How Many Subs Do I Need?
How many subs do I need has no head-count answer, because output depends on the cab, the room, the music and the level. What is fixed is the physics: each doubling of closely coupled identical subs adds up to 6 dB, each doubling of distance outdoors costs 6 dB, and below tuning the ceiling is set by total displacement. Size the rig from those.
Why per-head ratios fail
Charts of "subs per hundred people" ignore the variables that matter. Two 18" cabs can differ by several dB in efficiency depending on loading (scoop, horn or reflex) and driver. Outdoors you lose the boundary reinforcement a room gives you. And a dub or DnB session needs far more sub output relative to the tops than speech or background music. See scoop vs reflex vs horn.
Coupling: the numbers
Identical subs placed within a fraction of a wavelength of each other, in polarity and fed the same signal, sum coherently: pressure doubles, which is +6 dB. Half of that is the doubled electrical input; the other 3 dB is mutual coupling, each cone working into a higher radiation load. Feed two cabs from the same total amplifier power instead and you get +3 dB. Real rigs land a little under the ideal figure, but the rule holds:
| Cabs (same drive per cab) | Ideal gain over one | Total Vd, WM1850/8 |
|---|---|---|
| 1 | 0 dB | 1493 cm³ |
| 2 | +6 dB | 2986 cm³ |
| 4 | +12 dB | 5972 cm³ |
| 8 | +18 dB | 11944 cm³ |
Vd is Sd × Xmax (1244 cm² × 12 mm). Below the box tuning, excursion is the limit, and doubling Vd also adds 6 dB to that ceiling. Coupling breaks down when cabs are spread across a wide stage: path-length differences cause cancellation and uneven bass across the floor. Keep subs grouped. See sub arrays and cardioid and phase and polarity.
Distance and venue
In free field, level from a compact source falls 6 dB per doubling of distance. Moving the dance floor from 10 m to 20 m costs as much as halving the number of subs. Indoors, room boundaries and modes reinforce low frequencies, unevenly; outdoors there is no such help, so the same target level at the same distance needs more cabs.
Sub-to-top balance
"Two subs per top" is shorthand, not a rule. What matters is that the sub section reaches the level and balance you want without running into limiting before the tops do. A practical check:
- Set the tops at your working level on typical material.
- Bring the subs up to the balance you want.
- Watch limiter and clip indicators. If the sub channels limit long before the tops, you need more displacement: more cabs, or more efficient loading.
Leave headroom. A rig that sits on its limiters all night sounds compressed and runs its coils hot. See voice coils and heat and why sub drivers fail.
Build in matched pairs
Every doubling needs identical cabs with identical drivers to couple cleanly, so build to one proven design and add in pairs. That makes driver cost per cab the number that scales:
- WM1850/8: 1800 W AES, £275, 15.3p per rated watt. Four cabs: 7200 W AES of handling and 5972 cm³ of displacement for £1100 in drivers.
- WM18X451: 1800 W RMS, 115 mm copper coil, double spider, for scoops and folded horns; £345, 19.2p per rated watt.
Wiring four 8 Ω drivers in series-parallel keeps an 8 Ω load on one channel; two in parallel gives 4 Ω. See impedance and wiring and planning your first sound system. Both drivers can be heard in our listening room by appointment.
Safety
More subs means heavier, taller stacks and higher sound levels. Stack on level, solid ground, keep the public off and away from stacks, and protect your own hearing and the audience's. See stacking speakers safely.
Sources: inverse-square law and coherent summation of coupled sources (L. L. Beranek, Acoustics); Wavemark WM1850/8 spec sheet and WM18X451 product data. Gains shown are ideal; real rigs typically fall slightly short.
Questions
How many subs do I need for 100 people?
No fixed number holds up; cab efficiency, room, indoor or outdoor use and music style all change it. Start with a matched pair and add identical cabs in pairs if the sub channels hit their limiters before the tops do.
What is a good sub to top ratio?
Ratios like two subs per top are shorthand. The test is whether the sub section reaches your target balance without limiting before the tops. For bass-heavy music that usually means substantially more sub capacity than tops.
Does adding a second sub double the volume?
Two identical subs stacked together and driven with the same signal per cab give up to 6 dB more output: 3 dB from doubled power and 3 dB from mutual coupling. That is a clear step up, but less than a perceived doubling of loudness, which takes around 10 dB.
Should subs be stacked together or spread out?
Together. Closely grouped subs couple and give more even coverage. Spreading them across a wide stage creates path-length differences that cancel at some positions, giving loud and quiet spots.