Probe Microphone Research Articles

Chapter 6: What Else Can I Do with This Equipment?

AbstractIf there is sound in the ear canal, you can measure it with a probe microphone in the ear. The following are a few examples of how you might use your real-ear probe microphone measures beyond verifying hearing aid fittings, signal processing, and function of features. A process to simulate hearing loss to educate and support family members and patients is described.

Chapter 5: Setting the Hearing Aid Response and Verifying Signal Processing and Features with Real-Ear Probe Microphone Measures.

The real-ear probe microphone system provides a powerful tool to individual hearing aid fittings accounting for your patient's hearing and ear canal characteristics. The primary treatment for hearing loss is audibility, returning an audible signal across frequencies and input levels given the constraints of the hearing loss. This chapter will provide detailed information on the measures needed to individualize the hearing aid fitting and will present various clinical scenarios that will allow you to work with this information and see how you apply this knowledge clinically. You will explore the verification of signal processing and features that allow you to support your patients.

Chapter 4: Introduction and Getting Ready for Real-Ear Probe Microphone Measures.

Probe microphone measurements are an essential step in an individualized hearing aid fitting. These measurements allow audiologists to account for the individual's hearing and ear canal acoustics when programming hearing aids. An evidence-based hearing aid fitting includes matching the measured output of the hearing aids to targets for each input level and frequency. This allows the audiologist to confidently counsel the patient that the acoustic fitting is accurate, and the next step is for the individual to use the amplification during all waking hours to adapt to the newly amplified sounds. This also avoids mistakes such as overamplification or insufficient gain, which can endanger the patient and/or lead to a compromised fitting.

On the use of ground-board mounted microphones for outdoor noise measurements

Outdoor noise measurements made by a microphone mounted into or on an acoustically rigid thin board lying on the ground can be affected to some degree by diffraction from the edges of the board due to the thickness of the board or the finite impedance of the surrounding ground. This can result in the sound pressure level varying over the top surface of the board and deviating from the 6 dB increase (relative to the incident sound field) expected if the board was very thin and the surrounding ground was also acoustically rigid. In this paper, an experimental and numerical investigation is carried out to assess the variation in the sound pressure level over the top surface of the ground-board from sound incident at several different angles. The experiments utilise a probe microphone flush-mounted into a 0.4 m diameter circular thin metal ground-board slightly protruding from a simulated grass-covered field in an anechoic chamber. The experiments are repeated on an outdoor grass-covered field and compared. The effect of different installation methods such as the flush-mounted probe microphone, inverted microphone, and a side-set microphone are also investigated. The variation of the sound pressure level on the top surface of the ground-board due to diffraction from the edges of the board is shown. It is also shown that the presence of the microphone body begins to affect the noise measurements from a frequency of 3 kHz. The results presented in this paper suggest that narrow-band noise measurements using a microphone mounted on a circular ground-board installed on a grass-covered surface can deviate by more than 3 dB from the nominal 6 dB increase above the incident field level.

Comparing criteria for deviation from hearing aid prescriptive targets in children

Objective To determine if a stricter criterion for paediatric hearing aid fitting for proximity of fit-to-target of <3 dB root-mean-square (RMS) error produces better audibility and outcomes compared to the current <5 dB criterion, and to examine the relationship between aided audibility and RMS error by degree of hearing loss. Design We evaluated the influence of unaided hearing level on the relationship between RMS error and aided audibility. We assessed the effect of RMS error category (<3, 3–5, >5 dB) on aided audibility, speech recognition, expressive vocabulary, and morphosyntax. Study sample The study included 2314 hearing aid verification measurements from 307 children with hearing aids. Results Children who met a <3 dB criterion had higher aided audibility than children who met no criterion (>5 dB error). Results showed no differences in speech recognition or vocabulary by error category, but children with <3 dB error demonstrated better morphosyntax than children with 3–5 and >5 dB RMS error. Conclusions Fittings that are close to prescriptive targets provide a more positive outcome for children with hearing aids. Using probe microphone measures to adjust hearing aids to within 3 dB may benefit language abilities in children.

