Sound Reflection Research Articles

Review: Echo’s Chambers: Architecture and the Idea of Acoustic Space

Review: <i>Echo’s Chambers: Architecture and the Idea of Acoustic Space</i>

Investigation of Acoustic Efficiency of Wood Charcoal in Impedance Tube for Usage in Sound-Reflective Devices

Charcoal is an environmentally friendly, biodegradable, and economical material. This material is usually produced by slow pyrolysis—the heating of wood or other substances in the absence of oxygen. The aim of this study was to investigate the acoustic efficiency of charcoal and design an acoustic diffuser that utilizes charcoal. Samples of different types of tree charcoal—birch (Betula pendula), pine (Pinus sylvestris), and oak (Quercus robur)—with different thicknesses were used for the acoustic efficiency measurements. The sound absorption and sound reflection properties of charcoal were investigated. The bulk density of charcoal was measured. In this study, an impedance tube with two microphones was employed as the measurement method. The results of the impedance tube measurements showed that the charcoal samples had high sound reflection coefficients, the highest value of which was 1. The 50 mm samples of birch had a high bulk density of 473 kg/m3. The sample of 50 mm thick oak had the best reflection coefficient at 0.99. Reflection depended on the surface’s acoustic properties, and the sound reflection coefficient increased with the increase in the density. Charcoal measurements, due to the high reflection coefficient of the material, were used for the design of a sound diffuser, which included wooden perforated plates filled with cylindrical elements of wood charcoal.

Effect of The Volume of Banana Fiber As A Polymer Composite Amplifier With Polyester Resin Matrix On The Sound Absorption of Acoustic Materials

Research has been carried out on the banana fiber effect as a polymer composite amplifier with polyester resin matrix on the sound absorption of acoustic materials. Sound Absorber material is made with a variation in the volume of banana fiber that is different from polyester resin. The volume of banana fiber and polyester resin used is 15% Fiber: 85% Polyester Resin, 20% Fiber: 80% Polyester Resin, 25% Fiber: 25% Fiber: 75% Resin Polyester, 30% Fiber: 70% Polyester Resin, 35% Fiber: 65% Polyester Resin. So when made composite then the amount of banana fiber used and polyester resin is 100%. Research methods are carried out in the collection of banana fiber, composite manufacturing and testing of sound absorption properties using impedance tubes. The testing of coefficient sound reflection and also sound absorbance coefficient testing is done using sound level meter and signal generator by reference to principle the method of tube impedance via approach to the acoustic box. The banana fiber composite result and polyester resin are able to reflect the sound of highest which is 0.44 in a frequency 500 Hz with fiber 15%. While lowest sound reflection coefficient is 0.06 at a frequency of 8000 Hz and fiber 35%. While in the test the highest value absorbance coefficient that absorbs sound is = 0.94 at fiber volume 35%, and frequency 8000 Hz, while the lowest sound coefficient is 0.56 in fiber 15% and frequency 500 HZ

Thermally Induced Optical Reflection of Sound (THORS) in Ambient Air: Characterization and Temporal Dynamics

Thermally induced optical reflection of sound (THORS) provides a means to manipulate sound waves without the need for traditional acoustically engineered structures. By photothermally exciting a medium, with infrared light, a barrier can be formed due to abrupt changes in compressibility of the excited medium. Discovery and initial characterization of the THORS phenomenon utilized air saturated with ethanol vapor as the absorbing medium and a CO2 laser, operating at 9.6 µm, as the excitation source to achieve acoustic reflection efficiencies of 25-30% of the incident wave. In this work, we demonstrate for the first time, the ability to generate THORS barriers in ambient air (i.e., without the need for ethanol vapor). Employing atmospheric water vapor as the absorbing medium and a modulated, multiline carbon monoxide laser, operating at 5.5 ± 0.25 µm, THORS barriers capable of acoustic and ultrasonic reflection-suppression efficiencies greater than 70% are readily generated. To achieve these significant reflection-suppression efficiencies, the temporal dynamics of THORS barriers in ambient air were characterized using 300 kHz ultrasonic pulses incident on the barriers, revealing three different operational regimes. In the first regime, a single laser pulse generates a transient THORS barrier that lasts tens of milliseconds and exhibits minimal acoustic reflectivity. In the second regime, multiple laser pulses interact with the water vapor prior to complete relaxation of the THORS barrier from the previous excitation pulse, resulting in an additive response and reflectivity/suppression efficiencies as great as 72%. Finally, in the third regime, non-modulated continuous wave (CW) excitation of the water vapor occurs resulting in no measurable acoustic reflectivity/suppression from the THORS barrier. This work characterizes these different regimes and the optimal modulation timing to generate efficient continuous acoustic barriers using THORS.

