First step is to determine the size of the woofer. Woofers are generally grouped by nominal overall diameter, which is easy enough to measure. You just measure edge to edge of the outside of the chassis, and round down to the nearest inch. This works for most sizes, except around 5″-6″ where some manufacturers will state 5.25″ or 6.5″ rather than rounding off to the nearest inch. The speaker below has an outside diameter of 460mm, which is 18.1 inches – and for purposes of selecting a replacement, we consider this to be an 18″ woofer. When it comes to fitting a replacement with minimal hassle, a second measurement is also quite critical, the bolt circle diameter, or BCD. Whilst this tends to be fairly standard, it can vary by a few millimetres, particularly between pressed steel and cast alloy chassis, and can vary enough for the bolt holes to not line up. If you bolt holes don’t line up, replacing the driver ends up being a lot more work. The BCD is measured from the middle of the hole on one side to the middle of the hole on the opposite side, and ideally you want the BCD to be within around 2-3mm to avoid issues with fitment of the driver. Most manufacturers tend to follow a similar bolt pattern, but over the years one or two manufacturers have opted for non-standard bolt hole configurations, so this is well worth checking to save yourself a lot of hassle.

Thankfully most cast alloy chassis tend to have similar BCD, as do most pressed steel chassis. So the times you are most likely to encounter issues would be if you try to replace a pressed steel chassis driver with a cast alloy driver, or vice versa. The two different type of chassis are easy to distinguish, pressed steel chassis are generally sheet steel of around 1-2mm thickness that has been pressed into shape. Cast alloy chassis are thicker, sometimes 3-4mm or more around the edge of the driver, and thicker on the legs. For purposes of doing a drop in replacement, its generally much simpler to stick with the same type of chassis, as you have a much higher chance of everything lining up nicely with no modifications required.

Cast alloy chassis	Pressed Steel chassis

We’re nearly done on dimensions, a couple of final things to check are the hole cut-out size, and the overall depth. You are most likely to have issues with the hole cut out size if you switch from cast alloy chassis to pressed steel chassis, as the hole cutout (the hole in the cabinet) can vary by 3-5mm. If you upgrade from a pressed steel chassis to a cast alloy chassis, there is a good chance the hole cutout might be a little bit too small, and you may need to enlarge it. The other way around, from cast alloy to pressed steel may present bigger issues, as the hole may be too big, not leaving sufficient material around the driver for mounting screws. Most manufacturers specify the baffle cutout diameter in their specifications, so this is relatively easy to check.

The overall depth often isn’t something you need to worry about , however, if you are looking for a woofer for front loaded horn, rear loaded horn (scoop) or a bandpass design you may find the cabinet has been designed to accommodate a specific driver, and if you are upgrading to a modern driver, particularly a long excursion neodymium magnet woofer – its quite possible the magnet may be too long to fit the space – so please check the depth before committing to a purchase. If you are looking for drivers for specialist applications, this needs a lot more research than a standard design.

Once you have BCD, and baffle cut-out sizes, you can move on to the next stage: https://blog.bluearan.co.uk/selecting-a-replacement-speaker-part-2-power-handling-and-impedance/

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/