Probe-Mic Measures for CROS and BICROS Fittings

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This is an interesting question that has bounced around for several years. It's something that a group of us from Walter Reed Medical Center talked about in a paper presented at the ASHA convention in 1980! Granted, just because I've thought about it for 25 years doesn't make me an expert, but I'll give it my best shot.

Let's start with the CROS fitting. In the classic case, the CROS candidate has unaidable hearing in one ear, and normal hearing in the other ear (in practice, many of these patients have a mild high frequency loss in the "good" ear, but we'll save that condition for BICROS). For this discussion, let's say the unaidable ear is the left, and the good ear is the right. The purpose of the fitting, then, is make the aided ear canal SPL of sounds delivered to the right (good) ear the same as the unaided SPL of the same signals measured in the left (bad) ear. In other words, eliminate the head-shadow effect. So, it seems to me that your REAR fitting target for the right ear would be the REUR left ear values of the signal presented at a 270 degree azimuth (directly toward the left ear; this is the worse case situation for this patient). When the patient is then listening to signals presented from directly in front, the amplified signal and unaided signal in the right ear would be very similar (assuming his right and left ear geography was similar, and it usually is), so these inputs should sound "normal."

The protocol that I would suggest:

• Calibrate and configure probe-mic equipment to conduct measurements in substitution method rather than pressure method.
   
• Place probe-tube in the left (bad) ear. Place probe-mic loudspeaker at 270 degree location (directly toward left ear).
   
• Use a modulated speech-shaped signal or real speech presented at 65 dB SPL.
   
• Measure the REUR for the left ear. Leave the REUR curve on the screen to use as fitting target.
   
• Move the measurement probe-mic to the right ear (do not move loudspeaker). Place the CROS instrument on the head. Conduct REAR measurement.
   
• Modify the frequency response until the REAR is a close match to the stored REUR curve.
   
• Assuming a linear fitting is used, the match-to-target should not vary as a function of the input level, so it would not be necessary to test for other inputs.

For a BICROS, there are two fitting goals: eliminate the head shadow effect for signals presented from the bad side, and provide gain to the "better" ear for inputs presented from all locations. One part of the fitting would be the same as that of the CROS arrangement, except that we would no longer use the patients REUR for the target, as the REAR at the better ear needs to also reflect compensation for the hearing loss. The primary testing for the BRCROS would be conducted the same as for a routine monaural fitting. That is, determine a match to REAR targets for a family of inputs (e.g., 50, 65, and 80 dB SPL). Because the fitting to the better ear for typical listening situations (e.g., speech presented from directly in front of the patient) would have a priority over the "CROS effect," I would conduct that part of the fitting first.

The protocol I would suggest:

• Calibrate and configure probe-mic equipment to conduct measurements in pressure or modified-pressure method.
   
• Place probe-tube in the right (better) ear. Place probe-mic loudspeaker at 0 degree location (or 45 degree if recommended by manufacturer).
   
• Based on pure-tone thresholds, determine NAL-NL1 REAR fitting targets for inputs of 50, 65, and 80 dB SPL.
   
• Use a modulated speech-shaped signal or real speech presented at 50 dB SPL. Adjust channel specific gain to obtain best match.
   
• Present signals at 65 and 80 dB SPL. Adjust compression parameters to obtain best match.
   
• If hearing aid has special features such as directional technology or digital noise reduction, assess and verify at this time.
   
• Calibrate and configure probe-mic equipment to conduct measurements in substitution method rather than pressure method.
   
• Place probe-mic loudspeaker at 270 degree location (directly toward left ear).
   
• Use a modulated speech-shaped signal or real speech presented at 50 dB SPL, and measure REAR.
   
• Adjust gain of transmission to obtain best match (note: frequency shaping is not possible, as the hearing aid on right ear has been programmed correctly for primary listening situation). Testing at other levels shouldn't be necessary, as compression parameters already have been adjusted.

Final Note:

I've outlined the general thought process and technique, but some of the specific measures concerning the pressure versus substitution methods can be a little tricky, and do vary from manufacturer to manufacturer. I'd first check with your favorite manufacturer to see if they have written a protocol specific for your equipment.