The acoustic streaming effects of sonic black hole

In this paper, the acoustic streaming effects of Sonic Black Hole (SBH) are deeply studied by using the numerical and simulation method. By solving Navier–Stokes equations of compressible fluid, we analyze the flow characteristics of acoustic medium in SBH excited by sound wave, and further discuss the acoustic streaming effects and sound wave capture mechanism of SBH. The phenomenon of simultaneous reduction of medium and high-frequency sound reflection and transmission is analyzed. Under the excitation of medium- and high-frequency sound waves, the interaction between fluid medium flow and SBH structure leads to the uneven internal velocity distribution in the sound propagation direction, which can lead to the phenomenon of sound wave capture and deceleration. For low-frequency sound waves, the velocity distribution of sound medium is uniform, and there is almost no change in velocity gradient, resulting in the uniform distribution of sound pressure without sound absorption. These theoretical and numerical results are in good agreement with the experimental results in the literature, and also verify our results. Finally, by improving the structure, increasing the complexity of sound medium flow and the gradient change in the direction of sound propagation, the sound absorption and insulation ability of the SBH can be further improved. This study reveals the sound transmission mechanism of SBH, which can provide a new idea for the suppression of low-frequency sound waves in SBH.

Mitigation assessment of complex noise sources - an acoustical camera approach

A complex noise source is a set of multiple noise sources operating in proximity. Segregation of individual sub-sources based on sound characteristics and loudness is often the goal but can be challenging. This case study presents one such complex noise source, a high-speed offset printer in a large-scale printing facility. This printer is an assembly of several modules spanning approximately 40 feet in length. The sound field in the printer area was complex because of many mechanical components propagating sound through air (air-borne), through equipment casing (structure-borne), and acoustical reflections from nearby surfaces. Localization of dominant sources is required to adequately design mitigation. To identify sub-sources a sophisticated acoustical camera was used for this study. An acoustical camera (SoundCam) is an advanced acoustical measurement device with an array of 64 microphones and an optical camera. The acoustic signals from the microphones are processed and visualized as sound levels contours in real-time as an optical image or video. Visualization of printer sound levels helped to identify dominant sub-sources and areas of sound reflection. Once the dominant sub-sources were identified, the sound levels were measured. This helped in developing targeted noise controls in comparatively short duration for a complex noise source.

Effective roughness on the sea surface for determining variability characteristics of reflected sound waves

In sound wave propagation in the sea, it is important to evaluate the characteristics of sound reflection from the sea surface. The amplitude and phase of reflected sound waves fluctuate because of the changing sea surface with waves. In this study, using the finite difference time domain method and experiments in a water tank, we evaluated the variability characteristics of reflected sound waves on the sea surface generated by the Bretschneider-Mitsuyasu spectrum observed on the deep offshore coast of Japan. We introduced the concept of effective roughness of the sea surface and evaluated the variability characteristics of reflected sound waves on the sea surface. We confirmed the Rice distribution could express the amplitude variability, and the energy ratio γ is determined by the Rayleigh roughness parameter defined by the effective roughness

Broadband acoustic meta-porous layer for reflected wave manipulation and absorption

Acoustic metasurfaces (AMs) used for reflected sound wavefront manipulation are generally designed based on the generalized Snell’s law (GSL) at a single frequency, suffering from inferior broadband properties. Herein, an acoustic meta-porous layer (AMPL) with periodic structures is designed to realize reflected wavefront manipulation and effective sound absorption over a wide frequency band. The AMPL is constructed by four periodically arranged units, each consisting of porous elements inserted by acoustically rigid partitions, forming a linear reflected phase-shifting within 0 to 2π maintaining in a target frequency range of [1000,3000]Hz. To predict the reflected response of the element of the AMPL, an analytical model is proposed based on the transfer matrix method and is numerically validated. The sound reflection and absorption performance of the AMPL is investigated numerically and experimentally. The scattering sound pressure fields under normally and obliquely incident sound waves at different frequencies demonstrate the broadband reflected wavefront manipulation capability, including negative reflection and surface wave conversion. Compared to a uniform porous foam, an effective absorption is achieved by the AMPL due to the surface wave excitation at an identical thickness of 40 mm, with an averaged absorption coefficient greater than 0.9 in [800,3000]Hz. The results suggest the proposed structure could be a promising candidate of broadband noise absorption metamaterial for practical applications.

