15" vs 18" Subs: The Numbers
Below the box tuning, a sub's maximum output is set by how much air it can displace: Sd × Xmax. Compare a matched pair from one maker's range and an 18" gives about 3.3 dB more excursion-limited output than the 15", for nearly the same output per amp in the passband. The rest of the argument is cab size and logistics.
A matched pair: B&C 15SW115 vs 18SW115
Same family, same 116 mm (4.5") coil and near-identical motor strength, so the comparison isolates cone size. Figures are B&C's published 8 Ω data.
| Parameter | 15SW115 | 18SW115 | Difference |
|---|---|---|---|
| Sd | 855 cm² | 1210 cm² | +41.5% |
| Xmax | 13.5 mm | 14 mm | +0.5 mm |
| Vd (Sd × Xmax) | 1154 cm³ | 1694 cm³ | +46.8% (+3.3 dB) |
| Bl | 30 Tm | 30.3 Tm | ≈ same |
| Mms | 200 g | 275 g | +37.5% |
| Fs | 35 Hz | 32 Hz | −3 Hz |
| Qts | 0.24 | 0.30 | |
| Vas | 110 L | 187 L | +70% |
| Sensitivity (1 W/1 m) | 96 dB | 97 dB | +1 dB |
| Net weight | 11.1 kg | 11.9 kg | +0.8 kg |
| Mounting depth | 193 mm | 242 mm | +49 mm |
Displacement sets the ceiling
At a given frequency, far-field pressure is proportional to the volume velocity of the cone, so excursion-limited SPL scales with Vd. 1694 / 1154 = 1.47, which is 20·log₁₀(1.47) = +3.3 dB for the 18". To match it, the 15SW115 would need 1694 / 855 = 19.8 mm of Xmax against its rated 13.5 mm.
The demand grows fast as you go down. For constant SPL, cone displacement rises 12 dB per octave: four times the excursion at 30 Hz as at 60 Hz. That is why the gap between 15s and 18s shows up most at the bottom of a dub or DnB mix and hardly at all at 80 Hz. See Xmax and excursion.
Two 15s vs one 18
Two 15SW115 give 1710 cm² and 2308 cm³, against 1210 cm² and 1694 cm³ for one 18SW115: +2.7 dB of displacement to the pair, plus twice the thermal capacity. The cost is two drivers, two boxes (or one large one) and a 4 Ω load per pair in parallel. Like for like on cone area, a pair of 15s beats a single 18; one 18 beats one 15.
The "15s are faster" claim
Above resonance a driver is mass-controlled: cone acceleration is Bl·i / Mms, and radiated pressure follows Sd × acceleration. So output per amp in the passband tracks Bl·Sd / Mms:
- 15SW115: 30 × 855 / 200 = 128
- 18SW115: 30.3 × 1210 / 275 = 133
That is 0.3 dB apart, in line with the 1 dB sensitivity difference. The heavier cone is not slower: the larger Sd pays for its mass. What audiences hear as "tight" or "slow" bass is set by the high-pass behaviour of the whole system: box alignment and Q, port tuning, the crossover and DSP filters, and room modes. Those add group delay at the bottom of the band regardless of cone size. A 15" often sounds punchier because it is usually run higher, into the kick range, in a smaller box tuned higher.
Cab size
For a given alignment, box volume scales with Vas. Here the 18" has 70% more Vas, so expect a materially larger reflex box, and folded horns and scoops scale up with the mouth area needed to load the larger cone. Driver weight is close (0.8 kg between these two neodymium models); the extra mass in an 18" rig is plywood. Check mounting depth before a swap: 49 mm separates these two. See scoop vs reflex vs horn and horn loading explained.
Choosing
- Bottom octave at level (roughly 30 to 60 Hz, roots, dub, DnB): 18s, or more 15s. Vd decides it.
- Kick and upper bass (roughly 60 to 150 Hz): 15s in compact boxes are a common and efficient choice. See mid-bass drivers.
- Don't mix 15s and 18s on the same band. Different Sd, Mms and box tuning give different phase and roll-off through the crossover region. Give each size its own band, amp channel and crossover point. See crossover basics.
Power, build and price: Wavemark 18s
| Wavemark WM1850/8 | Wavemark WM18X451 | B&C 18SW115 | |
|---|---|---|---|
| Power | 1800 W AES / 3600 W peak | 1800 W RMS / 3600 W max | 1700 W nominal / 3400 W continuous |
| Voice coil | 125 mm | 115 mm (4.5"), copper | 116 mm |
| Sd / Xmax / Vd | 1244 cm² / 12 mm / 1493 cm³ | T/S on request | 1210 cm² / 14 mm / 1694 cm³ |
| Motor | Ferrite, vented | Ferrite, double spider | Neodymium |
| Price | £275 | £345 | £444 (Thomann UK) |
| Price per rated watt | 15.3p | 19.2p | 26.1p |
Price per watt uses each maker's headline continuous rating; the standards differ (AES, RMS, B&C "nominal"), so treat it as a guide, not a lab comparison. The 18SW115 has more Xmax and a lighter neodymium motor; the WM1850/8 has the larger coil and Sd for £169 less. Both Wavemark drivers can be heard in our listening room before you buy.
More on selection: how to choose an 18" sub driver.
Sources: B&C Speakers published data for the 15SW115 and 18SW115 (8 Ω), checked October 2026; Thomann UK listed price for the 18SW115 8 Ω, checked 11 October 2026; Wavemark WM1850/8 spec sheet and WM18X451 product data. Displacement and mass-controlled-region relationships: L. L. Beranek, Acoustics. Calculated values rounded.
Questions
How much louder is an 18 inch sub than a 15 inch?
Excursion-limited output scales with displacement (Sd × Xmax). For B&C's matched 15SW115 and 18SW115, that is 1154 cm³ against 1694 cm³, so the 18 gives about 3.3 dB more at frequencies where both are at Xmax. In the passband below that limit, their output per amp is within about 1 dB.
Are 15 inch subs faster than 18s?
Not because of cone size. In the mass-controlled range, output per amp follows Bl × Sd / Mms, which comes out within 0.3 dB for the matched B&C pair. Perceived tightness is set by box alignment, tuning, crossover and DSP filters and the room.
Are two 15s better than one 18?
On displacement, yes: two 15SW115 move 2308 cm³ against 1694 cm³ for one 18SW115, about 2.7 dB more, with double the thermal capacity. The trade is more boxes or a bigger box and a 4 ohm parallel load.
Can I run 15s and 18s on the same sub band?
It is hard to get right. Different Sd, moving mass and box tuning give different phase and roll-off, so they cancel in places through the overlap. Run each size on its own band with its own crossover point and amp channel.