“Buzz-saw” noise: Prediction of the rotor-alone pressure field

Public expectations of lower environmental noise levels, and increasingly stringent legislative limits on aircraft noise, result in noise being a critical technical issue in the development of jet engines. Noise at take-off, when the engines are at high-power operating conditions, is a key reference level for engine noise certification. “Buzz-saw” noise is the dominant fan tone noise from modern high-bypass-ratio turbofan aircraft engines during take-off. Rotor-alone tones are the key component of buzz-saw noise. The rotor-alone pressure field is cut-off at subsonic fan tip speeds; buzz-saw noise is associated with supersonic fan tip speeds, or equivalently, high power engine operating conditions. A recent series of papers has described new work concerning the prediction of buzz-saw noise. The prediction method is based on modelling the nonlinear propagation of one-dimensional sawtooth waveforms. A sawtooth waveform is a simplified representation of the rotor-alone pressure field. Previous validation of the prediction method focussed entirely on reproducing the spectral characteristics of buzz-saw noise; this was dictated at that time by the availability of spectral data only for comparison between measurement and prediction. In this paper, full validation of the method by comparing measurement and prediction of the rotor-alone pressure field is published for the first time. It is shown that results from the modelling based on a one-dimensional sawtooth waveform capture the essential features of the rotor-alone pressure field as it propagates upstream inside a hard-walled inlet duct. This verifies that predictions of the buzz-saw noise spectrum, which are in good agreement with the measured data, are based on a model which reproduces the key physics of the noise generation process. Validation results for the rotor-alone pressure field in an acoustically lined inlet duct are also shown. Comparisons of the measured and predicted rotor-alone pressure field are more difficult to interpret because the acoustic lining significantly modifies the sawtooth waveform, but there remains good agreement with the measured spectral data. The buzz-saw noise prediction code used to generate the simulations in this paper has been used by the Rolls–Royce Noise Department since 2004.     

A simple mathematical model for TTS

Published data on Temporary Threshold Shift (TTS) suggest that in many cases the rms pressure at threshold during and after exposure to noise varies in a simple exponential manner, and that the ultimate shift of pressure threshold for exposure to steady noise is dependent on the mean square pressure of that noise. This response could occur if some part of the hearing mechanism were heated by exposure to noise and were at the same time subject to Newtonian cooling, and if the change in the pressure threshold were proportional to the change of temperature. This model can explain the shapes of many growth and recovery curves given in dB, why time constants found for recovery from TTS appear greater than those for growth and why threshold shifts on ears with elevated thresholds appear smaller than those for ears with low thresholds. Because of individual variation, averaged dB results mask the nature of the processes involved. Hence, for a better understanding of TTS, individual ears should be studied separately, and, if possible, measurements should be made in rms Pa instead of dB.     

‘Best-case’ noise performance of feedback interferometers, potential for micromachine applications

We present a theoretical analysis of the noise performance of phase measuring feedback interferometers. We first analyse the operation of this kind of instrument and note that under certain circumstances bistability can occur. The bistable region should be avoided if possible when using feedback interferometry for high-accuracy measurement with low-input powers, as it can give rise to very high-noise levels. We then go on to investigate the effects of shot noise (on the interferometer output signal) and thermal noise (in the feedback loop), and relate these to the phase measurement accuracy of the interferometer. A ‘best-case’ calculation indicates that phase noise of about 0.005λ is possible at input powers in the nanowatt region. In practice, we expect that noise levels will be higher than predicted (particularly at high-input powers) due to the effects of vibration and air turbulence.     

Time frequency analysis of sound from a maneuvering rotorcraft

The acoustic signatures produced by a full-scale, Bell 430 helicopter during steady-level-flight and transient roll-right maneuvers are analyzed by way of time–frequency analysis. The roll-right maneuvers comprise both a medium and a fast roll rate. Data are acquired using a single ground based microphone that are analyzed by way of the Morlet wavelet transform to extract the spectral properties and sound pressure levels as functions of time. The findings show that during maneuvering operations of the helicopter, both the overall sound pressure level and the blade–vortex interaction sound pressure level are greatest when the roll rate of the vehicle is at its maximum. The reduced inflow in the region of the rotor disk where blade–vortex interaction noise originates is determined to be the cause of the increase in noise. A local decrease in inflow reduces the miss distance of the tip vortex and thereby increases the BVI noise signature. Blade loading and advance ratios are also investigated as possible mechanisms for increased sound production, but are shown to be fairly constant throughout the maneuvers.     

