Measuring the real-ear to coupler difference transfer function with an insert earphone and a hearing instrument: are they the same?

OBJECTIVE: The purpose of the study was to compare the real-ear to coupler difference (RECD) measured with an insert earphone and two models of hearing instrument. DESIGN: The RECD was obtained from one ear of 18 normal-hearing subjects by subtracting the 2-cc coupler (HA1 and HA2) response from a real-ear aided response, using a conventional probe-tube microphone system. The measurements were made with a conventional ER-3A earphone and two models of behind-the-ear hearing instrument (Unitron US80, Unitron, Kitchener, Canada; and Widex Diva, Widex, Vaerloese, Denmark). RESULTS: The procedures were very reliable, with mean differences on retest of less than 1 dB. There were statistically significant differences between the mean RECDs obtained using an insert earphone compared with those obtained with each hearing instrument (p < 0.05). The differences were greatest when using the HA2 2-cc coupler. For example, the maximum difference in mean RECD between the insert earphone and the Widex Diva was 6 dB and 11 dB when using the HA1 and the HA2 2-cc coupler, respectively. CONCLUSIONS: The RECD is dependent on the acoustic impedance of the sound source, the coupling system, and the coupler and ear. The acoustic impedance may be different for an insert earphone and a given hearing instrument. Therefore, the RECD measured with an insert earphone may not always accurately represent the difference in performance of a hearing instrument measured in the real ear and the 2-cc coupler.     

Deriving the real-ear SPL of audiometric data using the "coupler to dial difference" and the "real ear to coupler difference"

OBJECTIVE: The purpose of the study was to compare the measured real-ear sound pressure level (SPL) of audiometer output with the derived real-ear SPL obtained by adding the coupler to dial difference (CDD) and real-ear to coupler difference (RECD) to the audiometer dial reading. DESIGN: The real-ear SPL and RECD were measured in one ear of 16 normally hearing subjects using a probe-tube microphone. The CDD transform and the RECD transfer function were measured in an HA1 and an HA2 2-cc coupler using an EAR-LINK foam ear-tip or a customized earmold. The RECD transfer function was measured using the EARTone ER 3A and the Audioscan RE770 insert earphone. RESULTS: The procedures were very reliable with mean differences on retest of less than 1 dB. The mean difference between the measured and derived real-ear SPL was generally less than 1 dB and rarely exceeded 3 dB in any subject. CONCLUSIONS: The CDD measured for an individual audiometer and the RECD measured for an individual ear can be used to derive a valid estimate of real-ear SPL when it has not been possible to measure this directly.     

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.     

Customized acoustic transform functions and their accuracy at predicting real-ear hearing aid performance

OBJECTIVE: The purpose of the study was to evaluate the validity of predicting the real-ear aided response by adding customized acoustic transform functions to the performance of a hearing aid in a 2-cc coupler. DESIGN: The real-ear hearing aid response, the real-ear-to-coupler difference (RECD/HA2), and field to behind-the-ear microphone transfer functions were measured in both ears of 24 normally hearing subjects using probe-tube microphone equipment. The RECD/HA2 transform function was obtained using both insert earphones and with the hearing aid/ pressure comparison method. An RECD/HA2 transfer function was also obtained with a customized earmold, ER-3A foam tip, and an oto-admittance tip. RESULTS: Validity estimates were calculated as the difference between the derived and measured real-ear response. The derived response was generally within 5 dB of the measured real-ear response when it incorporated an RECD/HA2 transform function obtained with a customized earmold for the specific ear in question. Discrepancies increased when the RECD/HA2 transfer function was obtained from the same subject but the opposite ear. There were significant differences between the RECD/HA2 transform function obtained with customized and temporary earmolds. As a result, the derived response incorporating these transforms differed significantly from the measured real-ear response obtained with the customized earmold. The insert earphone and the hearing aid RECD/HA2 transfer function were equally valid. CONCLUSIONS: The derived response may be used as a substitute for in situ hearing aid response procedures when it incorporates acoustic transform functions obtained with a customized earmold from the specific ear in question.     

