Invention of the telephone
The carbon microphone was independently developed around 1878 by David Edward Hughes in England and Emile Berliner and Thomas Edison in the US. Although Edison was awarded the first patent in mid-1877, Hughes had demonstrated his working device in front of many witnesses some years earlier, and most historians credit him with its invention.
Hydrophone
The first hydrophones consisted of a tube with a thin membrane covering the submerged end and the observer's ear on the other end. The design of effective hydrophones must take into account the acoustic resistance of water, which is 3750 times that of air; hence the pressure exerted by a wave of the same intensity in air is increased by a factor of 3750 in water. The American Submarine Signaling Company developed a hydrophone to detect underwater bells rung from lighthouses and lightships. The case was a thick, hollow brass disc in diameter. On one face was a thick brass diaphragm which was coupled by a short brass rod to a carbon microphone.