Simulating the open-loop transfer function as a means for understanding acoustic feedback in hearing aids

Suppressing unstable acoustic feedback in hearing aids will first require knowledge of the open-loop transfer functions of such systems. Reported herein is a mathematical technique for simulating the open-loop transfer function of an in situ eyeglass-type hearing aid. In particular, a computer program was developed that characterized the hearing aid as a serial connection of two-port blocks, each representing one individual component of a hearing aid. Included, for example, were two-port blocks representing the microphone, amplifier, receiver, sound tubes leading to the eardrum (including the ear canal itself), earmold vent, and external pathway from the vent outlet back to the microphone. The computer program was validated by replicating laboratory data derived from an experiment involving a nonstandard manikin fitted with a nonstandard artificial ear. Next, the open-loop transfer function of an eyeglass-type hearing aid in situ on the manikin was simulated via the computer program. Unfortunately, those computer-generated data were not replicated in the laboratory due to the difficulty encountered in actually measuring the open-loop transfer function. Nevertheless, investigators were able to utilize those data to predict, within +/- 25 Hz, the "squeal" frequency of unstable acoustic feedback.     

A technique for simulating the amplifier-to-eardrum transfer function of an in situ hearing aid

There are numerous articles wherein mathematical models of various parts of an in situ hearing aid have been reported. Such parts include, for example, the microphone, receiver, cylindrical tubes carrying sound to the eardrum and out through the earmold vent, and the external path from the vent back to the microphone. This article extends these earlier works to include the hearing-aid amplifier. In particular, a mathematical technique for characterizing the amplifier in combination with the receiver is reported. Cascade parameters of a two-port model of one particular amplifier/receiver combination are obtained by this method. The cascade-parameter data and the method of obtaining this data are verified by two different experimental procedures. One procedure involves both computing and measuring the input driving-point impedance of the amplifier/receiver combination. In the second procedure, the amplifier-to-eardrum transfer function of a hearing aid incorporating this same amplifier/receiver combination and mounted on an artificial ear is both computed and measured. Experimental and computed values of this transfer function for three different earmold geometries are in reasonably close agreement. The amplifier/receiver model reported herein will be used in future studies of acoustic feedback in hearing aids.     

Occluded-ear simulator with variable acoustic properties.

Ear simulators were designed to replicate acoustical characteristics of the average adult ear. Due to variability of ear-canal geometry and eardrum impedance among individuals, the possibility of any one person exhibiting such "average" characteristics--especially if that person is a child and/or has a conductive pathology--is remote. Thus, ear simulators have been of only peripheral value when prescribing a hearing aid (a high output impedance device) to fit the acoustical requirements of a particular patient. Reported herein is development of a programmable artificial ear (PAE) that can account for individual differences in ear-canal geometry and eardrum impedance. It consists of a 2.0-cc coupler, microphone, amplifier, computer, PAE code, and a computer card and/or software for digitization and Fourier transformation. Required input data includes ear-canal dimensions, eardrum impedance, and output impedance of the hearing aid being tested. Sound-pressure recordings produced in the 2.0-cc coupler by the hearing aid are adjusted by the computer to what they would have been had the recordings been made at the eardrum of a particular patient wearing the same hearing aid. Good agreement was observed between experiment and theory for one test case involving a totally occluding miniature earphone.     

A computer simulation of hearing aid response and the effects of ear canal size

The response of a hearing aid is affected by many factors which include the head and outer ear, the microphone, amplifier, and receiver used in the hearing aid, the properties of the ear canal and the eardrum, and acoustic feedback through the vent. This article presents a computer simulation of an in-the-ear (ITE) hearing aid that includes all of the above factors. The simulation predicts the pressure at the eardrum for a frontal free-field sound source. The computer model was then used to determine the effects on the hearing aid response due to variations in the size of the ear canal. The simulation indicates that, for an unvented hearing aid, changes in the size of the ear canal shift the overall sound-pressure level at the eardrum but have only small effects on the shape of the frequency response. The situation is more complicated when a vent is present, however, since changes in the size of the ear canal that cause apparently small perturbations in the acoustic feedback signal may, nonetheless, have large effects on the overall system response.     

