Investigation of the noise emitting zones of a cold jet via causality correlations

Within the framework of lighthill's acoustic analogy the causality method proposed by Ribner and Siddon is used to identify equivalent noise sources inside a cold jet. An exploration of a few cross-sections shows that a two-dimensional investigation suffices in a first approach for integrating the source function provided the upper frequency limit does not exceed a Strouhal number approximately equal to 0·5. Furthermore the transverse distribution of the source term shows the jet region located on the microphone side to be dominant; the effective diameter of the source region is comparable with that of the nozzle. It is shown that in a direction with an angle of 30° to the jet axis, the “shear noise” is dominant (about 70% of the noise measured in the same frequency range). The noise emanates essentially from the transition region and from layers located between $\text{r}\text{D}\text{= ±0·25}$ and $\text{r}\text{D}\text{= ±0·375}$. This analysis is suitable for frequency range bounded above by St = 0·54. For the direction with an angle of 45° to the jet axis comparable results are obtained in a frequency range also limited at St = 0·54. However, this range contains only 40% of the total acoustic energy. The source region of the “shear noise” (near 70% of the total energy) and that of the “self-noise” remain always in the transition region located at 4–11 D. Radially the main part of the noise originates from the layers located on the microphone side between $\text{r}\text{D}\text{= 0·25}$ and 0·375. For the direction with an angle of 60° to the jet axis the “shear noise” is no longer measurable and the calculated “self-noise” represents only a few percent of the noise measured. For an acoustically excited jet (white noise filtered between St = 0·39 and St = 0·52, 320 Pa at the nozzle) another type of correlation appears which is believed to be related to a coherent structure travelling inside the jet at 0·75 v_j. The study of the source term shows that this structure must be related to noise originating from the nozzle outlet.     

Active noise control in finite length ducts

A simple technique for the active control of sound in ducts, initially suggested by Olson and May [1], is investigated in detail. A simple, “virtual earth” principle, feedback loop is used to drive the sound pressure to a minimum at a microphone placed close to a loudspeaker in the duct wall. This produces a reflection of downstream travelling plane waves. A detailed investigation of the loudspeaker near field has enabled the optimum position of the microphone to be identified. The system is shown to be especially effective at the frequencies of the longitudinal duct resonances, where the acoustic response of the duct produces a high loop gain. Results are presented which show a reduction of up to 20 dB in the amplitude of low frequency broadband noise at a position downstream of the cancelling source.     

Predicting the sound emission from air-conditioning and ventilating systems

Air-conditioning noise is, of course, only one part of the background noise spectrum for a space, but it has become significant over the last decade as more high velocity systems are being installed, and as people have become more sensitive to internal noise sources in buildings well insulated from the external environment. Research has been carried out to investigate the nature and level of sound emitted in rooms served by air-conditioning systems. An initial survey measured the sound spectra in 74 university lecture rooms. Fans, motors, vee-belt drives and an airflow system were then installed to serve one lecture room. It was found that the sound pressure level in the room could be predicted by the equation:

$\text{L}\text{=40}\text{v}\text{17}\text{+1}$

or:

$\text{L}\text{=37.4}\text{N}\text{2110}\text{+1}$

where $\text{L}$ is the sound pressure level in dB(A), $\text{v}$ is the air velocity in the main duct and $\text{N}$ is the fan speed (rpm). Further field studies are being undertaken on many systems in buildings to find out if a general formula can be applied in practice.     

气流管道有源降噪研究

如何消除气流的影响是管道有源降噪的难点之一，本文分析了湍流对有源降噪系统的影响，并开发了一种抗湍流传声器探管，最后在某种通风系统的进气管段进行了降噪试验，气流速度为２０ｍ／ｓ时，在６０－６３０Ｈｚ频带取得了１５ｄＢ（Ａ）的降噪效果。     

