Acoustic Intensity Method Research Articles

Vector acoustic intensity analysis of noise from the Delta IV Heavy launch vehicle during liftoff

The noise sources within a turbulent rocket plume are not well understood, let alone the radiation from multiple rocket nozzles. Even less is known about the noise sources during launch vehicle liftoff. This paper seeks to simultaneously address these noise source analysis challenges using data collected during the NROL-82 Delta IV Heavy (DIVH) launch from Vandenberg Space Force Base. The three-core DIVH’s liftoff noise was measured by a ground-based four-microphone array at a distance of 330 m. Hart et al. [Proc. Mtgs. Acoust. 45, 040003 (2022)] previously used this array with a cross-correlation technique to identify the overall noise source radiation locus as it switched from the flame trench exit to about 55 m downstream of the nozzle exit plane. Here, vector intensity is used to localize the frequency-dependent noise source axial distribution using a variant of the phase and amplitude gradient estimator method for acoustic intensity [Thomas et al., J. Acoust. Soc. Am. 137, 3366–3376 (2015)]. The results help clarify a complex noise source generation process that is both time and frequency-dependent and involves both free and impinging jet noise phenomena.

Azimuth Estimation of Multi-LFM Signals Based on Improved Complex Acoustic Intensity Method

The complex acoustic intensity method is one of the common methods used for the azimuth estimation of single-vector sensors. However, this method establishes a relationship between frequency and azimuth, which limits its practical applicability for multiple linear frequency modulation (LFM) signals with overlapping frequency domains. In this paper, the time–frequency distribution information of the LFM signal is combined with the complex acoustic intensity method, and more signal parameter information is used to expand the application scenario of the single-vector sensor. The proposed method first processes the time–frequency graph of the signal to obtain a stable and clear time–frequency distribution, and then obtains the acoustic intensity distribution of the signal using the time integration of the energy on the ridge of the signal to estimate the target orientation more stably. The simulation results show that the root mean square error of azimuth estimation is less than 1° when the SNR is greater than 0 dB. Furthermore, a pool experiment was carried out to verify the effectiveness of the proposed method.

Precision enhancement in source localization using a double-module, three-dimensional acoustic intensity probe

The three-dimensional (3D) acoustic intensity probe is not popularly used for the source localization of acoustic source because of the large error even though the technique has been well known. In particular, the measured direction-of-arrival strongly depends on the frequency, which is a detrimental drawback that hampers the use of this method. Such spectral bias error, due to the discrete positioning and spacing of sensors surrounding the acoustic center, fluctuates in a nearly regular spectral pattern when a 3D acoustic intensity probe configured in a tetrahedral shape is used. In this work, two error compensation methods are tried: The first method is to adopt a double-module configuration, which is formed by the connected assembly of two intensity modules, but with different locations of acoustic centers for a spatial averaging effect. The second one is the 1/3-octave band averaging of the measured intensity spectrum within the effective measurement range. Because two probe modules are attached, they concatenate 1–3 sensors in the overlap position. Four double-module probes different in assembly configuration of involved two single-modules are tested: modules having a same acoustic center but twisted by 60°, inverted modules sharing one microphone and twisted by 60°, symmetric modules having the joint angle of 60° in the sharing edge, symmetric modules to a common face thus sharing three microphones. Measurements are conducted in an anechoic chamber by changing the source positions for 4 azimuth and 3 elevation angles. The results reveal that the maximum mean bias error of about 4° can be obtained by the double-module 3D acoustic intensity method, depending on the module configuration and angular source position. This contrasts to the fluctuating direction-of-arrival within about ±10° error range, which is obtained by each single-module in a concatenated double-module. In particular, when the band averaging technique is applied to the measured data by the probe module composed of the inverted single modules, the maximum bias error can be reduced to less than 1° in both azimuth and elevation angles.

Initial characterization of infrasound sources using the phase and amplitude gradient estimator method for acoustic intensity

The phase and amplitude gradient estimator (PAGE) method for vector acoustic intensity [Thomas et al., J. Acoust. Soc. Am. 137, 3366–3376 (2015)] has been used previously to improve source characterization over broad frequency ranges. This paper describes initial applications of the PAGE method to the infrasound region for outdoor sources using multi-microphone probes. Measurement challenges include wind noise, which reduces signal-to-noise ratio and coherence, low-frequency phase and amplitude mismatch between microphones, and determining an appropriate microphone spacing to maximize bandwidth. Analysis challenges include phase unwrapping above the spatial Nyquist frequency, source statistical stationarity, and balancing frequency resolution with averaging across finite record lengths. This paper discusses how these challenges are being addressed for specific sources of infrasound, namely, wind turbines and large rocket motors. [Work supported by NSF.]