Variations in the feedback of hearing aids

Variations in the loop response of hearing aids caused by jaw movements, variations in acoustics outside the ear, and variations of vent size have been identified. Behind The Ear (BTE) and In The Ear Canal (ITEC) hearing aids were considered. The largest variations among the variations of the acoustics outside the ear, except when the hearing aid was partly removed, were found with the ITEC when a telephone set was placed by the ear. The variations of the loop response caused by changes in vent size were compared with the variations of a theoretical model of the feedback path. The theoretical model was also used to compare the feedback of different designs of the vent that gives the same acoustic impedance at low frequencies. The calculated feedback was less with the short vents (12 mm) than the long vents (24 mm).     

Power flow transmission influenced by vibration source impedance for a compound system

I.IntroductionVeryofteninengineeringpracticemachinesareusuallyinstalledonflexiblestructures.Buildingfloors,shipdecks,aircraftfuselages,carortrainChassisarebutafewexamplesthatfallintothiscategory.Themachinetransmitsvibrationtothereceivingstructureandresultsinstructure-bornesoundtransmission.Especiallywhenaheaviermachinerunningathighspeedismoulltedonaflekib1efoundation,thesourceandthereceiverwillpresentsimilardynamiccharacteristicsattheirinterface.Asaresult,theconventionalanalysiscannotbeappli…     

The hearing aid feedback path: mathematical simulations and experimental verification

Acoustic feedback in hearing aids has received little attention in the literature. Feedback occurs when stability conditions of the open-loop transfer function of an in situ hearing aid are violated. Solving the feedback problem will first require knowledge of the open-loop transfer function. Included in the open-loop transfer function is the acoustical path by which sound emanating from the earmold vent returns to the microphone (i.e., the feedback path). Reported herein are two different mathematical procedures for simulating transfer functions of the feedback path of an eyeglass-type hearing aid. In one procedure the vent exit was modeled as a point source of sound located on a flat plane, while it was treated as a point source on a sphere in the other. Results of laboratory experiments indicate that the mathematical models accurately predict those acoustic phenomena for which they were intended: point sources on plane and spherical baffles. Results of manikin experiments showed both models to be less accurate for simulating the feedback path around the human head. The maximum difference between experiment and theory was 6 dB at one frequency. Surprisingly, the flat-baffle model produced better agreement with experimental results than did the sphere model.     

The effect of superior-canal opening on middle-ear input admittance and air-conducted stapes velocity in chinchilla

The recent discovery of superior semicircular canal (SC) dehiscence syndrome as a clinical entity affecting both the auditory and vestibular systems has led to the investigation of the impact of a SC opening on the mechanics of hearing. It is hypothesized that the hole in the SC acts as a "third window" in the inner ear which shunts sound-induced stapes volume velocity away from the cochlea through the opening in the SC. To test the hypothesis and to understand the third window mechanisms the middle-ear input admittance and sound-induced stapes velocity were measured in chinchilla before and after surgically introducing a SC opening and after patching the opening. The extent to which patching returned the system to the presurgical state is used as a control criterion. In eight chinchilla ears a statistically significant, reversible increase in low-frequency middle-ear input admittance magnitude occurred as a result of opening the SC. In six ears a statistically significant reversible increase in stapes velocity was observed. Both of these changes are consistent with the hole creating a shunt pathway that increases the cochlear input admittance.     

Boundary element modeling of the external human auditory system

In this paper the response of the external auditory system to acoustical waves of varying frequencies and angles of incidence is computed using a boundary element method. The resonance patterns of both the ear canal and the concha are computed and compared with experimental data. Specialized numerical algorithms are developed that allow for the efficient computation of the eardrum pressures. In contrast to previous results in the literature that consider only the "blocked meatus" configuration, in this work the simulations are conducted on a boundary element mesh that includes both the external head/ear geometry, as well as the ear canal and eardrum. The simulation technology developed in this work is intended to demonstrate the utility of numerical analysis in studying physical phenomena related to the external auditory system. Later work could extend this towards simulating in situ hearing aids, and possibly using the simulations as a tool for optimizing hearing aid technologies for particular individuals.     

