In vitro considerations in the production of dimensionally accurate earmoulds. II. The earmould material

In the first part of this work, the accuracy of impression materials was studied. In the present paper, the dimensional accuracy of a range of hard and soft polymeric earmould materials was studied and found to be a function of choice of material, processing equipment, curing cycle, time after processing, and choice and storage time of impression material. It was noted that generally speaking the overall shrinkage of a two-stage specimen is approximately equal to the sum of the impression shrinkage plus processing shrinkage. Clinical trials are now being conducted to assess the real ear performance of earmoulds as a function of material accuracy and flexibility.     

A comparison of the effects of different methods of impression build-up on earmoulds

An investigation was carried out into effects of three types of impression build-up (patting down of impressions, special earmould-maker build-up and the multistage impression technique) on the dimensions, static pressure seal, degree of acoustic seal and the subjective tightness and comfort of earmoulds. Patting down the impression significantly improved the degree of acoustic seal provided by earmoulds without making them feel tighter or less comfortable. However, special build-up was much more effective than patting down and the multistage impression technique was slightly more effective than special build-up in improving the degree of acoustic seal. The improvement in acoustic seal provided by both multistage and specially built-up earmoulds can usually be obtained without an unacceptable level of discomfort. Patting down the impression did not improve the chance of obtaining a static pressure seal. Special build-up of the impression by the earmould-maker significantly increased the proportion of earmoulds which provided a static pressure seal but an even higher proportion of earmoulds made from multistage impressions provided a seal. Dimension results indicated that an increase in earmould-maker build-up of the minor axis at the beginning of the canal segment of the impression would improve the acoustic seal provided by specially built-up earmoulds and that earmoulds with rounder tips are more likely to provide a static pressure seal than earmoulds with more elliptical tips. The better the impression material fills the ear canal, the rounder the tip of the impression, and the rounder the tip of the earmould made from the impression.     

耳模和声管长短的声学效果评估

目的：了解耳模和声管长短对声学效果的影响。方法：任选外耳道结构正常的受试者30人（60耳）,并按耳印在第二峡部的平均直径≥4．0mm者为甲组,＜4．0mm者为乙组。采用丹麦产Aurical真耳分析仪,运用台台湾产恒升486笔记本电脑在Noah平台下操作,应用同一固定的助听器,按常规真耳测试方法分析耳模及声管在长短不同的A、B、C三种类型下的声学效果,即A型为耳模与声管等长且距离鼓膜约1．0cm,C型耳模及声管均比A型短0．5cm,B型的声管与A型等长而耳模与C型等长,实验中分别测量A、B、C型耳模在250、500、750、1000、1500、2000、3000、4000、6000Hz时的介入增益值,即A、B及C值,并将测量结果进行SAS软件分析,结果：甲组在250、500Hz处的A、B值较C表显著性差异（P＜0．05）,在3000、4000、6000Hz处A值与C值均有高度显著性差异（P＜0．01）,B值与C值均有显著性差异（P＜0．05）,在750、1000、1500和2000Hz处A、B和C值无明显变化（P＞0．05）;乙组中A、B值较C值仅在250、500Hz有显著性差异（P＜0．05）,在其它频率三者间无显著性差异（P＞0．05）。结论,当外耳道较为宽大时,增加声管的长度可以提高低频和高频的增益,当外耳道较为狭窄时,增加声管的长度仅能提高低频的增益。     

Perceived sound quality in a hearing aid with vented and closed earmould equalized in frequency response.

Nine normal-hearing subjects listened to speech and music in a hearing aid, either through a vented earmould or a closed earmould. The complex frequency responses of the two systems were made equal by compensating the aid and closed mould combination with a digital filter. The subjects rated the perceived sound quality of the systems on seven perceptual scales and a scale for overall impression. The results of the ratings support the hypothesis that there is no difference in perceived sound quality between vented and closed earmoulds that are equalized in frequency response, provided that the perceived loudness is the same in both cases.     