Comparison of real-ear to coupler difference values in the right and left ear of adults using three earmold configurations

OBJECTIVE: The purpose of the study was to compare real-ear to coupler difference (RECD) values in the right and left ear of adults using three earmold configurations. DESIGN: The RECD was obtained from both ears of 18 normal hearing adults by subtracting the HA2 2-cc coupler response from the real-ear response using an ER-3A insert earphone and a swept pure tone on the Audioscan RM500 probe-tube microphone system. The measurements were made with a personal earmold, foam eartip, and oto-admittance tip. RESULTS: The mean difference between the right and left RECD was close to 0 dB for all earmold configurations and was not statistically significant on a repeated-measures analysis of variance (p > 0.05). In 90% of participants, the difference between ears was generally less than 3 dB at 0.5 to 4 kHz. CONCLUSIONS: Cooperative participants with non-occluding wax and normal middle ear function (on tympanometry) show small differences in RECD between the right and left ear, irrespective of the earmold configuration. The study has yet to be extended to the clinical setting where subject cooperation and earmold fit may differ from the present study. In the meantime, the findings from the present study indicate that where an RECD can be obtained from only one ear of a participant, it is probably best to use this to derive real-ear SPL of both ears instead of relying on average age appropriate corrections.     

Perforation of the tympanic membrane and its effect on the real-ear-to-coupler difference acoustic transform function.

It is not always possible to undertake extensive real-ear measurements, especially in infants and young children. An alternative approach is to estimate the real-ear SPL by use of an acoustic transform function such as the real-ear-to-coupler difference (RECD). This may be used to estimate the real-ear sound pressure level (SPL) obtained from an insert transducer or a hearing instrument. The aim of the present study was to investigate the effects of tympanic membrane perforation on the RECD transform function. Subjects in the study comprised two groups of 12 individuals aged between nine and 65 years. One group of subjects had a tympanic membrane perforation and was recruited to the study before admission for myringoplasty. There was no evidence of middle ear pathology in the remaining subjects who comprised the control group. An RECD transform function for an insert transducer was measured on each subject using the standard clinical protocol on the Audioscan RM500 real-ear measurement system. There was a statistically significant difference between the two groups; mean RECD transform value of the perforation group was 9-12 dB lower than the corresponding value in the control group at audiometric frequencies below 1.5 kHz. This difference is probably due to the perforation acting as a vent and allowing low-frequency acoustic energy to escape into the middle ear cavity. Use of an average RECD transform function to estimate real-ear SPL in subjects with a perforation will overestimate the SPL reaching the tympanic membrane. As a result, the derived real-ear SPL obtained by use of either an insert transducer or a hearing instrument will be overestimated. This has implications for the selection and verification of a hearing instrument. The difference in the mean RECD transform function between the control group and subjects with a tympanic membrane perforation supports the use of individually measured RECD values wherever possible.     

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.     

Audiometric earphone discomfort level and hearing aid saturation sound pressure level for a 90 decibel input signal (SSPL90) as measured in the human ear canal

The acoustical problems involved in matching the saturation sound pressure level for a 90 dB input signal (SSPL90) of a hearing aid to individual discomfort level were investigated. The real ear SPL (RE/SSPL90) produced by a supra-aural earphone used when measuring uncomfortable loudness (UCL), and RE/SSPL90 produced by three different hearing aids at 90 dB SPL input, were measured for nine subjects, using a miniature microphone technique, and compared to the corresponding coupler levels used when matching hearing aid maximum output to UCL. It was found that a hearing aid often gives about 5 dB, and sometimes 10 dB, higher RE/SPLs than the earphone, if the hearing aid output levels, as measured in a 2-cc coupler (IEC126), are equal to the earphone output levels as measured in a 6-cc coupler (NBS9A). It is recommended that a safety margin of at least 5 dB be used in the preliminary fitting when matching hearing aid SSPL90 to the patient's UCL, converted to dB SPL.     