Semi-empirical calibration of remote microphone probes using Bayesian inference

The empirical calibration of remote microphone probes (RMP), used to acquire wall-pressure fluctuations, can introduce spurious resonance into the sensor transfer function due to the difference in the pressure field inside the calibrator geometry over multiple calibration steps. Such spurious resonance subsequently propagates into the unsteady-pressure data at which the calibration is applied, hindering the accuracy of the measurements. Current post-processing methods for tackling these issues are often manual and strongly dependent on the operator’s expertise. In this study, we propose an original semi-empirical calibration method to remove spurious resonance in a less operator-reliant manner. The approach is based on fitting an existing analytical fluid-dynamical model for the propagation of pressure waves in the probe to the empirical calibration data using Bayesian inference. The proposed method is successfully applied to three datasets, from a simple probe recessed behind a pinhole to a more complex branching RMP. For all the configurations, spurious resonance is eliminated from the transfer function with a strongly reduced impact of the operator intervention while retaining the resonant features that are characteristic of the RMP. The affected frequency bands are then replaced using the underlying physical model. In this way, the detrimental impact of spurious resonance is removed from the measured wall-pressure spectra. Furthermore, the RMP parameters retrieved by the fit can also be used as inputs to corrective models, specifically to account for averaging effects due to the probe sensing area or for the impact of grazing flow or temperature variations on the transfer function.

An experimental investigation of ground board mounted microphones for outdoor noise measurement

This paper reports an experimental investigation on the use of ground board mounted microphones for making outdoor noise measurements. The experiments utilized a 400 mm diameter thin metal ground board into which a probe microphone was flush mounted at a number of different radii. The ground board was placed on the floor of an anechoic chamber and the noise level from a source at a fixed height was measured for a large number of different ground board orientations. These measurements were used to assess the variation in the sound pressure level over the ground board.

PROPELLER INFLUENCE ON SURFACE PRESSURE FLUCTUATION UNDER BOUNDARY LAYER INGESTION CONDITION

Research on propeller noise under boundary layer ingestion condition is essential for designing new aircraft concepts. Measurements of pressure spectra on a flat plate below a propeller give important information about the noise coming from the blades. These also are important for designing surface noise mitigation structures, such as acoustic black holes, meta-materials, and liners. This paper investigates the effects of a propeller on the flat plate surface pressure under boundary layer ingestion, using bottom of the wind tunnel channel as flat plate. Tripping devices on the wind tunnel bottom wall control the boundary layer thicknesses. The surface pressure is measured using an instrumented plate positioned underneath the propeller. The plate has 87 pressure taps to measure the unsteady and steady pressure using the remote microphone probe (RMP) technique. The results show the influence of the propeller inflow, the advance ratio, and the number of blades on the different regions underneath the propeller, and on coherent turbulent structures of the incoming flow. The boundary layer thicknesses affects the static pressure upstream of the propeller - creating high pressure regions. The propeller affects the turbulent structures downstream of the blades, increasing its correlation length.

Aeroacoustic investigation of airfoil at near-stall conditions

This paper presents a detailed aeroacoustic investigation of a controlled-diffusion airfoil at near-stall condition. The study aims at answering two research questions: identify the flow mechanism responsible for separation noise for an airfoil near-stall conditions and whether the noise is generated by a dipole for airfoil close to stall and can be quantified by Amiet's diffraction theory. The study uses synchronized particle image velocimetry, remote microphone probes, and far-field microphone measurements to perform experiments at two chord-based Reynolds numbers of about 150 000 and 250 000. The results show that when the airfoil is placed at a higher angle of attack, such as 15°, strong amplification of flow disturbance is seen, resulting in the rolling up of the shear layer in the aft region of the airfoil, forming large coherent structures. While these rollers play a central role in the increase in noise due to flow separation, the flapping of shear layer does not contribute to the separation noise. The present study conclusively shows that separation noise is dipolar in nature and that the quadrupolar contribution for low-speed airfoils at near-stall conditions can be neglected. However, the increase in flow disturbances measured close to the trailing edge of the airfoil implies that the assumption of small-amplitude disturbance is no longer valid, which is the central premise of the thin linearized airfoil theory. Outside the frequency range at which flow separation operates, Amiet's theory is able to predict the far-field noise even at high angles of attack.

A Novel Method to Determine the Maximum Output of Individual Patients for an Active Transcutaneous Bone Conduction Implant Using Clinical Routine Data.