Accuracy of measurement of acoustic rhinometry applied to small experimental animals

Nasal obstruction is one of the major symptoms of allergic rhinitis. In the study of the mechanism of nasal obstruction, experiments on animal are useful. In adult humans, acoustic rhinometry has been used to evaluate nasal obstruction by determining nasal cavity dimensions in terms of cross-sectional areas as a function of the distance from the nostril. We modified the equipment used on humans to assess dimensions of nasal airway geometry of small experimental animals. The purpose of this study was to investigate the accuracy of measurement of the modified acoustic rhinometry applied to small experimental animals using nasal cavity models and guinea pigs. Measurement of the nasal cavity models (made of cylindrical silicone tubes) showed that the acoustic rhinometry estimated 85.5% of actual area and 79.0% of actual volume. In guinea pigs, nasal cavity volume determined by the acoustic rhinometry was 73.7 +/- 20.0% of actual volume. The actual volume was estimated by impression material instilled into the nasal cavity of the animals (IM volume), and volume determined by acoustic rhinometry significantly correlated with IM volume. Furthermore, there was a significant negative correlation between the volume and nasal airway resistance in guinea pigs. Measurement of the nasal airway resistance is the method frequently used in the evaluation of the nasal obstruction in guinea pigs. These results suggest that acoustic rhinometry is useful in evaluating nasal obstruction in small experimental animals.     

Is the real-ear to coupler difference independent of the measurement earphone?

Direct measurement of real-ear hearing aid performance can be obtained using a probe tube microphone system. Alternatively, it can be derived by adding the real-ear to coupler difference (RECD) to the electroacoustic performance of the hearing instrument measured in a 2-cc coupler. Inherent in this derivation is the assumption that the RECD measured with one transducer can be applied to a coupler measurement performed with a different transducer. For the RECD procedure to be valid, it should be independent of the measurement transducer. The Audioscan RM500 is an example of a commercially available real-ear measurement system that incorporates a clinical protocol for the measurement of the RECD. The RECD can be measured on the Audioscan RM500 using a standard EAR-Tone ER-3A insert earphone or the Audioscan's own RE770 insert earphone. The aim of this study was to compare the RECDs obtained with these two earphones. The Audioscan RM500 was used to measure the RECD from the right ears of 18 adult subjects ranging in age from 22 to 36 years (mean 25 years). Measurements were made with the EAR-Tone ER-3A and RE770 insert earphone and three earmould configurations: (1) the EARLINK foam ear-tip; (2) a hard acrylic shell earmould with the same length of acoustical tubing as the foam ear-tip (25 mm); and (3) the shell ear mould with the appropriate length of tubing for a behind-the-ear (BTE) hearing aid fitting (approximately 35-45 mm). The results show that the mean RECD was around 3 dB higher at 1.5 kHz with the foam ear-tip when measured with the RE770 earphone than when measured with the ER-3A earphone. The same magnitude of difference was obtained with the shell earmould and 25-mm tubing; however, this increased to 9 dB when the tubing was increased to around 40 mm for a BTE fitting. The difference in mean RECD with the two earphones was statistically significant on a repeated-measures ANOVA for every earmould configuration (p<0.001). The results of this study demonstrate that the RECD procedure that uses an HA2 coupler and earmould is not independent of the measurement earphone. This has important implications for clinical practice.     

In situ measurements for determining the effect of additional boring of the earmold

To investigate the effect of earmould venting on the insertion gain of hearing aids in patients' ear canal, the authors sealed 35 earmould ventings with soft silicone after measurement under normal conditions and repeated the measurement. The size of the ventings varied from 0.8 mm to 3.5 mm in diameter. The results show the expected SPL-reducing effect of the ventings between 500 Hz and 1,000 Hz. Below 500 Hz the effect found was only slight, because of the low amplification in this frequency range. Above 1 kHz an increasing sound pressure level was measured as a result of resonance effects. This study illustrates the effect of earmould venting on insertion gain. To improve the hearing aid fitting the earmould venting should be used, deliberately and specifically, far more often than at present. The resulting effects of earmould modifications of real ear gain can only be determined by in situ measurements on the patient's eardrum. This method also enables resonance to be compensationed for in a controlled manner.     

Physical and subjective correlates of earmould occlusion.

Physical and subjective aspects of earmould occlusion are examined and related. In particular, measurements made in an ear-like coupler for various venting arrangements are compared with those obtained subjectively by a loudness balance procedure. The results demonstrate the effectiveness of an open (completely non-occluding) earmould at low-frequency sound reduction but indicate some of the factors which make its individual effect difficult to predict. It is further seen the group of hearing-impaired persons was less able to make systematic judgements on sound quality than was the control group of normally hearing persons.     

