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). If you were close enough to a sound anywhere near the scale of Krakatoa ’s eruption, like the ship Norham Castle, 40 miles away, the only thing you ’d “hear” was a shockwave rupturing your eardrum.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. Historically loud sounds may still have an estimated decibel value, but it ’s detached from what we consider “sound,” and usually comes from barometer measurements of pressure. If a train goes so fast it rips up the rails beneath it and starts tumbling as a mass of metal, gravel and flying luggage, you might not see it and go, “Wow, that ’s a really fast train,” but you definitely still have a problem. If you think of sound as a train, in a quaint and probably flawed analogy, it ’s not limited just by how fast the train itself can travel, but by the limitations of the medium it travels on: the rails. Sound stops being technically “sound” at 194 decibels, but it doesn ’t stop existing: It just has so much energy that it now carries the air along with it, and becomes a shockwave. At 194 decibels, the low pressure regions of that wave have zero molecules, making it a bit like absolute zero in sound terms.ĭoes this mean you ’d be surprisingly fine if you were close to Krakatoa? Oh hell no. When that vibration hits an eardrum or robot equivalent, it ’s registered as sound. Let me explain: Sound is pressure from a vibrating object passing through a medium, in this case air (water actually has a higher limit, at around 270 decibels). This is because the 194 decibel maximum doesn ’t come from the human ear, or recording instruments, or limitations of a scale, it comes from the literal limitations of sound. In strict scientific terms, though, even that eruption never made a sound louder than 194 decibels. Obviously, you ’d think that if you were only a mile away, it would be way past the 194 decibel threshold. For example, the loudest sound in history, the eruption of Krakatoa in 1883, was recorded at 170 decibels, 100 miles away. It doesn ’t seem to make sense on its face. If you ’re curious enough to start looking into the upper limits and the loudest sounds of all time, however, you ’re going to end up with a very anticlimactic number: 194 decibels.
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