Hearing aid technology and historical development.
Hearing aid technology and historical development. :
A hearing aid is an electroacoustic device which is designed to amplify sound for the wearer , usually with the aim of making speech more intelligible, and to correct impaired hearing as measured by audiometry. All components of a hearing aid developed in efficiency and reduced in size over the years. Hearing aid technology and historical development given below :
HISTORICAL DEVELOPMENT OF HEARING AID:
- 17th century.
- Development of modern hearing aids accelerated after creation of telephone.
- Non-electric hearing aids followed by Electric hearing aids.
- Historical perspective can be divided in five eras.
ACOUSTIC ERA (NON-ELECTRIC)( 17th century)
- Cupping of ear : 5 to 10 Db gain in mid and high frequencies.
- Collection of sound from a larger area than ear itself.
- Shielding of sounds coming from the rear: very effective noise reduction system.
- Ear trumpet, horn or funnel( 1673 and 1650):
- Large open end to collect as much as sound posible.
- Energy transferred to the ear via a gradual reduction in area along the length of trumpet.
- Wide and long trumpet-more effective.
- Coiling of the trumpet to make it smaller reported at around 1692 (Lybarger 1988).
- Ear trumpets have been hidden in hats, armchairs, fana and beards.
- Speaking tube:
- Horn shaped end attached to long tube that terminates in an ear piece.
- Speaker directly speaks in the horn end-signal to noise ratio much better than natural ear.
- The fist full scale manufacturer of hearing aids was Frederick Rein of London in 1800. He produced ear trumpets, hearing fans and conversation tubes.
CARBON ERA (ELECTRIC HEARING AID)
- Late 19thcentury and early 20thcentury.
- 1878, invention of the carbon transmitter for the teleohone, which-unlike Bell’s device-amplified the electrical signal. Miller Resse Hutchinson in the United Statas and Frederick Alt in Austria-tried to adapt telephones as hearing aids in the early 20thcentury.
- Akoulalliuon – 1899, the first carbon hearing aid.
- Akouphone and Acousticon – 1902.
- Use of carbon hearing aids continued till 1940s.
- Components-carbon microphone, magnetic receiver, a battery and cord to connect the components. Useful for mild or mpderate loss. Produced noisy and scratchy sond due to the action of carbon balls on yhe diaphragm. The frequency response range was limited.
- Idea to amplify different frequencies by different amounts emerged.
- Couplers, initially with 0.5 cc volume emerged.
- Carbon Amplifier invented-better frequency response.
VACCUM TUBE ERA:
- Vaccum tube electronic amplifier invented in 1907 and applied to Hearing aids in 1920.
- (Applying the three-element tube invented by Lee De Forest in 1907, Western Electric Co., in New York City, stated producing technically superior hearing aids in 1920).
- First vaccum hearing aid was “ Vactuphone” –Hanson in 1920.
- Very powerful amplifiers with 70 Db gain and 130 Dbspl output-fitting ranges increased.
- Better control over gain frequency responses shape than carbon hearing aids.
- Reduced internal noise.
- The main focus of improving the hearing aid was “ adjusting the frequency response to meet charecteristics of hearing impairment”.
- The earliest wearable vaccum tube hearing aid was Arthur Wengel’s “Stanley phone:. Walter Huth developed a similar vaccum tube hearing aid shortly thereafter. After 1938, Raytheon, the leading maker of vaccum tubes for the hearing aid aindustry, continued to decrease the size of the tubes, thus enabling hearing aids to continually decrease in size.
- Flaw- as big as a filling cabinet and needed two batteries.
- Size reduced and practical in 1930s, but large batteries untill 1944.
- In 1944 battry and microphone technology improved and became a one piece hearing aid with an ear level receiver( the carbon zinc battery that had been used in all hearing aids to date were replaced with small layer type “B” batteries made by Eveready. These batteries were smaller and had longer capacity.)
- Large pen shaped case to enclose electronics other than transducers.
- Microphone in broaches and receiver cord in pearl strings.
- Sub miniature vaccum tube heraing aids: In the late 1940s, manufacturers combined these tubes with two innovations from World War ll-printed circuit boards and button batteries-to produce more compact and reliable models.
- In 1946, radio ear put out a wearable vaccum tube heraing aid with a built-in telephone pick-up coil.
- Also in 1946, the crystal microphone was replaced with the magnetic mocrophone. The magnetic phone was not sensitive to humidity and high temperature like the previous crystal microphone technique is sufficient to allow for real-time processing of audio signals. High-speed array processors were introduced toward the end of the nineteen seventies and shortly afterwards an all-digital hearing aid was developed configured around one of these units.
- Although still too large to be wearable, that instrument has been used effectively as a master hearing aid in a series of experimental investigations on the prescriptive fitting and evaluation of digital hearing aids.