Practical considerations in the fabrication of earmoulds for young babies

The practicalities of fabricating earmoulds for young babies (under the age of 3 months) have been investigated. The results indicate that earmoulds can be satisfactorily fabricated provided there are changes in approach in the fabrication procedure. This applies in both the ear impression taking and in the format of the final earmould.     

Use of the 'real-ear to dial difference' to derive real-ear SPL from hearing level obtained with insert earphones

The electroacoustic characteristics of a hearing instrument are normally selected for individuals using data obtained during audiological assessment. The precise inter-relationship between the electroacoustic and audiometric variables is most readily appreciated when they have been measured at the same reference point, such as the tympanic membrane. However, it is not always possible to obtain the real-ear sound pressure level (SPL) directly if this is below the noise floor of the probe-tube microphone system or if the subject is unco-operative. The real-ear SPL may be derived by adding the subject's real-ear to dial difference (REDD) acoustic transform to the audiometer dial setting. The aim of the present study was to confirm the validity of the Audioscan RM500 to measure the REDD with the ER-3A insert earphone. A probe-tube microphone was used to measure the real-ear SPL and REDD from the right ears of 16 adult subjects ranging in age from 22 to 41 years (mean age 27 years). Measurements were made from 0.25 kHz to 6 kHz at a dial setting of 70 dB with an ER-3A insert earphone and two earmould configurations: the EAR-LINK foam ear-tip and the subjects' customized skeleton earmoulds. Mean REDD varied as a function of frequency but was typically approximately 12 dB with a standard deviation (SD) of +/- 1.7 dB and +/- 2.7 dB for the foam ear-tip and customized earmould, respectively. The mean test-retest difference of the REDD varied with frequency but was typically 0.5 dB (SD 1 dB). Over the frequency range 0.5-4 kHz, the derived values were found to be within 5 dB of the measured values in 95% of subjects when using the EAR-LINK foam ear-tip and within 4 dB when using the skeleton earmould. The individually measured REDD transform can be used in clinical practice to derive a valid estimate of real-ear SPL when it has not been possible to measure this directly.     

Effects of anatomical variations of the nasal cavity on acoustic rhinometry measurements: a model study

BACKGROUND: The goal of this study was to assess how anatomic variations of the nasal cavity affect the accuracy of acoustic rhinometry (AR) measurements. METHODS: A cast model of a human nasal cavity was used to investigate the effects of the nasal valve and paranasal sinuses on AR measurements. A luminal impression of a cadaver nasal cavity was made, and a cast model was created from this impression. To simulate the nasal valve, inserts of varying inner diameter were placed in the model nasal passage. To simulate the paranasal sinuses, side branches with varying neck diameters and cavity volumes were attached to the model. RESULTS: The AR measurements of the anterior nasal passage were reasonably precise when the passage area of the insert was within the normal range. When the passage area of the insert was reduced, AR measurements significantly underestimated the cross-sectional areas beyond the insert. The volume of the paranasal sinus had limited effect on AR measurements when the sinus ostium was small. However, when the ostium size was large, increasing the volume of the sinus led to significant overestimation of AR-derived areas beyond the ostium. CONCLUSION: The pathologies that narrow the anterior nasal passage result in the most significant AR error by causing area underestimation beyond the constriction. It also appears that increased paranasal sinus volume causes overestimation of areas posterior to the sinus ostium when the ostium size is large. If these physical effects are not considered, the results obtained during clinical examination with AR may be misinterpreted.     

Earphone termination and the response of behind-the-ear hearing aids.

An investigation was carried out on the effects on hearing aid response of placing the earphone in the earmould rather than in the case of a behind-the-ear hearing aid. Results from a number of physical configurations are reported and discussed and it is shown that it is possible to achieve a considerable reduction in the height of frequency response peaks. On the basis of the research reported some specific recommendations are made about future aid design.     

Factors affecting the use and perceived benefit of ITE and BTE hearing aids.

The aim of the present study was to investigate factors that might affect hearing aid use, satisfaction and perceived benefit. A further aim was to look at which variables affect the choice of hearing aid, in particular, an in-the-ear (ITE) versus a behind-the-ear (BTE) device. Twenty-nine elderly hearing-impaired people with a mild-to-moderate hearing loss were fitted with both an ITE and a BTE hearing aid with similar electroacoustic performance. Both hearing aids were linear with output compression limiting and were fitted in a randomized order. After wearing each device for a six-week period, subj ets were asked to select the hearing aid they preferred. Variables significantly related to hearing aid choice, use and perceived benefit included ease of management, accuracy with which the NAL-R insertion gain target was achieved, earmould comfort and the type of hearing aid the client preferred initially.     

