Chris Collins

Tools · music

Speaker and room placement

Place speakers, subwoofers and seats in a rectangular room by dragging them on a plan, and see the bass at the seat from a modal model of the room (20–300 Hz), speaker-boundary nulls from image sources, first-reflection points with their delays and levels, seat and sub-crawl heat maps, multi-seat spread, the stereo triangle, 38%, Cardas and other placement rules, and Bolt and Bonello room-ratio checks.

Added

Works with

How to use it

Below about 300 Hz, the room affects the sound at your seat more than the speakers do. It has resonances (modes) that make some notes boom and others vanish, and each nearby wall reflects the speaker’s sound back into it, cutting a notch. This tool models a rectangular room. You drag the speakers, subwoofers and seats around a floor plan, and the low-frequency response at the seat updates as you move them. It also finds the reflection points that treatment should cover, and checks the room’s proportions.

How to use it

  1. Room. Pick a preset or type the length (front wall to back wall), width and height, in metres or feet. Set the damping in one of two ways. A low-frequency RT60 gives every mode the same decay. Wall absorption (α for the front and back walls, the side walls, and the floor and ceiling) damps each mode according to the walls it strikes.

  2. Speakers. Choose 2.0, 2.1, 2.2 or 5.1. The mains are modelled as sealed boxes with a corner frequency fc and a Qtc (put in the figures from the speaker box designer, or 0 for flat). With a sub, the mains are high-passed and the sub low-passed with Linkwitz–Riley 4th-order filters at the crossover. You can also set the sub’s level, delay and polarity.

  3. Drag. Drag the speakers, the subwoofer(s) and the seats on the plan (front wall at the top). The Bass at the seat plot updates as you move them. You choose the signal: everything playing the same mono bass (the usual case for bass), one speaker alone, or just the subs. The table under the plan sets heights and exact positions.

  4. Heat maps.

    • Seat map colours every point of the floor at ear height by how uneven the response would be there, as the standard deviation of the curve in the band you choose.

    • Sub map is the "sub crawl" in reverse. Instead of putting the sub at the seat and crawling around the floor to find where the bass sounds best, it moves the sub over the floor and scores the response at your seats. Reciprocity makes the two give the same answer.

      With several seats (a row of three, two rows, or your own), the sub map can score the seat-to-seat spread too. Click the map to move the seat or the sub to that spot.

  5. Boundaries (SBIR). Image sources up to the third order show the dips that the speaker’s own reflections cut, from 20 Hz to 1 kHz. Switch surfaces on and off to see which wall causes which dip. The table gives each boundary’s distance and its null, c/(4d).

  6. Reflections. For each front speaker and each surface, the mirror method finds the first-reflection point. The table gives its position, the delay after the direct sound, the level from spreading, how far off the speaker’s axis the sound leaves, and what to put there: absorber, diffuser, ceiling cloud or rug. The points are also drawn on the plan. Plan the panels with the acoustic treatment tool.

  7. Guidance. The panel shows the stereo triangle (the angle at the seat, and the toe-in for each speaker) and the distance from each speaker to the front wall, side wall and floor with that boundary’s null. It also checks Roy Allison’s advice to keep those three distances different, and warns about seats in the middle of the room or against the back wall. Overlays and one-click presets cover these placement rules:

    • the equilateral triangle;

    • the 38% rule;

    • Cardas (speakers 0.276·W from the side walls and 0.447·W from the front wall);

    • the rules of thirds and fifths;

    • for 5.1, ITU-R BS.775 (fronts at ±30°, surrounds at ±110°).

  8. Room ratio. Plots your proportions on Bolt’s chart and counts the modes in each ⅓ octave for Bonello’s test. Buttons change the room to a well-known ratio at the same volume.

  9. 3D. An orbit view of the room with the speakers, the seats and the reflection paths.

The method

  • Modes. f = (c/2)·√nx/L)² + (ny/W)² + (nz/H)²) for rigid walls, with c = 343 m/s. The pressure at the seat is the sum over all the modes up to 450 Hz, plus the (0,0,0) "pressure chamber" term that gives a sealed room its gain at the very bottom: p ∝ Σ ψn(source)·ψn(seat) / (Λn(ω² − ωn² − 2jδnω, with ψ = cos(nxπx/L)·cos(nyπy/W)·cos(nzπz/H). This is the Green’s function of a rectangular room (Morse and Ingard, Theoretical Acoustics; Kuttruff, Room Acoustics, ch. 3). The result is in dB relative to the same speaker at 1 m in free space.

  • Damping. With an RT60, every mode decays as e−δt with δ = 6.91/RT60, and its Q is πf/δ. With wall absorption, δ = c/(8V)·Σ wiAi: a pair of walls counts in full when the mode has a non-zero index across it and half when the mode only grazes it. This is Kuttruff’s weighting, scaled so that an oblique mode decays at the Sabine rate. The weighting is why axial modes between hard walls ring longest.

  • Image sources (Allen and Berkley, 1979). Mirroring the speaker in each wall, and the mirror images in turn, up to three reflections. Each image is delayed by its path and scaled by 1/r and by √(1 − α) for each bounce. With every order included this would give the same answer as the modal sum. Limited to the first one to three orders, it shows the speaker-boundary interference that dominates from about 60 Hz to 500 Hz.

  • SBIR null. A reflection that travels 2d further than the direct sound cancels it where 2d is half a wavelength: f = c/(4d). For the actual seat, the null falls where the extra path is half a wavelength, c/(2·Δ).

  • Reflections. The reflection point lies on the line from the speaker’s image to the listener. The delay is Δ/c. The level is 20·log₁₀(direct/reflected path) for a hard surface. Reflections within about 20 ms and louder than −10 dB are the ones worth treating, and a reflection-free zone aims for −20 dB.

  • Unevenness is the standard deviation, in dB, of the response sampled at 24 points per octave over the band. Seat-to-seat spread is the standard deviation across the seats at each frequency, averaged over the band. The sub map’s default score adds the two (after Welti’s multi-seat optimisation).

  • Bolt area. Bolt’s 1946 chart of good ratios, in the form given in EBU Tech 3276: 1.1·w/h ≤ l/h ≤ 4.5·w/h − 4, with l/h < 3 and w/h < 2.

  • Bonello criterion (Bonello, JAES 29, 1981). The number of modes per ⅓ octave up to 200 Hz should never fall from one band to the next, and a band with fewer than five modes should not hold coincident modes. Here, modes within 0.5% of each other count as coincident.

The model has limits:

  • The room is rectangular and empty, with walls that do not flex.

  • The speakers are point sources, so they have no directivity or baffle step.

  • There is no furniture, open doorway or other room.

Real rooms with stud walls, sofas and openings are better damped, and act larger, than the model. Use the tool to compare placements and to find the reflection points, then measure the room with a microphone and the spectrum analyser to confirm. Above the Schroeder frequency (marked on the plot), the response depends on the speaker’s directivity and the reflections, not on single modes. For the modes and RT60 of a room with real materials, see room acoustics.

Privacy

Everything is calculated in your browser. Nothing is sent anywhere. Your last layout is kept in this browser’s local storage, and the share link carries the settings in the URL.

Runs entirely in your browser; nothing you type leaves this page.