By TJ Cornish, April 2015
There is a thought floating around that says that by using an amp that is "too small” - usually defined as having a power rating of less than the speaker’s peak rating, you risk damaging your loudspeakers. Is there any truth in this? Short answer, NO. You don’t need to read any further - seriously. Loudspeakers get damaged by overpowering, not underpowering.
Still here? OK then, we will go a little bit deeper.
The Basics of Loudspeaker Operation
A loudspeaker has a fixed magnet and a coil of wire attached to the cone. The amplifier sends an electrical signal through the coil of wire which creates a changing magnetic field that interacts with the fixed magnet, creating repelling and attracting forces that cause the cone to move in and out. A positive voltage on the coil causes the cone to move out, and a negative voltage on the coil causes the cone to move in. The larger the voltage on the coil, the larger the magnetic field, and the stronger the force that acts on the cone.
Loudspeaker Damage
A loudspeaker driver can be damaged in two ways:
Overheating - Over time, an excessive amount of power causes the voice coil to overheat and either deform, or open up as part of the wire breaks. This failure mode is common in low frequency drivers, as a lot of energy is required to produce low frequency sound, and modern music has a lot of low frequency content; however it is possible to overheat high-frequency drivers as well. The amount of time required to cause wire damage depends on the thermal capability of the driver. In the case of high-frequency drivers whose capacity may only be 25 or 50 watts, a short duration signal such as dropping a microphone or extreme feedback can still cause damage.
Overexcursion - A very powerful short duration signal to the driver forces it hard enough to either tear or crash into the magnet structure, causing mechanical damage to the cone or diaphragm. This failure mode is somewhat more common in high frequency drivers than in low frequency drivers, but can occur in any driver.
Both of these conditions are exacerbated when more amplifier power is available.
The Nature Of Music and Loudspeaker Ratings
Music is dynamic. All music has high frequency content, mid frequency content, low frequency content, as well as loud sections and relatively quieter sections. Some genres have more dynamics than others, but these principles are true for all types of music in varying degrees. We describe the volume dynamics of a song with the term crest factor - the ratio of the peak volume to the average volume of a song.
The masochistic math defining the crest factor is outside the scope of this high level article, but this isn’t a hard concept to visualize. Most music has a percussive element - the drum kit, piano, rhythm guitar - something to indicate tempo and beat. There are also more sustained sounds - held guitar chords, vocals, and keyboard pads that hold the music together. Percussive elements tend to be relatively high-energy events, but are short lived - they attack and dissipate quickly. The sustained sounds are usually quieter, but because they last longer can have also significant energy.
Loudspeaker ratings are setup to approximate this usage. There are usually three numbers: the continuous rating (sometimes called RMS), the program rating, and the peak rating. For most speakers, the program rating is twice the power of the continuous rating, and the peak rating is twice the program rating. For example, 400w continuous, 800w program, 1600w peak. Every doubling of amplifier power equals a change of 3dB, so the peak rating is 6dB louder than the continuous.
Amplifiers are rated in peak power, and have a lower continuous capacity, which matches well with what loudspeaker drivers can actually handle. A general rule of thumb is that the amplifier should be sized to either the program power or peak power of the driver, depending on how close to the edge you want to run (keeping in mind that there’s only a 3dB level difference between the peak and program power).
Myth #1 - A clipped waveform causes the speaker to bash back and forth causing damage
As an amplifier is driven past its linear capacity, it starts running out of voltage, causing the peaks of the waveform to flatten, and in extreme cases this can approach a square wave. This myth says that the sharp corners of the square wave equate to the cone changing directions infinitely fast, which creates stress, causing the cone to tear itself apart.
The fallacy here is that the shape of the waveform maps directly to cone position. This is not the case. The waveform describes the force acting on the cone, not the position of the cone. As the cone has mass, changes in position happen gradually, as the force must overcome the inertia of the cone to change direction or position.
This hypothesis has a further challenge - distorted waveforms and even full-blown square waves are very common in music anytime you have an electric guitar or keyboard synthesizer. If simple distortion caused damage, we’d be replacing drivers constantly.
This myth clearly holds no water. Loudspeakers are very happy to produce square waves indefinitely, so long as the signal level is within the drivers’ power handling capabilities.
Myth #2 - A clipped waveform creates high-frequency content which damages high-frequency drivers
This myth is partially true. An FFT analysis of a square wave reveals that harmonic frequencies are present that are not present in a sine wave of the same frequency, so in effect clipping does increase the high frequency content of the signal compared to the unclipped version. This additional high frequency energy is passed through the speaker’s passive crossover to the high frequency driver, which may not be able to handle the additional harmonic energy.
However, this is still effectively a myth, not so much because the physical principle isn’t true, but because of a human principle: If someone is dumb enough to run a small amp at clipping, they will do the same with a larger amp, causing damage even more quickly than with a small amp. In other words, if a person gets speeding tickets driving their Ford Focus, upgrading to a Porsche is unlikely to help.
Never Blow Up A Driver
In most cases, loudspeakers get damaged because the user runs their system beyond its capability. This is only made worse by larger amplifiers, not better. Your system’s reasonable maximum performance comes from using amplifiers sized to the program power of your speakers, and then preventing them from clipping. Setting up a limiter in your DSP can help with this, but there’s no substitute for being familiar with your rig and its capabilities, and using your senses - eyes, ears, and nose to make sure you’re out of the red. When in doubt, turn it down. Turning down just a couple dB will make a huge difference in the stress on your loudspeakers, and turning down 3dB will cut the power being delivered to your speakers by half.
Even better yet, consider self-powered speakers, or a manufacturer-integrated amp/speaker system where all of the system protection work is done for you.