Understanding power handling

Part 1 - Thermally limited power handling (the only one quoted by all the other manufacturers!)

There's a lot of misinformation and worry about matching amps and cabs and most of this worry is totally unnecessary.

If a speaker is rated at say 450W, like the 15"s in our cabs, then it has endured a test many hours long of being powered by an 1800W amp with the peak level reaching the full 1800W and the average level being 450W or higher. So the chance of causing thermal damage to a bass guitar loudspeaker is extremely minimal unless you have a huge amp and are really rather stupid in how you use it.

But why are we specifically talking about thermal damage?

This is because the power rating of a speaker is merely the thermal power handling of the voice coil, which is a simply a coil of wire like that in an electric heater. As over 95% of the power going into a speaker is wasted as heat that voice coil gets very hot, just like an electric heater. If too much power goes into that voice coil the coil can suffer thermal damage in the form of becoming misshapen (causing rubbing or the speaker even becoming locked in place) or even the insulation melting so it short circuits and stops working entirely.

Part 2 - excursion limited power handling (the one that really matters for bass!)

However you may have noticed, particularly when slapping big low notes, that the woofers in a bass guitar cab move forwards and backwards rather a lot. You might also, quite correctly, assume that if the woofers move too far they will be damaged.

A speaker produces sound because the electricity moving through the voice coil generates a magnetic field (it becomes an electromagnet), which pushes or pulls against the magnetic field generated across the pole pieces by the permanent magnet structure (the neodymium magnet in our speakers), and as the voice coil is connected directly to the woofer cone the movement of the voice coil thus moves the woofer cone.

So what limits how far the cone can move without damage?

Two methods of damage are related to the voice coil and motor structure, one is related to the cone and suspension.

The first thing is when the voice coil moves so far to the rear that it hits the magnet behind it. The second thing is when it moves so far to the front that it leaps all the way out of the gap between the pole pieces and won't go back again. The third is when it moves so far in either direction that the flexible cone surround or the flexible spider supporting the centre of the cone and the voice coil run out of flexibility and pull against the cone or voice coil.

When the voice coil hits the back plate it tends to distort the shape of the voice coil so it gets stuck or rubs (much like excessive temperature does), or the impact causes the cone become creased. When the voice coil jumps out of the gap the speaker can no longer produce any sound at all. When the suspension runs out of travel the suspension can become stretched out of shape or the cone creased, causing a higher distortion level in the sound, particularly at high levels and also changing the parameters of the speaker, usually making the lows more boomy.

So what does all this mean for you?

If you've been following thus far your next question would hopefully be "how do we know when a speaker will suffer damage due to over-excursion?"

Well if we look at the parameters of any speaker there are two specs of interest - Xmas and Xlim. Xmax is the maximum excursion the speaker can achieve with less than 10% distortion. Xlim is the maximum excursion the speaker can achieve without damage.

In a hi-fi speaker Xmax and Xlim tend to be very close so if you exceed Xmax you are likely to damage your speaker. In a pro audio speaker like a bass guitar woofer Xlim is usually almost twice Xmax. This means that it will take at least four times as much power to reach Xlim as it will to reach Xmax.

Despite this because most bass guitar speakers have rather low Xmax they spend a lot of their time operating beyond this point and thus if you demand high low frequency output you can easily push your speaker beyond Xlim if you do not take note of the ever increasing level of distortion.

The lengthy search which led us to our woofer models was centred around finding a woofer that had sufficient Xmax to maintain truly clean output at high SPL and have sufficient Xlim for it to be almost impossible to damage the woofers through over-excursion under normal non-abusive usage.

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