| 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. |
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| 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 |
"0
dB" 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. |
|
| Does Pressure and Voltage Directly Relate To 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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