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.
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.
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.
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 is 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.
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.
'Square Wave' Voltage
Excursion 'Clean' Voltage
60 Hertz 23.6vac
45 Hertz 23.8vac
30 Hertz 21.8vac
If your speakers are capable of handling significantly more than your amplifier can produce, driving them with a clipped signal will not likely hurt them.
If the speakers can handle 3 or 4 times the power that your amplifier can produce, there's virtually no way to damage your speakers (no matter how clipped the signal is).
If your speakers are rated for the same power handling as your amplifier is capable of producing cleanly, driving them with a clipped signal for extended periods of time may cause speaker damage and/or premature failure.
If your speakers are rated for the same power handling as your amplifier is capable of producing cleanly, driving them with a square wave signal for extended periods of time will likely cause speaker damage