The acoustic characteristics of foam-metal samples were determined using a normal impedance tube. A foam- metal liner was designed based on the absorption characteristics of the foam-metal and the known acoustic character of a low-speed fan. The acoustic performance of the liner was significant, especially when placed over the rotor, achieving up to 4 dB of broadbandattenuation. The foam-metal liner effect on the flow was noted, especially affecting the pressure near the wall and increasing the size and strength of the rotor tip vortex. Due to the characteristic of the low-speed fan the impact on performance parameters such as thrust and efficiency cannot be determined using the ANCF test bed. Future testing of foam-metal liners on high-speed fans should be performed and the impact on fan performance quantified.
As modern turbofan engines have incorporated increased bypass ratios and advanced fan designs, the broadband component of fan noise has grown in relevance. Therefore, while the attenuation of fan tones remains a major factor in engine nacelle acoustic liner design, the ability to simultaneously reduce broadband fan noise levels has become more attractive. With these observations in mind, a process for the design and evaluation of novel broadband acoustic liner concepts with limited fan source information was established in a previous study. 1 Specifically, a statistical fan source model was used within an acoustic duct propagation code to predict optimum impedance spectra for the Source Diagnostic Test (SDT) fan (see Figure1a). Acoustic liner modeling tools were then used to identify geometric liner parameters (within manufacturing constraints) necessary to produce impedance spectra that most closely matched the predicted optimum values. Initially, constant-depth, double-degree of freedom (DDOF) designs were explored. In pursuit of enhanced broadbandperformance, variable-depth, multi-degree of freedom (MDOF) designs were also considered. The impedance spectra for each of these configurations were then used with the propagation code to predict attenuation spectra that could be achieved with each liner. A broadband liner design was produced that was predicted to provide increased attenuation over conventional tonal designs (see for example, Figure 1b) for the full range of frequencies and operating conditions considered. Unfortunately, plans to carry the designs through fabrication, testing, and evaluation did not materialize.
In contrast to that revealed in the previous section, the use of an axial fan as primary noise did not lead to such a good response. The origin of this poor performance was the turbulence originated by the airflow (about 7 m/s) downstream of the duct. 3,12 The flow component turned out to be higher than the acoustic component over the studied frequency range. In the absence of flow effects, to obtain an attenuation of 10 dB or more, the coherence must be greater than about 0.95. This characteristic is drastically reduced in the presence of local turbulence at the microphones. These problems may be reduced by attaching foam protectors to the microphones, although coherence cannot always be recovered, 3 as the work here exposed. Consideration of the results obtained (see Fig. 15) shows coherence to be very poor; consequently, the ANC systemʼs performance (see Figs.16 and 17) was also poor and only seemed to work at the blade passage frequency and at the frequency excited by the metallic element joining the fan and the duct.
NASA Glenn Research Center, Cleveland, OH 44212
The broadband component of fan noise has grown in relevance with an increase in bypass ratio and incorpo- ration of advanced fan designs. Therefore, while the attenuation of fan tones remains a major factor in engine nacelle acoustic liner design, the simultaneous reduction of broadband fan noise levels has received increased interest. As such, a previous investigation focused on improvements to an established broadband acoustic liner optimization process using the Advanced Noise Control Fan (ANCF) rig as a demonstrator. Constant-depth, double-degree of freedom and variable-depth, multi-degree of freedom liner designs were carried through de- sign, fabrication, and testing. This paper addresses a number of areas for further research identified in the initial assessment of the ANCF study. Specifically, incident source specification and uncertainty in some aspects of the predicted liner impedances are addressed. This information is incorporated in updated predictions of the liner performance and comparisons with measurement are greatly improved. Results illustrate the value of the design process in concurrently evaluating the relative costs/benefits of various liner designs. This study also provides further confidence in the integrated use of duct acoustic propagation/radiation and liner modeling tools in the design and evaluation of novel broadband liner concepts for complex engine configurations.
of time with different slopes when the bias 𝑭 and the noise correlation time 𝛕 𝟏 increases. From Fig.1(a), it is shown that the first moment 〈𝒙(𝒕)〉 is suppressed as the bias 𝑭 decreases. However, from Fig.1(b), it is shown that the first moment 〈𝒙(𝒕)〉 actively and then repressively as the noise correlation time 𝛕 𝟏 increases.
