Since the introduction of Baha™ sound processors that were programmable via a computer interface (BP100), Cochlear has implemented wide band dynamic compression in the signal path of these devices as a method of maximizing the output force in the limited dynamic range of the bone conduction transducer used in these devices.
As a part of this amplification strategy, Cochlear employs low level expansion as a method of controlling the introduction of gain at soft level inputs to the sound processor.
The goal of wide dynamic range compression, or WDRC, is to apply the most gain for soft sound inputs so that soft sounds are audible, and mid-level inputs are comfortable and audible, and finally loud level inputs be no louder that what the user would experience if they had no amplification applied at all to the signal.
This WDRC approach functions as a surrogate for a manual volume control providing, instead, an automatic format. WDRC was introduced into hearing aids in the 1980’s with the introduction of the first ReSound hearing aids to the U.S. Market. A side effect of this WDRC approach was the comment that when attempting to hear a person talking in close proximity to them (across the table in a restaurant for instance) recipients could actually hear more clearly the person two to three tables away. In other words, the WDRC was working by providing more gain to the softest sound and that acted to fill in the space between utterances from the closer speaker – interfering with understanding.
The fix for this issue was to apply a low level expansion function with a knee point for activation at a low level to act as a squelch when soft sounds below the activation point were processed.
In the case of Baha sound processors this expansion knee point, or activation level, is set at 30dB SPL. The feature is on all of the time and cannot be de-activated.
When one attempts to measure thresholds at, or below, the knee point of the expansion, the feature provides no gain there. If one is trying to measure aided thresholds in the Baha-aided condition, the expansion feature will not allow thresholds to be measured much below the presentation level of 25dB HL. In the case of hearing profiles where there is normal bone conduction thresholds below the 25dB HL level (as often seen in children with conductive hearing loss secondary to atresia) the clinician attempting to measure the threshold is led to believe that there is a poor fit as the expansion feature truncates the gain of the device and threshold measures below the knee point of expansion are not present.
Due to this feature, Cochlear recommends that clinicians use speech-based measures applied at supra-threshold levels for soft (30 to 40 dB), and louder levels (55 to 60dB) in sound field with (and without) the Baha sound processor in order to measure outcome.
Finally, if the threshold one expects to measure is above the knee point of expansion, then aided threshold measure with the Baha sound processor on should be more representative of the expected outcome.
Questions or comments for Dr. Cire? Please leave them below.
We look forward to hearing from you.