Probe Microphone Research Articles

Methods of processing laser Doppler anemometry signals to extract sound field information

For several years laser Doppler anemometry (LDA) has been used to measure mean flow rates in gaseous and liquid flows. It is only recently that LDA has been developed as a technique for recording instantaneous particle velocities, enabling nonintrusive measurements to be made in sound fields. The paper describes a typical experimental setup and discusses various methods of digitally processing the LDA photodetector signals to extract the required velocity information. Particular emphasis is placed on the zero counting and Hilbert transformation techniques. The problem of signal dropout is also addressed. LDA measurements of acoustic particle velocity amplitudes are compared with acoustic velocity amplitudes deduced from probe microphone and impedance measurements. It is found that LDA measurements could be made not only in monofrequency sound fields but also in more complex sound fields containing a fundamental component and additional harmonics. Theoretical and practical limitations of the techniques are investigated and used to assess the viability of applying them to studies in the bores and mouthpieces of brass instruments.

Contralaterally evoked transient otoacoustic emissions

Contralaterally evoked transient otoacoustic emissions (CETOAEs) were recorded from 10 normal-hearing young adults (20 ears) in response to monaural, 11/s, 65 dB pe SPL clicks to the ear contralateral to the microphone probe. A burst of CETOAEs was observed 12–22 ms (average peak at 18.5 ms) after the contralateral click, and its mean level was −7.3 dB pe SPL, 4 dB above the averaged noise level. The frequency content of CETOAEs included a prominence around 1 kHz. In 40% of the ears examined CETOAEs were 3 dB or more above noise level in both replications of records from the same ear. To explain these results CETOAEs are suggested to reflect mechanical events induced by the crossed efferent system in the cochlea that was contralateral to the stimulated ear. The latency of the contralateral responses suggests that they may be related to the contralateral suppression effect observed with binaural stimulation. The latency of the response, coupled with the anatomical origin of the crossed efferent system at the superior olivary complex, suggest its involvement in the contralateral CETOAEs reported here.

Laser vibrometry. A middle ear and cochlear analyzer for noninvasive studies of middle and inner ear function disorders

A complete battery of audiometric methods is required for the differential diagnosis of different hearing disabilities (including puretone audiometry, impedance, stapes reflex, speech audiometry, brainstam evoked response audiometry, otoacoustic emissions, etc.). In many cases, a comprehensive diagnosis is not possible. Here we describe a new technique based on a laser-Doppler vibrometer that has the potential for non-invasive diagnosis not only middle ear disease but also cochlear pathologies. Disturbance of cochlear function can be ascertained because the input impedance of the cochlea acts as a mechanical load on the middle ear and therefore influences motion of the umbo. In the present study vibration of the umbo and eardrum were measured with a commercially available laser-Doppler vibrometer coupled directly into a standard surgical microscope. The use of the microscope allowed non-invasive measurements of vibrations without having to introduce reflecting material onto the tympanic membrane. Sound pressure was measured with a calibrated probe microphone placed near the tympanic membrane. The displacement response and the specific acoustic impedance of the umbo were calculated from the velocity and sound pressure measured. For normal hearing subjects, the amplitude of the umbo's displacement for frequencies from 0.1 kHz to 1 kHz was 1 nm at 60 dB SPL and decreased with a slope of 6 dB/octave for frequencies between 1 and 5 kHz. A strong correlation was found between the specific acoustic impedance of the umbo and hearing thresholds for hearing-impaired subjects (having otosclerosis or sensorineural hearing losses). The frequency response of the umbo proved to be a means for evaluating the function of both the middle ear and the cochlea under pathological conditions. The measurement technique described is also suitable for intraoperative investigation of the frequency response of the opened middle ear, as well as for the in situ frequency response of partial and total ossicular replacement prostheses.