→ Ground reflection: The effect of ground reflection has influence on the noise measurement. The ground effects are defined as the difference between the measured sound pressure level and the measured sound pressured level in free field conditions. The physical and geometrical explanation of ground effect was explained in chapter 8 of reference . In this experiment the emitted sound is recorded under full ground reflection conditions (i.e. measuring flush using a 40 cm metal plate laying on sand foundation, as described by ICAO Annex 16 ). Therefore the receiver is placed directly on a full reflecting surface (shown in A.3.1). This results in the difference in path length between the direct and reflected signal becoming zero. Under this condition, both direct and reflected signals arrive in phase and interfere constructively. To eliminate full ground reflection, the theoretical amplification of 6 dB is subtracted from the recorded noise levels for all the frequencies.
Sound is defined as any pressure variation that the human ear can detect ranging from the weakest sound to the levels that could impair hearing. Increasing environment noise pollution is a matter of great concern and of late has been attracting public attention. Sound produces the minute oscillatory changes in air pressure and is audible to the human ear when in the frequency range of 20Hz to 20kHz. When its level exceeds a certain threshold level, it may impair hearing ability of the living beings. Therefore acceptable sound levels are specified by ordinances of indian Government and product standards like NEMA TR-1 and CBIP Manual on Transformers.
Lighthill (1952, 1954) in his theory of aerodynamic sound, modelled the problem of sound generation by turbulence using an exact analogy where sound was radiated by a volume distribution of acoustic quadrupoles in an ideal acoustic medium. The strength of the quadrupoles in Lighthill’s stress tensor used in his theory are the unsteady components of the Reynold’s stress at low Mach numbers. Curle (1955) showed how the presence of boundary surfaces could be accounted for by adding dipole and monopole sources. A dimensional analysis of the equations showed that the intensity of sound generated by free turbulence (quadrupoles) scales as the eighth power of a typical flow velocity while that induced by unsteady surface forces (dipoles) scales as velocity to the power six. Some of the earliest studies on trailing edge noise were performed by Ffowcs Williams and Hall (1970), Chase (1972) and Amiet (1975 and 1976). Ffowcs Williams and Hall (1970) stated that it was necessary to consider the details of the potential field in the vicinity of a sharp edge of a body that acts as a scattering center. In such a case, dimensional analysis showed that the sound generated is dominated by edge scattering and the resulting noise scales more effectively than a quadrupole or a dipole. The intensity scales as typical flow velocity (U c ) to the power five and is given by 〈𝑝 2 〉 ∝ 𝜌
CFD simulations are needed to provide inputs in terms of mean ow and turbulence character- istics for the CAA and fRPM methods. The DLR in-house solver TRACE  was used to perform all CFD simulations. A MUSCL (Monotonic Upstream Scheme for Conservation Laws) method of second order accuracy based on Fromm's scheme was applied for the spatial discretization and an Euler Backward scheme of second order accuracy, for the time discretization. The SSG/LRR-ω full Reynolds Stress turbulence model  was used. The turbulence at the inlet boundary of the fan stage was dened by a turbulence intensity of 1% (k t = 1.4 m 2 /s 2 ) and integral turbulent length scale of 0.01 m as those numbers are roughly representative in terms of expected magnitude for outdoor and rig testing. Compared to Boussinesq-based two-equation Menter SST k-ω turbulence model (see Figures 2 and 3), the wake structure can be clearly identied in terms of TLS and TKE using the SSG/LRR-ω turbulence model. Upstream of the rotor blades, the ingested turbulence is nearly identical for both simulations. In the interstage region, the TKE in the wake is also quite similar, while some small dierences in the background TKE can be observed. The TLS of the Menter SST k-ω turbulence model appears to be smeared and is too low in large regions between the wakes. This has a potential inuence on the fan broadbandnoise, in particular the contribution of the background turbulence.
The receiver is thus exposed to both transmitted and diffracted noise. Whereas the transmitted noise only depends on barrier material properties , the diffracted noise depends on the location, shape, and dimensions of the barriers. The sound wave diffraction is not equal to all frequencies. Low frequency waves diffracted more than medium and high frequencies. 
