Incident Sound Pressure Level Research Articles

Enhancing the acoustic-to-electrical conversion efficiency of nanofibrous membrane-based triboelectric nanogenerators by nanocomposite composition

Acoustic energy is difficult to capture and utilise in general. The current work proposes a novel nanofibrous membrane-based (NFM) triboelectric nanogenerator (TENG) that can harvest acoustic energy from the environment. The device is ultra-thin, lightweight, and compact. The electrospun NFM used in the TENG contains three nanocomponents: polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), and multi-walled carbon nanotubes (MWCNTs). The optimal concentration ratio of the three nanocomponents has been identified for the first time, resulting in higher electric output than a single-component NFM TENG. For an incident sound pressure level of 116 dB at 200Hz, the optimised NFM TENG can output a maximum open-circuit voltage of over 120V and a short-circuit current of 30μA, corresponding to a maximum areal power density of 2.25W/m2. The specific power reached 259μW/g. The ability to power digital devices is illustrated by lighting up 62 light-emitting diodes in series and powering other devices. The findings may inspire the design of acoustic NFM TENGs comprising multiple nanocomponents, and show that the NFM TENG can promote the utilisation of acoustic energy for many applications, such as microelectronic devices and the Internet of Things.

Low frequency and broadband sound attenuation by meta-liner under grazing flow and high sound intensity

Acoustic liners are the most effective solution to attenuate the noise of ducts with flows but still suffer from narrow sound absorption bandwidth and heavy bulk. In this work, we present a new type of acoustic meta-liner structure, which is composed of perforated plates and coiled-up Fabry–Pérot (FP) channels with carefully designed equivalent length. By tuning the geometric parameters of the perforated plate and the optimal combination of the coiled-up FP channels, perfect impedance matching is achieved in a broadband range. The strong dissipation of sound energy could also be observed under different speeds of grazing flow and high incident sound intensity. It is analytically, numerically, and experimentally demonstrated that for the structure, over 90% sound absorption is achieved in the broadband range from 500 to 3000 Hz in the absence of flow and the condition of grazing flow with a speed of 30–98 m/s, coupled with a 90–130 dB incident sound pressure level. Moreover, the thickness of the proposed simply structured meta-liner is 44 mm, which is only 8/125th of the wavelength corresponding to 500 Hz. As a deep sub-wavelength liner, it exhibits potential application prospects in the field of fluid–solid coupled machinery such as aero-engine systems and ventilation duct systems.

Experimental investigations on the acoustical performance of open-celled metal foams under grazing flow

Open-celled metal foams offer a possible solution for implementing acoustic liners in non-traditional nacelle locations, such as directly over the turbofan rotor. Previously, we have shown that the normal incidence absorption performance of metal foams can be further improved by compressing them to reduce their effective pore size. In this paper, we focus on the acoustical performance of compressed metal foams under grazing flow conditions. Experimental tests were conducted using a grazing flow impedance tube set up within Spirit AeroSystems' acoustics lab. Metal foams with four different compression ratios were tested under various flow rates and incident sound pressure levels. The sound pressure measurements were then utilized to educe their effective impedance under grazing flow conditions. Our results show that a uniform through-thickness compression can improve the acoustic performance of open-celled metal foams under grazing flow conditions and can aid the further development of acoustic liners for over the rotor applications.

Combustion instability control performance of an improved Helmholtz resonator in the presence of bias flow

Combustion instability may appear due to the interference between heat release rate and acoustically-driven flow perturbation in combustors. In the present study, we experimentally studied the effects of incident sound frequency, cavity depth, and sound pressure level on the sound absorption performance of a Helmholtz resonator (HR) with an optimized neck under bias flow. The greater the bias flow rate, the wider the sound absorption frequency band. The insertion of the honeycomb with minor porosity into the neck caused the central sound absorption frequency to shift to the low-frequency regime. The optimal cavity depth of the HR was also shortened, which is beneficial to save space. Moreover, a proper bias flow increased the maximum sound absorption coefficient of the resonator. When the modified HR was installed in the intake section of a swirl spray combustor, the pulsating pressure in the combustion chamber and plenum were reduced by 78% and 56%, respectively. The CH⁎ chemiluminescence intensity was reduced by 96%. Compared with traditional HR, the improved HR suppressed the pressure and heat release pulsation more effectively. Even if the primary air volume flow rate changed from 4.5 m3/h to 9.5 m3/h, the improved HR ensured that the pulsating pressure amplitude was reduced by about 82% on average.

