Gary Seiffert

Acoustic absorption behaviour of an open-celled aluminium foam

Metal foams, especially close-celled foams, are generally regarded as poor sound absorbers. This paper studies the sound absorption behaviour of the open-celled Al foams manufactured by the infiltration process, and the mechanisms involved. The foams show a significant improvement in sound absorption compared with close-celled Al foams, because of their high flow resistance. The absorption performance can be further enhanced, especially at low frequencies, if the foam panel is backed by an appropriate air gap. Increasing the air-gap depth usually increases both the height and the width of the absorption peak and shifts the peak towards lower frequencies. The foam samples with the smallest pore size exhibit the best absorption capacities when there is no air gap, whereas those with medium pore sizes have the best overall performance when there is an air gap. The typical maximum absorption coefficient, noise reduction coefficient and half-width of the absorption peak are 0.96–0.99, 0.44–0.62 and 1500–3500 Hz, respectively. The sound dissipation mechanisms in the open-celled foams are principally viscous and thermal losses when there is no air-gap backing and predominantly Helmholtz resonant absorption when there is an air-gap backing.

Removal of electrostatically deposited powders using high intensity low frequency sound, part 1: Experimental deposition and removal

This paper describes the controlled removal of powder layers using high intensity low frequency sound. The research has an industrial application, the cleaning of electrostatic precipitator filters in coal-fired power stations. The aim of the work is to quantify the sound pressure level and frequency content required to overcome adhesive and cohesive bonding forces formed when powder is electrostatically deposited onto a metal surface. In this, the first of two papers, a repeatable method for electrostatically depositing powder layers on to a metal surface is described. The design, construction and calibration of a high intensity wave tube also are described. The measured sound pressure level required for removal is compared with prediction. Measurements in a reverberant acoustic field using commercially available acoustic cleaning systems also are reported.

Removal of Electrostatically Deposited Powders Using High Intensity Low Frequency Sound, Part 2: Quantification of Adhesive and Cohesive Forces Using Vibration

This is the second of two papers on the controlled removal of electrostatically deposited powder deposits. The research has an industrial application, the cleaning of electrostatic precipitator (ESP) filters in coal-fired power stations. The first paper focused on the removal of powder layers from a metal surface using sound. The aim of the work, reported in this paper, is to quantify the adhesive and cohesive bonding forces formed when powder is electrostatically deposited on to a metal surface by removing deposited layers using vibration. It is these forces that sound must overcome in order to remove deposited layers. It is hoped to establish a link between the layer de-bonding acceleration and de-bonding sound pressure level. Powder layers were electrostatically deposited on to one surface of an aluminium cube. The deposited layers then were removed, using a calibrated vibration table, in order to estimate powder bonding forces.

Vibrotactile Presentation of Musical Notes to the Glabrous Skin for Adults with Normal Hearing or a Hearing Impairment: Thresholds, Dynamic Range and High-Frequency Perception.

Presentation of music as vibration to the skin has the potential to facilitate interaction between musicians with hearing impairments and other musicians during group performance. Vibrotactile thresholds have been determined to assess the potential for vibrotactile presentation of music to the glabrous skin of the fingertip, forefoot and heel. No significant differences were found between the thresholds for sinusoids representing notes between C1 and C6 when presented to the fingertip of participants with normal hearing and with a severe or profound hearing loss. For participants with normal hearing, thresholds for notes between C1 and C6 showed the characteristic U-shape curve for the fingertip, but not for the forefoot and heel. Compared to the fingertip, the forefoot had lower thresholds between C1 and C3, and the heel had lower thresholds between C1 and G2; this is attributed to spatial summation from the Pacinian receptors over the larger contactor area used for the forefoot and heel. Participants with normal hearing assessed the perception of high-frequency vibration using 1s sinusoids presented to the fingertip and were found to be more aware of transient vibration at the beginning and/or end of notes between G4 and C6 when stimuli were presented 10dB above threshold, rather than at threshold. An average of 94% of these participants reported feeling continuous vibration between G4 and G5 with stimuli presented 10dB above threshold. Based on the experimental findings and consideration of health effects relating to vibration exposure, a suitable range of notes for vibrotactile presentation of music is identified as being from C1 to G5. This is more limited than for human hearing but the fundamental frequencies of the human voice, and the notes played by many instruments, lie within it. However, the dynamic range might require compression to avoid the negative effects of amplitude on pitch perception.

