Otometry – Illustrative Procedure

Illustrative Procedure (A simple scientific substitution method is employed)

Introduction

When discussing topics that readers are somewhat unfamiliar with, it is helpful to provide a step-by-step description and explanation to assist in understanding the procedure. This is certainly desirable in regards to otometry. However, such a “walk-through” procedure is not found in either of Victoreen’s books on otometry or his published articles. It most likely was provided during individual training sessions and assuredly resides in the files of those who used the procedure.

Unfortunately, a review of the literature comes up essentially empty handed. What was located is consistent for the first steps, but provides confusing determinations of the actual gain required (the final step). I am certain that when going through some of my old files in the garage, when looking for something else, I will find the substantial notes provided to me by Dr. Victoreen many years ago that explain this discrepancy.

Illustrated Otometry Procedure

The numerical data for this illustrative purpose has been redrawn and modified based on an actual subject, and would vary for each subject{{1}}[[1]]Melen, L.A. Otometry: An emerging prosthetic discipline prescription selection and fitting of hearing instruments. The Guthrie Bulletin, Volume 47, Summer 1977[[1]]. The descriptive comments come primarily from other Victoreen publications{{2}}[[2]]Victoreen, J.A. The prescription of hearing aid instruments, Audecibel, Spring 1968, pp. 49-56, 58, 60-61[[2]]{{3}}[[3]]Victoreen, J.A. Hearing instruments and loudness, Audecibel, 1972, pp. 190, 191-194, 196-198, 200-202[[3]]{{4}}[[4]]Victoreen, J.A. Basic otometric principles, Audecibel, Spring 1973, pp. 63-70, 72-76, 78-79[[4]].  The procedure is as follows:

• Measure unaided MCLP (Most Comfortable Listening Pressure) using the damped wavetrain (DWT) signal (Figure 8 – figure numbering continues from the previous posts on otometry).  If greater than +6 dB from the 72 dB most comfortable loudness pressure line (red dashed line), amplification is recommended.
• The earpiece intended for use is attached to a laboratory or test hearing aid and placed on the subject. (A laboratory standard aid is a normal hearing aid whose parameters can be modified as necessary by the manufacturer to fulfill the prescriptive measurements.  This “substitution” of a standardized hearing instrument in place on the ear and operating at an approximately most-comfortable-loudness level, eliminates consideration of insertion losses that vary from patient to patient when fitted with amplification.
• Present the DWT signals from a loudspeaker at the appropriate distance (28 inches on axis, but could vary, depending on calibration). The signal repeats at a rate of 3/second. Measure each ear separately, occluding the contralateral ear during testing (Figure 9).
  • Turn hearing aid on and obtain a MCL at 1000 Hz for a 72 dB SPL DWT signal (some adjustment of the hearing aid volume control is most likely necessary). This becomes the reference position.
    • Measure range of comfortable loudness (too loud and too soft – usually about a ±4 dB range, with MCL about in the middle (I have found it more accurate and faster if the subject manipulates the level of the DWT signal to achieve these measurements by turning the attenuator knob themselves). This step, measuring the range from too loud to too soft is often eliminated, with the MCLP measured directly.
    • Measure threshold of discomfort (Maximum Tolerable Pressure, or MTP) using the DWT signal (determines maximum deliverable pressure to not be exceeded)
    • Measurement of the minimum audible pressure (MAP) is required only if one wants to review the actual dynamic range, or if one wants to estimate the MCLP by bisecting the MAP and MTP, a method sometimes used but not recommended.
  • With the volume control of the hearing instrument set in the reference position, MCLPs at other frequencies are measured (Figure 10). Frequencies available with the DWT generator are 250, 500, 750, 1000, 1500, 2000, 2500, 3000, 4000, and 6000 Hz.
  • Plot these data on the otometric tabulation chart (Figure 11, top line). Note that the 72 dB at 1000 Hz (outlined in yellow) represents the reference position as described previously and illustrated in Figure 9. The values at the other frequencies represent MCLP with the hearing aid set to the reference position.

The second line of the prescription tabulation chart of Figure 11 represents the MCLP objectives with the low frequencies rolled off at 3-dB/octave below 1000 Hz (Figure 12).  Line 3 is the difference between lines 1 and 2. Line 4 values are taken from the 2cc coupler data of the measurement hearing aid used in the test (Figure 13), and line 5 (in blue) is the eventual hearing aid prescription gain for the different frequencies, added or subtracted, of lines 3 and 4.

• The laboratory test aid is removed from the ear, and with the volume control still at the reference position, the aid is measured into a 2cc coupler in a hearing aid test chamber, and the response is superimposed onto the otometric chart (same graphical scale – Figure 13).  In the substitution method, the test aid is referenced to 1000 Hz (MCL to 72 dB DWT input signal at 1000 Hz). The other frequencies must be referenced to the gain at 1000 Hz, the reference position. The gain values for the other frequencies are shown, as calculated from the reference position gain of 28 dB, the dashed line. These numbers represent the correction factors for the different frequencies and are reported on line 4 of the tabulation chart (Figure 11). However, and unexplained, some users of otometry plot the measured gain directly on line 4 (upper curve of Figure 14).   Victoreen states that the difference in dB between 72 dB and the value actually found at any one signal frequency expresses the correction needed at that frequency. It is on this line that confusion appears in the literature when recording the prescriptive gain recommended with otometry. Instead of making the relative calculation as described, some have taken the measured 2cc gain in dB and inserted those numbers directly into line 4. Recording this way results in gain values that appear excessively high, an example of which is shown in Figure 14.  If the values were taken directly from the measurement aid’s 2cc data, the gain representing the red line would result. If, however, the 2cc data were taken with the correction procedure of Figure 13, the blue line would represent the gain required for the prescription. A cursory examination of the two gain recommendations relative to the unaided MCLPs would suggest the gain of the blue line to be more appropriate, and certainly more likely for the fitted hearing aid’s volume control to manage.  It is assumed that because the response configuration is the same, that compensation for the correct comfort listening level can then be made by user adjustment of the volume control.  This may be fine if the hearing aid has a volume control and can be adjusted, and if it can manage the overall gain reduction required. Figure 15 provides a comparison of the tabulation data differences.
• Any deviation from the prosthetic objective in measured most comfortable loudness pressure values is then appropriately added or subtracted from the 2cc coupler frequency response of the hearing aid, and the prescription is written.

• Figure 16 illustrates the final step. The manufacturer has supplied the hearing aid to be fitted based on the hearing aid prescription. The hearing aid is now tested on the subject to determine how closely the measured MCLPs (green squares) approximates the target (red line).       For this subject, the measured results come close to the target (hearing aid was selected based on using the relative difference pressures for the gain prescription).       Additional adjustments to the hearing aid could be made if felt necessary, and depending on the ability of the hearing aid itself to be further adjusted.