In vitro considerations in the production of dimensionally accurate earmoulds. I. The ear impression

As the first part of a study on the fabrication of earmoulds, the dimensional accuracy and stability, as a function of time, of a wide range of polymer-based impression materials has been studied. The results demonstrate that addition cured silicones are the most promising materials in terms of their accuracy and dimensional stability.     

A comparison of three types of earmoulds in elderly users of post-aural hearing aids

The objective of this study was to assess the ease of handling, comfort, and general effectiveness of three types of earmould in patients who, due to their age, could be expected to experience handling difficulties. The three mould types were, meatal tip, skeleton and skeleton with the 'top prong' removed. The results indicated that the skeleton with the top prong removed was the best in all contexts but one. It was easiest and quickest to fit and least likely to be badly fitted. Only in feedback control did it perform less well. It was concluded that it should be routinely used for elderly patients fitted with low gain hearing aids.     

The early diagnosis of acoustic tumors using posterior fossa cisternography with iophendylate

Posterior fossa cisternography is a basic method for confirming the presence of acoustic and other tumors of the cerebellopontine angle. A modified outpatient procedure with use of new instrumentation is described here, and comparisons are made to the conventional hospital procedure. A series of patients selected for outpatient posterior fossa cisternography was studied using data processing techniques. Presenting symptoms and preliminary test results were compiled, and an impression established as to the likelihood of a tumor being present. Outpatient posterior fossa cisternography was performed, with examination under fluoroscopy in certain circumstances; surgery was performed in certain instances. The results of diagnostic procedures, surgical procedures, and follow-up studies were coded, analyzed by computer, and conclusions drawn.     

Spatial representation of basilar membrane movement with 3D computer graphics

A mathematical model of the cochlea was implemented on a computer. The basilar membrane motion was computed for single, two, and multi-tone stimuli as well as for musical sounds and vowels. The pattern of the travelling waves were presented in three-dimensional color computer graphics. The high performance 3D graphics system performs local hidden surface removal, 3D geometric transformations and supports local lighting models to generate truly realistic shading for complex 3D objects. An addressable 1280 by 1024 pixel matrix assures crisp, precise resolution of the finest detail in the graphic images. The spatial pattern of basilar membrane motion conveys an impression of the image of acoustic stimuli on the basilar membrane. Firstly, the motion pattern of the travelling wave to a single tone is presented. The superposition of several tones (two-tone, multi-tone) causes a superposition of the travelling waves along the basilar membrane whereby the place principle in the cochlear partition becomes more clearly recognizable. Sounds (flute and violin) and vowels (German "u" and "i") evoke a complex motion pattern on the basilar membrane. The realization of the chronological order of movements on the basilar membrane can be made by computer animation. This enables the analysis of the space-time patterns of complex acoustic stimuli.     

The real ear effect of adjusting the tone control and venting a hearing aid system

The effect of modifying the frequency response of a hearing aid system has previously been reported only on 2 cc couplers. Although it is recognized that coupler measurements do not accurately reflect real ear measures, they are frequently quoted when describing the effect of modifying a hearing aid system. To investigate the correlation, the real ear effect was assessed in 43 ears by measuring the effect on the insertion gain of adjusting the tone control of a hearing aid and acoustically venting the mould with a 2 mm parallel vent, singly and in combination. The results were considerably different from those measured in couplers. By comparing insertion gain with 2 cc coupler gain measurements, it was found that: (1) adjusting the tone control had a lesser mean effect than predicted; (2) venting the earmould with a 2 mm parallel vent produced a mean reduction in the frequency response at 0.75 to 1 kHz of 8 dB SPL; (3) by combining a 2 mm earmould vent and adjusting the tone control, a mean reduction in the frequency response of 10 dB SPL was produced at 0.75 to 1 kHz. In addition, the range of acoustical effects was considerable, from virtually none to greater than those predicted in coupler experiments. This was presumably due to variation in the anatomy of normal canals. It is concluded that for both research and clinical purposes, the effect of modifying the frequency response of a hearing aid system should be measured by insertion gain rather than predicted from laboratory results.