利用残差网络分析普氏蹄蝠耳朵对恒频声源定向的影响

为研究恒频蝙蝠耳朵与空间定位的关系,利用深度学习算法和仿蝙蝠静态双耳接收器,分析蝙蝠耳朵对恒频声源定向的影响。首先根据普氏蹄蝠耳朵模型设计不同双耳夹角和间距的仿生双耳接收器,并从多个空间方位采集声源发射的不同频率的恒频声呐信号,然后提取双耳同步采集信号的时频图并归一化作为输入特征,最后利用残差网络实现声源定向。实验结果表明,静态双耳接收器对恒频声源的定向误差平均值基本保持在3.5°以下,但高于动态单耳接收器的定向误差;定向精度与声源频率及声源所在空间方位有关,声源位于接收器水平方向±30°范围内时,定向精度相对较高;双耳夹角和间距也会影响定向精度,且前者影响较为显著。     

Measurement of the eardrum impedance of human ears

The determination of an acoustical impedance requires measurements of pressure and volume velocity. As no direct method is available for measuring velocity in an ear canal, a technique was developed which is based on pure pressure measurements. The ear canal is used as a measuring tube, the area function of which is also deduced from the pressure measurements. High-frequency measurements in living subjects involve many sources of errors. A criterion for deciding if a good measurement has been made is given. The technique of measurements is described, regarding both the use of probe tube microphones and the computer aided data recording. Finally, the results are presented, and some comments are given. A reliable interpretation of the results seems to be impossible because of lack of our knowledge of the middle ear function at high frequencies.     

Active cancellation of occlusion: an electronic vent for hearing aids and hearing protectors

The occlusion effect is commonly described as an unnatural and mostly annoying quality of the voice of a person wearing hearing aids or hearing protectors. As a result, it is often reported by hearing aid users as a deterrent to wearing hearing aids. This paper presents an investigation into active occlusion cancellation. Measured transducer responses combined with models of an active feedback scheme are first examined in order to predict the effectiveness of occlusion reduction. The simulations predict 18 dB of occlusion reduction in completely blocked ear canals. Simulations incorporating a 1 mm vent (providing passive occlusion reduction) predict a combined active and passive occlusion reduction of 20 dB. A prototype occlusion canceling system was constructed. Averaged across 12 listeners with normal hearing, it provided 15 dB of occlusion reduction. Ten of the subjects reported a more natural own voice quality and an appreciable increase in comfort with the cancellation active, and 11 out of the 12 preferred the active system over the passive system.     

A direct method for acoustic impedance measurement based on the measurement of electrical impedance of acoustic transmitter

A simple and straight forward method for acoustic impedance measurement is presented and evaluated. In this method a speaker is used as the signal source. The relationship between the electrical impedance of the speaker and its acoustical load is developed and studied. It is shown that the electrical current and voltage of the speaker relate to the acoustical pressure and volume velocity. The mechanical and acoustical impedances are therefore easily derived by measuring the electrical current and voltage of the circuit. The proposed method yield itself to the automatic computer measurement and can be used for the field and in situ measurements.Comparison of the measurement data with those from other methods proves the applicability and accuracy of the proposed method.     

2.45 GHz ECR离子源的微波阻抗匹配

ECR离子源的等离子体阻抗对其微波传输与阻抗匹配设计至关重要。在中国科学院近代物理研究所现有的2.45 GHz ECR 质子源上,对等离子体阻抗进行了测量。首先用水吸收负载代替等离子体负载测量得到了所用微波窗阻抗,然后根据质子源测量数据,推算得到了等离子体阻抗。实验结果表明,脊波导输出端阻抗与后续负载不完全匹配,等离子体阻抗随微波功率变化呈非线性。这些结果为ECR离子源过渡匹配和微波窗的设计提供了参考依据。Plasma impedance of an ECR ion source is important for microwave transmission and impedance matching design. Plasma impedance was measured indirectly with the 2.45 GHz ECR proton source at the Institute of Modern Physics, Chinese Academy of Sciences. In the test, we got microwave window mpedance by using water absorption load instead of plasma load, and the source plasma impedance was derived from the test data with the 2.45 GHz ECR proton source and microwave window impedance. The experimental results show that ridge waveguide output impedance and the subsequent load does not exactly match, plasma impedance variation is nonlinear with microwave power. The achievedresult is useful in the design of ridged waveguide and microwave window.     

