We're around
the term decibel (dB) all of the time and we know it has something
to do with level or volume, but many of us really don't understand
the term. Why is is? And why should we be concerned?
The decibel is
hard to understand because it was such a well-conceived technical
term that survived from the horse & buggy days through the days of
inter-planetary travel. [Of course I'm stretching this a bit to
make a point.] Actually It was first used in the early 1920s and
has survived over three-quarters of a century. Do we need a new
term? - No, we need to understand it better.
Why we need to
know the term will be obvious once you really understand the term.
If you've been hanging around audio for a while, study of the term
will immediately help you get better results. If you're new to
audio, you'll develop skills faster.
BASIC DEFINITION:
The simplest
way to explain the term would be that the decibel is the ratio of
two levels according to a scale where a certain percentage change
is one unit. the abbreviation of decibel is dB. But it is
essential to know at least seven more things about the term to
even start to understand it:
1 |
One dB represents approximately the smallest
volume change which can be heard if listened for carefully. One dB represents an
approximate change in pressure of 12%. |
2 |
Three dB
represents a small but noticeable change in volume. |
3 |
Six dB
represents a change in level of twice (or half) as much pressure. |
4 |
A certain
amount of dB represents a change of level (a change from what was before or from a
standard level). |
5 |
A certain
percentage change in level will always be the same amount of change in dB. |
6 |
Adding dB
represents a multiplication of level. |
7 |
"0dB" a is starting point in the comparison and does not mean "no level." |
But don't stop
here - you really need the "rest of the story."
HISTORY AND ORIGINAL USE:
The decibel
unit was originally invented by Bell Telephone Labs as the Bel and
given its name after Alexander Graham Bell. The decibel is 1/10 of
this original unit. The Bel unit was defined as a ratio of power
levels of 10 to 1 (ten times the power or one-tenth the power). In
Telephones, amplifiers are driving speakers and doing so over long
lines. To drive speakers there needs to be a power transfer. So if
you are in the business of driving speakers, you will analyze how
much power there is available and how much power was lost in
getting the signal to the speaker.
WHAT DOES POWER HAVE TO DO WITH VOLUME
More power
means more volume can be achieved in the speaker but these two
factors don't directly relate. Volume has to do with the amount of
Sound Pressure there is. The ear is a pressure sensitive device.
Power (in an electrical circuit) is not just the pressure but also
the flow. In electricity, the pressure is the voltage and the
amount of flow is the amount of current. If you put twice as much
voltage into a device (say a light bulb) there would be twice as
much current; because both the voltage doubled and the current
doubled, the power has been multiplied by a factor of four. In
order to double the pressure of the sound pressure wave out of a
speaker, you need 4 times the amplifier power to drive it. So an
amplifier with twice the power is not twice as loud.
DOES PRESSURE DIRECTLY RELATE TOP VOLUME?
For all
practical purposes, yes. Although the ear is not exactly pressure
sensitive, it is closer to being pressure sensitive than to being
anything else.
Many studies
have been done regarding how changes in perceived loudness
(volume) relate to level changes in dB. All of these studies
suffered from problems in getting exact figures in that the
personal opinion of listeners had to be consulted to get data for
the studies. The results of the studies did show that the
perceived change in loudness varied greatly (by some 30%
difference) depending on the starting volume, the frequency of the
sound and the complexity of the wave. [Howard W. Tremaine, The
Audio Cyclopedia, Second Edition, pages 17-18] Furthermore, the
greenest recording student listening to music played through a
console with a meter can quickly discover that an increase in
level of a certain number of dB is much easier to hear than a
reduction of level by the same amount of dB.
Someone (or
some people) interpreted test data and made a generalization that
a 10 dB change in level was twice (or half) the volume and many
texts compound this useless and deceiving assertion. In practice
the 6 dB change for full fidelity music represents twice (or half)
the volume much better than the confusing 10 dB. It has good
scientific basis in that twice the pressure is an increase in 6
dB. The following is presented as factors supporting this:
1 |
For centuries, composers and conductors have used a formula that it takes
four times the musicians to get twice the volume. If a composer/conductor wanted the
violins to be twice as loud, they would specify 4 times as many. This is four times
the power or a 6 dB volume increase. |
2 |
If 10 dB is twice the volume, than 10 people talking (10 times the power)
would be twice as loud as one person talking. Any day care worker can tell you that
10 kids are much louder than twice the volume of one kid. If 6 dB is twice the
volume, four kids would be twice the volume of one kid; you might have a chance of a day
care worker agreeing with you on this. |
SOME OLD
THINKING:
Statements such
as "Voltage ratios cannot be expressed in decibels because
decibels are, by definition, a ratio of power levels," ignores the
current use of the unit and published standard decibel notation.
What is the case here is that the use of the term in the
profession demands a new definition of the term. It is common in
language for the manner that the word is used in society to be the
final determining standard for what a word means, and technical
terms are no exception. When a technical term is being commonly
used by professionals differently than the dictionaries say it
should be, it is time for the dictionaries to be re-written.
When you were
little, it's possible that your teacher would not allow you to use
the word "can" in asking a question. She may not have let you
leave the room until you said, "May I go to the bathroom?" She was
simply trying to get you to use words correctly. Most dictionaries
of today allow you to use the word "can" when asking a question -
it is too common in society for the writers of dictionaries to
ignore. The same kind of thing has happened to the decibel. Except
for the final amplifier that drives a speaker, almost all
equipment used in recording and sound reproduction is voltage
sensitive. A change in voltage gain is a change in level and
something to be measured, observed, and used by recording
engineers and design engineers. The dB, as read on meters, as
specified for the level in and out of equipment, and given in
overload levels are voltage levels. The units dBV and dBu are
based on standard levels of voltage completely divorced of current
in the circuit or power levels.
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