Effect of acoustically-induced elastic softening on sound absorption coefficient of hollow glass beads with inner closed cavities.

This study investigated the effects of sound wave magnitude on the sound absorption characteristics of granular materials. The normal incidence sound absorption coefficients of the hollow glass bead packings were measured with different incident sound pressure levels. The measurement results exhibited spectral peaks of sound absorption coefficients at several frequencies, implying that the first peak was caused by the resonance of the hollow glass beads as a layer. Although the first peak in the sound absorption coefficients did not vary when an incident sound pressure was low, the first peak shifted to a lower frequency when the sound pressure exceeded a certain magnitude. These results indicate that the hollow glass beads are softened at high incident sound pressures. Furthermore, by modelling the velocity-dependent elasticity of the hollow glass beads, the velocity and elasticity in the hollow glass beads were simulated in the direction of the sound propagation. The simulation results indicate that the velocity and elasticity profiles depend on the sound wave magnitude. Finally, the velocities of the hollow glass beads under acoustic excitation were measured by inserting an accelerometer into the beads. The measurement results demonstrated that the velocity profile depends on the sound wave magnitude, which parallels the simulation results.

On spatially varying acoustic impedance due to high sound intensity decay in a lined duct

Direct numerical simulations (DNS) based on advanced computational aeroacoustic (CAA) methods are conducted to study the acoustic impedance of a uniformly distributed multi-slit liner under grazing incident sound waves at different frequencies and sound pressure levels (SPL), in the absence of mean-flow. The liner consists of a series of fifteen identical slit Helmholtz resonators. Numerical results show that a portion of the slit resonators in the upstream works in a non-linear regime and the others in the downstream work in a linear regime when high intense waves near the resonance are excited. It is a direct evidence that the acoustic impedance is non-uniform and spatially varying over the liner length due to the different local incident sound pressure levels. With the numerical data, two methodologies are adopted to determine the acoustic impedance of the liner, one is referred to as the Prony's method and the other one is based on an inverse method. The impedance value is validated by a time-domain linearized Euler equation (LEE) solver. Numerical results indicate that representing the liner with a homogenized impedance leads to considerable errors in the reconstructed sound fields when the liner experiences a transition from non-linear regime to linear regime. On the other hand, in cases where such a transition is absent, the homogenized constant impedance is adequate to estimate the acoustic response of the liner. Therefore, a spatially piece-wise function is derived and calibrated by the DNS results to take into consideration both the resistance and reactance variation in stream-wise direction. Comparison shows that the prediction of the sound field is improved by the space-dependent impedance boundary.

Liner Impedance Eduction Under Shear Grazing Flow at a High Sound Pressure Level

This paper investigates the combined effects of high sound pressure level and grazing flow on impedance eduction for acoustical liners. Experiments are conducted in the grazing flow duct at ONERA (B2A). The impedance is then educed with an inverse method adapted to a shear flow. To take into account the effects of incident sound pressure level, a strategy for impedance eduction using a space-dependent impedance is considered. This strategy is applied to different experimental cases, and the results are compared with those obtained with a reference method in which the impedance is assumed constant.

Nonlinear sound absorption of ultralight hybrid-cored sandwich panels

Abstract A combined experimental, numerical and analytical approach is employed to investigate the nonlinear effect of incident sound pressure level (SPL) on the sound absorption performance of a novel ultralight sandwich panel with perforated honeycomb-corrugation hybrid (PHCH) core. Built upon the motion, continuity and heat conduction equations for compressible viscous fluids, the numerical model fully considers the nonlinear effects caused by incident sound wave with high SPL. The analytical model is constructed by using an approximation solution to the characteristic impedance of micro-perforated panel (MPP) absorbers with consideration of the compressibility of the fluid inside the perforation. The validity of both the numerical and analytical models is checked against experimental measurement results. The effects of facesheet thickness, corrugation thickness and core height on sound absorption are systematically explored. The proposed PHCH sandwich construction is ultralight. It has small total thickness and possess simultaneous load-bearing, energy absorption and sound absorption properties, showing great potential in multi-functional applications. This work provides a nonlinear analytical model and a nonlinear numerical method for the sound absorption of the ultralight hybrid-cored sandwich structures, which demonstrates their superior performance against sound with high SPL.