Effects of peak pressure and energy of impulses

Peak pressure has been one of the key parameters of impulse noise used to assess the hazard to hearing. It is used in most international noise exposure limits. France uses an A-weighted energy limit. There is a rough correspondence between peak pressure and the hazard to hearing for a given type of impulse noise. However, when the effects of different types of impulses are compared, this correspondence breaks down. One of the alternate measures of impulse intensity is weighted energy. Weighted energy is appealing for a number of reasons. It does not depend on details of the pressure-time history such as the peak pressure and the more common duration measures. It should be easier to integrate with continuous or intermittent noise standards. It would make it easier to use standard hearing protector attenuation to estimate the hazard when a specific hearing protector is worn. Results of previously published articles and reports will be discussed. These reports lead to the conclusion that weighted energy is a more potent determiner of hearing hazard than peak pressure if spectral effects are controlled.     

Noise from small air jets and a quiet valve

Noise measurements of air jets of from 0·0794 to 0·635 cm diameter, with jet exit velocity varying from 54 to 244 m/s, to frequencies of 100 kHz are presented. Results are compared to those previously obtained for larger nozzles; acoustical power spectral density curves are found to be similar to those for the larger nozzles at like velocities. Results of a noise survey conducted near a 0·127 m line size quiet vent valve having approximately 20 000 square jets, 0·127 cm on a side are presented and found to agree with the laboratory nozzle noise data. Noise above a jet velocity of 120 m/s was found to be quadrupole in nature, while below this velocity dipole surface sound was observed; this surface noise is the noise of quiet valves, which operate at low velocities. It is estimated that a quiet valve jet of 0·025 cm diameter, with a velocity near 60 m/s will exhibit a peak acoustical power spectral density at frequencies beyond the range of human audibility.     

Detection of high-frequency spectral notches as a function of level

High-frequency spectral notches are important cues for sound localization. Our ability to detect them must depend on their representation as auditory nerve (AN) rate profiles. Because of the low threshold and the narrow dynamic range of most AN fibers, these rate profiles deteriorate at high levels. The system may compensate by using onset rate profiles whose dynamic range is wider, or by using low-spontaneous-rate fibers, whose threshold is higher. To test these hypotheses, the threshold notch depth necessary to discriminate between a flat spectrum broadband noise and a similar noise with a spectral notch centered at 8 kHz was measured at levels from 32 to 100 dB SPL. The importance of the onset rate-profile representation of the notch was estimated by varying the stimulus duration and its rise time. For a large proportion of listeners, threshold notch depth varied nonmonotonically with level, increasing for levels up to 70-80 dB SPL and decreasing thereafter. The nonmonotonic aspect of the function was independent of notch bandwidth and stimulus duration. Thresholds were independent of stimulus rise time but increased for the shorter noise bursts. Results are discussed in terms of the ability of the AN to convey spectral notch information at different levels.     

Estimation of directivity and sound power levels emitted by aircrafts during taxiing, for outdoor noise prediction purpose

Integrated noise model (INM) is the most internationally used software to calculate noise levels near airports. Take off, landing or pass by operations can be modeled by INM, but it does not consider aircrafts taxiing, which, in some cases, can be important to accurately evaluate and reduce airports’ noise assessment.Aircraft taxiing noise emission can be predicted using other prediction tools based on standards that describe sound attenuation during propagation outdoors. But these tools require data inputs that are not known: directivity and sound power levels emitted by aircraft during taxiing.This paper describes methods used to calculate directivity indexes and sound power levels, based on field measurements made in Madrid-Barajas Airport (Spain). Obtained results can be used as inputs for general purpose outdoor sound prediction software, which will be able to evaluate noise at airports vicinity as industrial noise.Directivity and sound power levels have been estimated in octave and third octave band terms, for several aircraft families.     

