K.R. Holland

The effect of reverberation on the damping of bubbles

The measurement of an acoustic emission, or scatter, from a bubble is not difficult. However, an accurate interpretation of that signal in terms of the bubble dynamics may require careful consideration. The study presented here is at first sight relatively simple: comparison of the predicted and measured quality factors of injected bubbles. While the measurement is normally done by monitoring the decay of passive emissions from a bubble, this technique becomes difficult with smaller bubbles. Therefore an active technique is introduced, which removes all the frequency-dependent effects on the measurement (such as transducer response) bar one. That, critically, is the effect of the change in the bubble resonance (frequency and damping) which results from the loading on the bubble due to the reverberant field. The vast majority of theoretical treatments of bubble acoustics assume free field conditions, yet the environmental conditions rarely if ever match these. Therefore measurements of bubble damping are compared both with the established free field theory, and with a new theory relevant to the prevailing reverberant conditions (whether caused by tank surfaces, monochromatic neighboring bubbles, or both).

Minimally radiating sources for personal audio

In order to reduce annoyance from the audio output of personal devices, it is necessary to maintain the sound level at the user position while minimizing the levels elsewhere. If the dark zone, within which the sound is to be minimized, extends over the whole far field of the source, the problem reduces to that of minimizing the radiated sound power while maintaining the pressure level at the user position. It is shown analytically that the optimum two-source array then has a hypercardioid directivity and gives about 7 dB reduction in radiated sound power, compared with a monopole producing the same on-axis pressure. The performance of other linear arrays is studied using monopole simulations for the motivating example of a mobile phone. The trade-off is investigated between the performance in reducing radiated noise, and the electrical power required to drive the array for different numbers of elements. It is shown for both simulations and experiments conducted on a small array of loudspeakers under anechoic conditions, that both two and three element arrays provide a reasonable compromise between these competing requirements. The implementation of the two-source array in a coupled enclosure is also shown to reduce the electrical power requirements.

The measurement and prediction of sound waves of arbitrary amplitude in practical flow ducts

A study is presented that explores the influence of the peak to mean pressure ratio on the wave action occurring in highly acoustically reactive ducts with significant flow present. Particular practical applications concern predictions of orifice noise emissions from piston engine or compressor intake and exhaust systems, with the effect of acoustic resonances on both excitation and acoustic power transmission. The results indicate that any practical differences between linear and non-linear predictions remain negligibly small when the pressure ratio remains below 1.1, corresponding to 170 dB spl, provided that the influence of all frequency-dependent physical features is included. Above this level of excitation in any lengths of uniform pipe connecting other system components, some new observations demonstrate the extent to which compression wave steepening may be of practical significance in the spectral distribution of the power propagated.

Aeroacoustic sound generation in simple expansion chambers

A method is presented for measuring the aeroacoustic source strength in ducts with flow at frequencies at which the wave motion can be considered substantially one-dimensional. The method is based on coherent power flux measurements using pairs of microphones positioned both upstream and downstream of the source region. The method is applied to a flow excited expansion chamber with aeroacoustic source measurements presented for chambers with a range of flow velocities and chamber lengths. The results indicate locked-on flow tones are generated in the chamber. The frequency of these locked-on flow tones is compared with that predicted using describing function theory applied to resonators with a grazing flow as well as that of other literature.

Scan and Paint: Theory and Practice of a Sound Field Visualization Method

Sound visualization techniques have played a key role in the development of acoustics throughout history. The development of measurement apparatus and techniques for displaying sound and vibration phenomena has provided excellent tools for building understanding about specific problems. Traditional methods, such as step-by-step measurements or simultaneous multichannel systems, have a strong tradeoff between time requirements, flexibility, and cost. However, if the sound field can be assumed time stationary, scanning methods allow us to assess variations across space with a single transducer, as long as the position of the sensor is known. The proposed technique, Scan and Paint, is based on the acquisition of sound pressure and particle velocity by manually moving a P-U probe (pressure-particle velocity sensors) across a sound field whilst filming the event with a camera. The sensor position is extracted by applying automatic color tracking to each frame of the recorded video. It is then possible to visualize sound variations across the space in terms of sound pressure, particle velocity, or acoustic intensity. In this paper, not only the theoretical foundations of the method, but also its practical applications are explored such as scanning transfer path analysis, source radiation characterization, operational deflection shapes, virtual phased arrays, material characterization, and acoustic intensity vector field mapping.

