May 1999. The ongoing revolution in hearing aid technology reached another milestone this month with the introduction of the Behind-the-Ear (BTE) model of Sonic Innovations’ Natura™ digital signal processing hearing aid. The Natura, which was first introduced in the fall of 1998 in a Completely-in-the-Canal (CIC) case style, has held the interest of the hearing health care profession with its innovative design, but has been difficult to demonstrate to people.

Now with a behind the ear case style available, demonstrations are easy. The BTE hearing aid is connected to the ear using a disposable foam plug, allowing the patient to hear exactly what the hearing aid will sound like without waiting for a custom CIC case to be manufactured. This allows easy comparisons with other outstanding digital signal processing hearing aids, such as the Widex Senso, the ReSound digital 5000, the Siemens Prisma, the Phonak Claro, and others. Remember that no hearing aid is best for everyone, and it often comes down to a matter of personal preference. That’s why having the ability to demonstrate the hearing aids is so important.

To understand why the Natura is such an innovative hearing aid and why you might be interested in it, you need to go back to how the inner ear works. This will be old news to you if you have been a regular listener to Solveig’s radio show, "All About Ears", but let’s refresh your memory anyway. The inner ear contains thousands of hairlike nerve cells that are vibrated by sounds conducted to it by the middle ear. The hair cells are tuned to different frequencies, each overlapping the other somewhat.

The hair cells are arranged in layers with the outer hair cells being closer to the outside, sort of in front of the inner hair cells. The latest theories hold that the outer hair cells flex or stiffen to protect the inner hair cells from loud sounds, or to attenuate the louder sounds. You can think it of a gain adjustment like you have on your television set. You adjust the overall sound level so that you can hear the TV show without being blasted by the loud sounds. Sometimes you have to turn the set up or down to accommodate the different show segments. Commercials often have to be turned down, for example.

Well, the hair cells in a normal ear do the same thing. The inner hair cells tell the outer hair cells when the sound is too loud. This causes the outer hair cells to stiffen and reduce the amount of sound vibrations getting to the inner hair cells, thus lowering the volume.

[Audiologists can use this outer hair cell movement to check the hearing of newborns, by the way. They have an instrument that presents clicks to the baby’s ear, and then it listens for an “echo” signal. That “echo” is really the outer hair cells moving to change the gain. That small movement is enough to “hear” on the outside of the ear by the very sensitive instrument. No “echo” means that part of the ear isn’t working properly.]

[As another comment, your eyes also have a gain control. Have you ever stared at the TV too long and seen a reverse image of the TV when you looked away? Your eyes turned the gain down for the bright TV image, and when you looked at something else, the lower gain made a dark spot.]

In some kinds of hearing problems, one of the first things to go is the ability of the outer hair cells to control the amplitude of the sound. You sort of loose your automatic volume control. Loud sounds come blasting through and seem much too loud, while softer sounds can’t be heard at all. The audiologist measuring your hearing measures lots of things, but two of the measures are the quietest sounds you can hear (plotted on a chart, this becomes your audiogram), and the loudest sounds that you can stand (called the Uncomfortable Level, or UCL.)

What does all of this have to do with the Natura hearing aids, you ask? The design of the Natura is based on this model of human hearing. The Natura has nine active channels covering the frequency from 500 to 8000 Hz in half-octave bands. These bands correspond to the measurement points on your audiogram. Each channel is adjusted for your hearing loss (the quietest sound you can hear) and your UCL. Sounds quieter than you can hear are amplified up to where you can hear them, and sounds louder than you like are reduced in level until they are comfortable. The nine channels are adjusted individually, and continuously, so that all sounds are audible and comfortable, all the time.

The Natura works like you ear’s hair cells, compensating for the damage in your ear. It provides expansion to signals below the knee point, exaggerating the differences in amplitude between sounds to keep everything from just blending together, while at the same time, compressing sounds above the knee point to make sure nothing gets too loud.

The Natura uses fast acting compression in each of its narrow bands, responding to the environment in near real time. It is designed to respond to speech cues and other sounds without a delay. This fast compression and release over the nine bands avoids smearing of the sounds and provides clearer, cleaner sounds. The Natura bands have 30dB slopes, providing good separation of the signals.

The Natura uses a tiny hybrid circuit that contains all the circuitry including the dsp chip. This tiny circuit is what allowed Sonic Innovations to introduce the Natura in a CIC case style first. Their CIC has all of the advantages a CIC, namely ease of use with the telephone, good localization, minimal wind noise, and near invisibility.

The BTE also is interesting because it is quite small and powerful. It will allow some very hard of hearing people to experience the advantages of Sonic Innovations’ high-speed, multiple band compression and expansion technology in a very compact package. And this of course brings us back to the excitement that kicked off this article. The BTE allows anyone to hear this technology without waiting for a custom CIC to be made for their ears.

Natura 2™ and Natura 2 SE

(May, 2000). The Natura 2™ and Natura 2 SE DSP Hearing aids are the second generation of hearing aids from Sonic Innovations. The Natura 2 uses fast acting, narrow band expansion technology to actually reduce the volume of low-intensity sounds while still amplifying louder quiet sounds like soft speech. This reduces the background noise and makes listening more comfortable.

The Natura 2 has tones to identify different programs, to help the wearer tell which program he is using. Also, the Natura 2 has a tone to alert the user to a battery that is about to run out of power. Like the Natura, the Natura 2 and 2 SE have 9 independent frequency channels. (At the present time, the push button to change programs is not available on CIC case styles, however both programs are available and can be used by the audiologist to try different programs for the patient.)

The Natura 2 SE features noise reduction technology that actually recognizes speech signals and reduces other, non-speech sounds, to emphasize the speech. The amount of noise reduction can be personalized for you by your audiologist when the hearing aid is programmed. (This programming can be changed anytime you wish by plugging the hearing aid back into the programming computer in your audiologist's office.) The noise reduction amount can be different at different frequencies to help emphasize speech signals over out-of-speech-band signals. Since the Natura 2 SE has two programs selectable by push button on all but the CIC models, different amounts of noise reduction can be programmed into the different programs.

These Natura 2 and Natura 2 SE hearing aids use an extremely small hybrid circuit that contains all of the hearing aid's circuitry. The result is the ability to make a very small CIC hearing aid. So if you have been told that your ear canal is too small for a CIC hearing aid (which is true for some people, especially children), consider the Natura line of hearing aids. Since they can be manufactured in smaller sizes than most other hearing aids, they might fit you even if other brands do not.

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