The maximum output provided by a bone conduction (BC) device is one of the main factors that determines the success when treating patients with conductive or mixed hearing loss. Different approaches such as sound pressure measurements using a probe microphone in the external auditory canal or a surface microphone on the forehead have been previously introduced to determine the maximum output of active transcutaneous BC devices that are not directly accessible after implantation. Here, we introduce a method to determine the maximum output hearing level (MOHL) of a transcutaneous active BC device using patients' audiometric data. We determined the maximum output in terms of hearing level MOHL (dB HL) of the Bonebridge using the audiometric and direct BC threshold of the patient together with corresponding force levels at hearing threshold and the maximum force output of the device. Seventy-one patients implanted with the Bonebridge between 2011 and 2020 (average age 45 ± 19 years ranging from 5 to 84 years) were included in this study. The analyses of MOHLs were performed by (1) dividing patients into two groups with better or worse average audiometric BC threshold (0.5, 1, 2, 4 kHz), on the ipsilateral side or (2) by separating the MOHLs based on better or worse frequency-by-frequency specific audiometric BC thresholds on the ipsilateral (implanted) side. When using a frequency-by-frequency analysis obtained average ipsilateral MOHLs were in the range between 51 and 73 dB HL for frequencies from 0.5 to 6 kHz in the group with better audiometric BC threshold on the ipsilateral ears. The average contralateral MOHLs in the group with better contralateral hearing were in the range from 43 to 67 dB HL. The variability of the data was approximately 6 to 11 dB (SDs) across measured frequencies (0.5 to 6 kHz). The average MOHLs were 4 to 8 dB higher across frequencies in the group with better audiometric BC threshold on the ipsilateral ears than in the group with better audiometric BC threshold on the contralateral ears. The differences between groups were significant across measured frequencies ( t test; p < 0.05). Our proposed method demonstrates that the individual frequency-specific MOHL on the ipsilateral and contralateral side of individual patients with a transcutaneous BC device can be determined mainly using direct and audiometric BC threshold data of the patients from clinical routine. The average MOHL of the implant was found 4 to 8 dB higher on the ipsilateral (implanted) side than on the contralateral side.

Experimental and numerical investigation of the sound field produced by a shrouded UAV propeller

This paper investigates the effect of an acoustically rigid shroud on the tonal noise produced by an unmanned aerial vehicle (UAV) propeller. Enclosing the propeller with a shroud alters the steady loading on the propeller blades, thereby changing the steady loading tones radiated from the sources on the propeller surface. In this paper it is shown that the time-average rotating pressure field on the inner surface of the shroud also makes a significant contribution to the tonal noise. The aerodynamics of a shrouded and an isolated propeller was investigated using computational fluid dynamics (CFD) simulations. The tones produced by the steady loading and thickness sources on the propeller, and the time-average rotating pressure field on the surface of the shroud were calculated using an acoustic analogy approach and data from the CFD simulations. These calculations show that the tonal noise radiated from the loading sources on the inner surface of the shroud make a significant contribution to the tonal noise levels and that the contribution from the loading sources on the other surfaces of the shroud are negligible. A probe microphone was used to measure the time-average rotating pressure field on the inner surface of the shroud and these measurements showed good agreement with the CFD simulation. These measurements were then used to calculate the contribution of the loading sources on the inner surface of the shroud to the total noise measured experimentally. High resolution acoustic measurements in the near-field of a shrouded and an isolated propeller were also taken. The near-field measurements were then projected to the acoustic far-field and these levels are compared with numerical predictions. The results show that the effect of the shroud is to decrease the tonal noise for observers below the rotational plane of the propeller.

Real-ear Unaided Gain in Wrestlers with Unilateral Cauliflower Ear

Background and Aim: Real-Ear Unaided Gain (REUG) can reveal the outer-ear resonance which reflects the effects of body and ear factors. Deformity in the outer ear may change the real-ear unaided responses. Cauliflower ear is a common ear deformity in wrestlers. This study aimed to compare the REUG of normal and cauliflower ear in wrestlers to investigate the frequencies at which the gain may be inaccurate. Methods: Ten male wrestlers with the age ranged from 21 to 31 years participated in this study. The study had within-subject design and only one group. The REUGs was recorded for both ears, normal and cauliflower ears, and the results analyzed. Results: The mean of REUG values were significantly different between the deformed and normal ears. There were two distinct peaks in the REUG curve of the deformed ear, while there was a single peak in that of the normal ear. Conclusion: The differences in the REUG at 2–6 kHz between the deformed and normal ears was seen. Keywords: Cauliflower ear; real ear unaided gain; probe microphone measurement; ear canal resonance

Analysis of the remote microphone probe technique for the determination of turbulence quantities