On the prediction of impact noise,V: The noise from drop hammers

In earlier papers in this series, the concepts of “acceleration” and “ringing” noise have been studied in relation to impact machines, and values of radiation efficiency have been obtained for the various types of structural components. In the work reported in this paper the predicted and measured noise radiation from a drop hammer, both in full-scale and in $\text{1}\text{3}\text{-}\text{scale}$ model form, were examined. It is found that overall noise levels (L_eq per event) can be predicted from vibration measurements to within ± 1·5 dB, and to within ±2·5 dB in one-third octave bands. In turn this has permitted noise reduction techniques to be examined by studies of local component vibration levels rather than overall noise, a method which provides considerable enlightenment at the design stage. It is shown that on one particular drop hammer, the noise energy is shared surprisingly uniformly over four or five sources, and that when these have been reduced, the overall noise reduction is severely limited by the “acceleration” noise from the “tup” or “hammer” itself. As this is difficult to eliminate without a basic change in forging technology, it follows that “tup” enclosure or modification of the sharpness of the final “hard” impact are the only means available for any serious noise reduction. Also indicated is the reliability of using model techniques, suitably scaled in frequency and impulse magnitude, in developing machinery with impact characteristics.     

Aerodynamic sound production in low speed flow ducts

Measurements of spoiler aerodynamic noise, generated in a low velocity flow duct and radiated from an open exhaust termination, have been made in the form of sound power spectra. The individual $\text{1}\text{3}$ octave power measurements are satisfactorily collapsed (within ± 3 dB) with the aid of derived theoretical scaling laws. Non-dimensional spectra are presented which permit generalized predictions of flow noise for bluff bodies, including splitter attenuators, mounted in low speed flow ducts.     

An adaptive controller for multivariable active noise control

The control of vibration through the mounts of rotating machines can be achieved by actively generating cancelling forces from shakers located close to the mounts. The cancelling waveforms cannot simply be an antiphase copy of the original waveform as each shaker affects the vibration at mounts other than the one at which it is cancelling. This paper describes an approach to this multivariable control problem which measures all shaker to sensor transfer functions to give a shaker transfer function matrix, $\text{M}\text{(f)}$. The shakers are driven by voltages $\text{V}\text{(f)}$ given by $\text{V}\text{(f) = ?M}{}^{\mathrm{?1}}\text{(f)}\text{U}\text{(f)}$ where $\text{U}\text{(f)}$ is the vector of accelerations at the mounts. The controller then repeats the algorithm, this time operating on the residual accelerations at the sensors. The system is therefore adaptive and can cope with slowly changing noise spectra. Cancellations of better than ?25 dB have been achieved.     

Experiments on effective source locations and velocity dependence of the broad band noise from a rotating rod

Effective acoustic source positions (observed from the far field) have been located for the broad band noise from a cylindrical rod rotated about its mid-point by measuring the cross spectral density function of two microphone signals on the axis of rotation. Local source position Strouhal numbers could thereby be calculated. On the basis of acoustic power measurements it was demonstrated that the noise may be normalized on a rod tip Strouhal number basis, and that the velocity exponent is nearly constant when plotted against this parameter. The results indicate that vortex shedding like that for stationary cylinders in a cross flow (occurring along the outer $\text{1}\text{3}$ of the rod for a rotational speed of 1000 rpm) is responsible for the high levels of broad band noise in a major peak region. Sources influenced by harmonics of the rod passing frequency were found for frequencies lower than the vortex shedding ones. At higher frequencies broad band noise was found to be emitted from the rod tip area.     

Energetic analysis of an actively controlled one-dimensional acoustic waveguide

The aim of this work is to show the energetic behavior when an active noise controller is applied in a one-dimensional waveguide, namely an ideal duct under the first critical frequency. In order to model the duct, a spectral element method, which is shown to be more practical for analyzing pipe networks than other commonly used analytical models, was used. The model used here consists of a duct with two sources, the primary source at one end of the duct, and the secondary source at the middle section. The error sensor was placed downstream from secondary source, and the other end of the duct was open with no flange. Three optimal control methods were applied: minimization of the potential energy density, minimization of the active intensity, and minimization of the total acoustic power radiated by the sources. It was observed that the three control methods achieved the same final result, and when the volume velocity of the secondary source was driven to the optimal volume velocity, neither the primary source nor the secondary source radiated any acoustic power. Furthermore, the controlled duct was equivalent to a duct opened-ended at the secondary source position with radiation impedance equal to zero.     