MEMS directional acoustic sensor with charge amplifier based electronic readout

MEMS acoustic sensors based on the operating principle of the Ormia ochracea fly provide a unique way to determine the direction of sound. A typical sensor consists of two wings coupled by a bridge and the entire mechanical structure attached to a substrate using two torsional legs. The sensor mechanical structure has two different vibration modes which can be coupled by placing their resonant frequencies close to each other. The acoustic wave impinging on the sensor induces displacements which are detected using comb finger capacitors at the edges of the wings. Previously, this has been accomplished using the universal capacitive readout MS3110, which is prone to static variations of the comb finger capacitors. In this work, a charge amplifier circuit was developed to independently measure the displacements of the wings under sound excitation. The ability to simultaneously read these displacements is needed to determine the bearing of sound using a single MEMS sensor, in contrast to previously reported works that needed either a calibrated microphone or a second MEMS sensor to determine the incident sound pressure level. Measurements showed that both the phase difference and amplitude difference of oscillating wings were strongly dependent on the incident angle of sound.

A compact and low-frequency acoustic energy harvester using layered acoustic metamaterials

As an environmental energy, air-borne sound can be harvested for the long-term local energy supply of wireless sensors and portable devices. A compact acoustic energy harvesting (AEH) system is proposed, which consists of a beam-based PZT transducer and a dual-layer of acoustic metamaterial denoted as the layered acoustic metamaterial (LAM). The pressure amplification effect inside LAM can substantially increase motions of the PZT transducer. Therefore, in order to effectively improve the electromechanical conversion efficiency, the first bending mode of the PZT transducer is designed to generate the resonating vibration at the frequency related to the pressure amplification effect. An impedance-based theoretical model is developed to investigate the conditions for maximizing the pressure amplification effect. Experimental results show that the LAM sample in the proposed AEH system enhances the sound pressure level by 18 dB at 318 Hz. Under the condition of the incident sound pressure level of 100 dB, the maximum open-circuit output voltage of the proposed system reaches 72.6 mV, which is 4.2 times greater than that of the same PZT transducer without LAM.

Acoustic energy harvesting using piezoelectric generator for railway environmental noise

High-speed trains have a sustained high-noise level for long periods during operation. Although such high-noise levels are effective for acoustic energy harvesting, a practical design for an acoustic energy harvesting system from a high-speed train is lacking. In this study, the design of an energy harvesting system was implemented utilizing noise from a high-speed train during practical operation. We investigated the noise generated from a high-speed train and derived the characteristics of the main noise sources. The results confirmed that low-frequency noise of 50–200 Hz was generated in the passenger, cab, and between car sections. Results from this investigation were used to design a Helmholtz resonator for a target noise of 174 Hz based on a theoretical model. Moreover, numerical simulation was conducted using sound source speakers to investigate vibrations in the walls of the resonator. Finally, energy harvesting experiments were conducted using various types of piezoelectric elements such as rectangular and circular plates. Experimental results indicate that approximately 0.7 V was generated for an incident sound pressure level of 100 dB using a large rectangular plate. Such power level is sufficient to power a variety of low-power electric devices.

Sound absorption of a porous material with a perforated facing at high sound pressure levels

A semi-empirical model is proposed to predict the sound absorption of an acoustical unit consisting of a rigid-porous material layer with a perforated facing under the normal incidence at high sound pressure levels (SPLs) of pure tones. The nonlinearity of the perforated facing and the porous material, and the interference between them are considered in the model. The sound absorptive performance of the acoustical unit is tested at different incident SPLs and in three typical configurations: 1) when the perforated panel (PP) directly contacts with the porous layer, 2) when the PP is separated from the porous layer by an air gap and 3) when an air cavity is set between the porous material and the hard backing wall. The test results agree well with the corresponding theoretical predictions. Moreover, the results show that the interference effect is correlated to the width of the air gap between the PP and the porous layer, which alters not only the linear acoustic impedance but also the nonlinear acoustic impedance of the unit and hence its sound absorptive properties.

