Squeak, can you explain in detail how underpowering a subwoofer can cause damage? I've never understood that except unless it clipped or distorted. Thanks.
Very easily. Take a woofer and power it free-air (i.e. no enclosure, just sitting on a workbench). It won't take but probably 10% of the "rated" power handling before you begin reaching the mechanical limits of the driver. Try powering it with the rated RMS, or anything even remotely close, in a free-air situation and you will absolutely damage the subwoofer on the first stroke. Even if the subwoofer has a stiff suspension, when running free-air the driver will reach it's mechanical limits with relatively small amounts of power.
Extreme example? Yes....but it's an easy way to prove the point that the enclosure has everything to do with the mechanical power handling of the subwoofer.
Enclosure apply pressure to the subwoofer, acting as an additional suspension to the subwoofer and increasing the amount of power it takes to make the subwoofer move mechanically. This is the reason, for example, that sealed boxes are called "acoustic suspension" enclosures. This, in turn, means that you can apply more power to the subwoofer before it begins reaching it's mechanical limits. This is the reason the subwoofers can handle so little power when in a free-air or infinite baffle enviornment. Because in those situations, they are relying primarily on their own suspension, and have no added stiffness from being in an enclosure. This is also the reason less power is needed when using large enclosures and more power can be mechanically handled when in smaller enclosures.
The larger the enclosure, the lower the mechanical power handling ability of the subwoofer, meaning that it will take less power for the subwoofer to reach full excursion. Reason being that the larger enclosure has more compliance (i.e. less stiffness), which allows the subwoofer to have more movement with less power when compared to a smaller enclosure size.
Take a look at this white paper from Adire about the Brahma's power handling abilities in different sized enclosures:
http://www.adireaudio.com/Files/BrahmaPowerHandling.pdf
Notice how in the larger enclosures it takes much less power to push the driver to full excursion than it does in smaller enclosures. In fact, in many enclosures the Brahma will reach it's full excursion potential with much less than the "rated" power handling of the driver.
The inverse example of this is a driver being driven in a ported enclosure at the tuning frequency of the enclosure. At the tuning frequency, the port is doing most of the "work", the pressure inside the enclosure is very high (meaning very high mechanical power handling), and the subwoofer has very little excursion. This is the reason that SPL competitors can push tens of thousands of watts through a subwoofer and not breach their mechanical limits...because the mechanical power handling is extremely high at this point.
Another word of caution, and another reason why enclosure design and safety measures must taken when using ported enclosures; below the tuning frequency ported enclosures will "unload", meaning they no longer have control over the subwoofer and it basically acts as if it were free-air. This "typically" begins to occur about 1/2 octave below the tuning frequency. Now, since the driver is basically "free-air", it will take very little power to reach the mechanical limits of the driver (see first paragraph of this post). Which means it will be very easy to damage the subwoofer in this situation. This is the reason subsonic filters are important when using most ported enclosures, especially if you are using an enclosure with a high tuning frequency.
Sorry for the long post....I didn't intend on this turning into a freaking book.....