Towards a better understanding of temporary threshold shift of hearing

It was thought that temporary threshold shift of hearing due to exposure to noise might be more easily understood if the shifts were considered in terms of the rms pressure rather than in decibels. Therefore, the forms to be expected if the rate of shift of the pressure threshold were proportional to the difference between itself and the ultimate threshold were calculated and compared with a limited selection of published data. Good agreement with data for individuals during growth of TTS, and for one example of intermittent exposure to noise, was found and moderately good agreement with recovery. Agreement with averaged data, particularly for intermittent exposures, was poor, possibly because averaging the widely disparate figures obtained for individuals masks the true effects. It is also shown that the maximum ultimate TTS due to exposure to noise may be simply related to the mean square pressure of that noise.Further consideration of the mass of published work is needed, but this study suggests that at least some facets of TTS can be simply described in terms of exponential pressure shifts.     

Responses of "lower-spontaneous-rate" auditory-nerve fibers to speech syllables presented in noise. I: General characteristics.

Responses of auditory-nerve fibers in anesthetized cats to nine different spoken stop- and nasal-consonant/vowel syllables presented at 70 dB SPL in various levels of speech-shaped noise [signal-to-noise (S/N) ratios of 30, 20, 10, and 0 dB] are reported. The temporal aspects of speech encoding were analyzed using spectrograms. The responses of the "lower-spontaneous-rate" fibers (less than 20/s) were found to be more limited than those of the high-spontaneous-rate fibers. The lower-spontaneous-rate fibers did not encode noise-only portions of the stimulus at the lowest noise level (S/N = 30 dB) and only responded to the consonant if there was a formant or major spectral peak near its characteristic frequency. The fibers' responses at the higher noise levels were compared to those obtained at the lowest noise level using the covariance as a quantitative measure of signal degradation. The lower-spontaneous-rate fibers were found to preserve more of their initial temporal encoding than high-spontaneous-rate fibers of the same characteristic frequency. The auditory-nerve fibers' responses were also analyzed for rate-place encoding of the stimuli. The results are similar to those found for temporal encoding.     

Distribution of spectral density for underwater noise in the shelf zone of the Pacific Ocean

The results of measurements for the distribution of spectral density for underwater noise in the shelf zone of the Pacific Ocean 21 km from Shikotan Island at the depth of 130 m within the frequency range 1.9–11000 Hz at the wind speed 0–40 m/s are given. Measurements were conducted within the bands of 1/3- and 1/2-octave filters over six months. In the case of a realization length of 33 min and smaller, the distribution of the instant pressure values for underwater noise can be considered normal. The distribution of deep slow variations for the intensity of underwater noise at all frequencies differed from the normal one. The values for the asymmetry parameter and the coefficient of excess are given for the variation distribution of spectral density for underwater noise.     

Method of control of the noise level of road vehicles in use

The elaboration of the method of control of noise generated by moving road vehicles has been made possible by using previous measurements of external noise produced by each of those particular vehicles. The results have made it possible to lay down the requirements for the design of specialised instrumentation capable of making such measurements. The advantage of this development is that it makes it possible for inexperienced technical staff to obtain the required acoustic measurements. The previously employed methods of noise prediction have required preceisely defined measuring conditions at the measuring site. The measurements, besides being laborious and time-consuming, give little information concerning the technical condition of the vehicle. On the contrary, with the method presented in this paper it is possible to measure not only the sound pressure level of the general external noise of the vehicle but also the band levels in the two frequency bands which are characteristic of the working engine at a definite speed of rotation, thus giving a preliminary estimation of the vehicle's noisiness. Defining the limiting values for noise level and band levels will enable excessively noisy vehicles to be eliminated.The results of measurements obtained with the above technique for various groups of vehicles are presented in this paper.     