Spatial hearing: the psychophysics of human sound localization

Part 1 Introduction: auditory events and auditory space systems analysis of the auditory experiment remarks concerning experimental procedures (psychometric methods, signals and sound fields, probe microphones). Part 2 Spatial hearing with one sound source: localization and localization blur the sound field at the two ears (propagation in the ear canal, the pinna and the effect of the head, transfer functions of the external ear) evaluating identical ear input signals (directional hearing in the median plane, distance hearing and inside-the-head locatedness) evaluating nonidentical ear inputs signals (interaural time differences, interaural level differences, the interaction of interaural time and level differences) additional parameters (motional theories, bone-condition, visual, vestibular and tactile theories). Part 3 Spatial hearing with multiple sound sources and in enclosed spaces: two sound sources radiating coherent signals (summing localization, the law of the first wavefront, inhibition of the primary sound) two sound sources radiating partially coherent or incoherent signals (the influence of the degree of coherence, binaural signal detection) more than two sound sources and diffuse sound fields. Part 4 Progress and trends since 1972: preliminary remarks the physics of the external ear (transfer functions of the external ear, area function and termination of the ear canal, analysis of transfer characteristics) evaluation of monaural attributes of the ear input signals evaluation of interaural attributes of the ear input signals (lateralization and multiple auditory events, summing localization and the law of the first wavefront, binaural localization, signal detection, and speech recognition in the presence of interfering noise, models of binaural signal processing) examples of applications (the auditory spatial impression, dummy-head stereophony). Part 5 Progress and trends since 1982: preliminary remarks binaural room simulation and auditory virtual reality binaural signal processing and speech enhancement the precedence effect - a case of cognition.

Probe Microphone Placement for Real Ear Measurement

Probe microphone measurement systems allow determination of sound pressure level (SPL) within the external auditory meatus (EAM). EAM geometry and middle ear impedance contribute significantly to the acoustic properties of the EAM, and consequently sound pressure distribution along the EAM is not uniform. Accurate measurement of SPL at the tympanic membrane (TM) requires probe microphone placement within 6 to 8 mm of the TM. A method of probe placement using the acoustical properties of the EAM (Sullivan, 1988) was investigated in 6 adults. Sullivan's acoustic probe placement method was found to overestimate probe distance from the TM, but with minor modification the acoustic method could be used to place the probe so that real ear measurements accurately predicted SPL at the TM. The difference between TM SPL and SPL at probe position was compared for the acoustic method and a constant insertion depth (25 mm) method. More accurate estimates of TM SPL were obtained with the acoustic method.

Ein Meßsystem zur ÜberprÜfung des akustomechanischen Übertragungsverhaltens von Mittelohrimplantaten

A measurement system was developed that permits objective comparisons of the sound conduction of middle ear implants. The implants are fitted into a mechanical middle ear model which approximates the impedances of the eardrum and the inner ear. A defined signal within the frequency range of 0-5 kHz is provided by a miniaturized loudspeaker at the input to the model and is measured by a probe microphone. Displacement of an artificial stapes footplate at the output of the model is measured by a fiberoptic probe with a sensitivity of 5 nm. The transmission function is calculated as the quotient of the output and the input signal. This system can be used to evaluate the sound-transmitting properties of different middle ear implants excepting other influences, such as surgical techniques. This work details the measurement system and demonstrates basic influences on the sound-transmission of middle ear implants.

Auditory threshold sensitivity of the human neonate as measured by the auditory brainstem response

The absolute auditory sensitivity of the human newborn infant was investigated using auditory brainstem response thresholds (ABR). ABRs were elicited with clicks and tone-bursts of 0.5, 1.5, 4.0 and 8.0 kHz, embedded in notched noise, in healthy, full-term human neonates and young adults with known, normal-hearing sensitivity. Stimuli were calibrated using a probe microphone positioned near the tympanic membrane in the ear canal of each subject to control for differences in resonance characteristics of infant and adult ear canals. ABR thresholds were also characterized relative to group psychophysical thresholds (nHL) and relative to individual psychophysical threshold or sensation level (SL) for the adult subjects. Infant ABR thresholds measured in p.e. SPL for all stimuli are elevated by to 3–25 dB relative to adult thresholds. Threshold elevation is greatest for the high-frequency stimuli. Result are consistent with neural immaturity for high-frequency stimuli in the auditory system of human neonates.

