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, we suggest you assume the power rating of an active speaker is Program power, which is typically double the published AES Power 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 the original driver was never quite up the job, and was always likely to fail at some point anyway. Whatever the reason, if you want to avoid it happening again, it’s 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 disco speaker is typically 300W-500W – the driver and cone are balanced for good mid-response, and reasonable bass response down to a cut-off point. Whilst there are super high power 12″ available, they tend to have very large voice coils, heavy cones, and are designed for sub-bass applications only, they have poor mid response, and are usually a little inefficient, so whilst they will handle 1000W, they convert less of the power into useful sound, the end result is not good. So we suggest not chasing massive power upgrades, just take a small step up from the old driver, and choose a woofer that fairly efficient.

With 18″ woofers, power upgrades are a little easier, most 18″ woofers already have a large heavy voice coil, and fairly heavy cone, so stepping up in power with these is less noticeable in terms of mid-range response, as you shouldn’t be playing those frequencies through your 18″ anyway. However a BIG change in 18″ woofer can result in a slightly muffled, undefined bass if your old 18″ woofer had a lighter cone designed for high efficiency.

Choosing a replacement with broadly similar power rating is more likely to maintain the overall tonality of your speaker, with most woofers as the power rating increases, the efficiency/sensitivity decreases a little 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, and you’re ready to buy – there is still one final hurdle – the T/S Parameters – click here for step 3 which deals with the mechanical properties of a speaker.

Want to read a little bit more about power? We wrote a short article on the speakerwizard website that answers a few common questions: https://speakerwizard.co.uk/whats-up-with-the-watts/

Most novices tend to focus on power handling when choosing a replacement speaker, unfortunately this is often the least relevant specification to try to match up. The mechanical properties of a speaker will often have more effect on the final result in terms of tonality and efficiency than the power handling. The mechanical properties govern how the speaker interacts with the cabinet, and can give some insight into expected performance. This part can get quite complicated, and is where cabinet designers can spend days trying difference combinations of cabinet shapes, and sizes and different drivers to get the optimum results to fine tune their design.

If you check the specifications of most drivers on the market, they will include the Thiele-Small or T/S Parameters. (Read More Here) – these provide some numbers to help quantify the mechanical properties of a speakers, things like cone weight, cone stiffness, resonant frequency, etc

When selecting a replacement, it’s best to replace with a driver with similar mechanical properties, however, it’s very rare to have this information supplied by the manufacturer, so often an educated guess is required. Which mechanical properties are most important depend on the type of cabinet, and the desired results. We’ve worked out some rough guidelines to help you check if the driver you have selected is a good choice.

For Bass Speakers – look at Fs (Resonant Frequency), Qts and Vas – ideally these want to be similar to the part you are replacing.

For Mid-Bass speakers (such as a 12″ or 15″ in a full range speaker) – you have to look at a lot of factors – but avoid HIGH power woofers 900W or more with very large voice coils (4″) as these will tend to be heavy and sluggish and have poor mid-range response. You need a sensible Fs of 60Hz or lower to have any sensible bass response, high sensitivity (98dB or higher) and good frequency response extending up to 2Hz or higher.

Click here for guidelines on selecting a woofer for an 18″ Bass Reflex Cabinet