The Relationship of
Voltage, Loudness, Power and Decibels
What a boring title! On the contrary, I think you will
find the information contained herein to be both enlightening
(maybe even surprising) and certainly valuable. This paper
will help you to better understand how the major component
parts of your stereo system interact. In particular, we
will explore the elusive relationship between speaker output
(volume) and amplifier output (power).
Loudness, to a great degree, is subjective. What is loud
to one person is not to another. Thus we need a measure
that allows us quantify what we hear. The unit of measure
most common is the decibel (dB). Most audiophiles
are familiar with the term, but just what does it actually
mean? Without getting too technical, the dB is a unit of
measure for sound intensity or level. It is a logarithmic
scale developed to express wide ranging quantities on a
simple scale. Because it is logarithmic, it can be a little
bit confusing. As an example, a doubling of volume is not
equal to twice the number of dB. The chart below will help
you to better understand the dB and how it relates to what
you hear, and what is happening throughout your system as
volume levels change. But first, we need to look at the
other end of the equation.
Power, in this discussion, refers to watts. Power
defined is the rate at which energy is converted or dissipated,
as in the case of an amplifier driving a loudspeaker. It
is important in our discussion in defining the general relationship.
A simple chart will help to understand the relationship
of the terms we have introduced:
Looking at the chart, and starting at the left, we can
see than an increase of 3dB results in a voltage
increase 1.4 times the original, a doubling of power, and
yields only a subjective increase in loudness only 1.23
times the original. To get a doubling of loudness, it is
important to note that an increase of 10dB is necessary.
And to reproduce that volume through our loudspeakers, note
that we require ten times more power from the amplifier!
The implications of this comparison in a typical hi-fi
system is significant. Let us assume that you are listening
to your stereo at a comfortable volume of 8OdB. We will
further assume that this level requires 25 peak watts from
your amplifier. To just barely increase the volume, we might
increase loudness by 3dB. As we can see from the chart,
this will require a doubling of power from the amplifier,
which will now be churning out 50 peak watts to produce
a loudness of 83dB. If we want to double the apparent volume
from our original level we will need to create 9OdB of sound
pressure, requiring 250 peak watts.
It is obvious from the example above that adequate amplifier
power is necessary to provide an accurate portrayal of music.
This is especially true when attempting to reproduce realistic
levels, or when driving inefficient speakers. However, even
small efficient speakers may require copious amounts of
power to cleanly reproduce transient peaks. In the final
analysis, it is difficult to have too much power. While
a small amplifier of only a few watts output can produce
surprising average loudness, the dynamic peaks will usually
suffer unless the sensitivity of the speaker is very high.
Many individuals I have spoken with are concerned with
having "too much" power. The perception is that
an amplifier with an output greater than that recommended
by the speaker manufacturer would be likely to damage the
speaker. This is not necessarily so. Interestingly, an under
powered amplifier is more likely to cause speaker damage!
During my training at the JBL factory for transducer servicing,
I saw more speakers (tweeters/midranges) damaged from use
with under powered amplifiers than ones that succumbed to
being overdriven. Sound strange? Not really, once you understand
what is happening.
When an amplifier is over-driven, it "clips"
the wave-form. What was a clean sine wave becomes a distorted,
almost square, wave. A square wave is extremely difficult
for a speaker to reproduce, as it requires virtually instantaneous
starting and stopping of the diaphragm. At sufficient power
levels, the tweeter will simply die trying to reproduce
this wave-form. A given tweeter rated to handle 50 watts
of clean undistorted sine-wave power, will be capable of
handling only a fraction of that amount in square-wave input.
As you can see, clean, undistorted power is the key. A
25 watt amplifier, constantly driven to clipping, is more
dangerous than a 250 watt amplifier that is never taxed.
Of course, let reason prevail. I am not saying that speakers
can handle endless input, they cannot. However, extra power
does not mean that speaker damage is bound to occur. If
common sense is used, any size amplifier can be employed.
Many factors influence amplifier choice. Some of the more
important considerations are: speaker type (efficiency or
sensitivity), room size, the type of music you listen to
and, most importantly, how loud you like to listen and even
how far one sits from the speakers. In an extreme example,
one might own an inefficient loudspeaker, have a very large
room, and like to listen to pipe organ music at realistic
levels. This individual is going to require a tremendous
amount of power to satisfy his/her needs. The next listener,
with the same speaker, may have a much smaller room, and
prefers chamber music at background levels. Here, power
requirements might be only one tenth to one-fourth those
required in the previous illustration.
Requirements do vary widely. It is wise to take all factors
into account before making a decision regarding the amount
of power necessary for your situation.