Interesting note: Due to the fact that his wife and daughter were deaf, Alexander Graham Bell attempted to develop hearing aids by transforming sound waves into electrical impulses. That's how he invented the telephone. In 1905, a portable electric hearing aid was developed, consisting of a microphone worn on the neck, a battery and an amplifier. A year later, you would use a small box, about the size of a suitcase, called a vactuphone, equipped with a microphone connected to the ear by a cable. In 1925, the vactuphone was already miniaturized. It could now be carried in your pocket. In 1937, the sonotone developed, which was smaller and more effective.