Hearing aid coupler output level variability and coupler correction levels for insert earphones

HA coupler type SPLs were independently determined by two experimenters for five repeated measurements with and without replacement of two ER-3A and two EARTONE 3A insert earphones. Measurements were made using a B&K DB-0138 coupler configured as an HA-1, HA-2 earphone coupler and HA-2 earphone coupler with entrance through a rigid tube referred to as the DB-0138 coupler. The HA-1, HA-2, and DB-0138 SPLs were found to be very stable (+/- 0.2 dB) for all intra- and interexperimenter measurements for each insert earphone and coupler type from 125 to 8000 Hz. Averaged across both experimenters and all repeated measurements, the mean HA-1 and HA-2 coupler SPLs were similar for each insert earphone from 125 to 8000 Hz. The mean HA-1, HA-2, and DB-0138 coupler SPLs were similar for each insert earphone from 125 to 2000 Hz; however, from 3000 to 8000 Hz the DB-0138 coupler SPLs were higher than the HA-1 and HA-2 coupler SPLs for each insert earphone. This occurred because of the geometrical differences between the insert earphone to coupler connections and coupler types. The HA-1 minus DB-0138 and HA-2 minus DB-0138 coupler SPL differences, or coupler correction levels, could be explained by quarter-wave resonances and stepped-diameter tubing systems creating acoustic horn effects.     

Clinical protocols for hearing instrument fitting in the Desired Sensation Level method

A discussion of the protocols used particularly in the clinical application of the Desired Sensation Level (DSL) Method is presented in this chapter. In the first section, the measurement and application of acoustic transforms is described in terms of their importance in the assessment phase of the amplification fitting process. Specifically, the implications of individual ear canal acoustics and their impact on accurately defining hearing thresholds are discussed. Detailed information about the statistical strength of the real-ear-to-coupler difference (RECD) measurement and how to obtain the measure in young infants is also provided. In addition, the findings of a study that examined the relationship between behavioral and electrophysiologic thresholds in real-ear SPL is described. The second section presents information related to the electroacoustic verification of hearing instruments. The RECD is discussed in relation to its application in simulated measurements of real-ear hearing instrument performance. In particular, the effects of the transducer and coupling method during the RECD measurement are described in terms of their impact on verification measures. The topics of insertion gain, test signals, and venting are also considered. The third section presents three summary tables that outline the hearing instrument fitting process for infants, children, and adults. Overall, this chapter provides both clinical and scientific information about procedures used in the assessment and verification stages of the DSL Method.     

Attenuation provided by four different audiometric earphone systems

The attenuation provided by TDH earphones in MX-41/AR and P/N 51 cushions, Audiocup earphone enclosures and ER-3A insert earphones with ER3-14 foam earplugs was determined for 30 normally hearing subjects using a real-ear attenuation at threshold paradigm. The MX-41/AR and P/N 51 cushions provided about the same amount of attenuation which was less than the attenuation provided by the Audiocup enclosures. The ER-3A/ER3-14 provided the highest amount of attenuation. The MX-41/AR and ER-3A/ER3-14 attenuation values were in agreement with other studies using similar methodology. However, the attenuation provided by the Audiocup enclosures was considerably less, in the lower frequencies, than reported in two other studies. ANSI S3.1-1977 supra-aural earphone cushion attenuation values, which were determined using pure-tones presented in a free-field, should be replaced by earphone cushion attenuation values determined with 1/3 octave bands of noise presented in a diffuse sound field.     

鼓膜穿孔对真耳-耦合腔差的影响

目的:探讨鼓膜穿孔对真耳一耦合腔差(RECD)的影响.方法:34例(34耳)中耳功能及听力正常成人为对照组,30例(34耳)干性鼓膜穿孔患者为实验组,用真耳分析仪测试RECD.结果:实验组与对照组RECD值比较在1 kHz以下(含1 kHz)及4 kHz差异有统计学意义(P＜0.05),实验组比对照组要小;实验组的标准差变化较大,平均为4.4 dB,而对照组为1.4 dB;实验组RECD值与等效外耳道容积大小在0.75 kHz以下呈显著负相关(r=-0.70,P＜0.01),而1 kHz以上无相关性;鼓膜穿孔大小对RECD值无影响.结论:鼓膜穿孔患者RECD值在不同频率变化较大,选配助听器时应进行真耳测量以测试个体RECD,尽量不用平均值,适当增加低频的增益.     

