The electrical properties of a speaker are important when selecting a replacement speaker. Ideally you should always replace like for like with a driver of the same impedance, so if your old driver is 8 ohms, you should select an 8 ohm replacement, the same goes for 4 ohms. This is particularly important with active speakers, as the amplifier module in your active speaker will be designed to deliver the correct power into the correct impedance driver, and if you substitute a 4 ohm woofer where there was previously an 8 ohm woofer, you could end up damaging the amplifier, as well as overloading the new speaker with too much power.

Some speakers are labelled with their impedance, others are not. Its usually possible to measure the impedance if you have a working speaker - you should only ever measure impedance with the power switched off and the speaker disconnected, otherwise you could damage something. A normal household multi-meter can be used to measure the impedance, however you should be aware that you wont measure 4 ohm or 8 ohms, as this is a measure of impedance which is frequency dependent. Your multimeter will measure DC resistance, which is lower than impedance. A 4 ohm speaker will typically measure around 2.5-3 ohms DC resistance, and an 8 ohm woofer will measure around 5-6 ohms.

Power. Perhaps the most misunderstood thing in audio, not helped by the many different methods of measuring power, and the fact that many manufacturers try to inflate their power ratings to make their speakers seem better. We'll do an in-depth discussion into power ratings elsewhere, as its a complicated subject. For purposes of selecting a drop-in replacement for your existing speaker, we'll keep things simple.

Ideally you want to obtain the power rating for your existing speaker. If its an active speaker, it's fairly common for the manufacturer to specify the Program power or Peak power of the amplifier module. This isnt a great deal of help, so we suggest you assume the power rating of an active speaker is Program power, which is typically double the AES power rating we list on our website. So if your active speaker claims an 800W woofer, its very likely a 400W AES power woofer.

If you've blown your woofer, this could just be bad luck, it could be a bit too much heavy bass, or it could be that the original driver was never quite up to the job and was always likely to fail at some point anyway. Whatever the reason, if you want to avoid it happening again, its usually a good idea to select a replacement of the same power, or a little higher, so 450-600W if the original was 400W.

Why not just max out the power and go to 1000W or higher? The reason is that drivers are made differently for different applications. A mid-bass 12" woofer designed to go into a full-range portable speaker is typically 300W-500W, with the driver balanced for good mid response and reasonable bass extension. Very high power 12" drivers tend to have large voice coils and heavy cones, are often intended for sub-bass use only, and usually have poor mid response and lower efficiency. The result is often worse overall sound, not better.

With 18" woofers, power upgrades are a little easier. Most 18" woofers already have large voice coils and heavier cones, so stepping up in power is less noticeable in terms of mid-range response, as those frequencies should not be reproduced by an 18" driver anyway. However, a big change in driver type can still result in slightly muffled or undefined bass if your original woofer was designed for high efficiency.

Choosing a replacement with a broadly similar power rating is more likely to maintain the overall tonality of your speaker. As power ratings increase, efficiency and sensitivity often decrease slightly, which can affect the tonal balance of the overall system.

So assuming you've found a woofer that's the right size and the right power, there is still one final hurdle: the Thiele-Small parameters. Click here for Part 3, which deals with the mechanical properties of a speaker.

If you want to read a little more about power ratings, we wrote a short article on SpeakerWizard that answers some common questions: What's up with the Watts?