A polytrihedral dome for acoustic retroreflection, and its application to creative-arts practice-led research

This paper examines acoustic retroreflection from a polytrihedral dome that was used in creative-arts practice-led research as a unique, high voice support singing practice space. The polytrihedral dome is investigated with laboratory measurements, field recordings of singing practice, and by finite-difference time-domain (FDTD) simulation of sound reflections within the polytrihedral dome and other kinds of domes for comparison. Measurements and FDTD simulations show that the polytrihedral dome is retroreflective, especially in the 2 kHz octave band and higher, and provides high levels of voice support. Oral-binaural measurements of voice support exhibit a 4 dB/octave spectral slope from the 500 Hz to the 4 kHz octave bands. The power spectrum of singing is measured and used to weight voice support, yielding measured mean values of + 3.5 dB (maximum + 6 dB). This value is substantial partly because the on-axis singing energy is greatest in the 2 kHz octave band where retroreflection is particularly evident. The polytrihedral dome was installed at a remote acoustically pristine location for sustained use in singing practice. In-situ field measurements of singing practice show reduced voice levels (room effect) from the dome’s voice support, contrasted with increased voice levels (Lombard effect) at a second practice setting, a waterfall. Average voice output between the two environments as measured by a contact microphone differ by 9 dB while the level at the ears remains approximately the same. Using these approaches to investigate reflections in the polytrihedral dome, this paper documents retroreflective acoustic design that, on a larger scale, might ameliorate sound focussing problems in domed room designs.

Predicting reflection patterns from binaural activity maps using deep neural networks

A new model architecture is presented to predict room acoustical parameters from a running binaural signal. For this purpose, a deep neural network architecture is combined with a precedence effect model to extract the spatial and temporal locations of the direct signal and early reflections. The precedence effect model builds on the modified BICAM algorithm [Braasch, J. Acoust. Soc. Am. 140, EL143], for which the 1st layer auto-/cross correlation functions are replaced with a Cepstrum method. The latter allows a better separation of features relating to the source signal's early reflections and harmonic structure. The precedence effect model is used to create binaural activity maps that are analyzed by the neural network for pattern recognition. Anechoic orchestral recordings were reverberated by adding four early reflections and late reverberation to test the model. Head-related transfer functions were used to spatialize direct sound and early reflections. The model can identify the main reflection characteristics of a room, offering applications in numerous fields, including room acoustical assessment, acoustical analysis for virtual-reality applications, and modeling of human perception. [Work supported by the National Science Foundation under Grant No. IIS-1909229.]

Low-Frequency Low-Reflection Bidirectional Sound Insulation Tunnel with Ultrathin Lossy Metasurfaces

We report both numerical and experimental constructions of a tunnel structure with low-frequency low-reflection bidirectional sound insulation (BSI). The designed tunnel was constructed from a pair of lossy acoustic metasurfaces (AMs), which consists of six ultrathin coiled unit cells, attached on both sides. Based on the generalized Snell′s law and phase modulations for both AMs, the tunnel with the low-frequency BSI was constructed based on sound reflections and acoustic blind areas created by the AMs. The obtained transmittances were almost the same for sound incidences from both sides and were lower than −10 dB in the range 337–356 Hz. The simulated and measured results agreed well with each other. Additionally, we show that the low-reflection characteristic of the tunnel can be obtained simultaneously by thermoviscous energy loss in coiled channels of the unit cells. Finally, an interesting application of the designed tunnel in an open-window structure with low-frequency low-reflection BSI is further simulated in detail. The proposed tunnel based on the ultrathin lossy AMs has the advantages of ultrathin thickness (about λ/35), low-frequency low-reflection BSI, and high-performance ventilation, which may have potential applications in architectural acoustics and noise control.

Effect of 3D-Printed PLA Structure on Sound Reflection Properties.

