- Too Little Power:
- As you have probably heard,
some people say that too little power can blow
speakers. Well... How can I say this... BULL S***
- Too little power will only
cause the maximum output level to be low. Abuse
and the defective 'wing nut' (an idiot) connected
to the volume control blow speakers with low
powered amplifiers. If driving a speaker with low
power would cause them to fail, speakers would
fail every time you lower the volume on the head
unit. I will try to explain what happens when
speakers are driven with clipped signals but
remember... you get what you pay for. :)
- Note:
- This page deals mainly with
speaker damage that involves thermal damage of
the voice coil. Speakers can also be damaged
mechanically by driving it beyond what the
suspension can handle. Mechanical damage is
generally caused by driving the speaker with too
much power but it can also be done when a speaker
is in a ported enclosure and is driven with
frequencies below the port tuning frequency. Most
of the damage I've seen has been thermal damage
to the voice coil.
- Damaging Woofers:
- When a woofer is driven with a
high powered amplifier to high levels, there will
be a significant amount of current flowing
through the voice coil. Since the voice coil has
resistance, there is a voltage drop across the
speaker's voice coil (which the amplifier
appreciates greatly :-). This means that there
may be a great amount of power being dissipated
(in the form of heat) in the voice coil. When a
speaker is driven with lots of clean power, the
cone moves a great deal (in proportion to the
output voltage from the amplifier). For speakers
with vented pole pieces (or other types of
venting), this movement forces a lot of air to
flow in the magnetic gap (area where the voice
coil rides). When the woofer moves out of the
basket, the chamber that's under the dust cap and
around the voice coil expands (increases in
volume) which pulls cool air into the magnetic
gap. When the woofer moves the other direction,
the chamber size is reduced and the hot air is
forced out of the vent in the pole piece. This
air flow cools the voice coil. If a relatively
low powered amplifier is driven into clipping (to
a full square wave for a lot of people), a
relatively large portion of the time, the voltage
delivered to the voice coil no longer resembles a
sine wave as it would with an unclipped signal.
While the amplifier's output is clipped, the
voice coil is not being
motivated to move as far as it should for the
power that's being delivered to it and therefore
is likely not being cooled sufficiently (since
the speaker is driven by a linear motor, the voltage
applied to the voice coil determines how far the
voice coil moves from its point of rest). At
points a, b, d, e, f and h the voltage is
changing causing the voice coil to move in the
gap and therefore pull in fresh cool air. At
points c and g, the voice coil may still be
moving a little due to momentum but may not be
moving enough to cool properly. Remember that
during the clipped portion of the waveform
current is still flowing through the voice coil.
Since the displacement of the voice coil (and
therefore the airflow around the voice coil) is
no longer proportional to the heat being
generated, the voice coil can overheat. This
excess heat may cause the
voice coil former to be physically distorted
and/or melt the insulation off of the voice coil
wire and/or cause the adhesives to fail
(especially if the speaker is rated to handle no
more than the power that the amp can produce
cleanly). If your speakers are rated (honestly)
to handle the maximum 'clean' power that your
amplifier can produce, slight
clipping isn't generally a problem. Severe
clipping is more likely to cause a problem.
- Severe Clipping (square wave):
- It always amazes me when I hear
some idiot driving down the road and the audio is
clearly distorted (is that possible :-). Many people drive their amplifiers into
what could be called a square wave output (white
line below). When an amplifier is pushed that
hard, it is actually possible to drive the
speaker with twice as much power as the amplifier
can cleanly produce into the speaker. As you can
see below, the yellow sine wave is the maximum
'clean' output that the amp can produce. When an
amplifier is pushed way too hard, the signal will eventually
look like the white line. The effective voltage
of the white line is ~1.414 x the yellow line.
This means the the total power driven into the
speaker by the clipped (square wave) signal is double
the power delivered by the 'clean' signal (yellow
line). This means that the power is double but
the cooling of the voice coil will not increase
in proportion with the power increase (since the
voice coil isn't moving as much as it needs to be
for the given power dissipation). This will lead
to the voice coil overheating. If we compared the
output of a 100 watt amp (the one that's
clipping) to a 200 watt amp, the 200 watt
amplifier would be able to push the speaker as
much as 40% farther than the 100 watt amp
(depending on the frequency of the signal). This
extra travel (in each direction from its point of
rest) would result in added airflow around the
voice coil.
- Note:
- The RMS voltage of a pure sine
wave is equal to the
peak
voltage multiplied by 0.707. The RMS voltage of a pure square wave it equal
to the peak voltage. For 2 waveforms with equal
amplitude (as shown above), the RMS voltage of
the square wave is 1.414 times the voltage of the
sine wave. If we use the example of the 100 watt
amp which can produce a sine wave of 20 volts
RMS, we can see that the output power at hard
clipping is double the power it can produce
cleanly.
- Clean
Signal Calculations:
- P
= E^2/R
- P
= 20^2/4 (4 ohm speaker)
- P
= 400/4
- P
= 100 watts RMS
-
- Square
Wave Signal Calculations:
- P
= E^2/R
- P
= 28.28^2/4 (the RMS voltage is 1.414 times the
RMS voltage of the sine wave)
- P
= 800/4
- P
= 200 watts RMS
-
- If you need more info about
peak, peak-peak and RMS waveforms, click here.
-
- Note:
- As I've said somewhere else on
the site, there's actually no such thing as RMS
power. The proper term is average power. I use
the term RMS power anywhere that RMS voltage is driven into a resistive load. The
purists just hate when someone uses the term RMS
power. Since the term RMS power is used by most
everyone in the industry, I'll use it here also.
It may not be technically correct but it's less
confusing for some.
- In the following table I drove
a speaker with an amplifer in full clip (square
wave) at 3 different frequencies. In each case I
measured the voltage across the speaker terminals
with a true RMS volt meter. To get equal power
from the square wave signal AND the 'clean'
signal, I took the voltage reading from the
square wave signal and multiplied it by 1.414 and
used that voltage (peak voltage) for the clean
signal. The RMS readings for the square wave and
the sine wave were the same. Both the square wave
and the sine wave would produce approximately the
same power dissipation in the voice coil and
therefore the same heat output. As you can see,
the square wave signal did not move the cone as
far as the 'clean' sine wave signal. Since the
cone has more excursion with the clean signal,
the voice coil will have more air flow which will
result in more cooling (especially for woofers
with vented pole pieces). The woofer was a long
excursion unit made by Eminence. The excursion
measurements are all in one direction. The
voltages below were the peak AC voltages as read
on the scope. The output voltages vary with
frequency due to the impedance variations of the
speaker at the different frequencies.
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