Abstract: The influence of urban morphology of low-density built-up areas on spatial noise level attenuation of flyover aircrafts is investigated at a mesoscale. Six urban morphological parameters, including Building Plan Area Fraction, Complete Aspect Ratio, Building Surface Area to Plan Area Ratio, Building Frontal Area Index, Height-to-Width Ratio, and Horizontal Distance of First-row Building to Flight Path, have been selected and developed. Effects of flight altitude and horizontal flight path distance to site, on spatial aircraft noiseattenuation, are examined, considering open areas and façades. Twenty sampled sites, each of 250m*250m, are considered. The results show that within 1000m horizontal distance of flight path to a site, urban morphology plays an important role in open areas, especially for the buildings with high sound absorption façades, where the variance of average noise level attenuation among different sites is about 4.6 dB 2 at 3150 Hz. The effect of flight altitude of 200ft-400ft on average noise level attenuation is not significant, within about 2 dB at both 630Hz and1600Hz in open areas. Urban morphological parameters influence the noiseattenuation more in open areas than that on façades. Spatial noiseattenuation of flyover aircrafts is mainly correlated to Building Frontal Area Index and Horizontal Distance of First-row Building to Flight Path.
There have been several explorations of the effect and yet complete agreement on the cause of seat dip attenuation does not yet exist. The reports of Sessler and West and of Schultz and Watters concluded that the effect seemed mainly due to a vertical resonance in the gaps between the rows of seats. This frequency domain explanation has been followed by Bradley  who argued for both vertical and horizontal resonances. However, frequency domain models do not explain every aspect of the seat dip effect. For example, the attenuation changes over time in the very early sound field . Ishida et al  were the first to explain seat dip effect in the time domain: many small reflections from the seats and floor produce a complicated impulse response immediately after the arrival of the direct sound from the stage. The seat dip attenuation is simply what results when this impulse response is Fourier Transformed. More recent measurements have supported this idea . An array of seats can thus be thought of as a diffraction grating for low frequency sound: the seat dip is an interference pattern.
Man’s desire for pollution free atmosphere needs control of air pollution and noise pollution. The principal sources of noise in automotive engines are intake noise, radiator noise, combustion noise, exhaust noise etc. Out of these exhaust noise is predominant and it is to be controlled. Noise pollution affects human beings physiologically and psychologically. The cardiac patient may get cardiac arrest due to excessive dose of noise for a longer period of time. In this paper one reactive muffler for a multi-cylinder diesel engine is designed and modified. More attenuation is given by making some changes in its configurations.
Data taken on the PXI needs to be processed before it can be usefully interpreted. Matlab is used to analyse the data. It is theoretically possible to modulate the probe beam with broadbandnoise and get all the required information from post-processing of this signal. The noise could just be filtered at each frequency of interest and delay, attenuation and conditional variance could then be calculated. This type of process would allow a broad range of information to be inferred from one single shot measurement. However, it was necessary to take data at discrete frequencies by modulating the probe beam at stepped frequencies and taking data at each frequency. Single-shot measurement proved to be too noisy due to limited amounts of data.