An experimental study of a broadband active attenuator for cancellation of random noise in ducts

A broadband active attenuator for cancellation of plane wave duct noise has been demonstrated. Reductions of 16–20 dB RMS were achieved with random noise over the bandwidth 30–650 Hz, and 20 dB with transient noise. A two-microphone and two-loudspeaker unidirectional coupler arrangement was employed in the attenuator, with loudspeaker motional feedback, and a transversal filter which corrected residual amplitude and phase errors.     

Adaptive fuzzy neural network control on the acoustic field in a duct

Base on the principle of the superposition of waves, active noise control is achieved by adaptively tuning a secondary source which produces an anti-noise of equal amplitude and opposite phase with primary source. This paper presents the study on the acoustic attenuation in a duct by using the combination of fuzzy neural network with error back propagation algorithm to control secondary source. The most important advantage of fuzzy inference system is that the structured knowledge is represented in the form of fuzzy IF-THEN rules. But it lacks the ability to accommodate the change of external environments. Combining neural network with fuzzy system can help in this tuning process by adapting fuzzy sets and creating fuzzy rules. The performance of attenuation and control error can be measured by the microphone placed in the downstream of duct. The results of this study, show that the acoustic attenuation by 40 dB for pure-tone noise and nearly 30 dB for dual-tones noise are obtained.     

Attenuation of traffic noise indices Leq and LN

Analysis of the attenuation of $\text{L}{}_{\mathrm{eq}}$ and $\text{L}{}_{\mathrm{N}}$ values with distance over grassland from a line of moving traffic indicates that the noise sources effectively radiate energy in the horizontal plane with a distance term different from the commonly assumed inverse square law. This requires an adjustment to the angular corrections for partial screening which now become dependent on the noise index being measured. The attentuation of traffic noise by a wooden fence structure around a suburban house has also been investigated. Shielding effects caused by various components were isolated as the fence structure was sequentially demolished. The shape of the sound field behind the finite length of front fence was deduced, for the various $\text{L}{}_{\mathrm{N}}$ values, by using the amended angular corrections.     

参量阵扬声器在管道噪声控制中的研究*

为了解决管道有源噪声控制中声反馈造成的系统复杂度和计算量的增加,文中引入参量阵扬声器作为次级声源,利用其强指向性减小控制系统的声反馈。为了验证该方法可行性,本文分别在直管和L管中,对600 Hz单频噪声和频率范围为500 Hz~1000 Hz的窄带噪声进行了管道有源噪声控制,同时测量了参量阵扬声器的管内声场和降噪范围。结果表明,参量阵扬声器声反馈小,在没有声反馈补偿的条件下对单频噪声的降噪效果基本达到了声反馈补偿条件下普通扬声器的降噪效果,对窄带噪声的降噪效果稍差。此外,通过测量管道声场和降噪量,确定了参量阵扬声器的降噪区域为误差传感器下游整个管道,降噪面积为管道整个截面。这说明参量阵扬声器作为次级声源降低了系统的复杂度和算法的计算量,并取得了较好的降噪效果。     

Multiple pure tone noise prediction

This paper presents a fully numerical method for predicting multiple pure tones, also known as “Buzzsaw” noise. It consists of three steps that account for noise source generation, nonlinear acoustic propagation with hard as well as lined walls inside the nacelle, and linear acoustic propagation outside the engine. Noise generation is modeled by steady, part-annulus computational fluid dynamics (CFD) simulations. A linear superposition algorithm is used to construct full-annulus shock/pressure pattern just upstream of the fan from part-annulus CFD results. Nonlinear wave propagation is carried out inside the duct using a pseudo-two-dimensional solution of Burgers? equation. Scattering from nacelle lip as well as radiation to farfield is performed using the commercial solver ACTRAN/TM. The proposed prediction process is verified by comparing against full-annulus CFD simulations as well as against static engine test data for a typical high bypass ratio aircraft engine with hardwall as well as lined inlets. Comparisons are drawn against nacelle unsteady pressure transducer measurements at two axial locations as well as against near- and far-field microphone array measurements outside the duct.     