Transverse pressure distributions in a simple model ear canal occluded by a hearing aid test fixture

The sound field in a model ear canal with a hearing aid test fixture has been investigated experimentally and theoretically. Large transverse variations of sound pressure level, as much as 20 dB at 8 kHz, were found across the inner face of the hearing aid. Variations are greatest near the outlet port of the receiver and the vent port. Deeper into the canal, the transverse variations are less significant and, at depths greater than 4 mm, only a longitudinal variation remains. The model canal was cylindrical, 7.5 mm diameter, and terminated with a Zwislocki coupler to represent absorption by the human middle ear. The outer end of the canal was driven by the receiver in the hearing aid test fixture, with the acoustic output entering the canal through a 1 mm port. The hearing aid was provided with a 20-mm-long vent, either 1 or 2 mm in diameter. The sound field inside the canal was measured using a specially designed 0.2-mm-diam probe microphone [Daigle and Stinson, J. Acoust. Soc. Am. 116, 2618 (2004)]. In parallel, calculations of the interior sound field were performed using a boundary element technique and found to agree well with measurements.     

Late-onset auditory deprivation: effects of monaural versus binaural hearing aids

Performance on tests of pure-tone thresholds, speech-recognition thresholds, and speech-recognition scores for the two ears of each subject were evaluated in two groups of adults with bilateral hearing losses. One group was composed of individuals fitted with binaural hearing aids, and the other group included persons with monaural hearing aids. Performance prior to the use of hearing aids was compared to performance after 4-5 years of hearing aid use in order to determine whether the unaided ear would show effects of auditory deprivation. There were no differences over time for pure-tone thresholds or speech-recognition thresholds for both ears of both groups. Nevertheless, the results revealed that the speech-recognition difference scores of the binaurally fitted subjects remained stable over time whereas they increased for the monaurally fitted subjects. The findings reveal an auditory deprivation effect for the unfitted ears of the subjects with monaural hearing aids.     

Speech-reception threshold in noise with one and two hearing aids

The binaural free-field speech-reception threshold (SRT) in 70-dBA noise was measured with conversational sentences for 24 hearing-impaired subjects without hearing aids, with a hearing aid left, right, and left plus right, respectively. The sentences were always presented in front of the listener and the interfering noise, with a spectrum equal to the long-term average spectrum of the sentences, was presented either frontally, from the right, or from the left side. For subjects with only moderate hearing loss, PTA (average air-conduction hearing level at 500, 1000, and 2000 Hz) less than 50 dB, the SRT in 70-dBA noise in both ears is determined by the signal-to-noise ratio even if only one hearing aid is used. For larger hearing losses the SRT appears to be partly determined by the absolute threshold. In conditions with a high noise level relative to the absolute threshold, in which case for both ears the SRT is determined by the signal-to-noise ratio, a second hearing aid, just as a monaural hearing aid, generally does not improve the SRT. However, in the case of a high hearing level, or a low noise level, in which a monaural hearing aid is profitable, the use of two hearing aids is even more profitable. In a separate experiment, acoustic head shadow was measured at the entrance of the ear canal and at the microphone location of a hearing aid. It appeared that, for a lateral noise source and speech frontal, the microphone position of behind-the-ear hearing aids has a negative effect on the signal-to-noise ratio of 2-3 dB.     

