The influence of RECD transducer when deriving real-ear sound pressure level

OBJECTIVE: The main aim of the present study was to compare the derived and directly measured real-ear hearing instrument performance for a range of commonly used hearing instruments. A secondary aim was to compare the real-ear to coupler difference (RECD) measured using the ER-3A insert earphone and a selection of hearing instruments. DESIGN: The real-ear SPL was measured for four models of hearing instrument in 20 adult participants using an Audioscan RM500 real-ear system. This was compared with the derived real-ear SPL obtained by adding the RECD (measured using the ER-3A insert earphone) to the 2-cc coupler response of each hearing instrument. Measurements were made at 1/12 octave intervals from 0.2 to 6 kHz, using both the HA1 and HA2 2-cc coupler. In addition, the RECD was measured using four models of hearing instrument for comparison with the ER-3A insert earphone values. RESULTS: The procedures were very reliable with mean differences on retest of less than 1 dB. Repeated-measures analysis of variance revealed statistically significant differences between the measured and derived real-ear SPL (p < 0.001) for several models of hearing instrument. The derived responses using the HA1 coupler yielded good accuracy, whereas the HA2 yielded less accuracy. For three models of hearing instrument, the maximum difference was between 5 and 10 dB when using the HA2 coupler. The mean RECD measured with the ER-3A insert earphone and HA2 coupler was not always equivalent to the RECD measured with the hearing instruments. CONCLUSIONS: The accuracy of the derived real-ear response obtained using an RECD, measured with an ER-3A insert earphone, is very good when an HA1 is used for the coupler component of the RECD. The accuracy diminishes somewhat with the HA2 coupler, especially for undamped hearing instruments. The accuracy of the derived real-ear response is very good when the RECD is measured using the hearing instrument and the HA1 or the HA2 coupler.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

New version of the TEN test with calibrations in dB HL

OBJECTIVE: To develop a new version of the threshold-equalizing-noise (TEN) test for the diagnosis of dead regions, with levels calibrated in dB HL rather than dB SPL, and with levels corresponding to the dial readings on the audiometer. DESIGN: The spectral shape of the noise required to give equal masked thresholds at all frequencies, when expressed in dB HL, was derived by two calculation methods and by empirical measurements of the electrical output of audiometers using TDH50 earphones and TDH39 earphones. To reduce the loudness of the noise and to minimize distortion generated in the audiometer and/or earphone, the noise was bandlimited between 354 and 6500 Hz. In addition, the noise was synthesized using a method that leads to a low crest factor (ratio of peak to root-mean-square value). This further reduced audiometer/earphone distortion and allowed higher levels per ERBN; ERBN is the equivalent rectangular bandwidth of the auditory filter at 1 kHz, as determined in young, normally hearing subjects. The test tone frequencies were limited to the range 500 to 4000 Hz. Subjects with normal or near-normal hearing were tested by using a noise level of 60 dB HL/ERBN to assess whether the noise did lead to equal masked thresholds in dB HL for all audiometric frequencies from 500 to 4000 Hz. Thresholds in the TEN were measured by means of manual audiometry with a 2 dB final step size. RESULTS: The mean masked thresholds were almost constant across frequency when expressed in dB HL and were within 0.5 dB of the noise level per ERBN. For a single noise level, the test takes approximately 5 minutes per ear to administer. CONCLUSIONS: The new TEN test has the following advantages over the original version (which used levels calibrated in dB SPL): (1) All levels are expressed in dB HL. Thus, absolute thresholds only need to be measured once. (2) Calibration is such that both the noise level/ERBN and the test tone levels correspond to the values indicated on the audiometer. This makes the test simpler to apply and reduces the likelihood of errors. (3) The noise bandwidth is restricted, and the noise has a low crest factor. This allows the noise level/ERBN to be increased while avoiding distortion, excessive loudness, and possible further damage to hearing.     

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.     

Assessment of FM systems with an ear canal probe tube microphone system

A technique is described to measure the real-ear performance of an FM system using an ear canal probe tube microphone device. The method involves placement of the FM microphone next to the monitoring (compression) microphone of the probe tube assembly to produce a constant sound pressure level input to the FM system. With the probe tube in the ear canal, a hearing aid alone is measured with a 60 dB SPL input and the FM system attached to the hearing aid (personal FM system) is assessed with an 80 dB SPL input to account for the higher input levels that occur due to the 6 inch distance between the speaker's mouth and the FM microphone. This technique permits a rapid comparison of the real-ear response of the hearing aid and the FM system.     

四川地区成年人真耳-耦合腔差值分析

目的：了解四川地区成年人真耳耦合腔差值（RECD）的平均值。方法：对95例四川地区成年人进行双耳RECD测定．并进行性别及左右耳间的比较及与欧美地区成年人的RECD平均值比较。结果：四川地区与欧美地区成年人的RECD平均值差异有统计学意义。结论：在助听器验配过程中,使用RECD值时不能盲目采用欧美地区成年人的平均值标准。     

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.     

Evaluation of a probe-tube insertion technique for measuring the real-ear-to-coupler difference (RECD) in young infants

A common strategy for measuring the real-ear response of the real-ear-to-coupler difference (RECD) in the pediatric population is to insert a probe-tube separately from the eartip. This strategy is at times difficult to implement while attempting to obtain the measurement from a young infant. An RECD probe-tube insertion technique that involves connecting the probe-tube to an eartip with plastic film for simultaneous insertion was examined on 30 infants. Repeated measurements were completed on each infant to obtain within-session test-retest reliability data. Probe-tube insertion depth was also examined across participants to provide a guideline for the infant population. Findings indicate that reliable RECD values can be obtained in infants when the probe-tube is extended approximately two to four millimeters (mm) beyond the eartip or 11 mm from the entrance to the ear canal. Clinical implications of this work are discussed.