6 working group to draft a new framework on noise barriers assessment. All main reports are public available on the SOPRANOISE and CEDR webpages.

Thinned array ultrasonic imaging of debonding defects in discontinuous impedance bonded structures

Ballastless track is a typical bonding structure with discontinuous impedance. Under the influence of heavy load, deteriorating environment and other situations, the circumstance of debonding occurs frequently and seriously affects the safety of train operation. In this work, a high-precision array ultrasound fast imaging method is proposed. Based on the propagation path and time of sound wave in the medium obtained by ray tracing method, a theoretical model of sound velocity of discontinuous impedance bonding structure is established when the difference in sound velocity among dielectric layers is taken into account. High degree of freedom sparse array based on real number coding as well as synthetic aperture focusing technology (SAFT) imaging method is used to improve detection efficiency. The results of the experiments carried out on ballast less track structure are shown below. The ray tracing method can accurately calculate the propagation path and propagation time of ultrasonic wave and improve the detection accuracy. The optimized thinned array pattern has narrowed the main lobe width and low side lobe gain, which improves the detection efficiency and sound field directivity. The efficiency of thinned array SAFT imaging method is improved by 30.9% when the imaging error of debonding defects is within ±5%, which provides a theoretical support fordetecting such debonding defects.

Research on ultrasonic sparse DC-TFM imaging method of rail defects

Rail damage is usually the main factor that affects the safe operation of trains, which may even lead to catastrophic accidents, such as train derailment and capsizing. It’s important to accurately detect the internal damage of the rail in real time. Ultrasonic testing is a main technology for the detection of rail damage in China, but there are some problems of misdetection and omission due to irregular rail shape, poor coupling, difficulty in automatic alignment, and inconvenient adjustment of the incident angle of the ultrasonic wafer, which results in a damage review rate of rail-defect detection lower than 78% in China. In this study, the phased array total focus method (TFM) imaging was applied to rail-damage detection, and the directivity correction-based sparse TFM imaging method (DC-TFM) was proposed to solve the problems of the existing phased array TFM imaging methods, such as inaccuracy and long computing time. The directivity function of array elements was used to correct the sound field intensity of different pixels to improve the accuracy of the TFM imaging. The experimental results show that the accuracy of the TFM imaging of defects is improved by 28% with correction of the sound field directivity. A sparse array designed based on the particle swarm optimization (PSO) algorithm was used to replace the full matrix data for the DC-TFM imaging, ensuring the imaging quality and reducing the imaging computing time. When the sparse rate is 75%, the array performance indicator (API) and the signal-to-noise ratio (SNR) are reduced only slightly, but the imaging computing time is shortened substantially by more than 40%, which indicates that the proposed sparse DC-TFM method can effectively improve the speed of rail-damage detection.

Selectable single-mode guided waves for multi-type damages localization of plate-like structures using film comb transducers

ABSTRACT The dispersion and multimode natures of guided waves bring forth difficulties in signal analysis, limiting the application of traditional transducers. In this paper, we report the design, fabrication and characterisation of transducers composed of a polyvinylidene fluoride (PVDF) film with comb electrodes that can excite selectable single-mode guided waves for high-precision damage diagnosis and localization. The frequency response of the PVDF comb transducers was analysed with respect to the geometric parameters of the comb electrodes, and the sound-field directivity was determined by measuring the in-plane and out-of-plane displacements of the far field around the transducers by means of a double-wave mixed interferometer. In experimental measurements, a surface damage was located using such a PVDF comb transducer to excite single A0 and S2 mode guided waves. In addition, quarter omnidirectional PVDF comb transducers were fabricated and deployed on the corners of a square 7075 aluminium alloy plate for locating multi-type damages. It is found that the proposed quarter omnidirectional PVDF comb transducers are capable of locating multi-type damages with high accuracy. The PVDF comb transducers has the capability of the guided waves mode selection and flexibility, which is expected to achieve high-precision, long-time and online multi-type damages detection and monitoring.

Effects of periodic suction-blowing excitation on the aerodynamic sound generated by a laminar flow past a square cylinder using the direct numerical simulation approach

This paper analyzes the effects of a periodic suction-blowing excitation on the aerodynamic sound generated by a laminar flow past a square cylinder using the direct numerical simulation approach. The periodic suction-blowing excitation has been prescribed on the top and bottom surfaces of the square cylinder. The proper orthogonal mode decomposition (POD) technique has been used to find information about important modes associated with disturbance pressure fields. The POD technique separated the contribution of the dominant lift dipole equivalent sources and the drag dipole equivalent sources to the disturbance pressure field for the no-excitation case. The POD technique also revealed that the periodic suction-blowing excitation introduced an additional monopole equivalent sound source and a drag dipole equivalent sound source due to periodic enhancement and reduction of the body’s effective cross-sectional area. Modifications in the sound field due to changes in excitation amplitude, forcing frequency, and the phase delay between the excitation and vortex shedding process have been studied in detail. Although no significant changes in the flow field were noticed due to a small amplitude of excitation, the directivity of the sound field was significantly altered. The sound fields have been classified into five distinct zones for different periodic suction-blowing excitation frequencies. The beats of sounds were noted when the forcing frequency of excitation and the Strouhal frequency associated with vortex shedding were sufficiently close. It is observed that the in-phase excitation in which either blowing or suction is applied on both surfaces of a cylinder at a particular instant introduces a significant bias in the sound field directivity. The interaction between the lift dipole equivalent sources due to vortex shedding and the monopole and the drag dipole equivalent sources due to excitation introduces a bias in the sound field directivity. As a result, a dominant sound field is observed either in the top-left or in the bottom-left parts of the domain.

