![]() In other words, while the dBm is a measure of power, the dBu is a measure only of voltage. ![]() Because exact load impedances are less of an issue in modern audio systems, the new term dBu (u meaning unloaded) was introduced to signify a voltage level of 0.775 Volts, regardless of the load impedance. Today, the term dBm is often abused to signify a signal level of 0.775 Volts, but unless the load impedance is exactly 600 ohms, this is incorrect. In the world of telephones and 600 ohm line impedances, 0 dBm tended to mean a signal of 0.775 Volts applied to a load of 600 ohms - or 1mW. This is of little direct relevance in the world of modern audio, but was vitally important in the pioneering days of telephone when small amounts of electrical power needed to be transmitted over long distances. While dBs express only general ratios, the dBm is a fixed value where 0dBm equates to 1 milliwatt of power. The same is true of specifications such as dynamic range: A 100dB dynamic range means that the largest signal a circuit can handle is 100,000 times bigger than the smallest signal it can handle. Some of the more common ratios are shown in the table below.įrom the table, it can be seen that a voltage amplifier having a gain of 60dB amplifies the input signal 1000 times. If a signal is lower than optimum, it is read as minus so many dBs, whereas if the signal is too high, it is shown as plus so many dBs. For example, the record level meter on a tape machine is always set so that the optimum recording level is shown as 0dB, regardless of what that means in terms of magnetic flux at the record head. We can pick any power or voltage to be our 0dB level and then express all other values relative to that. The method of calculating dBs for both voltage and power ratios is shown in the accompanying box. I say convenient because the nature of the decibel makes it logarithmic, and it just so happens that our ears are also logarithmic in the way they perceive sound level. The first hurdle is to grasp that the dB doesn't have to relate to any fixed level of signal it is simply a convenient way of expressing the ratio between two signal levels. The name decibel means a tenth of a bel, the bel part being named after that well‑known inventor of telephones, Alexander Graham Bell (hence the capital B in dB). Most of us accept that our VU meters are calibrated in decibels, or dBs, but even the most experienced engineer can start to fumble when asked to explain exactly what they are, and how they are related to the likes of dBu, dBm, dBv and dBV.
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