Extension of the phase and amplitude gradient estimation method for acoustic intensity to narrowband sources

The phase and amplitude gradient estimation (PAGE) method [D. C.Thomas et al., J. Acoust. Soc. Am. 137, 3366-3376 (2015)] has proven successful in improving the accuracy of measured energy quantities over the traditional p-p method in several applications. For example, the PAGE method has successfully increased the bandwidth over which magnitude and phase calculations are accurate for broadband sources with smoothly varying phase. This is partially accomplished by unwrapping the phase relationship in order to get valid phase information above the spatial Nyquist frequency. However, narrowband sources may not have sufficient coherent bandwidth information for a phase unwrapping algorithm to unwrap properly. To test the limits of the PAGE method on narrowband sources, sine waves, sawtooth waves, and bandlimited white noise have been used in various scenarios. In one-dimensional tests of these signals, the PAGE method provides correct magnitude and direction for frequencies up to the spatial Nyquist frequenc...

Influence of beat phenomenon on direction of arrival estimation based on a single vector hydrophone

The vector hydrophone can measure sound pressure and orthogonal components of particle velocity co-locately in space and simultaneously in time. DOA (direction of arrival) estimation based on a single vector hydrophone has attracted extensive attention. Beat phenomenon can cause confusion in the DOA estimation results. In this paper, the impact mechanism of beat phenomenon to DOA estimation by cross spectrum method and average acoustic intensity method is explored through theoretical study and simulation. The result shows that beat phenomenon causes the change of sound pressure and vibration velocity amplitude with time and leads to DOA estimation results of these two methods are instability eventually. First, DOA is estimated by the average sound intensity method, if the temporal resolution is smaller than the minimum envelope period, the DOA estimation will change over time; Second, DOA is estimated by the cross spectral method, if the FFT frequency resolution is greater than frequency difference, it is unable to distinguish the frequency of these two signals. Subsequent operations will also be confused by two signal energy, causing error estimates. In the end, the solutions and application examples are proposed. The conclusion could be applied to improve the DOA estimation performance of a single vector hydrophone.

Experimental validation of acoustic intensity bandwidth extension by phase unwrapping.

The phase and amplitude gradient estimator (PAGE) method for active acoustic intensity uses pairwise microphone transfer functions to obtain the phase gradient, which improves the calculation bandwidth over the traditional weighted quadspectral method. Additionally, for broadband sources, the PAGE theory indicates that the transfer function phase can be unwrapped to further extend the usable frequency range to beyond the spatial Nyquist frequency. Here, two experiments demonstrate intensity bandwidth extension by more than an order of magnitude using phase unwrapping. First, plane-wave tube results show accurate one-dimensional intensity calculations with the microphones separated by five wavelengths, 30 times the traditional limit. Second, two-dimensional measurements of a laboratory-scale jet with a four-microphone probe yield physically reasonable calculations at frequencies 15 times the traditional limit.

An application of a parametric transducer to measure the acoustical properties of a living green wall

Greening of urban spaces provides a number of environmental benefits. Living Green Walls (LGW) is a most typical example of greening which is also known for its ability to absorb unwanted noise. However, this ability of LGW to absorb noise is rather hard to quantify, because there is a lack of reliable experimental methods to measure it in-situ. This work reports on a new method to measure the absorption coefficient of LGW which makes use of a highly directional parametric transducer and acoustic intensity method. This method is tested under controlled laboratory conditions and in a typical street environment. The results of these experiments demonstrate the ability of the method to measure the absorption of a LGW. It also enables us to quantify the effects of the plant type and moisture content in the soil on the ability of the LGW to absorb sound. The proposed method has certain benefits over ISO354-2003 and CEN/TS 1793-5:2003 standard methods.