Sound power flux measurements in strongly exited ducts with flow

This contribution describes new robust procedures for the measurement of sound power flux at appropriate axial positions along a duct with flow, using pairs of flush wall mounted microphones, or pressure transducers. The technology includes the application of selective averaging, order tracking, and optimized sampling rate methods to identify the small fraction of the total fluctuating wave energy that is being propagated along the flow path in a reverberent, or highly reactive duct system. Such measurements can also be used to quantify the local acoustic characteristics that govern the generation, transfer, and propagation of wave energy in the system. Illustrative examples include the determination of the acoustic characteristics of individual silencing elements installed in IC engine intakes and exhausts both on the flow bench and during controlled acceleration or run down on a test bed, where the wave component spectral levels approached 170 dB.

Aft fan noise reduction with a lined afterbody

Aft fan noise is becoming a more dominant source as engine bypass ratio is increased and improved methods are for required for its control. Acoustic linings are an effective means of reducing internally generated noise and bypass liners are especially effective in attenuating aft fan noise. Previous calculations have shown that as the fan noise emerges from the bypass exhaust nozzle, a significant part of the sound field radiates directly out through the shear layer but there is also another propagation path, along which the sound field is first reflected from the hard ‘afterbody’ before it propagates out to the far field. It has been suggested that if the afterbody were acoustically lined, it would reduce the strength of that reflected field and hence the far field noise level. Here we describe results from some preliminary tests on a rig, using scaled, locally reacting, linear SDOF acoustic linings to simulate the conventional, internal bypass liners and also the new external Afterbody Liner (AL). The measured data from this ‘No-flow’ rig indicate that up to 3 dB PWL insertion loss can be achieved with an AL in addition to that achieved with conventional bypass liners. Calculations agree well with the measured PWL insertion loss spectrum although the measured SPL reductions are not so well predicted at some angles. This novel application of conventional liner technology to the external afterbody of an engine nacelle may provide some significant reductions in aft broadband fan noise levels although it should be emphasised that the results obtained so far are without flow.

Extraction of Turbofan Combustion Noise Spectra Using a combined Coherence-Beamforming Technique

A new processing technique to extract turbofan engine combustion noise called 3S-Array is presented. It has been developed using a multiple coherence technique with data acquired in the in-duct and external sound fields of a jet engine. In-duct sensors are located in the combustion chamber and in the nozzle of the engine, and external data is acquired using an array of microphones. A beamformed signal focused on the nozzle of the engine is generated with the data from the external array. Jet noise and influences of the room on the array output are reduced using this focusing technique, which is referred to as Focused Beamformed Output (FBO). Results show that using this new 3S-Array technique with two of the in-duct sensors and the focused beamformed signal as the third one, provides a better estimator of combustion noise than the 3-signal coherence technique alone, or the Coherence-Output Power Spectra (COP), both of which are reported in the literature as methods for the extraction of combustion noise from the radiated noise spectrum.

Further Development of Velocity-based Airborne TPA: Scan & Paint TPA as a Fast Tool for Sound Source Ranking

The interior noise of a car is a general quality index for many OEM manufacturers. A reliable method for sound source ranking is often required in order to improve the acoustic performance. The final goal is to reduce the noise at some positions inside the car with the minimum impact on costs and weight. Although different methodologies for sound source localization (like beamforming or p-p sound intensity) are available on the market, those pressure-based measurement methods are not very suitable for such a complex environment. Apart from scientific considerations any methodology should be also “friendly” in term of cost, time and background knowledge required for post-processing. In this paper a novel approach for sound source localization is studied based on the direct measurement of the acoustic particle velocity distribution close to the surface. An airborne transfer path analysis is then performed to rank the sound pressure contribution from each sound source. The method called “Scan & Paint TPA” makes use of only one probe that is swept along the surface. The reciprocal transfer functions are measured by a second sweep with the same probe and a monopole sound source in the driving position. A new methodology for applying “Scan & Paint TPA” in a complex acoustic environment is given along with an experimental validation in a car interior.

Fan noise propagation within curved bypass ducts with 3D features

This paper focuses on aft radiated fan noise propagating through the bypass duct: the effect of duct geometry on noise propagation is examined. The acoustic benefits of a novel highly curved bypass geometry are compared with those of a typical modern turbofan engine bypass geometry. Both 2D and 3D finite element models are used to examine the effect of duct geometry and other features on noise propagation, including modal scattering within the duct. An analytic radiation model is also used to examine the effect of these features on sound directivity patterns in the far-field.