A123 同心円筒キャビティーを有する円形ダクトからの放射音場

Reduction of noises emitted from ducts or openings of machineries is one of the most important problems on the environmental view point. In this paper, the sound radiated from a cylindrical flanged duct having a co-axial cavity at the exit of the duct is investigated. The structure of sound filed around the exit is analyzed numerically by using a finite difference method for various depths and radii of the cavity. Also, experimental investigations are made to obtain the sound pressures and sound intensities around the exit by employing a 3D-intensity probe microphone system. The present study shows that the cavity affects the sound field remarkably and particular configulations of the cavity generate distinctive sound fileds and reduce sound pressure level significantly.

SOUND LOCALISATION IN A HABITAT: AN ANALYTICAL APPROACH TO QUANTIFYING THE DEGRADATION OF DIRECTIONAL CUES

ABSTRACT Although much research has been done to describe the degradation of sound signals propagating in natural habitats, the directional cues of sound have so far been neglected. This paper describes a first approach to quantifying the degradation of directional cues in sound propagating parallel to the ground in a grassland habitat of orthopteran insects. A matched pair of probe microphones measured the sound amplitude and phase close to the ears of grasshopper carcasses for 12 evenly spaced directions of sound incidence. The degradation was found to increase with frequency and distance from the sound source and to decrease with distance from the ground. The acoustical data were used to predict how well animals with different auditory systems can determine the direction of the sender. At one position in the habitat, the predictions were compared with the pattern of phonotactic responses of live grasshoppers. Amplitude cues appear to degrade much faster with distance than phase cues. Animals exploiting phase cues may therefore maintain a reasonable directional hearing when the amplitude cues no longer make sense. The pressure-difference-receiver type of ears responds to phase differences, and these ears may be particularly suited to overcoming the degradation of directional cues. This suggests that the possession of such ears may be an adaptation not only to small body size (relative to wavelength), but also to the acoustic properties of the habitat.

Probe microphone instrumentation for determining soil physical properties: testing in model porous materials: J. M. Sabatier, D. C. Sokol, C. K. Frederickson, M. J. M. Romkens, E. H. Grissinger & J. C. Shipps, Soil Technology, 8(4), 1996, pp 259–274

Probe microphone instrumentation for determining soil physical properties: testing in model porous materials: J. M. Sabatier, D. C. Sokol, C. K. Frederickson, M. J. M. Romkens, E. H. Grissinger & J. C. Shipps, Soil Technology, 8(4), 1996, pp 259–274

Directional hearing by mechanical coupling in the parasitoid fly Ormia ochracea.

Sound localization is a basic processing task of the auditory system. The directional detection of an incident sound impinging on the ears relies on two acoustic cues: interaural amplitude and interaural time differences. In small animals, with short interaural distances both amplitude and time cues can become very small, challenging the directional sensitivity of the auditory system. The ears of a parasitoid fly Ormia ochracea, are unusual in that both acoustic sensors are separated by only 520 microns and are contained within an undivided air-filled chamber. This anatomy results in minuscule differences in interaural time cues (ca. 2 microseconds) and no measurable difference in interaural intensity cues generated from an incident sound wave. The tympana of both ears are anatomically coupled by a cuticular bridge. This bridge also mechanically couples the tympanana, providing a basis for directional sensitivity. Using laser vibrometry, it is shown that the mechanical response of the tympanal membranes has a pronounced directional sensitivity. Interaural time and intensity differences in the mechanical response of the ears are significantly larger than those available in the acoustic field. The tympanal membranes vibrate with amplitude differences of about 12 dB and time differences on the order of 50 microseconds to sounds at 90 degrees off the longitudinal body axis. The analysis of the deflection shapes of the tympanal vibrations shows that the interaural differences in the mechanical response are due to the dynamic properties of the tympanal system and reflect its intrinsic sensitivity to the direction of a sound source. Using probe microphones and extracellular recording techniques, we show that the primary auditory afferents encode sound direction with a time delay of about 300 microseconds. Our data point to a novel mechanism for directional hearing in O. ochracea based on intertympanal mechanical coupling, a process that amplifies small acoustic cues into interaural time and amplitude differences that can be reliably processed at the neural level. An intuitive description of the mechanism is proposed using a simple mechanical model in which the ears are coupled through a flexible lever.