Far-field acoustical measurements during QM-2 solid rocket motor static firing

The five-segment Space Launch System solid rocket motor was recently tested at Orbital ATK. Far-field acoustical measurements were performed at angles between 80° and 120° relative to the rocket exhaust at a distance of roughly 2500 m from the rocket, approximately 800 nozzle diameters. The angular aperture allows for evaluating spatial variation in acoustic properties and a comparison with similar tests in the past, including the 2015 test of the same rocket motor. Although terrain variations introduce uncertainty, an approximate 10 dB change in level is seen throughout the aperture, consistent with previous studies. In addition, at low frequencies a high degree of correlation is seen. Near the peak radiation direction high levels of derivative skewness indicate significant shock content and crackle. This dataset also presents the opportunity to test a new method for processing acoustic vector intensity [Thomas et al., JASA 137, 3366-3376 (2015)]. Comparison with the traditional method shows an increase in usable bandwidth of more than an order of magnitude.

Comparison of holography, beamforming, and intensity-based inverse measurements on full-scale jet noise sources. Part I: Numerical investigations

The reduction of high-performance fighter aircraft noise, which is of particular concern due to its potential negative impact on communities near air bases and damage to military personnel hearing, is facilitated by improved understanding of the jet noise sources. To this end, near-field acoustical holography, beamforming, and acoustic vector intensity methods have been investigated for the source imaging of full-scale jets. In the first of a two-part investigation, a numerical experiment is performed to test the accuracy of each of these methods, using a simulated measurement array similar to a physical array from an actual full-scale measurement. Extended, partially correlated sources are generated over a range of frequencies using a wavepacket ansatz, and array measurements are simulated in the geometric near field. Source reconstructions are obtained using each of the methods. These equivalent sources are then propagated to the mid and far field. The various reconstructions are compared to simulated benchmarks in order to highlight the advantages and disadvantages of each method, and to evaluate the frequency ranges over which they provide accurate results at the source and in the mid and far fields. [Work supported by USAFRL through ORISE.]

Phase and amplitude gradient method for the estimation of acoustic vector quantities.

An alternative pressure-sensor based method for estimating the acoustic intensity, the phase and amplitude gradient estimation (PAGE) method, is presented. This method uses the same hardware as the standard finite-difference method, but does not suffer from the frequency-dependent bias inherent to the finite-difference method. A detailed derivation of the PAGE method and the finite-difference method is presented. Both methods are then compared using simple acoustic fields. The ability to unwrap the phase component of the PAGE method is discussed, which leads to accurate intensity estimates above previous frequency limits. The uncertainties associated with both methods of estimation are presented. It is shown that the PAGE method provides more accurate intensity estimates over a larger frequency bandwidth.

CAM 곡선 개선에 의한 차량용 공조기의 소음 저감 평가

The noise in a vehicle is an important factor for customers purchasing a car. Particularly, reduction of the noise that is generated from HVAC(heating, ventilation and air conditioning) is very important since it has considerable effects on interior noise. In general, identification of noise source is crucial to reduce noise level. The complex acoustic intensity method is widely used to obtain the accurate measurement and identification of noise source. Therefore, in the previous study, noise source of HVAC was identified through experimental approach using the complex acoustic intensity method. In this study, we are intended to confirm reduced level of noise by comparing the result between before and after modification of cam curve that is based on identified noise source of HVAC. It is found out that noise source of HVAC are motor and cam area using the complex acoustic intensity method in the previous study. We performed experiments to compare noise level between before and after modification of cam curve. Especially, it can be seen that complex acoustic intensity method using both active and reactive intensity is vital in devising a strategy for comparison to noise level. Also, the vector flow of acoustic intensity was investigated to identify sound intensity distributions and energy flow in the near field of HVAC.

복소음향인텐시티법을 이용한 HVAC의 소음원 검출

The relation between the vibration induced from machinery and the radiated sound is complicated. Acoustic intensity method is widely used to obtain the accuracy of noise measurement and noise identification. In this study, as groundwork, the complex acoustic intensity method is performed to identify noise source and transmission path on different free space point source fields. As an industrial application, the complex acoustic intensity method is applied to HVAC to identify sound radiation characteristics in the near field. Experimental complex acoustic intensity method was applied to HVAC, it is possible to identify noise sources in complicated sound field characteristics which noise sources are related with each other, and certificate the validity of complex acoustic intensity. Especially, it can be seen that complex acoustic intensity method using both of active and reactive intensity is vital in devising a strategy for identification of noise. Also, the vector flow of acoustic intensity was investigated to identify sound intensity distributions and energy flow in the near field of HVAC.

The estimation of the room acoustic characteristic using the acoustic intensity method

When a sound radiates in rooms, a lot of reflection sounds are generated. From estimation of the direction where the room reflection sound comes from, we can understand the diffusion situation in the room acoustic field. By using the acoustic intensity method, we can measure the strength and the direction of the sound. In this paper, we estimate the direction of the reflection sound in the time-space by the acoustic intensity method and show the acoustic characteristic of the room.

