Choosing loudspeakers for a room

Loudspeakers are usually chosen by brand, price and a demonstration in someone else’s room. None of those predicts how they will sound in yours. Decades of research have identified what does: how evenly a loudspeaker sends sound in every direction, whether the loudspeakers around the room match, and whether they can reach the levels the room needs without strain.

Directivity: the part you hear in a room

In a room you hear the direct sound and, a few milliseconds later, the same loudspeaker’s sound reflected off the walls, floor and ceiling. So a loudspeaker’s off-axis sound matters almost as much as its on-axis sound.

The measurement standard for this is ANSI/CTA-2034 — the “spinorama”. It takes 70 measurements around the loudspeaker and reduces them to a set of curves: on-axis, a listening window (the average within ±30° horizontally and ±10° vertically), the early reflections, the total sound power and the directivity index between them. Blind listening tests have shown a consistent preference for loudspeakers with flat, smooth on-axis response and a smoothly changing or constant directivity index (Toole, 2015); Olive’s regression model built on these measurements predicted listeners’ preferences closely.

Practically: ask for spinorama-style data, not a single frequency-response curve with a wide tolerance. A loudspeaker whose off-axis curves look like scaled-down copies of its on-axis curve will behave predictably in a treated room.

Matching across the room

Sounds move between loudspeakers. If the centre sounds different from the left and right, dialogue changes character as it pans; if the surrounds differ from the fronts, effects change as they travel. Broadcast and listening-test standards ask for matched channels — ITU-R BS.1116 asks for front loudspeakers matched within 1 dB between 250 Hz and 2 kHz — and Dolby asks for overheads timbre-matched to the main loudspeakers. The simplest way there is one family of loudspeakers throughout, with identical models across the screen wall.

Will it play loud enough?

Cinema soundtracks are mixed to a reference level with a lot of headroom above it. SMPTE RP 200 sets the cinema reference at 85 dBC per screen channel, and THX describes reference level as 85 dB SPL with 20 dB of headroom — so a screen channel may be asked for peaks around 105 dB at the seat, and the low-frequency effects channel is replayed 10 dB higher in its band. Smaller rooms are commonly calibrated lower: Dolby’s home guide uses 79–82 dB SPL(C) at the main seat for its −20 dBFS test signal, and ATSC’s guidance for production rooms steps the reference down from 85 dB to 76 dB as room volume falls.

The level at the seat falls with distance. Outdoors, every doubling of distance costs 6 dB; indoors the reflections make up part of that, but the rear row of a deep room still hears noticeably less than the front. So the question is not a loudspeaker’s wattage. It is: from its sensitivity, its maximum output and the distance to each seat, can it produce the required peaks without distortion — with margin left over?

Example (free-field, before room gain): a loudspeaker that can produce 115 dB at 1 m gives about 103 dB at 4 m — 2 dB short of a 105 dB peak. In the room it may just make it; with no margin, it will compress on the loudest scenes.

In-wall, on-wall and free-standing

Mounting changes the bass. A loudspeaker standing out from the wall gets a cancellation notch from the reflection off the wall behind it; one built into the wall, or flush with a baffle wall, avoids that notch entirely. That is why theatres with a screen wall often build the front loudspeakers into an absorbent baffle behind the screen. The placement chapter covers the physics.

Sources

1. 01Toole, “The Measurement and Calibration of Sound Reproducing Systems”, JAES 63(7/8), 2015 (open access)Spinorama curves and what listeners prefer.
2. 02Olive, “A multiple regression model for predicting loudspeaker preference using objective measurements: Part II”, AES 117th Convention, 2004
3. 03ANSI/CTA-2034 — Standard method of measurement for in-home loudspeakers
4. 04ITU-R BS.1116-3 — Methods for the subjective assessment of small impairments in audio systemsLoudspeaker matching in critical listening rooms.
5. 05EBU Tech 3276 Supplement 1 — Multichannel listening conditions
6. 06SMPTE RP 200-2002 — Relative and absolute sound pressure levels for motion-picture multichannel sound systems
7. 07THX — What is reference level?
8. 08ATSC A/85 — Techniques for establishing and maintaining audio loudness (reference levels by room volume)
9. 09UNSW School of Physics — dB: what is a decibel? (inverse-square law)

External links open in a new tab. Figures quoted in this chapter are taken from these sources; a Science of Sound measurement is only ever presented as one, with its file, date and instrument.