Application of the short-term Leqto the identification and quantification of several sources constituting an environment

A technique applicable to situations with different noise sources is developed on the basis of a series of limited data (short-term $\text{L}{}_{\mathrm{<mtext>eq</mtext>}}$).This technique makes acoustic monitoring of a site possible, identifying and evaluating the respective contributions of the different components of the environment studied.     

Velocity tuned resonances as multi-doppleron processes

We consider multi-photon processes for a Doppler broadened two-level system. Alternative absorption and emission from two oppositely travelling waves is interpreted as a multi-quantum absorption (or emission) process with quanta $\text{h}\text{?}$kv. These are called Dopplerons and the analogy with the RF work is established. The calculations also display a resonance caused by slow atoms.     

High energy xps using a monochromated Ag Lα source: resolution,sensitivity and photoelectric cross sections

Resolution, sensitivity and calibration data are presented for a novel high energy XPS source, monochromated Ag Lα radiation (hv = 2984.3 eV). Adequate resolution is attainable for good signal/noise spectra, whilst values for experimental sensitivity factors agree well with theoretical cross section values calculated by Nefedov. This allows an evaluation of ESCA 3 Mk. II transmission function up to 3000 eV, which appears to obey an approximate E^{$\text{?1}\text{2}$} dependence. Monochromated Ag Lα (linewidth 1.3 eV) overcomes the problem of broad natural linewidths for high energy sources, such that chemical state information can be gained. Various new core level and Auger peaks are developed, a notable feature being the 1s core level and KLL Auger transition capability from Al through to Cl. Improved sensitivity is experienced for elements whose major peaks occur in the 1500–3000 eV BE range, whilst there is no serious reduction of sensitivity in the conventional XPS energy range.     

Environmental acoustic quality in Jeddah urban sites

Surveys of physical exposure to noise at urban sites in Jeddah city indicate that noise from road traffic is very intensive. Relatively high instant sound levels (90 dB(A) and higher) were recorded on a numbr of congested, as well as freely-flowing traffic, roads. Values of the statistical indicators $\text{L}{}_{\mathrm{<mtext>10</mtext>}}$, $\text{L}{}_{\mathrm{<mtext>50</mtext>}}$, $\text{L}{}_{\mathrm{<mtext>90</mtext>}}$ and $\text{L}{}_{\mathrm{<mtext>dn</mtext>}}$ were determined for indoor domestic noise and comparisons with current western standards show that the levels determined exceeded the limits of dissatisfaction given by those standards. Overall sound pressure levels measured inside typical university offices indicate that the presence of individual room-units of air-conditioning impairs the acoustic quality in those environments. The results of a social survey on noise perception in residential areas are in good agreement with the above findings, indicating that 89 per cent of people interviewed were substantially disturbed by traffic noise. The results of this research, however, demonstrate the necessity for the application of a traffic noise control programme on Jeddah main roads and also the need for attention to be paid to the indoor acoustic quality of homes and offices.     

Origin of 1ƒ noise

$\text{1}\text{?}$ noise dependence on temperature for both n- and p-type silicon has been measured. The formula for calculation of $\text{1}\text{?}$ noise is presented. The formula has been derived by presumption that only lattice scattering gives $\text{1}\text{?}$ noise. A good agreement between calculations and experimental results is found.     

An experimental study of swinbanks' method of active attenuation of sound in ducts

The concept of attenuating sound propagating down a duct by means of an antiphase copy of the sound was first put forward forty years ago, but to date no practical system has been produced. The principal problem is to introduce the antiphase signal in such a way that it propagates only in the direction of propagation of the original sound and to ensure that this property can be maintained over a useful frequency range.A suitable arrangement of secondary sound sources has recently been proposed by Swinbanks and this paper describes experiments carried out in an attempt to realise a practical system.A unidirectional array of secondary sources has been successfully constructed around a rectangular duct, using loudspeaker drive units and electronic delays. Sound propagating in the direction of these sources was sampled and a control signal applied to the sources which in turn acted to significantly reduce the amplitude of the sound. Pure tones at frequencies around 150 Hz have been attenuated by more than 50 dB but results with band-limited noise have been less successful. Further work is suggested which should result in a device having significant advantages over conventional splitter silencers, at low frequencies.