The Rayleigh integral is always positive in steadily operated combustors

The Rayleigh index has been used for decades by a large number of researchers as an indicator to determine if a flame is driving or damping thermoacoustic interaction mechanisms. The use of the Rayleigh criterion has found applications in rocket combustors, gas turbine combustion technology and basic combustion research. The global Rayleigh index or integral is obtained by integrating the product of heat release rate and pressure fluctuations over space and time. Depending on the phase between pressure oscillations and heat release rate response, the oscillations can be enhanced or damped. It is commonly assumed in literature that the sign of the Rayleigh index from steady state data can be used to determine if the thermoacoustic feedback loop is stabilizing or destabilizing. However, we show in this paper that under fairly general conditions, a correctly measured Rayleigh index is always positive if evaluated from statistically stationary data. This proves to be true even if the heat release rate response to pressure fluctuations is in phase opposition to those pressure fluctuations. This is shown in a straightforward manner by substituting the wave equation with a heat release rate source term into the Rayleigh index. This was verified experimentally on a fully premixed combustion system by measuring the flame chemiluminescence using a photo multiplier and pressure fluctuations using a microphone placed sufficiently close to the flame to ensure acoustic compactness for the frequency range of interest. A large range of operating conditions have been tested, spanning linearly stable and unstable stationary thermoacoustic states, respectively corresponding to resonance or a limit cycle driven by the inherent stochastic forcing from the turbulent combustion noise. The experimental results corroborated the analytic finding: the Rayleigh index is found to be positive for all frequencies and all operating conditions.     

海面波浪谱对深海低频环境噪声的影响

提出了一种适用于深海低频环境噪声的波浪谱,通过声压谱和波浪谱的理论关系,分析了深海低频噪声在百赫兹以下的谱特征,解释了不同频段噪声谱的主要产生机理。将深海传播条件下海面波浪谱与海面风速相结合,利用波浪发声理论得到一种低频海洋环境噪声理论表示方法。仿真结果表明,波浪谱决定着辐射噪声谱的强度和斜率,本模型得到的理论噪声谱可以对低频海洋环境噪声进行预报。2016年的深海实验观测数据分析显示,统计的环境噪声谱级在1 Hz至100 Hz频段范围内大于70 dB,并且噪声谱在低频段呈倒“N”型,在34 Hz处为噪声谱的谷值,噪声级为70 dB,在50 Hz处为噪声谱的峰值,噪声级为92 dB,通过理论计算和实验对比,相关系数为0.95,理论结果和实验测量对比结果符合较好。      

Differences in voice quality between men and women: Use of the long-term average spectrum (LTAS)

The goal of this study was to determine if there are acoustical differences between male and female voices, and if there are, where exactly do these differences lie. Extended speech samples were used. The recorded readings of a text by 31 women and by 24 men were analyzed by means of the Long-term Spectrum (LTAS), extracting the amplitude values (in decibels) at intervals of 160 Hz over a range of 8 kHz. The results showed a significant difference between genders, as well as an interaction of gender and frequency level. The female voice showed greater levels of aspiration noise, located in the spectral regions corresponding to the third formant, which causes the female voice to have a more “breathy” quality than the male voice. The lower spectral tilt in the women's voices is another consequence of this presence of greater aspiration noise.     

The effects of the ocean surface wave spectrum on low frequency ambient noise in deep water

An ocean surface wave spectrum which is used for low frequency ambient noise in deep water is proposed. It explains the mechanism of low frequency ambient noise from the theoretical relation between the spectrum of sound pressure and wave. Combining the surface wave spectrum and local wind speed in deep water, a theoretical expression of low frequency ambient noise is obtained with wave generated noise theory. Simulation results show that the wave spectrum is crucial to the intensity and the spectral slope of radiated noise spectrum,and the theoretical noise spectrum could be used to predict the ambient noise in deep water.The predicting results axe verified through the experimental data recorded by an ocean bottom seismometer that was deployed on the floor of deep water in April 2016. It is observed that the statistical noise levels from the experimental data for frequencies from 1 Hz to 100 Hz are larger than 70 dB, and the low frequency ambient noise spectrum follows the shape of inverted"N",the valley of noise spectrum is at 3-4 Hz, and the noise intensity is 70 dB. The peak of noise spectrum is at 50 Hz, and the noise intensity is 92 dB. The correlation coefficient is 0.95 between the model spectrum and measured data.     