Sound transmission to and within the human ear canal.

Sound transmission to the eardrum from various points in the external ear was measured by means of probe microphone technique. Twelve human subjects participated, and three directions of sound incidence were included. For the major part of the audio frequency range the transmission to the eardrum proved independent of direction from points at the centerline of the ear canal, including the entrance (open or blocked). The results further suggested that the region with independent transmission extends some millimeters outside the entrance plane. The transmission from the free field to the eardrum was divided into a directional-dependent part and two directional-independent parts: (1) the transmission from the free field to the blocked entrance, (2) a pressure division between the radiation impedance and the ear-canal input impedance, and (3) the transmission along the ear canal. All parts of the transmission were seen to be highly individual. The first part was shown to be uncorrelated with any of the other parts, whereas mutual dependence of parts (2) and (3) resulted in a smaller variation in the combined transmission than for the parts in separate. The standard deviation between subjects for head-related transfer functions (HRTFs) measured at the eardrum, the open entrance, and the blocked entrance was studied, and the lowest values were found for the blocked-entrance HRTFs. It is concluded, that the blocked entrance is the most suitable point for measurements of HRTFs and for binaural recordings, since sound at this point includes the complete spatial information, and in addition to that the minimum amount of individual information.

Acoustic properties of the chinchilla pinna and ear canal.

Measurements of the acoustic transfer function (ATF) of the pinnae of 4 chinchillas were compared with the auditory‐evoked potential (AEP) hearing thresholds of 16 chinchillas measured in free field and with insert earphones. The ATF was measured in anesthetized chinchillas in a far‐field condition in a semi‐anechoic room. Probe microphone measurements were collected just outside the pinna and at the tympanic membrane. The ATF exhibited a broad resonance between 2 and 6 kHz with about a 10 dB gain. The chinchillas were monauralized and had a chronic electrode implanted in the left inferior colliculus. The animals were awake and restrained during the AEP testing. The AEP measurements were averaged from 512 presentations of a toneburst at 0.5, 1, 2, 4, and 8 kHz. AEPs collected with insert earphones were calibrated with a probe microphone positioned near the tympanic membrane. The differences in the AEP hearing thresholds measured in the different configurations exhibited a 10 dB resonance at 4 kHz. The agr...

Development and Operation of a System to Monitor Occupational Noise Exposure Due to Wearing a Headset

Abstract Although regulations limiting noise exposure in the workplace apply to individuals who wear communication headsets as part of their employment, no standard method for measuring this type of noise exposure has been available. A system for conducting this type of measurement has been developed. This system is indirect in that the electrical signal to the headset is measured after the transfer functions of the headset, ear, and a dosimeter microphone are applied to the signal. The resulting electrical signal is then inputted to a conventional dosimeter. While this approach requires that the transfer function of the headset be determined before a field measurement, it eliminates the need for a probe microphone in the wearer's ear. The transfer function of the headset (its effectiveness in generating sound) is measured using an acoustic manikin head. A significant number of headset brands and models have now been tested with this system in the laboratory, and the system is currently being used in the ...

Probe microphone instrumentation for determining soil physical properties: testing in model porous materials

An acoustic technique for evaluating soil physical properties is described and tested in model porous materials. A probe microphone is used to measure the acoustic signal attenuation and phase speed. Emphasis is on the probe construction, insertion in soils and reduction of acoustic data to predict tortuosity and an effective air-flow resistivity of the soil. Well-known capillary-tube models of porous materials which incorporate these two porous material properties are used to invert the acoustic data. The probe microphone, associated hardware, data acquisition and analyses are implemented on a personal computer. The system can provide real time prediction of the soil properties in the field. Measurements in three materials: glass beads, washed sand and loess soil are analyzed and discussed. The instrumentation works well in the glass beads and washed sand tested; however, in the loess soil tested some restrictions occur.