Acoustic mechanisms that determine the ear-canal sound pressures generated by earphones

In clinical measurements of hearing sensitivity, a given earphone is assumed to produce essentially the same sound-pressure level in all ears. However, recent measurements [Voss et al., Ear and Hearing (in press)] show that with some middle-ear pathologies, ear-canal sound pressures can deviate by as much as 35 dB from the normal-ear value; the deviations depend on the earphone, the middle-ear pathology, and frequency. These pressure variations cause errors in the results of hearing tests. Models developed here identify acoustic mechanisms that cause pressure variations in certain pathological conditions. The models combine measurement-based Thévenin equivalents for insert and supra-aural earphones with lumped-element models for both the normal ear and ears with pathologies that alter the ear's impedance (mastoid bowl, tympanostomy tube, tympanic-membrane perforation, and a "high-impedance" ear). Comparison of the earphones' Thévenin impedances to the ear's input impedance with these middle-ear conditions shows that neither class of earphone acts as an ideal pressure source; with some middle-ear pathologies, the ear's input impedance deviates substantially from normal and thereby causes abnormal ear-canal pressure levels. In general, for the three conditions that make the ear's impedance magnitude lower than normal, the model predicts a reduced ear-canal pressure (as much as 35 dB), with a greater pressure reduction with an insert earphone than with a supra-aural earphone. In contrast, the model predicts that ear-canal pressure levels increase only a few dB when the ear has an increased impedance magnitude; the compliance of the air-space between the tympanic membrane and the earphone determines an upper limit on the effect of the middle-ear's impedance increase. Acoustic leaks at the earphone-to-ear connection can also cause uncontrolled pressure variations during hearing tests. From measurements at the supra-aural earphone-to-ear connection, we conclude that it is unusual for the connection between the earphone cushion and the pinna to seal effectively for frequencies below 250 Hz. The models developed here explain the measured pressure variations with several pathologic ears. Understanding these mechanisms should inform the design of more accurate audiometric systems which might include a microphone that monitors the ear-canal pressure and corrects deviations from normal.     

Localization of virtual sound sources with bilateral hearing aids in realistic acoustical scenes

Sound localization with hearing aids has traditionally been investigated in artificial laboratory settings. These settings are not representative of environments in which hearing aids are used. With individual Head-Related Transfer Functions (HRTFs) and room simulations, realistic environments can be reproduced and the performance of hearing aid algorithms can be evaluated. In this study, four different environments with background noise have been implemented in which listeners had to localize different sound sources. The HRTFs were measured inside the ear canals of the test subjects and by the microphones of Behind-The-Ear (BTEs) hearing aids. In the first experiment the system for virtual acoustics was evaluated by comparing perceptual sound localization results for the four scenes in a real room with a simulated one. In the second experiment, sound localization with three BTE algorithms, an omnidirectional microphone, a monaural cardioid-shaped beamformer and a monaural noise canceler, was examined. The results showed that the system for generating virtual environments is a reliable tool to evaluate sound localization with hearing aids. With BTE hearing aids localization performance decreased and the number of front-back confusions was at chance level. The beamformer, due to its directivity characteristics, allowed the listener to resolve the front-back ambiguity.     

Effects of normal and pathologic eardrum impedance on sound pressure in the aided ear canal: a computer simulation

Reported herein are results of computer simulations of aided sound spectra in ears with normal and pathologic eardrum impedance. The computer technique used in this study has been reported elsewhere [D. P. Egolf, D. R. Tree, and L. L. Feth, J. Acoust. Soc. Am. 63, 264-271 (1978)]. Consequently, to develop reader confidence in the computer scheme, its application to real ears was first tested. This was accomplished by (1) comparing computed spectral data with in-the-ear measurements and (2) comparing real ear minus 2-cc coupler data-both computer generated--with an idealized difference curve published elsewhere [R. M. Sachs and M. D. Burkhard, unpublished rep. no. 20022-1, Industrial Research Products, Inc., Elk Grove Village, IL (1972)]. Results indicate that the wide variation in eardrum impedance among normals evidenced in other studies produces a corresponding wide variation in aided spectrum shape. Likewise, simulations utilizing two sets of pathologic eardrum impedance data obtained from the literature show that aided sound spectra in such ears are likely to be significantly different from those occurring in normal ears. These findings suggest, as others have concluded, that there may be a substantial variation in spectrum shape among individuals wearing identically the same hearing aid--even if those individuals have normal hearing. In conclusion, questions are raised about the use of real-ear simulators and the need for a comprehensive computer-based model of an entire hearing aid.