Sound Absorption Characteristics of Porous Steel Manufactured by Lost Carbonate Sintering

This paper investigates the sound absorption characteristics of porous steel samples manufactured by Lost Carbonate Sintering. Measurements of the normal incidence sound absorption coefficient were made using an impedance tube for single-layer porous steel discs and assemblies comprising four layers of porous steel discs. The sound absorption coefficient was found not to vary significantly with pore size in the range of 250-1500 m. In general, the absorption coefficient increases with increasing frequency and increasing thickness, and peaks at specific frequencies depending on the porosity. An increase in porosity tends to increase the frequency at which the sound absorption coefficient reaches this peak. An advantage was found in using an assembly of samples with gradient porosities of 75%-70%-65%-60% as it gave higher and more uniform sound absorption coefficients than an assembly with porosities of 75%.

Reciprocal auditory attenuation affects looking behaviour and playing level but not ensemble synchrony: A psychoacoustical study of violin duos

Evidence suggests that musicians may be more susceptible to developing a hearing impairment due to increased exposure to loud sounds over the lifespan. Hearing impairments can affect musical performance behaviours, yet research suggests they do not significantly affect ensemble synchrony unless the hearing loss is severe or profound. This study investigated the effect of reduced auditory feedback on ensemble synchrony, looking behaviour and playing level. Four violinists, with self-reported normal hearing, formed two duos in acoustically-isolated rooms separated by a glass window. Each player received feedback from their own and their co-performer’s playing attenuated by 0, 10, 20, 30 and 40 dB. Video recordings of their looking behaviours were coded and signed asynchronies were identified in the audio files. The strongest effects found were bi-directional changes to playing levels as a result of auditory feedback levels, which increased when a player’s own feedback was reduced and reduced when co-performer feedback was attenuated. Violinists’ looking behaviour was found to increase when co-performer feedback was attenuated by 20 dB or more relative to their own, such that they glanced more frequently and looked for longer towards their partners. There were no effects of auditory attenuation on ensemble synchrony, even with 40 dB attenuation. The results indicate that “self-to-other” sound level ratios are more likely to prompt compensatory musical performance behaviours than an individual’s hearing ability.

Comparison of high‐intensity sound and mechanical vibration for cleaning porous titanium cylinders fabricated using selective laser melting

Abstract Orthopedic components, such as the acetabular cup in total hip joint replacement, can be fabricated using porous metals, such as titanium, and a number of processes, such as selective laser melting. The issue of how to effectively remove loose powder from the pores (residual powder) of such components has not been addressed in the literature. In this work, we investigated the feasibility of two processes, acoustic cleaning using high‐intensity sound inside acoustic horns and mechanical vibration, to remove residual titanium powder from selective laser melting‐fabricated cylinders. With acoustic cleaning, the amount of residual powder removed was not influenced by either the fundamental frequency of the horn used (75 vs. 230 Hz) or, for a given horn, the number of soundings (between 1 and 20). With mechanical vibration, the amount of residual powder removed was not influenced by the application time (10 vs. 20 s). Acoustic cleaning was found to be more reliable and effective in removal of residual powder than cleaning with mechanical vibration. It is concluded that acoustic cleaning using high‐intensity sound has significant potential for use in the final preparation stages of porous metal orthopedic components.

Uncertainties in the Two-Stage Reception Plate Method for Source Characterisation and Prediction of Structure-Borne Sound Power

To obtain the transmitted structure-borne power from a vibrating machine into a supporting/connected structure, three quantities are required in some form: source activity (either the free velocity or the blocked force), source mobility and receiver mobility. The three quantities can be measured directly, or indirectly using a reception plate method. Whilst direct measurements can be precise, they require extensive data acquisition and processing. The reception plate method is simpler and less precise and therefore yields an engineering grade of accuracy. This paper reports on a collaborative investigation, towards developing an industrial standard for source characterization using the reception plate method. The method yields data as frequency band-averaged values and also as equivalent single values. These simplifications result in uncertainties when obtaining the source quantities and therefore in predictions of the structure-borne sound power in installed conditions. The causes of these uncertainties are considered. PACS no. 43.40