From the first time a human cupped their hand over their ear to more clearly hear a sound the need for a device that enabled better hearing capabilities was demonstrated. Since the earliest of human life the need for some individuals to obtain instruments to maximize their quality of hearing existed. Now, in the digital age in which we live, those with hearing difficulties can place a tiny device within their ear that is barely visible to others which enables some to hear for the first time and others to improve the quality of their hearing. However, hearing aids have come a long way. There have been many contributors and many different forms of this device have been created as antecedents to the digital hearing aid which we are now familiar with. Construction on this much needed device began as early as the seventeenth century and has improved steadily throughout time. But looking back to the history of this product hearing aids originated as a very different instrument in comparison to what we, today, can even imagine. The history of hearing aids can be roughly divided into five main periods: (1)
• Acoustic Era
• Carbon Hearing Era
• Vacuum Tube Era
• Transistor Era
• Microelectric/ Digital Era
Figure 1. Principle Hearing Aid Technology Eras
The first and most primitive period was the acoustic era in which objects such as horns, trumpets, and speaking tubes were used to amplify sound (2). As early as the thirteenth century, animal horns were adopted as a device to aid humans to hear more clearly. The idea of the horn stuck through several centuries until in 1673 Dekker illustrated a very simple funnel device. Later in 1692, Nuck created a more complex device that consisted of a trumpet with a coiled section between a horn and the ear tip (3). More work was completed to further progress on trumpet in the seventeenth and especially the eighteenth century. A “tin trumpet” that appeared in an issue of the William V. Willis & Co. catalog (1930) was stated to be “the best known and most used of all hearing devices.”(4)
It is obvious that these first, primitive hearing aids are much different than what we currently see, however in the seventeenth and eighteenth century these were the first devices invented to help those with hearing disabilities. It was an innovative notion that held much promise for the future. Acoustic hearing devices remain being patented today and have the longest history of any hearing aid (5).
In 1876 Alexander Graham Bell’s invention of the telephone brought about significant changes for the hearing aid industry. Although the magnetic microphone used in Bell’s invention did not amplify sound, the idea aided Blake and Hughes in 1878 in their discovery of carbon transmitters that did provide the amplification necessary to making the technology used to invent the telephone adaptable to hearing aids (6).
“The first practical, commercially available, wearable carbon hearing aid in the U.S. was made by Miller Reese Hutchinson in 1902, although Hutchinson had received a patent as early as 1899. His invention became the basis for “Acousticon” hearing aids”. (7)
The simple carbon hearing aid consisted of three major components:
• A carbon microphone
• A magnetic receiver
• A battery
• Plus: cords needed to connect the main parts
“Sound waves would hit the carbon microphone. The microphone would then push the carbon balls (granules) that were inside of the cups in the carbon block thus reducing resistance and increasing the current flow. The diaphragm moved out in response to fainter sound waves. The current flowing through the diaphragm, carbon shot, and carbon block produced an electrical representation of sound which was amplified by this process”(8). When further amplification was needed more microphones were attached to the diaphragm, sometimes up to four microphones (9).
There were several disadvantages, however, to the carbon hearing aid:
• They would not operate if they were lying flat
• Only helped people with mild to moderate hearing loss.
• Produced noisy and scratchy sound due to the action of the carbon balls on the diaphragm.
• The frequency response range was limited.
Because of the limitations to the carbon hearing aid, carbon amplifiers were developed in the 1930’s. “The vibration of the receiver diaphragm, instead of producing amplified sound in the ear canal, is used to drive another microphone, or carbon cell, that further increases the signal delivered to the receiver”. (10)
This model produced a better frequency response; however, upon the arrival of Vacuum-tube hearing aids the carbon aid was replaced (11).
This four decade period was crucial to the furthering of the hearing aid industry.
Vacuum Tube Era
The Vacuum Tube Era was brought about significant changes in the hearing aid. It made possible greater amplification, wider response range, and reduced internal noise (12).
The first hearing aid using a vacuum tube was the “Vactuphone,” invented by Hanson in 1920. It was produced by Western Electric Co., a leading producer of hearing aids, and distributed by Globe Phone Co. beginning in October 1921 (13).
In 1923 and 1928 when Tillyer and Kranz owned patents to hearing aids with carbon microphones and vacuum the main focus of improving the hearing aid was “adjusting the frequency response to meet characteristics of hearing impairment” (14).
In 1924 further progress was made with the invention of a large multitube hearing aid by E.A. Myers, known as the Radioear. The carbon microphone was replaced in this model by a moving coil microphone which was superior because it eliminated internal noise. In addition, a rubber diaphragm was used in this device which gave a smoother response (15).
In 1936, a similar non-wearable device using a Rochelle salts crystal microphone was produced. Later, in 1937, a carryable crystal microphone vacuum tube hearing device was produced by Sonotone. A Wearable Vacuum tube hearing aid was not available until 1937. The Rochelle Salts Crystal microphone was crucial to the development of wearable vacuum tube hearing aids because according to the inventor Sawyer of Brush Development Company, “this type of microphone had small size, high output, good frequency response, and high impedance well suited to work into the grid of a vacuum tube” (16).
The earliest werable vacuum tube hearing aid was Arthur Wengel’s “Stanleyphone.” Walter Huth developed a similar vacuum tube hearing aid shortly thereafter. After 1938, Raytheon, the leading maker of vacuum tubes for the hearing aid industry, continued to decrease the size of the tubes, thus enabling hearing aids to continually decrease in size (17).
