Gunfire Volume Bird Song Telegraph Signals Volcanic Explosion Distance 
Answer: Telegraph Signals
The decibel as we now know it, a logarithmic unit used to measure the intensity of a sound source and/or the power of other energy sources like cellular transmissions, was created by Bell Telephone Laboratories in 1924 when researchers there proposed that telephone signal strength should be measured in Transmission Units (TUs).
One TU was defined such that the number of TUs was ten times the base-10 logarithm of the ratio of measured power to a reference power level. In 1928, they formally changed the name from TUs to decibels, being one tenth of a newly defined unit for the base-10 logarithm of the power ratio. It was named the bel, in honor of Alexander Graham Bell.
The decibel wasn’t the first attempt to measure transmission power, however, and what became the decibel and our accepted way of measuring everything from how loud a rock concert is to how strong a cellular signal our phones receive, all started with early attempts to quantify signal loss in telegraph and telephone circuits. Before the TU/decibel, there was the Miles of Standard Cable (MSC) measurement.
One unit of MSC corresponded to the smallest detectable change as caused by a drop in power over a one mile length of standard telephone cable (approximately 19 gauge) at a frequency of 5,000 radians per second (795.8 Hz). The decibel was a significant improvement over the MSC standard because the measurement was relative to the power source and could be applied across all mediums (not just telegraph and telephone wires).
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