How room grades placement.

How close the speakers are to the nearest boundary, expressed as the first-null cancellation frequency from speaker-boundary interference (SBIR).

A driver close to a wall creates a comb-filter null at f = c / (4 · d) where d is the distance from the driver to the wall. If that null lands in the speaker’s bass range, you get a perceived hole in the bass that no DSP cure can fully fix — the path-length cancellation is geometric.

• Good

  First-null frequency above 200 Hz

  Null sits above the bass band — no perceived bass hole

• Note

  First-null 100 – 200 Hz

  Null edges into the upper-bass — minor scoop possible

• Warn

  First-null 60 – 100 Hz

  Null lands in the kick-drum band — bass thins on transients

• Fail

  First-null at or below 60 Hz

  Deep-bass null — geometric cancellation no DSP cure can fully fix

Whether the listening seat is clear of a wall-reflection null in the low-mid range — the SBIR cancellation pattern shifts toward the listener as well as toward the speaker.

Same physics as speaker-side SBIR, but the reflective surface is the wall behind the listener. A listener pressed against the rear wall has a strong null near f = c / (4 · d_listener) that scoops the low-mids. Moving the seat 0.5 m can shift the null out of the critical band entirely.

• Good

  First-null frequency above 200 Hz

  Seat clear of the cancellation — full low-mid energy at the listener

• Note

  First-null 100 – 200 Hz

  Slight low-mid scoop — moving the seat 0.5 m may resolve it

• Warn

  First-null 60 – 100 Hz

  Audible low-mid hole at the chair — reposition the seat off the wall

• Fail

  First-null at or below 60 Hz

  Deep null at the seat — listener sits inside the cancellation

Whether the room’s modal distribution avoids stacked resonances — the Bonello criterion for "smooth modal density" in the low frequencies.

When two or more room modes pile up at the same frequency, you get a bass resonance peak that’s audible as boominess. Bonello’s 1981 JAES paper showed that "good-sounding" rooms have a monotonically rising mode count per 1/3-octave band below 200 Hz — the eye/ear test still holds.

• Good

  Mode count rises monotonically across 1/3-octave bands below 200 Hz

  Smooth modal density — Bonello criterion satisfied

• Note

  One band-to-band count violation

  Minor dip — barely audible as bass colouration

• Warn

  Two or more band-to-band violations

  Stacked modes — audible boominess at the affected frequencies

How equal the distance from each speaker to its nearest sidewall is, as a coarse proxy for early-reflection symmetry at the listener.

Asymmetric early reflections pull the phantom centre toward the louder side. In a geometrically asymmetric room the listener can compensate via toe-in, surface treatment, or measured correction — but the geometry itself doesn’t predict the perceptual outcome. When the listener confirms imaging balance via pink-noise centre-image test, this check defers to that confirmation.

• Good

  L/R sidewall residual < 25%

  Within reference convention — early reflections roughly symmetric

• Note

  L/R sidewall residual 25 – 40%

  Minor L/R difference — usually inaudible after toe-in

• Warn

  L/R sidewall residual 40 – 50%

  Imaging may pull toward the closer wall

• Fail

  L/R sidewall residual ≥ 50%

  Severe asymmetry — geometric mitigation recommended

• Note

  Imaging confirmed by listener (Toole pink-noise test) or REW-verified

  Downgrade — geometric asymmetry present but acoustically tuned

• Note

  Surface profile is "dead" (heavy absorption)

  Downgrade — heavy absorption attenuates the early-reflection energy this check is looking for

Angle subtended by the two speakers at the listening position, in degrees. Reframed from the pre-revision equilateral-residual metric to angular subtense, which speaks directly to the perceptual outcome (image width and centre-image stability).

ITU-R BS.775-1 §3 specifies ±30° from listener axis (60° total subtense) as the reference layout for two-channel stereo. Below 30° the phantom centre collapses to mono; above 90° the listener is geometrically inside or behind the speakers (topology error). Listener confirmation can’t override this — the metric IS the perceptual outcome.

• Good

  Subtense 50° – 70° at the listener

  Within the 60° reference window (ITU-R BS.775-1 §3)

• Note

  Subtense 40 – 50° or 70 – 80°

  Slightly off the 60° reference — image width or centre stability mildly off

• Warn

  Subtense 30 – 40° or 80 – 90°

  Listener too far back / speakers too wide

• Fail

  Subtense below 30° or above 90°

  Phantom centre collapses (mono fusion) / listener inside or behind the speakers

Speaker rotation classification (straight / crossed-in-front / at-listener / crossed-behind / facing-away) plus L/R rotation asymmetry in degrees.

Multiple toe-in schools are defensible: Toole-school straight or behind-listener, Geddes-school crossed-in-front, Genelec-school crossed-at-listener, Linkwitz-school deliberate-asymmetric for asymmetric rooms. The engine doesn’t know speaker directivity — so when the listener confirms imaging balance, the asymmetry penalty suppresses entirely and only the school classification matters.

