MacBook Pro 15,1 — custom speaker DSP graph

A modified PipeWire filter-chain graph for the built-in speakers of the MacBook Pro 15,1 (Intel T2) running Linux, plus a script to install it.

This is not a driver. The T2 kernel/ALSA stack exposes the raw speaker PCM; WirePlumber (via t2-linux-audio's 51-t2-dsp.conf) renames it to alsa_output.platform-sound.RawSpeakers, hides it, and splices this graph in front of it. The graph does the crossover, voicing EQ, dynamics, and FIR correction that the T2's own DSP does under macOS.

Origin

Forked from configs/15_1/graph.json in lemmyg/t2-apple-audio-dsp (which itself borrows FIR filters and structure from chadmed/asahi-audio).

All structural elements are unchanged: node names, FIR .wav paths (/usr/share/t2-linux-audio/15_1/), capture.props / playback.props, target.object = alsa_output.platform-sound.RawSpeakers, 4-channel FL/FR/RL/RR output, allowed rates 48000/44100, and the capture.volumes mapping that ties the sink volume slider to the loudness-compensation stage.

Signal chain

in ─▶ equalizer ─▶ virtualbass ─▶ multiband_compressor ─▶ limiter ─▶ ell/elr ─▶ copyL/R ─┬▶ convLT/convRT ─▶ tlim ─▶ out
     (LSP x16)     (bankstown)   (LSP mb_comp x8)      (fastLookahead) (loud_comp)        │  (tweeter FIR)   (limit)
                                                                                          └▶ convLW/convRW ─▶ wlim ─▶ out
                                                                                             (woofer FIR)    (limit)

Changes vs. upstream 15_1/graph.json

Stage	Upstream	This fork	Purpose
Pre-EQ	none	LSP para_equalizer_x16_stereo, g_in 0.5	Warm/bass-forward voicing curve
Gain staging	n/a	EQ g_in padded to 0.5 (≈ −6 dB); the ~3 dB net loss restored at multiband_compressor.g_out 1.4, with band thresholds al_* scaled to match	Run the EQ + virtualbass cooler; recover level only after the compressor detectors, right before the limiter
EQ band 0	n/a	48 dB/oct high-pass @ 60 Hz (ft_0 2, s_0 3)	Kill everything below the woofer's usable range — −3 dB at 60 Hz, ≈ −30 dB by 40 Hz
Dynamics	single-band compressor_stereo	mb_compressor_stereo, 8 bands (xover 60/80/100/130/160/200/500 Hz), Modern mode	Per-band peak control that doesn't duck the mids on a bass beat; the woofer range is split finely (five bands 60–200 Hz) so the 60–80 and 80–100 Hz octaves can be clamped harder than the rest
Woofer FIR gain	1.0	1.15 (convLW / convRW)	Small trim only; the low-end drive now lives in the EQ bass bells (upstream, so it passes through the compressor + limiters instead of being an uncontrolled post-gain)
Bass EQ bells	n/a	31.5–200 Hz boosted ~+2.3 dB above the base warm tilt to offset the FIR gain reduction	Same woofer output level, but dynamically governed
Post-FIR limiters	none	wlim (−2 dB) after woofer FIR, tlim (−1 dB) after tweeter FIR	Hard ceiling on the actual driver signal — excursion / clip backstop

Everything else is byte-identical to upstream.

Design rationale

Goal: mild-volume music should sound warm and full; bass-heavy material should not distort the woofers or duck the midrange.

• Warm at low volume is handled two ways:

  • ell / elr (loud_comp_mono) is a true ISO-226 equal-loudness compensator. The sink volume slider feeds ell:volume / elr:volume (cubic, −65→0 dB), so bass/treble lift automatically increases as you turn the volume down and recedes as you turn it up.
  • The static EQ bells (31.5–125 Hz) add a fixed warmth tilt. Note LSP's g_* ports are linear amplitude, not dB — g_3 = 3.26 is ≈ +10 dB, offset by g_in 0.5 (≈ −6 dB). This is a hot bass shelf on purpose; the dynamics stages below exist to keep it safe when loud. The bass boost lives here rather than in the woofer FIR gain (kept near unity at 1.15) so it passes through the compressor and limiters and is dynamically controlled, instead of being a fixed post-everything gain that only wlim can catch.

• Gain staging. g_in on the EQ is padded to 0.5 so the boosted bands and virtualbass's saturation stages run with headroom rather than near/over 0 dBFS. The signal path is 32-bit float end-to-end (real clipping only happens at the ALSA sink), but a cooler operating point keeps virtualbass from being over-driven and keeps every plugin's internal detectors honest. The ~3 dB net level loss is put back at multiband_compressor.g_out (1.0 → 1.4) — after the band detectors, immediately before the main limiter — and the band thresholds al_* were scaled by the same factor so the compressor behaves exactly as before, just at a lower internal level.

