You might think you don’t need a hearing aid, and you might be right. But in general, hearing loss eventually comes to all of us. In fact, you progressively lose hearing every year, which is why kids can have high-pitched ringtones their parents can’t hear.
You’d think hearing aids would be pretty simple, right? After all, we know how to pick up sounds, amplify them, and play them back. But there’s a lot more to it. Hearing aids need to be small, comfortable, have great battery life, and cram a microphone and speaker into a small area. That also can lead to problems with feedback, which can be very uncomfortable for the user. In addition, they need to handle very soft and loud sounds and accommodate devices like telephones.
Although early hearing aids just made sound louder and, possibly, blocked unwanted sound, modern devices will try to increase volume only in certain bands where the user has hearing loss. They may also employ sophisticated methods to block or reduce noise.
A Brief History
Hearing loss is nothing new. Ear trumpets appeared around the 17th century. These were just simple sound baffles that directed sound to your ear and, perhaps, cut some noise out that wasn’t in the trumpet’s direction.
The modern hearing aid dates back to the akouphone in 1895. [Miller Hutchison] developed the device for a friend who was deaf from a bout of scarlet fever. It was bulky — sitting on a table top — and used a carbon microphone, but it did work. He was also able to sell several models to royalty, many of whom suffered from hereditary deafness. This included Denmark’s Queen Alexandra, who, reportedly, was very impressed with the results.
Around 1902, [Hutchison] changed the device’s name to the acousticon, making it more portable with battery power. Despite impressive marketing, not all medical professionals were sold. If you were totally deaf, the device did nothing, unsurprisingly. In addition, the bulky batteries required frequent replacement, and the frequency response was poor.
It was still better than nothing, and the invention also led to the massacon and akoulalion that converted sound into vibration for the profoundly deaf. He later sold the rights for the acousticon to [Kelley Turner], who would not only improve the device, but also use the technology to launch the dictograph, which was a well-known office machine for many years.
Modern Times
The Zenith Miniature 75 (photo by [France1978] CC-BY-SA-2.0).
Amplified hearing aids appeared around 1913, but they were still large boxes. By 1920, the vactuphone used vacuum tubes to perform amplification. At “only” seven pounds, the vactuphone was considered quite portable.
Keep in mind that portable hearing aids in the 1920s was a relative term. Typically, you’d have a unit carried in a bag or hung around your neck. World War II brought advances in minaturization which benefited hearing aids like the Zenith Miniature 75.
Transistors, of course, changed everything, including hearing aids. The Sonotone 1010, which appeared in 1952, used both transistors and tubes. Early transistor units were known to fail early due to moisture and heat. Silicon transistors and encapsulation helped.
Naturally, all of these hearing aids were analog as were the earliest IC-based devices. However, with the advent of ICs, it was possible to use digital techniques.
Patent drawing from 1984 — Hardly portable!
The path to digital hearing aids was difficult. In the 1970s, large computers could program digital elements in hearing aids to tune the device to set frequency bands and gains.
By 1980, several groups were experimenting with real digital hearing aids, although many of them had wireless links to real computers. A fully digital hearing aid first appeared in a 1984 patent, but it wasn’t tiny. Since then, things have gotten smaller and more capable.
Physical Form
Hearing aids went from table-top devices, to boxes hanging on necks. Getting smaller devices allowed for small boxes that hug the back of the ear with the earpiece into the ear canal.
With even smaller devices, the entire apparatus can be placed in the ear canal. Many of these go so deeply into the ear that they are largely invisible. There are also hearing aids that can surgically attach to your skull using a titanium post embedded in the bone. This can transmit sound even to people who can’t hear sound directly since it relies on bone conduction.
Other places to find hearing aids are built into thick glasses frames. Doctors with hearing problems can opt for stethoscopes with integrated hearing aids.
Modern hearing aids sometimes have rechargeable batteries. Otherwise, there will be some kind of small battery. There was a time that mercury cells were common, but with those banned in most places, many aids now take zinc-air batteries that deliver about 1.4 V.
We hear from an 8th grader that you can make hearing aid batteries last longer by peeling the sticker from them and waiting five minutes before installing them. Apparently, giving them a little time to mix with the air helps them.
What’s Next?
On the market today are hearing aids that use neural networks, have Bluetooth connections, and use other high tech tricks. We’ve looked at the insides of a hearing aid and why they cost so much before. If you want to roll your own, there is an open source design.
This articles is written by : Nermeen Nabil Khear Abdelmalak
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By Nermeen Nabil Khear 
