How many subwoofers does a home cinema need?

Why one is rarely enough

At low frequencies, sound reflects between the walls, floor and ceiling and sets up standing waves, or room modes. In some places the bass builds up; in others it cancels. With a single subwoofer, the bass can sound right at one seat and thin or boomy at the next, and no amount of equalisation fixes a cancellation.

Several subwoofers in symmetrical positions excite the room’s modes more evenly, so the bass varies less from seat to seat. Equalisation then has a much easier job, because it is correcting one response that most seats share rather than a different response at each seat.

What RP22 recommends

Two or four subwoofers, laid out symmetrically.
Positions at the midpoints of the walls where possible.
Four in preference to two, for more even bass across the seats.

RP22 also sets targets for how much the bass may vary from seat to seat below the transition frequency, where room modes dominate:

Bass, below transition	Level 1	Level 2	Level 3	Level 4
Response at the reference seat	±5 dB	±4 dB	±3 dB	±2 dB
Variation from seat to seat	–	±4 dB	±3 dB	±2 dB
Bass extension (−3 dB, in room)	35 Hz	30 Hz	20 Hz	18 Hz

Minimum values. Source: CEDIA/CTA‑RP22 v1.2 (2023), Table A.1.

How much output

RP22 sets a target for the combined output of all the subwoofers at the listening position, with the room’s own gain included:

LFE, all subwoofers	Level 1	Level 2	Level 3	Level 4
Minimum	109 dB	112 dB	115 dB	118 dB
Recommended	114 dB	117 dB	120 dB	123 dB

dB SPL, C-weighted, at the reference seating position. Source: CEDIA/CTA‑RP22 v1.2 (2023), Table A.1.

The subwoofers carry more than the LFE channel. With bass management, the low frequencies of every other channel are sent to them as well, which RP22 notes adds around 4 dB to what they need to deliver.

Output also adds up. At low frequencies, where they combine in phase, two identical subwoofers each driven with the same power produce up to about 6 dB more at the seats than one. Four subwoofers share the work further, so each runs further from its limits. Try it with the calculator.

Placement and set-up

With two subwoofers, the usual symmetrical positions are the midpoints of the front and rear walls, or of the two side walls. With four, the midpoints of all four walls. The room’s layout, doors and seating decide what is practical, so treat these as the starting point for the design.

Once they are in place, the subwoofers need to be aligned with each other and with the main loudspeakers: level, delay and equalisation, set from measurements at the seats. Most cinema processors can do this for several subwoofers at once.

The Cinema Pro 18S

The Cinema Pro 18S is designed to be used this way, in pairs or fours, with the Cinema Pro 8 and Cinema Pro 6 handing over to it at their high-pass point. Each is a sealed 18-inch subwoofer reaching 124 dB at 50 Hz. In a solid room at 4 m, two bring the 115 dB cinema reference level to the seats down to 30 Hz, which is RP22 LFE Level 2, and four take it to 20 Hz, Level 3.