Possibilities of Evaluating the Effectiveness of Noise Barriers in Slovakia

This paper presents environmental acute problems from increasing noise levels caused by automobile and rail transport. Noise barriers are considered to be an effective element in reducing noise in densely populated urban areas. However, do these barriers have the correct height, shape, material design, and construction? In the materials and methods, the basic characteristics of noise barrier, sound absorption, and sound insulation are described. Further, measurement methods of the insertion loss of outdoor noise barriers of all types using the direct and indirect method according to standard ISO 10847 and the Adrienne method according to standards EN 1793-5 and EN 1793-6 are presented. The measurement results of insertion loss of the selected noise barrier obtained by the indirect method showed a value of 19.1 dB(A) of insertion loss. This result was compared with simulation by our own software based on the CNOSSOS methodology, giving value of 19.6 dB(A), which is a good correlation. The Adrienne method was used for determination of the airborne sound insulation index of another noise barrier, giving a value of the single-number rating of the sound insulation of the barrier of 28 dB. In discussion, according to EN 1793-2, this value assigns an insulation index of category D3, which means that the barrier is high quality and has satisfactory airborne sound insulation. The advantages and disadvantages of both methods according to STN ISO 10847 and STN EN 1793-6 were discussed. We concluded that the main advantage of the method Adrienne—TN EN 1793-6 is that it can be used in situ under direct sound field conditions and can directly evaluate the sound insulation index of the whole barrier structure.

Sparse Array Ultrasonic Lamb Wave TDTE Imaging Method

The current Lamb wave topology imaging methods are limited to the ideal model of through-hole defects, ignoring the influence of mode coupling. However, blind hole defects, such as structural plate corrosion and cracks, are the most likely defects in practical applications, and Lamb waves are bound to appear mode-coupled at these defects. The use of the full matrix data calculations of the direct and concomitant sound fields is time-consuming and limits the industrial application of the ultrasonic Lamb wave time-domain topological energy (TDTE) imaging method. A TDTE imaging method of a sparse ultrasonic Lamb wave is proposed in this paper to solve the aforementioned two problems. The sparse array is designed based on a genetic algorithm, then the direct sound field and the accompanying sound field are calculated using the sparse array data. The mode conversion of the Lamb wave is considered when calculating the accompanying sound field, and then the accompanying sound field is reversed in time so that the direct sound field and the accompanying sound field are adaptively focused at the defect. The influence of Lamb wave mode conversion and dispersion on the imaging results is overcome. Finally, the TDTE values of the direct sound field and the accompanying sound field are used as the imaging pixel values to improve the imaging accuracy. The experimental results show that for asymmetric blind hole defects, the TDTE imaging method is more accurate than the total focus (TFM) imaging method; the sparse array designed by the genetic algorithm has good sidelobe characteristics and can effectively reduce sidelobe noise assuming the array aperture is unchanged. The TDTE imaging results based on the sparse array data show that the imaging efficiency is greatly improved compared with the full array when the receiving array is four elements under the condition of ensuring the imaging quality.

SOPRANOISE - update and analysis of noise barrier database including new current results

In the frame of the SOPRANOISE project (funded by CEDR in the Transnational Road Research Programme 2018) work package 2 focused first on providing theoretical and practical background information on measurement of the acoustic performance of noise barriers due to a state-of-the-art regarding correlations and possible trends between diffuse (EN 1793-1, EN 1793-2) and direct sound field methods (EN 1793-5, EN 1793-6). After that, the objective of this research was to extend and update the database of the European noise barrier market developed during the QUIESST project, including more detailed analyses on single-number ratings as well as third-octave band measurement results. The data collected and the analysis performed show relevant facts and figures about acoustic performances of noise barriers measured under diffuse and direct sound field conditions, together with a better understanding of the respective significance, similarities and differences of these standardized methods, improving data analysis and correlations between these methods. This paper gives a general overview of the data collected, summarising the main results of the statistical analyses performed. Overall results and comparisons between results of measurements performed under diffuse and under direct sound field conditions are shown. Finally, conclusions and possible outlook of the research are presented.

