Back in January, I posted here on how the Phone Clip+ provides much improved sound quality over its predecessor for the listener on the far end of the call. To review, the Phone Clip+ provides excellent noise reduction so that the voice quality for the call recipient is clear and pleasant even when the hearing instrument wearer is in noisy surroundings, like in the car. Since then, there have been numerous questions about how that noise reduction actually works. Are we applying NoiseTracker II noise reduction? Is it our Adaptive Directional algorithm? Did we drop in something from our colleagues at Jabra? The answer to all these questions is no.
The engineers at ReSound developed a new noise reduction algorithm specifically for outgoing calls from the Phone Clip+. This noise reduction method requires two microphones, which the original Phone Clip does not have. Well-known (at least in the engineering world) adaptive filtering techniques are used to subtract the noise from the output of the front microphone1.
This noise reduction algorithm makes the assumption that the front microphone picks up both the signal of interest, which in this case is the voice of the hearing aid user, and the interfering background noise. A further assumption is that the rear microphone only picks up the interfering noise. Then, adaptive filtering is applied to make the noise from the rear microphone equivalent to the noise from the front microphone. This filtered noise is subtracted from the front microphone signal, leaving a clear signal of interest.
Obviously, the assumption that the rear microphone only picks up noise is not true. The way this issue is solved is by estimating direction of arrival of sounds, similarly to what directional hearing aids are able to do. Using this information, the filter applied to the rear microphone noise is only updated in the frequency bands where sound is estimated to come from the rear. As a consequence, the filter can “tune in” to noise sources from the rear and remove them, but leave signal sources from the front intact.
We have already reported on the perceptual improvements for call recipients with Phone Clip+, but what is the measured improvement in signal-to-noise ratio? This was tested by placing a phone accessory (Phone Clip, Phone Clip+ with and without noise reduction, and other manufacturer’s streamer) on a Head and Torso Simulator (HATS) in the position a hearing aid user would wear it. The HATS includes a speaker in the mouth of the manikin which can be used to present the signal-of-interest. The HATS was placed in varying noise backgrounds, and the signal received from the phone accessory by a Bluetooth compatible device was analyzed at a remote location from the test room. The following graph shows results for “Pub noise” background noise.
First of all, it is obvious that any of the noise reduction processing provided by the devices tested provided a signal-to-noise ratio benefit compared to no noise reduction. However, the Phone Clip+ provided an additional 3 dB improvement in SNR over both the Phone Clip as well as another manufacturer’s streamer.
Wireless streaming of the phone signal to both ears has been shown to be an unbeatable way for hearing instrument users to hear on the phone2. But phone calls are two-way communication. It’s not much help if you can hear the person you are calling but they can’t hear you well. The Phone Clip+ takes the benefit of a phone accessory a step further by ensuring the best possible quality for the person on the other end of the call.
- Haykin S. Adaptive Filter Theory, 4th edition. Prentice Hall, New Jersey, 2002.
- Picou E, Ricketts T. Efficacy of hearing aid based telephone strategies for listeners with moderate-to-severe hearing loss. Ear & Hearing 2013; 24:59-70.