Performance of custom-fit versus fixed-format hearing aids for precipitously sloping high-frequency hearing loss

This study compared the real-ear response provided by custom-fit hearing aids to the closest matching fixed-format disposable hearing aids in patients with precipitous high-frequency hearing loss. Laboratory and field measures of aided performance were obtained to compare patient performance with the custom-fit and fixed-format hearing aids. In addition, coupler versus real-ear response differences were compared for the two hearing aid types. The results revealed that relatively close approximations to the real-ear aided responses of the custom-fit instruments were possible for most participants using seven fixed acoustic formats. No significant differences in mean performance between the two instrument types were observed for aided speech recognition or field ratings of aided performance, although mean patient satisfaction was lower for the disposable hearing aids. The real-ear to coupler difference was greater for the disposable hearing aid than for the custom-fit instruments, presumably owing to its deeper insertion into the ear canal.     

Real-Ear to coupler difference in patients with ear drum perforation.

The aim of this study is to investigate the effects of ear drum perforation on real-ear to coupler difference (RECD) in adults. RECD was measured using a probe tube microphone system in 22 patients with ear drum perforations. Twenty-two normal subjects served as controls. For normal subjects, RECD was in good agreement with the values reported in the literature. For the perforated ears, the RECD was up to 8 dB smaller in the frequency range from 0.5 to 1 kHz. There was no significant difference at frequencies below 0.25 kHz and above 1 kHz. A much larger intersubject variability was found in the experimental group. The mean intersubject standard deviation was 4.4 dB in the experimental group as contrasted with 2.2 dB in the control group. Neither the equivalent ear canal volume nor the perforation size appeared to be correlated with the degree of RECD reduction over lower frequencies. These results strongly suggest the need for individual RECD measurements, rather than using the average normal RECD, to appropriately compensate for the reduced transmission of lower-frequency sounds in fitting hearing aids for patients with ear drum perforations.     

Insertion gain measurements using two low-powered analogue hearing aids

Differences between insertion gain measurements and coupler measurements were investigated using two different hearing aids, to assess the accuracy of hearing aid fitting methods which involve the use of real-ear-to-coupler differences (RECDs). The hearing aids used were the NHS BE19 and the Lavis X11. They were allocated alternately to 80 new adult hearing aid users, and insertion gain measurement was carried out with the allocated aid on all subjects, and with both hearing aids on 36 subjects. Coupler measurements were also carried out with both hearing aids, at two different gain settings. Comparison of the coupler measurements and the insertion gain measurements suggests that the differences in insertion gain produced by different hearing aids cannot be predicted accurately from the differences in coupler gain measurements produced by the same aids. Although the use of RECDs measured using insert earphones may accurately predict the real-ear performance of some models of hearing aid, this method may not produce equally satisfactory results with other models.     

Effect of earmold fit on predicted real ear SPL using a real ear to coupler difference procedure

OBJECTIVES: The goal of Experiment I was to quantify the SPL entering the ear canal via a secondary pathway created by a vent in the earmold and/or a slit leak around the earmold. The goal of Experiment II was to determine the validity of a real ear to coupler difference (RECD) procedure under conditions that are likely to produce errors (e.g., when hearing aid gain in the low frequencies is minimal and large negative RECD values occur as a result of venting or a loosely fitting earmold). DESIGN: In Experiment I, the SPL entering the ear via the secondary pathway was measured in 61 hearing-impaired children and 13 normal-hearing adults. In Experiment II, traditional probe microphone measures of real ear SPL were compared to the SPL predicted using the RECD procedure in five normal-hearing adults with loosely fitting earmolds. RESULTS: Results of Experiment I indicated that sound entered the ear canal unattenuated at 250 and 500 Hz, regardless of earmold fit, vent size, or subject age. In Experiment II, the largest differences between traditional probe microphone measures of SPL and predicted measures were noted when hearing aid gain was 0 dB and large negative RECD values were present. When hearing aid gain was minimal and the RECD was in the -10 to -22 dB range, predicted values underestimated the real ear SPL by an average of 14 dB. CONCLUSIONS: Although the results of this study apply only to a limited range of conditions found in clinical practice, in those cases, the errors may influence clinical decisions about the type of hearing aid fitted and the amount of gain provided. Potential solutions to this problem are discussed.     

