Imagine you\u2019re comparing money, not sound. If today you have one dollar more than you did yesterday, that\u2019s a linear difference \u2014 an absolute amount. But if today you have twice as much money as yesterday, that\u2019s a ratio \u2014 a proportional change.<\/p>\n\n\n\n
The decibel<\/strong> works on that second principle. It doesn\u2019t tell you how many volts, watts, or pascals you have in absolute terms; it tells you how much more or less you have compared to something else, on a logarithmic scale. Doubling your \u201cmoney\u201d (or power) equals a +3 dB<\/strong> increase, ten times as much equals +10 dB<\/strong>, half as much equals \u22123 dB<\/strong>, and one-tenth as much equals \u221210 dB<\/strong>.<\/p>\n\n\n\n For the decibel to have meaning, we need a known reference point, or anchor. For instance, the anchor for measuring voltage in professional audio systems is 0.775 volts. We signify that anchor value as 0 dBu. <\/em>The small \u201cu\u201d after the dB signifies that we are measuring voltage in reference to an anchor of 0.775 V. If we double that voltage, to 1.55 Volts, we now have 6 dB more voltage, or a value of +6 dBu<\/em>. <\/p>\n\n\n\n Each quantity we measure refers to a specific anchor value, and the letter following \u201cdB\u201d identifies which anchor point we are using. Here are some common anchor points and their notation:<\/p>\n\n\n\n 0 dB SPL<\/strong> (Sound Pressure Level) is referenced to air pressure measuring 20 micropascals (20 \u00b5Pa), the threshold of human hearing at 1 kHz.<\/p>\n\n\n\n 0 dBu<\/strong> refers to a voltage of 0.775 volts RMS (across any impedance), used in modern professional audio electronics.<\/p>\n\n\n\n 0 dBV<\/strong> refers to a voltage of 1 volt RMS (across any impedance), used in consumer electronics and audio electronics.<\/p>\n\n\n\n 0 dBm<\/strong> refers to a power level of 1 milliwatt, originally standardized for telephone circuits with a 600-ohm impedance.<\/p>\n\n\n\n 0 dBFS<\/strong> refers to the maximum signal amplitude in a digital system. <\/strong>All other levels are negative relative to 0 dBFS.<\/p>\n\n\n\n The decibel sprang from the early telephone industry\u2019s need to quantify signal loss over long transmission lines. In the 1920s, engineers at Bell Laboratories needed a practical method to quantify the power lost between two points in a telephone circuit. They coined the term bel<\/strong>, named after Alexander Graham Bell, to describe a 1:10 ratio of power. If 1 watt of power was lost in one case and 10 watts in another, the difference was 1 bel. The bel proved too large for typical measurements, so the decibel (one-tenth of a bel) became the standard. This method of relating two numbers is useful for comparing vastly different energy levels, including voltages, power, and sound pressure levels for audio measurements.<\/p>\n\n\n\n Sound energy and electrical power vary across enormous ranges, so expressing them in linear units is unwieldy. For instance, a whisper and a jet engine differ in air pressure by a factor of 10 million. The decibel compresses this span into a scale of roughly 0 to 140 dB by using ratios expressed as a logarithmic relationship. Decibels simplify the discussion of very large values.<\/p>\n\n\n\n This logarithmic relationship coincidentally mirrors how our ears perceive loudness as proportional changes in intensity. Doubling the power of a signal increases its level by 3 dB, which we hear as only a slight increase in loudness. 10 dB, a 10x increase in sound power, sounds twice as loud. That alignment between physical energy and human perception makes the decibel both mathematically elegant and psychologically meaningful.<\/p>\n\n\n Our previous article covers more details about wh<\/a>at l<\/a>oudness is<\/a>, the difference between peak and RMS metering, different ways of measuring sound, and different methods for achieving loudness when mixing and mastering. So in this article, we’ll continue deep-diving into the other aspects. <\/p>\n\n\n\n In modern audio devices\u2014microphones, preamps, mixing consoles, outboard processors, interfaces, even guitar pickups\u2014the circuitry is designed for voltage transfer, not power transfer. We use the decibel notations for voltages, dBu and dBV, to describe the signal level. dBV has been adopted for consumer electronics, while dBu is used for professional equipment. Fader markings, VU meters, and even digital meters represent voltages.<\/p>\n\n\n\n A microphone typically outputs a few millivolts and plugs into a mic preamp, where the signal level is boosted to line level, about 1.23 volts. The signal increases from 0.001 volts to 1.23 volts, a 1000x increase in level. In decibels, this increase is described as a 60 dB change. <\/p>\n\n\n\n For the math-curious, here is how to calculate the decibel value for a voltage compared to a reference level. For professional audio signals, we calculate the ratio based on a reference of 0.775 V. <\/p>\n\n\n\n Voltage Level (dB) = 20 \u00d7 log<\/strong>10<\/sub><\/strong>(V\/V<\/strong>0<\/sub><\/strong>)<\/strong> where V<\/em> is the voltage being measured, and V<\/em>0<\/sub> is the reference to which V<\/em> is being compared (0.775 volts in our case). <\/p>\n\n\n\n For example:<\/strong><\/p>\n\n\n\n A condenser mic outputs 20 mV from a singer\u2019s voice. 20 \u00d7 log10<\/sub>(1.23 \/ 0.02) = 35.8 dB <\/p>\n\n\n\n Note that the above result in dB does not use a suffix (dBu<\/em><\/strong> or dBV<\/em><\/strong>), because we are only comparing two numbers, and the result is simply a ratio expressed as dB. If we directly describe a voltage, like 0.775 volts in the context of professional audio gear, we would write 0 dBu, which describes a voltage level as compared to the reference level for dBu. <\/p>\n\n\n\n Quick Tip:<\/strong> It\u2019s handy to memorize that for voltages a 6 dB increase means the voltage has doubled, while a 20 dB increase represents a tenfold change, and a 60 dB increase represents a 1000x change. In practice, we use a calculator to determine dB, and we have created a web-based calculator for this purpose.<\/p>\n\n\n\nDecibel, What a Strange Name<\/h2>\n\n\n\n
![\"Table](\"https:\/\/blog-uploads.imgix.net\/2025\/11\/sound-pressure-levels-spl-table-whisper-to-jet-engine.png?auto=compress%2Cformat\")
<\/figure><\/div>\n\n\nUsing dB to Express Voltage<\/h2>\n\n\n\n
Voltage: The Language of Signal Flow<\/strong><\/h3>\n\n\n\n
To reach line level, defined as +4 dBu (aka 1.23 V RMS, 0 VU), the preamp must raise 0.02 V to 1.23 V. In decibels, this gain calculation is:<\/p>\n\n\n\n