• Fail

  Speakers face away from the listener (rotation > 90°)

  Topology error — drivers point away from the seat

• Warn

  L/R asymmetry > 20°

  Unusual mismatch unless deliberate

• Note

  L/R asymmetry 15 – 20°

  Within Linkwitz asymmetric-compensation range

• Note

  Behind-listener school with asymmetry ≤ 15°

  Toole-school valid setup (less toe-in for wider stage)

• Good

  Otherwise (symmetric or within tolerance)

  Geometry consistent with at least one of the defensible toe-in schools

• Good

  Imaging confirmed by listener or REW-verified

  Asymmetry penalty suppressed — Linkwitz / Genelec / Cardas compensation acknowledged

For 2.1 / 2.2 systems: whether the subwoofer is placed for even bass excitation across the listening area, per the Welti & Devantier multi-sub layout principles.

A single sub in a corner maximises efficiency but couples strongly to room modes — great for the corner, lumpy everywhere else. Welti & Devantier (Harman, JAES 2006) showed that mid-wall or asymmetric placements yield far smoother modal excitation across multiple seats. For a one-seat listener with a one-sub system, geometry matters more than the corner-loading shortcut.

• Good

  Sub ≥ 0.8 m from any corner AND ≥ 0.5 m off the room midplane

  Asymmetric placement — smoother modal excitation across the room

• Note

  Sub meets one of the two criteria but not both

  Better than corner-loading but not optimal

• Warn

  Sub within 0.5 m of a corner

  Strong room-mode coupling — great efficiency, lumpy response

• Warn

  Sub directly in a corner

  Maximum coupling — corner-loading shortcut; expect uneven seats

Whether the room volume sits in a reasonable range for the speaker class — a sanity-check guardrail, not a precision metric.

A pair of full-range floor-standers in a 12 m³ closet over-pressurises the room and excites every mode; a pair of standmounts in a 90 m³ great hall can’t move enough air to fill it. The detailed cone-area vs volume math lives in bass-headroom-vs-volume; this check is the coarse class-vs-room flag.

• Good

  Room volume inside the class-tolerated band

  Reasonable speaker-to-room sizing — no pressure-overload or under-fill

• Note

  Room volume within 25% of the band edge

  Marginal — close call either way

• Warn

  Room substantially undersized for the speaker class

  Over-pressurises the room and excites every mode

• Warn

  Room substantially oversized for the speaker class

  Can’t move enough air to fill the space

Whether the amp + speaker pair can hit reference SPL at the listener distance — same physics as the Signal-side SPL headroom check, but anchored to the geometric path rather than a default 3 m assumption.

The Signal SPL headroom check uses a fallback distance when the seat isn’t set. Here, with full layout, we use the exact Euclidean listener-to-speaker distance — which is typically larger than the perpendicular distance editors record — and re-score against Toole’s 95 dB (apartment) / 105 dB (reference) targets.

• Good

  Peak SPL at the seat meets or exceeds the distance-scaled target

  Headroom for dynamic-range material at the actual listening position

• Note

  0 – 3 dB below target

  Comfortable everyday level with limited reserve

• Warn

  3 – 6 dB below target

  Headroom-limited — transients compress on dynamic content

• Fail

  More than 6 dB below target

  Under-powered for the geometry — large peaks will clip

Whether the speaker’s published low-edge frequency response reaches the room’s pressure-zone region, where the room reinforces bass for free.

Every rectangular room has a lowest standing-wave mode f_room = c / (2 · L_max). Below f_room the room transitions toward pressure-zone reinforcement (typically 6–12 dB/oct of free bass). A speaker that reaches into that band gets the room’s gift; one that rolls off above f_room sounds thin no matter the cabinet.

• Good

  Speaker (or sub) reaches the room mode — gap ≤ 0 octaves

  Pressure-zone reinforcement engaged — free bass below f_room

• Note

  Gap of 0 – ½ octave above f_room

  Just shy of the pressure-zone gift

• Warn

  Gap of ½ – 1 octave

  Missing the room’s bottom-end reinforcement — sounds lean

• Fail

  Gap greater than 1 octave

  Bottom octave entirely absent from the in-room response

Whether the speaker has enough cone area to move air proportional to the room’s volume. Big rooms need more cone area for the same bass SPL — straight conservation of acoustic energy.

Sound pressure from a piston radiator at low frequencies scales with displacement volume V_d = S_d · X_max. Below the lowest room mode, in-room SPL scales roughly with V_d / V_room. The engine carries S_d (driver area sum) but not X_max — the coarse bins absorb that limitation honestly.

• Good

  Cone area ratio ≥ 10 cm²/m³

  Generous cone area for the volume — full dynamic headroom at low frequencies

• Note

  Ratio 5 – 10 cm²/m³

  Adequate — bass scales to room without obvious strain

• Warn

  Ratio 3 – 5 cm²/m³

  Marginal — low-bass peaks will audibly compress

• Fail

  Ratio below 3 cm²/m³

  Cone area can’t move enough air for the volume