Experimental Results of Converted NAH Method

Among sound source localization methods, the acoustic intensity (AI) method is widely used in industry. On the other hand, nearfield acoustic holography (NAH) method, which is also a localization method, has not spread to industry use because of its difficulty in measuring high frequency measurement. For high frequencies, the NAH method requires a very large number of measurement points, which results in a prolonged measurement time and very large size. I have proposed a theoretically converted NAH method(1),(2), which uses the extra spatial-domain spectrums in the backpropagating computation of NAH method. This method can measure the sound field using a reasonable number of measurement points even for high frequencies. As a result, the workload of measurement is reduced to the same level as that using acoustic intensity method. In this paper, the theory of the proposed method is explained, and some experimental results are presented to verify this method.

334 改良近距離音響ホログラフィ法の実験結果について

As a localization method, the Acoustic Intensity method is widely used. On the other hand, the Nearfield Acoustic Holography method is rarely used in actual industry though this method has high performance. There are some reasons about this. I am proposing converted Nearfield Acoustic Holography method. The purpose of this method is to lighten the workload of measurement. The workload of measurement is one of main reason that the Nearfield Acoustic Holography method is not used widely. In this paper, the experimentations are performed. And as a result of it, it is verified that the Nearfield Acoustic Holography method can be measured by same workload with the Acoustic Intensity method.

The Separation of Multipath Reflection Waves in Water

In the past, the standing wave method was used to we measure the acoustic characteristic of underwater objects which includes complex sound pressure reflectance, a normal-incident absorption coefficient and normal impedance. The standing wave method is not suitable for performing measurements in a low-frequency range, because the length of the sound tube should be at least more than one wavelength. The 2-hydrophone method was proposed to rectify this drawback. But in the 2-hydrophone method, it is necessary to measure the sound velocity precisely. In this study, we inspect the 3-hydrophone method, where it is not necessary to measure the velocity of sound in water. We also applied the acoustic intensity method to separate multipath waves in water. We proved successful separation of multipath reflection waves using the 3-hydrophone method and the envelope intensity method. The effectiveness of those methods was verified.

Measurement of sound power using the acoustic intensity method—a consultant's viewpoint

An overview of observations from almost 15 years of practical experience with acoustic intensity measurements is given. The often difficult working conditions of acoustic consultants mean that we tend to make demands on the measuring methods and the instrumentation that presently cannot easily be met. Therefore the instrumentation should be improved in order to increase both the dynamic and the frequency range. Further development of ‘real-time’ control of measurement quality is also desirable i.e. the possibility of surveying the quality of the result during or immediately after the measurement is performed.

Acoustic intensity and structural power flow methods for scientific visualization

Interpreting large data sets that result from the application of high-resolution computational and experimental methods to the solution of structural acoustics problems is a difficult, time-consuming task. The use of scientific visualization techniques expedites the process of data interpretation by condensing these large quantities of data into small packets of visual information. As generally applied, scientific visualization techniques are used to produce color coded plots of the acoustic pressure magnitude and of the structural motion. The focus of this paper is to show that the vector quantities, acoustic intensity and structural power flow, are very useful for the visualization of the complex phenomena that result when a vibrating structure interacts with its surrounding acoustic fluid. Results will be shown for applications to both two- and three-dimensional data sets resulting from the analysis of acoustic radiation from a submerged elastic shell. Algorithms for the required computations will also be given.

Study on Active Booming Noise Control through Car Interior Sound Field Analysis.

With the aim of reducing “booming noise”, which is one of the major noise problems for 4-cylinder-engine vehicles, a basic study on active noise control (ANC) has been carried out. First, the interior sound field was examined in terms of its fluctuation characteristics, which eventually led to the confirmation that an adaptive technique is necessary. The multiple-error filtered-x LMS (MEFX) algorithm was then found to be effective through an “off-line” shaker test on a real car. Secondly, the acoustic intensity method was applied to analysis of the sound field inside a car. Actuator (speaker) positioning was considered based on this analysis of standing wave and traveling wave contribution. In addition, the controller parameters' influence on control performance was estimated by means of computer simulation. Guidelines were developed for applying this technique to automobiles. ANC was found to be capable of reducing booming noise by more than l0 dB.