In this video, you'll learn everything
you probably need to know about decibels. But if you're new here, welcome. My name
is Kyle. If you want to learn audio production online, subscribe to Audio
University. The decibel is one of the most confusing concepts in audio
production. Trying to wrap your mind around every detail all at once might
leave you feeling overwhelmed. The truth is: for most people, learning a few key
principles about the decibel will be sufficient for working in the audio
industry. the first thing you should know is that decibels can represent large
ratios using simple numbers. That's one reason the decibel is so useful for
describing sound pressure. Humans are capable of perceiving a vast range of
sound pressure - from very subtle to very extreme. Discussing sound pressure in
absolute units can be challenging. The pascal is an absolute unit of
measurement that describes air pressure in terms of force per square meter. The
limits of human perception of sound pressure range from 20 micropascals to
20 pascals - a ratio of 1 to 10 million! It's much easier to discuss sound
pressure in terms of decibels. This chart shows a side-by-side comparison of the
two. You can see sound pressure in pascals on the left and sound pressure
level in decibels on the right. Decibels present these quantities in a way that
is much easier for us to interpret. In audio production, the decibel is also
useful because it presents sound pressure on a scale that's much more
relevant to the way humans perceive loudness. Most of us are accustomed to
thinking in linear terms. For instance, twice as many letters produce a word
that's twice as long. This mode of thinking, however, does not hold true for
the way humans perceived loudness. Thinking in linear terms, you might
suspect that twice the amplifier power produces a sound that's twice as loud.
This is not the case. The way humans perceive loudness is more similar to a
logarithmic scale than a linear scale. In a linear scale, each step is the same
size. For example: 10 - 20 - 30 - 40 (where each step is an addition of 10). In a
logarithmic scale, each step has the same proportion.
10 - 100 - 1,000 - 10,000 (where each step is multiplied by ten). As
sound pressure increases, more and more power is required to create the same
perceived increase in loudness. The decibel accounts for this fact by
presenting absolute units on a scale that better relates to human hearing. A
decibel is a way of describing the ratio between two quantities. Saying, "The drums
are 120 dB", is an incomplete statement. That's because the decibel alone has no
value. It's only when you draw a comparison to something else that the
decibel has meaning. You could instead say, "The drums are 20 dB louder than the
guitar". This statement is more meaningful because it draws a comparison
between the level of the drums and the level of the guitar. The level of the
guitar in this case is the 0 dB reference point. In the case of a fader
on a mixing console, unity gain is the 0 dB reference point. Pushing the fader
above 0 dB adds gain to the signal. Pulling the fader below 0 dB
attenuates, or reduces, the signal. Decibel values above unity are
represented by positive numbers and decibel values below unity are
represented by negative numbers. For many applications in audio production, there
are standardized reference points that define 0 dB in absolute terms. Each
application has its own suffix: dBm, dBu, dB SPL, etc. Here's a helpful chart that
defines the 0 dB reference point in common audio applications. This chart
brings meaning to the various types of decibels. The formula for decibel
calculations will make a lot more sense if you first understand the relationship
between decibels and Bels. This is the formula for finding the ratio between
two power quantities in Bels. "Power 1" is the reference power and "Power 2" is
the measured power. Let's try finding the ratio in Bels
between 10 watts and 100 watts. The formula would look like this. Dividing
100 by 10 gives us the ratio between the power quantities, which is 10. Using the
log function of the calculator, we can find that the log of
10 is 1. Therefore, the ratio between 10 watts and 100 watts is 1 Bel. Now that
you've learned the formula for calculating ratios and Bels, let's take
a look at how to adjust that formula to calculate ratios in decibels. A decibel
is 1/10th of a Bel. For comparison's sake, let's use the same power quantities we
used before: 10 watts and 100 watts. This time, finding the ratio in decibels. To
start, divide 100 by 10 to find the ratio, which is 10. Use a calculator to find the
log of 10, which is 1. Multiply 1 by 10 to convert bells to decibels. The ratio
between 10 watts and 100 watts is 10 decibels, or 1 Bel. For many, the most
confusing aspect of the decibel is deciding which formula to use in a given
situation. The formula for decibels is: 10 * log(Power2/Power1). In
some situations, however, you might see this formula: 20 * log(Value2/Value1). Why are different formulas used and how do you choose
which formula is appropriate in a given situation?
The decibel is a power related ratio. When finding the ratio between two
quantities of acoustical power or electrical power, use the basic formula
we learned in the last section. The decibel can be used to represent the
ratio between non-power quantities, such as sound pressure and voltage. When
calculating voltage and sound pressure levels, use this formula. The reason we
multiply by 20 is explained by the power equation. In this equation, W is power, E
is voltage, and R is resistance. Notice that voltage is squared in this equation.
Power is proportional to voltage squared. Quantities that are not power must be
made proportional to power. Another way to come to the same answer would be to
use this formula, squaring each voltage value before finding the ratio. Anytime
you're comparing power quantities in decibels, multiply the Bel formula
by 10. Anytime you're comparing non-power quantities in decibels
multiply the Bel formula by 20. It's hard to get a straight answer when
asking someone, "How many decibels is a doubling?". Well, that's because it depends
on what's being doubled. As you just learned, non-power quantities must be
made proportional to power in order to be represented in decibels. Following
that logic, the decibel value used to describe a doubling of a power quantity
is different from the decibel value used to describe a doubling of a non-power
quantity. It's helpful to remember the decibel value for representing a 2:1
and 10:1 change in power, voltage, and sound pressure level. Use this chart
for reference. Everything discussed in this video can be found in the post I
wrote on AudioUniversityOnline.com. I'll put a link to that post in the
description of this video. If you got value out of this video, hit the "Like"
button, share the video, and subscribe to Audio University. Thanks for watching!