3D printing technique is currently one of the promising emerging technologies. It is used in many areas of human activity, including acoustic applications. This paper focuses on studying the sound reflection behavior of four different types of 3D-printed open-porous polylactic acid (PLA) material structures, namely cartesian, octagonal, rhomboid and starlit structures. Sound reflection properties were evaluated by means of the normal incidence sound reflection coefficient based on the transfer function method using an acoustic impedance tube. In this study, various factors affecting the sound reflection performance of the investigated PLA samples were evaluated. It can be concluded that the sound reflection behavior of the tested PLA specimens was strongly affected by different factors. It was influenced, not only by the type of 3D-printed open-porous material structure, but also by the excitation frequency, the total volume porosity, the specimen thickness, and the air gap size behind the tested specimen inside the acoustic impedance tube.

Characterisation of a Standard Fan Test Facility to Accommodate Reflection of Sound During Noise Measurements

Axial flow fans are fundamental to air-cooled condensers. While they provide the air flow for heat transfer, they also generate noise, which is a growing environmental concern. Sound reflections are characterised in an existing fan performance test facility, which does not conform to fan noise measurement standards. Corrections to the measured SPL spectrum is deduced along with the uncertainty in noise measurements. Corrections are compared to a reflection correction standard relating to buildings. This methodology can be repeated for other fan test facilities which do not comply with noise measurement standards.

Reconstruction of half-space boundary impedance and sound source direct radiation based on reflection coefficient estimation

When implementing the reconstruction of the sound field directly radiated from a source located in a half-space, the half-space basis functions with boundary impedance as a parameter, need to be formulated. And the boundary impedance is usually obtained via &lt;i&gt;in situ&lt;/i&gt; sound impedance measurement techniques. In the reconstruction method based on the expansion in half-space spherical wave basis functions, a hologram surface and a single reference microphone placed in the near-field are used to collect sound pressures. The sound pressure at the reference microphone is first reconstructed and the error of the reconstructed pressure relative to the measured pressure is then calculated. The sound pressure reflection coefficient corresponding to the minimum error is chosen as the measure of the reflection coefficient at the measurement point. Thus, this method is applicable to reconstructing the directly radiated sound pressures without the knowledge of boundary impedance, free from the in situ sound impedance measurement techniques necessary for conventional methods. The purpose of this work is to discuss the various parameters affecting the accuracy of reconstruction. Moreover, the boundary impedance is reconstructed based on the estimation of the reflection coefficient, so that a sound impedance measurement technique implemented via the near-field acoustical holography is proposed. By taking the spherical source for example, numerical simulations are conducted to verify the proposed method of reconstructing the boundary impedance and the directly radiated sound pressures. The influences of reference microphone coordinate, the effective flow resistivity of the boundary, and the rate of decrease of porosity with depth of the boundary on the accuracy of reconstruction are quantitatively analyzed.

Reconstruction of half-space boundary impedance and sound source direct radiation based on reflection coefficient estimation

When implementing the reconstruction of the sound field radiated directly from a source located in a half-space, the half-space basis functions need to be formulated with boundary impedance as a parameter. The boundary impedance is usually obtained via &lt;i&gt;in situ&lt;/i&gt; acoustic impedance measurement techniques. In a reconstruction method based on expansion in half-space spherical wave basis functions, a hologram surface and a single reference microphone placed in the near-field are used to collect sound pressures. The sound pressure at the reference microphone is first reconstructed and the error of the reconstructed pressure relative to the measured pressure is then calculated. The sound pressure reflection coefficient corresponding to the minimum error is chosen as the estimated value of the reflection coefficient at each of the measurement points. Thus, this method is applicable to reconstructing the directly radiated sound pressures without knowledge of the boundary impedance, without the &lt;i&gt;in situ&lt;/i&gt; acoustic impedance measurements necessary for conventional methods. The purpose of this work is to discuss the various parameters affecting the accuracy of reconstruction. Moreover, the boundary impedance is reconstructed based on the estimation of the reflection coefficient. In this way, an acoustic impedance measurement technique implemented via the near-field acoustical holography is proposed. Taking the source to be spherical, numerical simulations are conducted to verify the proposed method of reconstructing the boundary impedance and the directly radiated sound pressures. The influences of reference microphone coordinates, the effective flow resistivity of the boundary, and the rate of decrease of porosity with depth of the boundary on the accuracy of reconstruction are quantitatively analyzed.