Measuring the statistics of the wall-pressure fluctuations (WPFs) is essential for understanding and evaluating laminar/turbulent boundary layers and flow-induced noise. The remote microphone probe (RMP) technique is one of the most successful experimental approaches to measuring WPFs. Despite its extensive use in academic and industrial research, the scientific literature currently lacks an evaluation of the RMP influence on the determination of turbulence quantities, such as the WPF spectrum, the convection velocity, and the turbulence coherence length. To fulfill this necessity, this paper analyses the effects of the RMP on the determination of turbulence quantities, including comparisons between the results obtained using RMPs and flush-mounted microphones (FMMs). The RMPs and the FMMs measure the WPFs under a turbulent boundary layer developed on a zero-pressure-gradient flat wall. This study shows that the turbulence quantities determined from RMP measurements are influenced by the RMP setup, i.e., tube length and inner diameter. The tube length affects the phase between RMPs, which impacts the turbulent structure convection velocity estimation, which can be determined correctly by considering the calibrated phase. Furthermore, the tube inner diameter influences the coherence between a pair of RMPs, affecting the calculation of the turbulence correlation length. This research also shows that the turbulence quantities determined from the RMP and the FMM measurements differ, mainly in the coherence between pairs of microphones.

Basic investigation of sound field inside and outside ear canal under ultrasound irradiation

In recent years, high-intensity airborne ultrasonic waves applied technology has been developed actively. Accordingly, there are concerns about the effects of sound wave exposure by high-intensity airborne ultrasonic waves. The sound wave intensity is different near the entrance of the ear canal and near the eardrum, because the wavelength is short in the ultrasonic region. Therefore, it is necessary to know accurately the sound pressure at the eardrum position in considering the effects of sound waves exposure by ultrasonic waves. However, it is difficult to measure sound pressure near the eardrum with a probe microphone because it may damage the ear canal and eardrum. Against this background, we have studied the sound pressure characteristics inside and outside the ear canal when airborne ultrasonic waves are exposed to the mannequin head (KEMAR) with a pseudo-ear canal created to imitate the dimensions of the human ear canal. And, we have attempted to estimate the sound pressure value near the eardrum using the sound pressure in the ear canal. In this report, we verified the sound field inside and outside the ear canal and the sound pressure near the eardrum, when ultrasonic waves (frequency:20 kHz) were irradiated to the mannequin head.

Spatial relationships between interaural differences in a room

The interaural ratio, as it occurs for a human head in a room, can be represented by a filter with a complex transfer function of the spatial location of the head. The log of this transfer function has real and imaginary parts, respectively, the interaural level difference (ILD) and the interaural phase difference (IPD). Because the filter is causal, the ILD and IPD spatial functions are Hilbert transforms of each other if the interaural filter is a minimum phase function of the head location. Interaural differences for a spherical “head” with tiny probe microphones, separated by a head diameter of 17.5 cm, were measured by Mr. Zane Crawford in an empty room having a volume of 63 $m∧3$ and a reverberation time of 0.4 s. The head was moved in increments of 2.54 cm over a two dimensional grid 102 cm by 23 cm. Measurements at 250 Hz showed that the ILD and IPD were almost perfectly related by Hilbert transforms. Measurements at 500 and 1000 Hz showed increasing deviations. Because a function is orthogonal to its Hilbert transform, the ideal ILD is orthogonal to the IPD. Therefore, at locations where the ILD is the most stable function of space, the IPD is the most unstable and vice versa.

Automated human temporal-bone experiments

Abstract Temporal-bone (TB) experiments are useful for the investigation of middle-ear and inner-ear transfer properties, and the analysis and development of active or passive middleear prostheses or related devices. To acquire the mechanical middle-ear (ME) response, the Laser-Doppler vibrometry (LDV) is utilized as a preferred method. In most setups, the exact measurement point chosen on the biological structure of interest as well as the direction of the corresponding vibration measurement is not precisely defined and of unknown reproducibility. Hence, an automated setup controlling coordinates of TB and measurement direction is developed and tested. To test the reproducibility, TBs were measured with ER7C and ER10C probe microphones employing a small industrial robot to position the TB to the ME points of interest and align it to the laser beam. The position and vibration data acquired at the same locations at different points of time were processed and compared. Measured transfer functions indicate noticeable differences at some frequency ranges. Nonetheless, the reproducibility of positioning to repeatedly measured points proved itself relatively accurate, if no manipulations in the setup are made. In conclusion, the current study shows that automated LDV TB measurement system is in principle capable to adapt to the changes in setup during intermediate preparation steps. With more acquired data, the setup stability and reproducibility can be more precisely verified, and additional improvements can be made.

