Will a New Driver Fit My Cabinet?

Will a new driver fit my cabinet? Answer it with the old driver out and the new one's datasheet in hand. Failed swaps come down to millimetres: a cut-out a few mm small, a motor that hits a brace, bolt holes on a different circle. Then check the Thiele/Small figures, because a driver that fits can still mistune the box.

How much 18s differ

Two 18" drivers, published figures:

Same nominal size, a 33 mm spread in cut-out and 40 mm in depth. Never assume an 18 is an 18.

Get the old driver out

Disconnect the cab from the amp. Undo the fixings, lift the driver by the basket (never the cone or dust cap) and note which lead went to which terminal before disconnecting. An 18" ferrite driver can weigh 20 kg: lay the cab on its back and use two people. With the driver out, everything can be measured.

The measurements

Cut-out

Measure the baffle hole across two diameters at right angles and compare with the datasheet's recommended cut-out. Too small: open it up with a router or jigsaw, keeping enough wood for the fixings. Much too large: the new flange may not cover and seal it, so fit an adaptor ring.

Baffle thickness

Sets the fixing length and, on rear-mounted drivers, how far the cone sits behind the front face. Check for a doubled baffle.

Depth behind the baffle

Measure from the back face of the baffle to the nearest obstruction: rear panel, horn wall, fold or brace. It must exceed the new driver's mounting depth with a margin, because panels flex under load. A driver that just touches is too deep.

Magnet and basket clearance

Measure the clear diameter around the motor, not only behind it. In scoops and folded horns the driver sits close to angled panels, and a wider ferrite motor can foul them. Keep the motor clear of port openings too. Typical brace layouts are in bracing and sealing cabinets.

Bolt circle and hole count

Measure centre to centre across opposite holes (the pitch circle diameter) and count the holes. A different circle or count means new holes and T-nuts, and filling the old ones. New holes must not break into old ones.

Front or rear mounting

Front-mounted drivers drop in from outside; rear-mounted drivers fit from inside the box, usually through a removable panel. The flange must suit the hole on the side it seats.

Terminals

Check the new terminals will not hit a brace and that the internal leads reach without strain. Mark polarity before disconnecting anything.

Fitting is not the same as working

Every cab is designed around the driver it was built for. A replacement with different Fs, Qts or Vas shifts the low-frequency response of a reflex box, changes horn loading and changes excursion near tuning: expect a peak, a thinner bottom end or more cone travel. Compare old and new T/S figures, and if they differ much, simulate the box with both before buying. See Thiele/Small parameters and cabinet tuning.

Wavemark drivers

• WM1850/8, £275: cut-out 392 mm, depth 202 mm, 20 kg. Full T/S published (Fs 42.9 Hz, Qts 0.644, Vas 120.9 L, Sd 1244 cm², Xmax 12 mm), so a swap can be simulated before you commit. 1800 W AES / 3600 W peak, 125 mm coil.
• WM18X451, £345: for scoops and folded horns, 1800 W RMS / 3600 W max, 115 mm copper coil, double spider. T/S data on request.

Both can be heard in our listening room by appointment. Matching a Wavemark driver to common cab types is covered in the replacement guide, and the fitting itself in replacing a speaker driver.

Sources: B&C Speakers published data for the 18SW115 (8 Ω), checked October 2026; Wavemark WM1850/8 spec sheet and WM18X451 product data.