Shortly thereafter the carbon zinc battery that had been used in all hearing aids to date were replaced with small layer type “B” batteries made by Everready. These batteries were smaller and had longer capacity. Using these batteries, Beltone produced a one-piece vacuum tube hearing aid in July of 1944 that brought about immediate acclaim (18). Size reduction alone was large progress for the hearing aid because it became more convenient than carrying around a large, heavy medal device or strapping it to one’s body. With all of these innovations vacuum tube hearing aids had higher gain, wider frequency, and lower distortion than the previous carbon hearing aidsIn addition, the vacuum tube era allowed the user to better control the device they were using (19).
In 1946, Radioear put out a wearable vacuum tube hearing aid with a built-in telephone pick-up coil. Today, almost all aids have built in telecoils (20). Also in 1946, the crystal microphone was replaced with the magnetic microphone. The magnetic phone was not sensitive to humidity and high temperatures like the previous crystal microphone (21).
The Vacuum Tube Era was a time of great progress for the hearing aid industry. Many advances were made during this period. These developments laid the foundation for hearing aids of our time. In addition, the study of audiology began during this period (22).
The Vacuum tube hearing aid used since the 1920’s had made great improvements and advances to the hearing aid and the amplification available in the device. The size of the hearing aids had also been reduced dramatically. Further reduction in size had not been possible up until the 1950, however, because of the two large “A” and “B” batteries needed to power the device (23).
In 1948, The Bell Labs invented the transistor. The transistor was quickly adopted in the making of hearing aids. The “A” battery had previously been needed to power the filaments of the vacuum tubes and the “B” battery to power their plates. Transistors replaced the need for both batteries. Transistors required lower operating voltages meaning one small battery was all that was needed after the introduction of the transistor (24).
The transition from the Vacuum tube period to the transistor era began with the invention of a Hybrid Vacuum Tube/ Transistor hearing aid. A transistor was used to substitute one of the three vacuum tubes. This made the battery life longer, in turn reducing the cost of hearing aids for consumers. The first transistor hearing aid was the Sonotone Model 1010, which began being distributed in December 1952 (25).
Fully Transistor hearing aids began distribution in 1953 (26). The Vacuum tube hearing aids were seldom purchased after the introduction of transistor aids to the market. Reductions in size continued into the 1950’s and 60’s.
Microelectric/ Digital Hearing Aids
The invention of the digital hearing aid began with the replacement of magnetic microphones with ceramic in 1967. It not only allowed for further size reduction, but it also gave a smoother and wider response range. Further progress was made with the miniature directional microphone in 1969. “A very important feature was its far lower sensitivity to mechanical vibration” (27).
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. A programmable hearing aid came along shortly after. In these aids the gain, output, and frequency response can be set by the user (28).
The first wearable digital hearing aid was invented by Audiotone. It had a converter that allowed the device to change from analog to digital and back again (29). It also had digital signal processing. An October 1983 edition of the Hearing Aid Journal titled this hearing device “A Wearable Digital Hearing Aid” (30).
How the digital hearing aid operates: (30), (31)
1. The microphone within the hearing aid acquires signals from noise.
2. These analog speech signals are digitized in real time by the Analog/Digital converter.
3. The central processing unit mathematically manipulates these digitized data according to programmed instructions…
4. The Digital/ Analog converter converts the output to analog form.
5. The analog output signal is used to drive the hearing aid receiver.
Today all major hearing aid manufacturer’s produce digital hearing aids. They are produced in many styles including Behind the Ear, In the Ear, and In the Canal (32). Digital hearing aids have many advantages over analog aids including digital noise reduction, digital feedback cancellation, and directional microphones combined with mutlichannel digital signal processing. Although digital aids are more expensive, their popularity has soared and popular thought tells us that digital hearing aids are likely to replace analog hearing aids.
(1) Sandlin, Robert. “Textbook of Hearing Aid Amplification” 2.
(2) Sandlin, Robert. “The Textbook of Hearing Aid Amplification” 3.
(3) Berger, K.W. “The Hearing Aid: It’s Operation and Development”
(4) Sandlin, Robert. “Textbook of Hearing Aid Amplification” 5.
(5) Hunt, Frederick. “Origins in Acoustics.”
(6) “Textbook of Hearing Aid Amplification”
(7) “Textbook of Hearing Aid Amplification”
(8) Bauman, Neil. “Hearing Aid Museum.” Carbon Hearing Aid General Information.
(9) Bauman, Neil. “Hearing Aid Museum.” Carbon Hearing Aid General Information.
(10) Bauman, Neil. “Hearing Aid Museum.” Carbon Hearing Aid General Information.
(11) “Textbook of Hearing Aid Amplification”12.
(12) “Textbook of Hearing Aid Amplification” 12.
(13) “Textbook of Hearing Aid Amplification” 12.
(14) “Textbook of Hearing Aid Amplification” 12.
(15) “Textbook of Hearing Aid Amplification” 12.
(16) “Textbook of Hearing Aid Amplification” 12.
(17) “Textbook of Hearing Aid Amplification” 13.
(18) “Textbook of Hearing Aid Amplification” 15.
(19) “Textbook of Hearing Aid Amplification” 15.
(20) “Textbook of Hearing Aid Amplification” 16.
(21) “Textbook of Hearing Aid Amplification” 17.
(22) Yost, W. “Fundamentals of Hearing.”
(23) Bauman, Neil. “Hearing Aid Museum.”
(24) Bauman, Neil. “Hearing Aid Museum.”
(25) Bauman, Neil. “Hearing Aid Museum.”
(26) Berger, K.W. “The First Electric Hearing Aids.”
(27) “Textbook of Hearing Aid Amplification”21.
(28) “Textbook of Hearing Aid Amplification”26.
(29) Kates, James. “Digital Hearing Aids.”
(30) “Textbook of Hearing Aid Amplification”31.
(31) Kates, James. “Digital Hearing Aids.”
(32) Pollack, M.C. “Amplification for the Hearing Impaired.”
(33) Bauman, Neil. “Hearing Aid Museum.”