Empirical study on the correlation between measurement methods under diffuse and direct sound field conditions for determining sound absorption and airborne sound insulation properties of noise barriers

In the frame of the SOPRANOISE project (funded by CEDR in the Transnational Road Research Programme 2018) the database of the European noise barrier market developed during the QUIESST project was updated with newly acquired data. This database gives the opportunity for an empirical study on the correlation between the different measurement methods for the acoustic properties of noise barriers (according to the EN 1793 series) to further investigate the interrelationships between these methods by using single-number ratings and third-octave band data. First a correlation of the measurement methods for sound absorption under diffuse field conditions (EN 1793-1) and sound reflection under direct sound field conditions (EN 1793-5) is presented. Secondly, a correlation of the measurement methods for airborne sound insulation under diffuse field conditions (EN 1793-2) and airborne sound insulation under direct sound field conditions (EN 1793-6) is shown. While for airborne sound insulation a distinct correlation is found due to the wide data range, for sound absorption no robust correlation can be found.

Directional sound field decay analysis in coupled spaces

Multi-exponential energy decays—showing a distinct curvature in the measured decay curve—are a well known phenomenon in coupled spaces if both spaces exhibit different reverberation times due to their specific absorption characteristics or volumes. Recently, Berzborn et al. (2019) investigated directional energy decay curves—calculated from a plane wave decomposition of the sound field—as an analysis framework for the angular distribution of energy during the decay process in reverberation rooms. In this contribution, we investigate the directional characteristics of the energy decay in two rooms connected by a door. Spherical microphone array measurements at two different receiver positions and coupling areas were performed. Results indicate a distinct anisotropic energy decay in the room with the shorter reverberation time, showing a decreased isotropy in the late decay process compared to the early part.

Directional sound field decay analysis in performance spaces

The analysis of the spatio-temporal features of sound fields is of great interest in the field of room acoustics, as they inevitably contribute to a listeners impression of the room. The perceived spaciousness is linked to lateral sound incidence during the early and late part of the impulse response which largely depends on the geometry of the room. In complex geometries, particularly in rooms with reverberation reservoirs or coupled spaces, the reverberation process might show distinct spatio-temporal characteristics. In the present study, we apply the analysis of directional energy decay curves based on the decomposition of the sound field into a plane wave basis, previously proposed for reverberation room characterization, to general purpose performance spaces. A simulation study of a concert hall and two churches is presented uncovering anisotropic sound field decays in two cases and highlighting implications for the resulting temporal evolution of the sound field diffuseness.

Investigation of a directional warning sound system for electric vehicles based on structural vibration.

Warning sound systems for electric vehicles with advanced beamforming capabilities have been investigated in the past. Despite showing promising performance, such technologies have yet to be adopted by the industry, as implementation costs are generally too high and the components too fragile for implementation. A lower cost solution with higher durability could be achieved by using an array of inertial actuators instead of loudspeakers. These actuators can be attached directly to the body of the vehicle and thus require minimal design modifications. A directional sound field can then be radiated by controlling the vibration of the panel via adjustments to the relative magnitude and phase of the signals driving the array. This paper presents an experimental investigation of an inertial actuator-based warning sound system. A vehicle placed in a semi-anechoic environment is used to investigate different array configurations in terms of the resulting sound field directivity and the leakage of sound into the cabin. Results indicate that the most efficient configuration investigated has the actuators attached to the front bumper of the vehicle. Using this arrangement, real-time measurements for different beamformer settings are performed to obtain a thorough picture of the performance of the system across frequency and steering angle.

Sound field separation method for motorcycle coherent sound sources

To solve the problem of main noise sources of motorcycles running at high speed, especially restoring the free sound field information of the target sound source in the non-free sound field, direct separation method with single holographic surface is proposed. According to the transfer function relationship between the theoretical and the measured sound pressure on the holographic surface based on the wave superposition algorithm, the individual radiation information of the target sound source in the coherent sound field is obtained. Through numerical simulation, it is found that the method could effectively separate the coherent sources. The radiation sound of motorcycles engine speed at 6000 r/min is measured, and the results show that the direct sound field separation method can effectively separate the coherent sound sources and intake noise radiate the higher pressure amplitude.

Calculation on the radiation noise of ceramic ball bearings based on dynamic model considering nonlinear contact stiffness and damping

Radiation noise (RN) is an important parameter for representing the characteristics of a bearing. By simplifying a ball bearing as a mass–spring–damping system, the differential equations for the vibration of an angular contact ceramic ball bearing (ACCBB) are established from the view of energy conservation considering nonlinear contact stiffness and damping. Then, the RN model of the bearing is constructed by Helmholtz's acoustic theory. The bearing noise is calculated using the proposed model, and the corresponding experiments are performed to confirm the model precision. The bearing noise consists of the noise at the characteristic frequency of each component; the rotational frequency is the primary contributor to the bearing noise. The radiation noise first increases with increasing rotational speed and the sound pressure level (SPL) has a high peak value at a critical speed of 28,000 r·min−1. Then, the noise decreases with increasing rotational speed. The directivity of sound field is discussed at different rotational speed. With increasing the rotational speed, the directivity level (DL) increases and the directivity angle (DA) skews towards the rotation direction. The research provides a theoretical basis to analyze the RN generated by ACCBBs at different rotational speeds.