**3. Digital hearing aid**

to devices that could amplify sound through an electric circuit. The history of the electronic hearing aid is thus very similar to that of the technological evolution of the microphone, amplifier, and battery cell. Between 1989 and 1900, the Akouphone Company produced the first electronic hearing aids called "*Akoulallion*" and "*Akouphone,*" introducing a carbon microphone technique. The carbon transmitter was able to use an electric current to amplify weak signals by 20–30 dB [9]. The *Akoulallion* was large and was used on a table, while the *Akouphone* was portable. Miler Reese Hutchison—one of the company's founding presidents—established the Hutchison Acoustic Company in 1903 and produced "*Acousticon,*" an improved electric hearing aid that was further miniaturized (**Figure 5a**). Users wore the microphone and main body, put the battery in a bag, and held the earphone in their hands. The Danish company *Oticon* and the German company *Siemens* are two current makers of hearing aids that begun operations

**Figure 5.** (a) Early electronic hearing aid with carbon components: *Acousticon* model A (Hutchison Acoustic Company, 1905). (b) Early vacuum tube hearing aid: *Vactuphone* (Globe Ear-phone Company, 1921). (c) Behind-the-ear hearing aid-

embedded transistor amplifier (Zenith Diplomat Company, 1956).

**Figure 4.** (a) and (b) Flower-vase-type hearing aid for gathering multiple speakers' voices (F. C. Rein Co.) in 1810.

during this period.

154 An Excursus into Hearing Loss

(c) Acoustic throne for European royalty in 1819.

After achieving this level of hearing aid output gain and miniaturization, scientists and engineers began looking for further ways to improve hearing-aid convenience. For example, analog hearing aids amplify sound from all frequency ranges, which means that both speech and unnecessary background noises are amplified equally. Another problem was that the small size of the devices caused the speaker and microphone to be too close, and acoustic feedback generated uncomfortable levels of amplified sound. In 1996, these problems began to be addressed when hearing aids entered the age of digital sound. The digital hearing aid converts sound into a binary digital signal, which can then be modified by computer software. This type of signal modification is called digital signal processing (DSP) and is the biggest advantage of the digital hearing aid.

**Figure 6** shows a computer interface through which the DSP installed in a hearing aid can be controlled after connecting the hearing aid to the computer. The most beneficial aspect

require any cords or cables. However, in modern society, wireless function is increasingly useful for connecting hearing aids to other electronic devices. For example, because holding a smartphone near one's ear is difficult when wearing a hearing aid, the wireless hearing aid can receive speech signals directly from the smartphone using Bluetooth technology. In the near future, users will be able to control the DSP from a smartphone app. Wireless hearing aids can also help when watching television. By wirelessly connecting to the TV, users can hear clear sound from a TV program without the sound needing to travel through the air. Thus, environmental noise can be minimized and reverberations in

Hearing Aids

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http://dx.doi.org/10.5772/intechopen.73527

Digital hearing aids will likely continue to evolve and add new features and functions. For example, a hearing aid with Global Positioning System will make it easier to be found if it gets lost or misplaced. Like other wearable electronics, if hearing aids incorporate functions for measuring heart rate, step counts, and burned calories, they will become a type of wearable

Against a background of various technological advances, the hearing aid has continued to respond to user expectations. Are users today satisfied with the current hearing aid? Countries in Europe and Japan have conducted large-scale market research surveys called *EuroTrak* and *JapanTrak* since 2009 [10]. According to JapanTrak 2015, 1306 people with hearing loss filled out their marketing surveys. Along with a corresponding survey in the US (*MarkeTrak IX* in 2014), we can discuss how satisfied hearing-aid users are in countries whose citizens have

**Figure 7** shows basic demographic information. The percentage of people with hearing loss is higher for countries with aging populations (i.e., Germany, Italy, and Japan). However, the percentage of people with hearing loss who own hearing aids is lower in these countries than

**Figure 7.** (a) Percentage of people with hearing loss in several high-tech countries. (b) Percentage of people using hearing

a room can be ignored.

device that supports lifelong health.

access to the latest technology.

**4. User subjective assessment of hearing aids**

aids to the number of people with hearing loss in each country.

**Figure 6.** Example of software used to adjust a hearing aid's digital signal processing.

of the DSP is the ability to adjust the output gain in each one or one-third octave frequency band. The computer program designs the shape of a digital filter and modifies the signal passing through the filter (multichannel analysis). Because sound energy in human voice is distributed in a frequency range of 200 Hz to 4 kHz, unwanted noise can be weakened by amplifying only the desired frequency range. Audiograms of people with hearing loss differ according to individual sensitivity to each frequency band (e.g., high tone deafness, horizontal deafness, convex deafness, or concave deafness). The multichannel output can thus limit compensation to the frequency bands for which a person has difficulty in hearing.

The multichannel output is also useful for suppressing acoustic feedback. Acoustic feedback occurs when the following happens simultaneously: (1) attenuation of the speaker's voice by the time it reaches the microphone is less than the amount of sound gain and (2) the phases of the original and feedback signals are mostly overlapped. ITC and CIC hearing aids have a vent to reduce the uncomfortable feeling that can occur in the occluded ear, and the output sound transmitted through the vent can cause acoustic feedback. The solution is a feedback canceller in the hearing aid that identifies the offending frequency-repeating amplification and reduces the sound gain in the corresponding band.

The other important role of DSP is to create a compression system for the output sound. If a hearing aid amplifies sound linearly, it makes already loud sounds excessively loud. Patients with sensorineural hearing loss hear sounds above a certain sound level louder than normal listeners (recruitment hearing). Thus, the sound of a closing door or a cry from a child can annoy hearing aid users. The compression system suppresses the amplification of sounds above a certain sound level and instead fits them within the restricted dynamic range of the user. This system can therefore avoid unpleasant sounds, normalizes the perceived loudness, and improves speech intelligibility.

Another advantage of digital hearing aids is their ability to work wirelessly. In their times, the BTE hearing aid only had a short cord, and the ITC and CIC hearing aids did not require any cords or cables. However, in modern society, wireless function is increasingly useful for connecting hearing aids to other electronic devices. For example, because holding a smartphone near one's ear is difficult when wearing a hearing aid, the wireless hearing aid can receive speech signals directly from the smartphone using Bluetooth technology. In the near future, users will be able to control the DSP from a smartphone app. Wireless hearing aids can also help when watching television. By wirelessly connecting to the TV, users can hear clear sound from a TV program without the sound needing to travel through the air. Thus, environmental noise can be minimized and reverberations in a room can be ignored.

Digital hearing aids will likely continue to evolve and add new features and functions. For example, a hearing aid with Global Positioning System will make it easier to be found if it gets lost or misplaced. Like other wearable electronics, if hearing aids incorporate functions for measuring heart rate, step counts, and burned calories, they will become a type of wearable device that supports lifelong health.
