Today, we all so familiar with the decibel (dB) unit, whether dealing with electro-acoustical measurements on audio meters or with numbers that define Sound Pressure Levels, such as knowing that an SPL of 120 dB refers to the human hearing threshold of pain, or that 1 dB is accepted as the smallest change of level that is generally perceptible to the human ear. But the dB story starts back in 1923, the “mile of standard cable” unit of measurement (referring to the efficiency of telephone lines over distance) was replaced by term “Transmission Unit or TU,” based on a logarithmic scale that was one-tenth of a "bel," named for Alexander Graham Bell. In 1928, Bell Labs' transmission research director Ralph V. Hartley announced that Bell Labs was proposed changing the TU unit to "decibel" — meaning one-tenth of a Bel. And the dB is still with us nearly 90 years later.