Investigation of nonlinearity parameter B/A of saturated, unconsolidated marine sediments via a statistical approach

In this talk, we present a theory and the supporting experiment for the nonlinearity parameter B/A of saturated, unconsolidated marine sediments. The model is composed of the quadratic nonlinearity of the equivalent suspension of grains and the interstitial fluid and the nonlinearity resulting from the random grain contacts. To capture the random nature of grain contacts, the number of Hertzian contacts on each grain and the distribution of contact forces between grains are treated statistically. Predictions of B/A are compared with measurements performed with the finite amplitude insert-substitution (FAIS) method. Dependence of B/A on external compression and the incident sound pressure level is also discussed. [This work was conducted in the Unmanned Technology Research Center (UTRC) sponsored by the Defense Acquisition Program Administration (DAPA) and the Agency for Defense Development (ADD) in the Republic of Korea.]

Bio-Inspired Miniature Direction Finding Acoustic Sensor.

A narrowband MEMS direction finding sensor has been developed based on the mechanically coupled ears of the Ormia Ochracea fly. The sensor consists of two wings coupled at the middle and attached to a substrate using two legs. The sensor operates at its bending resonance frequency and has cosine directional characteristics similar to that of a pressure gradient microphone. Thus, the directional response of the sensor is symmetric about the normal axis making the determination of the direction ambiguous. To overcome this shortcoming two sensors were assembled with a canted angle similar to that employed in radar bearing locators. The outputs of two sensors were processed together allowing direction finding with no requirement of knowing the incident sound pressure level. At the bending resonant frequency of the sensors (1.69 kHz) an output voltage of about 25 V/Pa was measured. The angle uncertainty of the bearing of sound ranged from less than 0.3° close to the normal axis (0°) to 3.4° at the limits of coverage (±60°) based on the 30° canted angle used. These findings indicate the great potential to use dual MEMS direction finding sensor assemblies to locate sound sources with high accuracy.

Effect of orifice shape on acoustic impedance

The effect on normal incidence acoustic impedance of a non-circular orifice shape is examined relative to a circular orifice. The impedance of an adjustable porosity perforate, formed from two identical perforates sliding over each other, is measured. As the orifice shape becomes more non-circular, the measured impedance is found to deviate from the predicted results for a circular orifice of the same area. Several isolated orifices of the same open area but different shapes are tested and compared with a circular orifice. Both low incident sound pressure levels using broadband noise and high incident sound pressure levels using sinusoidal tones are used to evaluate the impedance performance of these isolated orifices. One orifice mimics the unique shape produced by the adjustable perforate and results in a smaller attached mass (or mass end correction) compared with a round orifice. This is consistent with the perforate impedance results. The unique orifice shape does not appear to have measurable differences in acoustic normal resistance at high incident sound pressure levels. However, since the attached mass plays a key role in determining the peak absorption frequency of resonant liners, the reduction in attached mass relative to a circular orifice has implications where these types of orifices are used.

Nonlinear effects on tonal sound in lined ducts

Porous sound absorbers are a familiar and supposedly well-understood feature of the noise control palette. As in many other aspects of engineering, however, aerospace applications require a degree of mastery well beyond the common place: sound absorbers must be designed to be effective at high noise levels within strict mass and volume constraints. In the process of fine-tuning sound absorbers for jet aircraft, it was discovered that high incident sound pressure levels cause the performance of lined ducts to diverge from small-signal predictions. The underlying physics was not fully understood prior to the early 1980s, at which point NASA-sponsored research disclosed and quantified the primary mechanism. Further research by others continues building on that foundation down to the present. This paper gives a brief overview of the topic, summarizes one of the research efforts, and adapts its findings for lined duct applications.