Creation of an incus recess for a middle-ear microphone using a drill or laser ablation: a comparison of equivalent noise level and middle ear transfer function

PurposeStudies have assessed the trauma and change in hearing function from the use of otological drills on the ossicular chain, but not the effects of partial laser ablation of the incus. A study of the effectiveness of a novel middle-ear microphone for a cochlear implant, which required an incus recess for the microphone balltip, provided an opportunity to compare methods and inform a feasibility study of the microphone with patients.MethodsWe used laser Doppler vibrometry with an insert earphone and probe microphone in 23 ears from 14 fresh-frozen cadavers to measure the equivalent noise level at the tympanic membrane that would have led to the same stapes velocity as the creation of the incus recess.ResultsDrilling on the incus with a diamond burr created peak noise levels equivalent to 125.1–155.0 dB SPL at the tympanic membrane, whilst using the laser generated equivalent noise levels barely above the baseline level. The change in middle ear transfer function following drilling showed greater variability at high frequencies, but the change was not statistically significant in the three frequency bands tested.ConclusionsWhilst drilling resulted in substantially higher equivalent noise, we considered that the recess created by laser ablation was more likely to lead to movement of the microphone balltip, and therefore decrease performance or result in malfunction over time. For patients with greatly reduced residual hearing, the greater consistency from drilling the incus recess may outweigh the potential benefits of hearing preservation with laser ablation.

Visualization of a shock wave travelling inside a rectangular duct using the background-oriented schlieren method

The propagation along a narrow duct of a shock wave generated by the rupture of a membrane is studied by means of visualization using the background oriented schlieren (BOS) method. This optical technique, which has not been used before in this context, is non-intrusive and can be employed to produce a two-dimensional quantitative portrait of the density field inside a section of the duct. Images obtained using a high-speed camera confirm that the shock is, as expected, planar, with boundary layer effects near the interior duct walls observed as well. The quantitative aspect of this method is validated by comparing the amplitude of the shock as obtained from images with direct measurements of the acoustic pressure acquired by means of a probe microphone. Successive images of the shock as it travels also permit calculation of the shock speed.

Generating a physics-based quantitatively-accurate model of an electrically-heated Rijke tube with Bayesian inference

We perform 7000 experiments at 175 stable operating points on an electrically-heated Rijke tube. We pulse the flow and measure the acoustic response with eight probe microphones distributed along its length. We assimilate the experimental data with Bayesian inference by specifying candidate models and calculating their optimal parameters given prior assumptions and the data. We model the long timescale behaviour with a 1D pipe flow model driven by natural convection into which we assimilate data with an Ensemble Kalman filter. We model the short timescale behaviour with several 1D thermoacoustic network models and assimilate data by minimizing the negative log posterior likelihood of the parameters of each model, given the data. For each candidate model we calculate the uncertainties in its parameters and calculate its marginal likelihood (i.e. the evidence for that model given the data) using Laplace’s method combined with first and second order adjoint methods. We rank each model by its marginal likelihood and select the best model for each component of the system. We show that this process generates a model that is physically-interpretable, as small as possible, and quantitatively accurate across the entire operating regime. We show that, once the model has been selected, it can be trained on little data and can extrapolate successfully beyond the training set. Matlab code is provided so that the reader can experiment with their own models.

A coherence-based phase and amplitude gradient estimator method for calculating active acoustic intensity

The phase and amplitude gradient estimator (PAGE) method [Thomas, Christensen, and Gee, J. Acoust. Soc. Am. 137, 3366-3376 (2015)] has been developed as an alternative to the traditional p-p method for calculating energy-based acoustic measures such as active acoustic intensity. While this method shows many marked improvements over the traditional method, such as a wider valid frequency bandwidth for broadband sources, contaminating noise can lead to inaccurate results. Contaminating noise degrades performance for both the traditional and PAGE methods and causes probe microphone pairs to exhibit low coherence. When coherence is low, better estimates of the pressure magnitude and gradient can be obtained by using a coherence-based approach, which yields a more accurate intensity estimate. This coherence-based approach to the PAGE method, known as the CPAGE method, employs two main coherence-based adjustments. The pressure magnitude adjustment mitigates the negative impact of uncorrelated contaminating noise and improves intensity magnitude calculation. The phase gradient adjustment uses coherence as a weighting to calculate the phase gradient for the probe and improves primarily the calculation of intensity direction. Though requiring a greater computation time than the PAGE method, the CPAGE method is shown to improve intensity calculations, both in magnitude and direction.