Impact on hearing aid targets of measuring thresholds in dB HL versus dB SPL

Audiometric measurements are traditionally made in dB HL, which by definition are specified relative to the sound pressure level (SPL) in a coupler. Real-ear dB SPL is then estimated by applying an average ear transform to the coupler value. However, individual variation in ear canal acoustics and variations in transducer placement strongly influence the dB SPL of signals arriving at the eardrum. In this paper, data from 1814 ears are presented, showing that the distribution of eardrum dB SPL for a fixed signal level varies across ears and across frequency by as much as 40 dB. The impact of this variance upon hearing aid targets computed with the NAL-NL1 fitting algorithm is examined by comparing the targets obtained from using an average transform with those obtained when audiometric data in dB SPL are obtained by applying individually measured real-ear-to-coupler difference (RECD) values to dB HL thresholds. The impact can be considerable.     

Predictive validity of a procedure for pediatric hearing instrument fitting

In 1994, Moodie, Seewald, & Sinclair described the development of a clinical procedure for predicting real-ear hearing instrument performance in young children. The purpose of the present study was to determine the validity of this procedure for predicting the real-ear aided gain (REAG) and real-ear saturation response (RESR) of hearing instruments worn by children. To this end, both the REAG and RESR were measured, through probe-microphone measures, and predicted, using the Moodie et al. procedure. The findings confirmed that the 2-cc coupler-based procedure, with individualized acoustic transforms, described by Moodie et al., resulted in highly accurate predictions of real-ear hearing instrument performance for both REAG and RESR at five test frequencies. The implications of these findings for the clinical fitting of hearing instruments in infants and young children are discussed.     

A comparison of inter-aural attenuation with the Etymotic ER-3A insert earphone and the Telephonics TDH-39 supra-aural earphone.

One of the many reported advantages of the insert earphone over the supra-aural earphone is increased inter-aural attenuation (IA). Minimum values of IA determine the need for masking of the non-test ear in air-conduction audiometry. The aim of the present study was to measure inter-aural attenuation for the Etymotic Research ER-3A insert earphone (with deep and shallow insertion of the ear plug within the ear canal) and compare this with the supra-aural Telephonics TDH-39/MX41-AR earphone/cushion combination. Subjects were 18 adults ranging in age from 38 to 68 years (mean 50 years). Each subject had no hearing in one ear following translabyrinthine surgery for removal of an acoustic neuroma. The opposite ear had hearing thresholds better than 40 dB HL and an air-bone gap of less than 10 dB at any audiometric frequency. Pure tone air-conduction thresholds were obtained in the range 0.25-8 kHz. Deep insertion of the insert earphone was deemed to occur when the outside edge of the ear plug was flush with the entrance of the ear canal. Shallow insertion was deemed to occur when half of the ear plug (6 mm) was inside the entrance of the ear canal. IA was defined operationally as the difference between the good-ear and poor-ear not-masked air conduction threshold for a given audiometric frequency and earphone. The results show that the TDH-39/MX41-AR combination provides a median IA of approximately 60 dB with a lower limit of approximately 45 dB. Greater IA was obtained with the ER-3A insert earphone but this depended on the depth of insertion. With a deep insertion, the 1A values were some 15-20 dB greater than with the supra-aural earphone. Although frequency-specific IA values are provided, a simple rule of thumb is to apply masking to the non-test ear when the pure tone airconduction signal from the ER-3A insert earphone exceeds the bone conduction threshold of the non-test ear by 55 dB HL or more. If it is not possible to obtain a deep insertion depth this value should be reduced by 5 dB.