A Technique for Experimental Acoustic Modal Analysis

A technique developed to carry out experimental acoustic modal analysis using commercially available structural modal analysis software is described. This uses a finite difference calculation to determine the spatial variation in pressure. In order that the resulting function exhibits the same nodes and antinodes as the actual pressure distribution at resonance, a second-order finite difference calculation is performed to obtain the second spatial derivative. This is implemented in practice using a three side-by-side microphone probe with an analogue differential amplifier. The technique is verified by measuring the natural frequencies and mode shapes of a bare rectangular office. These results are compared with analytical calculations and output from a finite element model. The results show very good agreement for all modes in the frequency range of interest.

Acoustic characterization of rigid-frame air-filled porous media using both reflection and transmission measurements

Acoustic measurements in and above porous materials (500-μm-diam glass beads) have been used to determine rigid-frame porous medium parameters. The propagation constant for the acoustic signal propagating through the air in the pores was calculated from probe microphone measurements made inside the porous medium. Reflection measurements have been used to calculate the characteristic impedance of a thick porous medium. A model relating porous medium properties to the propagation constant and impedance was used to analyze the data from both the reflection and transmission measurements. Both reflection and transmission measurements are needed to determine the rigid-frame porous medium parameters. Porous medium parameters can be calculated using the model and the data. The calculated porous medium parameters from the transmission and reflection measurements are compared. Some possible explanations for the discrepancy and suggestions for further work are presented.

Dosimetry measurements using a probe tube microphone in the ear canal.

Federal and international standards recommended use of microphone placement either on or in the vicinity of the shoulder for dosimetry to minimize deviations from the undisturbed sound field. Probe microphone measurements from the ear canal were compared to shoulder and chest measures in order to investigate the validity of current dosimetry methodologies. Six subjects were monitored in an industrial setting. As expected, ear-canal levels exceeded other measures for all subjects. Shoulder and chest measures showed very low intersubject variability whereas ear-canal measures resulted in large intersubject variability. The ear-canal methodology has the potential to identify individuals whose external ear gain exceed the mean, putting them at increased risk of noise-induced permanent threshold shift (NIPTS). It is proposed that overall external ear pressure gain be used as an index to adjust exposure levels when predicting NIPTS using ISO 1999. A normative database of external ear pressure gain was constructed from 30 ears for this purpose.

Localization of real and virtual sound sources

Localization of real and virtual sound sources was studied using two tasks. In the first task, subjects had to turn their head while the sound was continuously on and press a button when they thought they faced the source. In the second task, the source only produced a short sound and the subjects had to indicate, by pressing one of eight buttons, in which quadrant the source was located, and whether it was located above or below the horizontal plane. Virtual sound sources were created using head-related transfer functions (HRTFs), measured with probe microphones placed in the ear canals of the subjects. Sound stimuli were harmonic signals with a fundamental frequency of 250 Hz and an upper frequency ranging from 4 to 15 kHz. Results, obtained from eight subjects, show that localization performance for real and virtual sources was similar in both tasks, provided that the stimuli did not contain frequencies above 7 kHz. When frequencies up to 15 kHz were included, performance for virtual sources was, in general, poorer than for real sources. Differences between results for real and virtual sources were relatively small in the first task, provided that individualized HRTFs were used to create the virtual sources, but quite large (a factor of 2) in the second task. The differences were probably caused by a distortion of high-frequency spectral cues in the HRTFs, introduced by the probe microphone measurement in the ear canal.

Primary and secondary sound transfer functions in an active noise control earplug

Active noise control in a headset or earplug typically attempts to lower the sound level at an error microphone located close to the ear. If the transmission path between the error microphone and the eardrum for the primary noise is different to the path for the secondary noise, a reduction at the error microphone may not lead to a reduction at the eardrum. In fact, if these paths are sufficiently different a reduction at the error microphone may actually lead to amplification at the eardrum. An example of a possible cause for differing transmission paths is the bone conduction effect that becomes noticeable in occluded ear canals. In this study, a probe microphone was inserted into the ear canal of human subjects. This microphone was used to measure the pressure near the eardrum for primary and secondary sound sources. The pressure was also measured at an error microphone of an active noise control earplug. These measurements will be presented and some of the implications for active noise control will be given.