particular high-frequency noise that is generated in imper- fectly expanded jets. BBSAN results from the interaction of turbulent structures and the series of expansion and com- pression waves which appears downstream of the convergent nozzle exit of moderately under-expanded jets. This paper focuses on the impact of the pressure waves generated by BBSAN from a large eddy simulation of a non-screeching supersonic round jet in the near-field. The flow is under- expanded and is characterized by a high Reynolds number Rej = 1.25 × 10 6 and a transonic Mach number Mj = 1.15. It is shown that BBSAN propagates upstream outside the jet and enters the supersonic region leaving a characteristic pattern in the physical plane. This pattern, also called signa- ture, travels upstream through the shock-cell system with a group velocity between the acoustic speed Uc − a∞ and the sound speed a ∞ in the frequency–wavenumber domain (U c is the convective jet velocity). To investigate these character- istic patterns, the pressure signals in the jet and the near-field are decomposed into waves traveling downstream ( p + ) and waves traveling upstream ( p − ). A novel study based on a wavelet technique is finally applied on such signals in order
shows the phase speed map at 60 s period. This wave is most sensitive to depths from 50 to 150 km and reveals features of mantle structure and lithospheric thickness, in contrast to the shallower sensitivity of maps in Figure 14. The cold, thick lithosphere beneath the cratonic core of the continent appears clearly as a fast anomaly in the central and eastern US, while the thinner lithosphere in the western United States appears as low velocities over a large area. The transition between the tectonic and cratonic lithosphere is similar in both maps, but the ambient noise map reveals more of a stair-step latitudinal structure rather than the more continuous variation with latitude found in the 3-D model prediction. The lowest velocities of the map are in the high lava plains of southeast Oregon and northwest Nevada, which is believed to be the location of the first surface expression of the plume that currently underlies Yellow- stone. Yellowstone itself is below the resolution of the maps presented in this study. However, a low velocity anomaly does appear in the maps derived from ambient noise tomography based on the Transportable Array component of EarthScope/USArray [Moschetti et al., 2007; Lin et al.,
Acoustic interaction between rotors is a common observation in turbomachinery noise measurements and has been discussed analytically by Cumpsty, 14 Holste & Neise, 15 Enghardt et al. 16 and numerically by Nallasamy. 17 Energy at two diﬀerent frequencies may interact to induce energy at a third. In these situations, the upstream energy source is a rotor-stator pair whose excited spinning modes impinge upon a second rotor found downstream. These interactions manifest themselves as sum and diﬀerence frequencies. However, the case where noise, which originates from an alternative noise source to the upstream rotor/stator pair, interacts with a downstream rotor-stator pair, is a relatively unexplored area. In Bennett & Fitzpatrick 13 it was shown, using an experimental rig, how it is also possible for a tone generated, not from an additional upstream rotor/stator pair, but instead from an upstream speaker, to interact with a downstream rotor/stator pair to produce interaction frequencies. In addition, analysis of data acquired from full scale aero-engine tests within a European test campaign (Silencer) demonstrated that noise generated from the combustor interacted with both the high pressure and low pressure turbines to generate noise at sum and diﬀerence frequencies which were measured in the hot jet pipe of the engine. In this paper, the case where broadband or narrowband noise, such as may originate from a combustor or within the turbomachinery itself, interacts with a rotor-stator pair, to produce noise at sum and diﬀerence frequencies, is examined.
Currently industry relies on database methods (normally from model rig data extrapolated to full scale) and by evolution of the past experience to new engine designs. Thus the approach could be summarised as an empirical one that relied heavily on simple scaling laws. As the symmetries underlying scaling laws are removed (e.g. by introduction of azimuthal variations of the flow by the use of chevrons) there was an increasing need for more sophisticated prediction methods. RANS cfd has proved a valuable resource in flow prediction and had been used to guide nozzle selection by considering predictions of turbulent kinetic energy and making subjective judgements on the likely resulting noise. However, the more formal predictions of Acoustic Analogy RANS based models had been of only limited use and the question as to the future usefulness of such methods was one of
The temporal properties of the synthetic turbulence are controlled by a stochastic field. The cases of frozen and evolving turbulence have been studied in details. In the case of frozen turbulence – where only convection effects are modelled – vortex particles are convected with the mean flow with constant strength. The resulting velocity field seen by an observer moving with the mean flow is a frozen pattern. In the case evolv- ing turbulence the temporal decorrelation present in turbulent flows has been included by updating the strengths of the vortices as they are convected with the mean flow. Langevin equations have been proposed to update the vortex strengths in time. It has been shown that standard Langevin equations capture the statistical properties of tur- bulent flows but lead to non-differentiable velocity fields. The lack of differentiability has proved to be an issue when coupling the stochastic method with the linearised Euler equations to predict broadband fan noise as spurious sound sources are introduced at high frequencies. A second-order Langevin model has been proposed and validated as an alternative to the standard Langevin equation. The second-order Langevin method can be interpreted as a filtering process to smooth the synthetic velocity field in time. It has been demonstrated that it reproduces accurately the statistics of the turbulence and, in contrast to standard Langevin equations, it is suitable to couple with high-order finite difference schemes.