Subwoofer Power Handling

[Kevin Haskins]

This is an often confusing topic. The specifications given by manufactures is confusing for the layman. To make a long story short, power handling numbers given for subwoofers is a guideline only. I've taken some time to update all of our documentation to include this information but I think making it a sticky is a good idea also, just to spread the information.

Subwoofer Power Ratings:

Power ratings on transducers are just guidelines for use. They are not guarantees of the fitness of a transducer to withstand a given power input. Why? Well, the actual power that can be safely handled by a subwoofer depends upon several variables, including the enclosure design, test signal or program material, amplifier details and ambient tempature.

Subwoofers by nature are inefficient devices. Most subwoofers are around 1-3% efficient, meaning that for every 100W of power delivered to the transducer, only 1-3W is actually converted into acoustical output. What happens to the other 97-99%? It is given off as heat and dissipated to the environment, primarily (> 95% of it) through convective heat loss as air passes over the voice coil & former.

Obviously, the movement of the coil & former is critical to the device being able to dissipate heat. Consequently, if you run a 1K test tone into the driver at high power under continuous conditions in a small sealed & stuffed box, it is easy to thermally damage a driver. If you use a 20Hz test-tone in free-air, the same driver may be able to withstand the same power indefinitely. For these reasons power ratings are just guidelines. Ultimately the user must understand the conditions under which the driver is going to be used and plan for an appropriate amount of power for the application. This is the same design paradigm that manufactures use. Note: Highly equalized systems quickly reach thermal limits. Careful design is required under heavy equalization especially with test-tones. Music or movies are typically 1/8th power. Test tones present a MUCH higher thermal load on transducers. Be careful testing drivers with test tones in high SPL conditions.

It is often best to be conservative with amplifier power. If you under-power the system you will clip the amplifier at the limits of output. That will MOST LIKELY not damage the transducer. Overpowering the transducer can quickly damage the device before the user has a chance to adjust the system levels. Also, amplifiers are capable of power output that exceeds their specifications under short duration transients. These short transients can be enough to damage the system if you are running it under full power conditions. The user is ultimately responsible for operating the transducer within it’s limitations.

So what is a consumer to do? We recommend building a subwoofer like you would a bridge. No responsible engineer would build a bridge to operate at its limit. You don’t design a bridge to support EXACTLY the weight that it will carry under use. You design a bridge so that it can EXCEED not only the worst-case load, but typically you would design it to have capacity beyond it’s maximum expected load. While a subwoofer isn’t a bridge and it’s failure wouldn’t be as catastrophic, it is a very good idea to build the system such that it has MUCH higher capacity than your highest output need. Only through good design and common-sense use will you prevent damaging the system. Since output levels required depend upon the room, program material, user preference etc… It is beyond the scope of this document to determine output needs.

Kevin Haskins
Exodus Audio