Room acoustics for better bass

Below a few hundred hertz, a small room has as much influence on bass as the subwoofer does. Reflections from the walls, floor and ceiling combine into resonances called room modes, which make some notes boom and others nearly disappear, and the pattern changes from seat to seat. Better bass starts with understanding that pattern: placement decides how the subwoofer and the seats sit within it, and treatment can calm the worst resonances.

Why does the room change bass so much?

Bass wavelengths are long: about 8.6 m (28 ft) at 40 Hz and 3.4 m (11 ft) at 100 Hz. In a living room, those waves reach every wall many times before they fade, so what arrives at your ears is the direct sound plus a dense mix of reflections. At some frequencies the reflections reinforce one another and build up a resonance; at others they cancel. A subwoofer that measures flat outdoors can easily vary by 20 dB or more across frequency once it is in a room, and by a similar amount between seats at a single frequency.

Acousticians describe this as two regions. At the lowest frequencies, individual room modes dominate and the room behaves like a set of separate resonators. Higher up, modes overlap so densely that the sound field is better described by reflections and reverberation. The boundary between them, often called the Schroeder or transition frequency, typically lands somewhere around 100–200 Hz in home rooms. Most subwoofer problems live below it.

Resonances, modes and standing waves

Each room mode has a frequency set by the room's dimensions and a fixed pressure pattern, a standing wave, with loud regions near the walls and quiet nulls at predictable fractions of the room. Three things follow from that pattern:

• A seat at a null hears little of that mode; a seat near a wall hears a lot of it.
• A subwoofer at a null barely excites the mode; one in a corner excites every mode.
• Because each mode has a different pattern, every seat has its own frequency response.

You can estimate where the modes fall from length, width and height with the free room mode calculator. That tells you which frequencies to watch, not how loud they will be at your seat.

Placement or absorption: which fixes what?

The two are often lumped together as "fixing the room," but they work in different ways and solve different problems.

Placement changes how the pattern is driven and heard

Moving the subwoofer changes which modes it excites and how strongly. Moving the seat changes where in each pattern you listen. Neither removes a mode, but together they can turn a deep null at the main seat into a mild dip, or move a boom away from it. Placement costs nothing and usually has the largest effect, which is why the subwoofer setup guide treats it as the first step. Several subwoofers in chosen positions go further, because their contributions can partly cancel a mode's pattern and even out the response between seats.

Absorption removes energy, if it is deep enough

Absorbers turn sound energy into heat, which lowers resonant peaks and shortens the time a note rings on. At bass frequencies that takes size. Porous absorbers need to reach well into the region where air moves fastest, roughly a quarter wavelength from the wall, which is more than a metre at 60 Hz. A 5 cm (2 in) foam panel works mainly at a few hundred hertz and above, and does little at 40–80 Hz. Effective bass treatment means thick porous traps, usually in corners, or pressure-based membrane and resonator traps tuned to the problem range.

Absorption helps most with peaks and long decay. It rarely fills a null caused by cancellation at a particular seat; that is a placement problem. See why bass disappears in some spots for the difference.

How do you measure room acoustics at low frequencies?

Listening alone is a poor guide at these frequencies, because your judgement of a single note depends on the note before it and on where your head is. A measurement plays known test tones and records how loud each one arrives. Keep three points in mind:

• One position is not the room. A response measured at one spot can change noticeably 30 cm away. Measure every seat you use, and around it.
• Change one thing at a time. Move the subwoofer or the seat, then measure again under the same conditions, so the comparison means something.
• Predictions are a guide. Calculated modes assume a rigid rectangular box. Doors, openings, sloped ceilings and flexible walls move and soften them.

Where to start

1. Learn what the room is doing: room modes explained and standing waves.
2. Estimate your room's modes with the room mode calculator.
3. Diagnose specific problems: bass dead spots.
4. Then work on the source: the subwoofer setup guide.

Sources

• Floyd E. Toole, Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms, Routledge.
• F. Alton Everest and Ken C. Pohlmann, Master Handbook of Acoustics, McGraw-Hill.
• Heinrich Kuttruff, Room Acoustics, CRC Press.