Transmission loss of a silencer using resonator arrays at high sound pressure level

Helmholtz resonator is generally used to reduce the low frequency noise. However, it has only high transmission loss in a narrow band at the resonance frequency. In order to obtain broad band transmission loss, studies of a silencer using various resonator arrays have been carried out. These previous studies have been limited to low incident sound pressure level conditions with no change of impedance with respect to pressure. In the case of high sound pressure, the resistance increases due to nonlinear behavior at the neck of a resonator. As a result, the desired noise reduction performance cannot be achieved due to the transmission loss decrease. To predict the transmission loss of a silencer using resonator arrays at high sound pressure level, impedance variation depending on the incident sound pressure level should be considered. Furthermore, resonator’s numbers and order of arrangement are significant design parameters for design of transmission loss shape.

Transparent Gradient-Index Lens for Underwater Sound Based on Phase Advance

Spatial gradients in refractive index have been used extensively in acoustic metamaterial applications to control wave propagation through phase delay. This study reports the design and experimental realization of an acoustic gradient index lens using a sonic crystal lattice that is impedance matched to water over a broad bandwidth. In contrast to previous designs, the underlying lattice features refractive indices that are lower than the water background, which facilitates propagation control based on a phase advance as opposed to a delay. The index gradient is achieved by varying the filling fraction of hollow, air-filled aluminum tubes that individually exhibit a higher sound speed than water and matched impedance. Acoustic focusing is observed over a broad bandwidth of frequencies in the homogenization limit of the lattice, with intensity magnifications in excess of 7 dB. An anisotropic lattice design facilitates a flat-faceted geometry with low backscattering at 18 dB below the incident sound pressure level. Three dimensional Rayleigh-Sommerfeld integration that accounts for the anisotropic refraction is used to accurately predict the experimentally measured focal patterns.

A Numerical Investigation on Sound Absorption Mechanism of Micro Resonator with Offset Slits

Acoustic liners are widely used in commercial aero-engine to suppress noise. In theoretical investigations, the liner geometry is often assumed as an array of symmetric micro resonator with orifice or slit at the center. However, in real application, orifices or slits distributed in micro resonator are offset. For better understanding of sound absorption mechanism of micro resonator with offset slits under high incident sound pressure level (SPL), direct numerical simulations (DNS) using high order low dispersion and low dissipation computational aeroacoustics (CAA) method are carried out. The simulations are first validated by experimental data, showing good agreement and establishing the relevance of the simulation methodology. Numerical simulations of resonators with single offset slit or two slits are then conducted. The two sound absorption mechanisms, namely viscous dissipation and vortex shedding, are discussed with detailed numerical data and analysis, which lead to quantitative parametric description of the energy partition between the two mechanisms as a function of both frequency and geometry. It is shown that offset slit can reduce vortex shedding and results in less sound absorption. The effects of more than one slit are, however, opposite; more vortex shedding occurs with more slits so that sound absorption is enhanced. This may potentially help guide liner design in practical applications.

Experimental study on self-powered synchronized switch harvesting on inductor circuits for multiple piezoelectric plates in acoustic energy harvesting

Self-powering mechanisms for synchronized switch harvesting on inductor circuits have been experimentally investigated to harvest low-frequency acoustic energy (∼200 Hz) using multiple lead zirconate titanate piezoelectric plates placed inside a quarter-wavelength straight-tube resonator. Unlike conventional synchronized switch harvesting on inductor circuits, the self-powered synchronized switch harvesting on inductor circuits can perform alternating current/direct current conversion without externally powered switch controllers, which is beneficial in real applications. In the acoustic energy harvester, the nonuniform driving force to piezoelectric plates generates the asynchronous charging effect in piezoelectric internal capacitances. Due to more energy dissipative components in the self-powered synchronized switch harvesting on inductor circuits, there exists a critical voltage above which the self-powered synchronized switch harvesting on inductor circuits outperform the standard circuit. With the incident sound pressure level of 112 dB at the frequency of 196 Hz, the output power of self-powered series-synchronized switch harvesting on inductor using three piezoelectric plates is measured as 0.796 mW. This is 13.2% higher than the standard circuit.