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For this reason, many people confused the reference of 0 dBm to be equal to 0.775 V. Because the power is always 0.001 watts, and the impedance was 600 ohms, this meant that the voltage across the system was exactly 0.775 V rms. When the unit was first devised, it was mainly used to measure 600 ohm telephone line.
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This unit of measurement was an industry standard in the 1940’s. This means that 0 dBm = 1 milliwatt, and it has no direct correlation to voltage or impedance. The term dBm expresses an electrical power level, which has the reference at 1 milliwatt. Regarding these 2 categories, there are several types of decibels which we will enumerate and explain in the following part of this article. We’ve talked about decibels and how they relate to volts and watts, but where is the loudness I’m used to talk about when I mention decibels ? That’s because decibels relate to 2 things : electrical signals and acoustical signals. When you double the voltage, you get 4 times the power. P = Power in watts E = voltage in volts and R = resistance in ohms.Īs you can see power is proportional to voltage squared. Why is it that when we double the power, we get a 3 db increase, but when we double the voltage we get a 6 db increase ? This is because of the relationship between power and voltage. If it is 1/10 the voltage, it is 20 dB less (-20 dB). Whenever one voltage is 10 times the other, it is 20 dB greater.If the voltage is a half the other, we are talking about 6 dB less (-6 dB). 4 times would be 12 dB, 8 times would be 18 dB an so on. Whenever one voltage is double the other, it is 6 dB greater.įor voltages we can draw the following conclusions : What is the ratio in dB of 100 volts to 10 volts ?.What is the ratio in dB of 10 volts to 5 volts ?.Let’s use the same examples as above on how to calculate decibels, but instead of watts we use voltages : If we got two voltage values, E 1 and E 2, we can calculate the relationship between the 2 values (in dB), using the following formula : The decibel relationship of power ratios is not the same for voltage ratios. We have used the decibel to express power ratios (watts), but it can also be used to express voltage ratios (volts). If it is 1/10 the power, it is 10 dB less (-10 dB). Whenever one power is 10 times the other, it is 10 dB greater.If the power is a half the other, we are talking about 3 dB less (-3 dB). 4 times would be 6 dB, 8 times would be 9 dB an so on. Whenever one power is double the other, it is 3 dB greater.These ratios I chose intentionally, because they reveal values which are significant and makes interpreting easier : What is the ratio in dB of 100 watts to 10 watts ?.What is the ratio in dB of 10 watts to 5 watts ?.Let’s give a practical example on how to calculate decibels using watts : We can define a Bel as a logarithm of an electrical power, acoustical power, or any other power ratio.įor example, the relationship between 2 power values P 1 and P 2, in Bels : This is a good thing, because our ears have a logarithmic sensitivity and for lower db values, there is no need for lots of digits. The core unit is a Bel, decibel is actually 0.1 Bels, aka dB. Well, what is a decibel, you might ask ? The decibel describes a ratio between two quantities, which most often, relate to power. When people hear about decibels, they immediately think about loudness, which is somewhat incorrect. The dBu decibel, and the other types also, are often a misunderstood unit of measurement, so let’s learn how to calculate decibels. Here, the velocity is the velocity of the oscillations of the medium, and not the velocity of the sound wave.How to calculate decibels and how to interpret their values The intensity of the sound wave is the power per unit area, and the power is the force times the velocity,