- 1982-high speed digital signal processing (DSP) chips was introduced.
- DSP chips were fast enough for limited real-time processing of speech signals. Because of their small size, these chips can be packaged in a unit small enough to be wearable.
- In 1988, the Knowles amplified receiver contributed to further size reduction. This is the first of the hearing aids that can be worn inside the ear canal.
- Further developments include feedback cancallation systems, applications of adaptive directionility, application of frequency transpositions, reduction in size of receiver with high outputs ( RIC hearing aids), remarkable improvements in programming interface, data logging systems; inter ear communication within hearing aids.
- Other developments ear-mould venting, piezo electric microphone, compression amplification.
TRANSISTOR AND INTEGRATED CIRCUIT ERA:
- 1952 transistors commercially available.
- By 1953 all hearing aids used transistors- remarkable reduction in battery consumotion.
- Head mounting of hearing aids possible, disadvantages of body type overcome.
- Barette hearing aids and spectacle hearing aids.
- BTE hearing aids became dominant till 1990s.
- First ITE in 1950s.
- 1964-Integrated circuits applied to hearing aids helped greatly in miniaturization-multiple transistor and resistors could be combined in one component.
- 1968-piezoelectric microphone was combined with a field effect transistor in a small metal can helped in smooth and wide frequency response.
- Directional microphone emerged.
- 1971 microphone technology further improved electret/FET microphone.
- Receiver and microphone volume decreased.
- 1980s –ITC.
- Finally CIC- with improvements in battery chemistry, amplifier efficiency and transducer size.
- 1986-digital controls and digital memory. Use of remote control possible.( In the late 1980s, several companies were applying digital signal-processing chips to hearing aids, initially in hybrid analog-digital models in which digital circuits controlled an analog compression amplifier.)
DIGITAL ERA Journal of Rehabilitation Research
- Digital body aid- not commercially successful.
- 1996-first digital BTE, ITE and ITC.
- Advent of the digital computer that these techniques started to take on a new, practical importance.
- The invention of the digital hearing aid began with the replacement of magnetic microphones with ceramic in 1967-size reduction, wider and smoother frequency response.
- Miniature directional microphone in 1969. “A very important feature was its far lower sensitivity to mechanical vibration.”
- A concomitant development of great consequence for the digital processing of audio signals was the development of analog-to-digital and digital-to-analog converters fast enough and with enough precision for the conversion of continuous audio signals to digital form, and vice versa.
- Quasi Digital hearing aid developed.
- Fully digital hearing aid developed in late 1970s with two major developments in technology-development of the array processor in which an array of numbers is processed simultaneously, instead of only one number at a time as in a conventional digital computer. The saving in processing time resulting from the use of this.
Now hearing aids are available in two types of Technology Features as follows -
A. Digital Technology - Sounds are processed digitally for better speech enhancement, noise reduction, feedback elimination etc. Nowadays all manufacturers used this technology for making hearing aids as digital technology is flexible with customized features. Latest invisible hearing aid models and other fashionable models like R.I.C, R.I.E, Open Fit,B.T.E, C.I.C, micro C.I.C, mini B.T.Es are made using this technology due to miniature of microprocessor chip.
B. Analog Technology – Sounds are processed as analog signal. Suitable for some types of hearing loss. Nowadays analog technology is used in to make body worn hearing aids and few B.T.E models.
Other Technological Advancement with Digital Technology :
Digital Technology further categorized according to the customization programming features of hearing aids manufactured.
** Micon Technology – Siemens hearing aids with most advance latest features used this technology for fitting hearing aids.
** XCL Technology – Siemens hearing aids with advance features used this technology for fitting hearing aids.
** Channel Free Technology – Bernafon hearing aids with most advance latest features used this technology for fitting hearing aids.
** Nano Technology – Used by most of the hearing aid manufacturer to minimize the hearing aid size.
** Camisha Technology – Widex and Sonic Innovations used this latest technology. This is sophisticated method of manufacturing individual hearing aid shells, ear-moulds and ear-pieces. Computer Aided Manufacturing of Individual Shells for Hearing Aids (CAMISHA) uses laser technology to make an impression of the hearing aid user’s ear canal and turns this data into a 3D computer model (def. courtesy Widex) .
** Wireless Technology – Used by most of the hearing aid manufacturer for establishing Bluetooth and FM connectivity with hearing aid and other compatible device. This technology is also used for synchronization of binural fitting of hearing aids.
Hearing aid technology and historical development now changing in rapid way.
Information collected by :
MR.ANKIT KR MISHRA, M.A.S.L.P, RCI REGISTERED AUDIOLOGIST AND SPEECH PATHOLOGIST.
Disclaimer – Data and photo compiled from books and Internet.