Investigation of a glottal related harmonics-to-noise ratio and spectral tilt as indicators of glottal noise in synthesized and human voice signals

The harmonics-to-noise ratio (HNR) of the voiced speech signal has implicitly been used to infer information regarding the turbulent noise level at the glottis. However, two problems exist for inferring glottal noise attributes from the HNR of the speech wave form: (i) the measure is fundamental frequency (f0) dependent for equal levels of glottal noise, and (ii) any deviation from signal periodicity affects the ratio, not just turbulent noise. An alternative harmonics-to-noise ratio formulation [glottal related HNR (GHNR')] is proposed to overcome the former problem. In GHNR' a mean over the spectral range of interest of the HNRs at specific harmonic/between-harmonic frequencies (expressed in linear scale) is calculated. For the latter issue [(ii)] two spectral tilt measures are shown, using synthesis data, to be sensitive to glottal noise while at the same time being comparatively insensitive to other glottal aperiodicities. The theoretical development predicts that the spectral tilt measures reduce as noise levels increase. A conventional HNR estimator, GHNR' and two spectral tilt measures are applied to a data set of 13 pathological and 12 normal voice samples. One of the tilt measures and GHNR' are shown to provide statistically significant differentiating power over a conventional HNR estimator.     

Investigation of the dose-response relationship for road traffic noise in Assiut, Egypt

A field study has been carried out in urban Assiut city, Egypt. The goals of this study are: (1) to carry out measurements to evaluate road traffic noise levels, (2) to determine if these levels exceeds permissible levels, (3) to examine people’s attitudes towards road traffic noise, (4) to ascertain the relationship between road traffic noise levels and degree of annoyance. The measurements indicate that traffic noise noise levels are higher than those set by Egyptian noise standards and policy to protect public health and welfare in residential areas: equivalent continuous A - weighted sound pressure levels (L_A eq) = 80 dB and higher were recorded, while maximum permissible level is 65 dB. There is a strong relationship between road traffic noise levels and percentage of highly annoyed respondents. Higher road traffic noise levels mean that the percentage of respondents who feel highly annoyed is also increased.     

The noise exposure of infants in incubators

The noise exposure of infants in incubators due to both services noise and self-generated noise has been measured in an investigation involving 45 incubators and 69 infants. Incubator services noise levels were consistent with those reported in previous surveys but the noise produced by the infants has been found to increase levels by approximately 8 dB(A) on average. Statistical distribution analysis of the noise levels has shown that energy content of the infant generated noise has maximum values between 90 dB(A) and 100 dB(A) and peak levels of 107 dB(A) have been recorded. The possibility of the measured sound pressure levels inducing cochlear damage is discussed and an assessment is made of incubator services noise which suggest a design level of 45 dB(A) for new incubators and a limiting sound level of 55 dB(A) during normal usage.     

Effect of unsteady pressure rise on flame propagation and near-cold-wall ignition

Thermodynamic pressure rise during combustion is a key feature in internal combustion engines. Yet, hardly any studies have been conducted to investigate the effects of transient pressure rise on flame propagation as well as on the ignition of the unburned gas. In this study, the effects of unsteady pressure rise were parametrically studied using a one-dimensional reacting flow model in which the thermodynamic pressure variation is an independent variable and thus its rate of rise can be controlled. It was determined that large rates of pressure rise can significantly increase the mass burning flux of a laminar flame and that this modification becomes more pronounced at higher pressure and temperature conditions. Furthermore, it was shown that the development of ignition near a cold wall, for mixtures that exhibit negative temperature coefficient behavior, is very sensitive to rate of change of pressure. The near-wall ignition behavior was found also to be rather sensitive to the prevailing pressures and temperatures whose values control whether ignition will occur in the main-gas or within the thermal boundary layer.