Modeling the inhomogeneous anisotropic coating of an elastic cylinder that provides minimal sound reflection

Modeling the inhomogeneous anisotropic coating of an elastic cylinder that provides minimal sound reflection

Schools Traffic Noise Pollution Levels Along Federal Roads in Muallim District, Perak, Malaysia

This article aims to identify the school traffic noise level along federal roads in Muallim District, Perak, namely SMK Khir Johari (SMKKJ), SMK Agama Slim River (SMKASR) and SMK Dato’ Zulkifli Muhammad (SMKDZ). Data were collected during school hours and on weekends, from 7 September to 13 September 2019 and three times a day, 7.00 -8.00 am, 9.00 -10.00 am and 12.00 -1.00 pm. Data was collected using Digital Sound Level Meter AS804, which was mounted on a tripod approximately one meter high from the road surface to prevent road sound reflection. Sound recordings were performed using decibel scale weights (dBA) and values for Max (Maximum Noise Level), Min (Minimum Noise Level), Leq (Average Overall Noise) with five minutes time delay and descriptive analysis. The findings show all stations values exceeded the standards set by the Malaysia Department of Environment (DOE) (60 dBA). SMKKJ shows the Min value is 57.7 dBA, the Max value is 94.5 dBA and Leq 76.1 dBA during working hours. In conclusion, the study showed that all stations recorded high noise during working hours and non-working hours surpass the standard set by DOE during the day. The need for the Malaysian Ministry of Education to assess and plan to reduce this noise level through short-term and long-term measures.

Low-Frequency, Open, Sound-Insulation Barrier by Two Oppositely Oriented Helmholtz Resonators.

In this work, a low-frequency, open, sound-insulation barrier, composed of a single layer of periodic subwavelength units (with a thickness of λ/28), is demonstrated both numerically and experimentally. Each unit was constructed using two identical, oppositely oriented Helmholtz resonators, which were composed of a central square cavity surrounded by a coiled channel. In the design of the open barrier, the distance between two adjacent units was twice the width of the unit, showing high-performance ventilation, and low-frequency sound insulation. A minimum transmittance of 0.06 could be observed around 121.5 Hz, which arose from both sound reflections and absorptions, created by the coupling of symmetric and asymmetric eigenmodes of the unit, and the absorbed sound energy propagating into the central cavity was greatly reduced by the viscous loss in the channel. Additionally, by introducing a multilayer open barrier, a broadband sound insulation was obtained, and the fractional bandwidth could reach approximately 0.19 with four layers. Finally, the application of the multilayer open barrier in designing a ventilated room was further discussed, and the results presented an omnidirectional, broadband, sound-insulation effect. The proposed open, sound-insulation barrier with the advantages of ultrathin thickness; omnidirectional, low-frequency sound insulation; broad bandwidth; and high-performance ventilation has great potential in architectural acoustics and noise control.

On the development of a virtual test bench to assess sound absorption of materials in an alpha chamber

A description is given of a virtual test-bench (VTB) designed by engineering center of Peter the Great St. Petersburg Polytechnic University to calculate a sound absorption coefficient of various materials. Developed VTB differs from known programs in that it allows a sound absorption coefficient of various materials to be determined with minimum involvement of an engineer. This VTB differs from other programs also by using infinite elements jointly with finite elements, which increases adequacy of the discrete model being used, and also the configuration of the boundary of an alpha chamber being used. The known programs use various phenomenological mathematical models of porous materials such as Johnson-Champoux-Allard (JCA) model. The VTB is based on fundamental mathematical models and statistical energy analysis (SEA) that allow describing adequately the established or transitional processes of sound absorption and reflection by a porous material the properties of which are not homogenized. The value of this VTB, which is created on the basis of a VA One software complex supplemented by a set of files of boundary conditions, files of solvers’ settings, secondary finite element (FE) models, is that VTB allows standardized calculations to be performed to determine the sound absorption coefficient of a material with minimum involvement of an engineer and the obtained result to be submitted for detailed processing. Developed virtual test-bench enables determination of a sound absorption coefficient for various materials within the entire finite frequency range. The result of the calculation is displayed as a graph of dependency of the material sound absorption coefficient on frequency.

Controlling the Coefficient of Reflection of Sound from a Plane Piezoelectric Plate by Selecting its Electrical Load

Controlling the Coefficient of Reflection of Sound from a Plane Piezoelectric Plate by Selecting its Electrical Load