• Bass beats don't distort is handled by multiband, not broadband, compression. A single-band compressor keyed off a kick drum applies gain reduction to the whole spectrum — vocals and mids pump on every beat, and loud bass can shut the woofers down across all frequencies. The 8-band multiband keeps each band responding only to its own energy. The seven bands below ~500 Hz — where over-excursion and boom live — are effectively limiters:

  Band	Range	cr	kn	al (≈ dB)	Note
  0	< 60 Hz	50	0.10	0.093 (−21)	catch band — mostly empty now that the EQ HPFs hard at 60 Hz
  1	60–80 Hz	50	0.06	0.078 (−22)	hardest clamp — lowest ceiling, highest ratio, widest (softest) knee so the 50:1 eases in
  2	80–100 Hz	30	0.12	0.095 (−20)	clamped harder than the rest, a step gentler than 60–80
  3	100–130 Hz	20	0.20	0.120 (−18)	midbass, as the old 90–200 band
  4	130–160 Hz	18	0.24	0.130 (−18)
  5	160–200 Hz	16	0.28	0.140 (−17)
  6	200–500 Hz	15	0.30	0.159 (−16)	low-mid body, as the old 200–500 band
  7	500 Hz+	5	0.40	0.284 (−11)	single gentle band above 500 Hz (was three: 500/1500/5000)

  (Every mb_compressor port is documented in mb-compressor-params.md.)

  Band 1 (60–80 Hz) and band 2 (80–100 Hz) carry the lowest ceilings and the highest ratios, so the two octaves that drive woofer excursion hardest are clamped ahead of everything else — their wide knees (kn down at 0.06 / 0.12, i.e. −24 / −18 dB) make that heavy ratio ramp in gradually rather than snap. Band 7 limits gently (cr 5) and does not move because of a kick drum. The EQ is left untouched, so anything below the thresholds — i.e. quiet listening — passes with its full warm tilt intact; only loud peaks are clamped.

• Woofers can't bottom out. The woofer FIR is near unity now (1.15), but loud_comp still adds bass gain after the main limiter, so the very last stage is unguarded. wlim / tlim are fastLookaheadLimiter instances placed after the convolvers, so they clamp the real signal the drivers see regardless of upstream gain. wlim at −2 dB is the mechanical-excursion backstop; tlim at −1 dB protects the tweeters and keeps the two paths time-aligned (equal lookahead latency — no comb filtering at the crossover).

• virtualbass (bankstown) synthesizes harmonics of the bass in the 60–150 Hz window, so the ear perceives low end the driver never has to physically produce — the psychoacoustic counterpart to the 60 Hz high-pass.

Tuning knobs

If the woofers still bottom out or anything distorts, in order of preference:

Where	Key	Now	Effect
wlim.control	limit	-2	Lower to -3 / -4 — hard woofer ceiling, dB
equalizer.control	g_1–g_5	1.82 / 2.48 / 3.26 / 2.61 / 1.75	The bass boost — lower all five proportionally for less low-end drive overall
convLW / convRW config	gain	1.15	FIR trim; leave it — adjust the EQ bells instead
multiband_compressor.control	al_0	0.093 (≈ −21 dB)	Lower = < 60 Hz band clamps sooner
multiband_compressor.control	al_1 / al_2	0.078 / 0.095	The 60–80 / 80–100 Hz ceilings — lower these two to pull the peak down further in those octaves
multiband_compressor.control	cr_1 / kn_1	50.0 / 0.06	60–80 Hz clamp — highest ratio, widest knee. Less aggressive: lower cr_1 toward 20. Sharper corner / less low-level squash: raise kn_1 toward 1.0
multiband_compressor.control	cr_2 / kn_2	30.0 / 0.12	80–100 Hz clamp — a step gentler than 60–80
multiband_compressor.control	cr_0	50.0	Already near brick-wall; leave it
multiband_compressor.control	at_0	4.0 ms	Lower toward ~3 ms if kick transients poke through (adds some LF harmonic distortion)

If the midrange sounds over-controlled / lifeless, raise al_3–al_7 (higher = those bands stay out of the way longer) or lower their ratios cr_3–cr_7 toward 2.0. Band 6 (200–500 Hz) at cr 15 reaches into low-mid body — if male vocals / snare sound boxy or thin, drop cr_6 back toward 8.

Gain staging. To run the EQ / virtualbass even cooler, lower equalizer.g_in further (e.g. 0.4, 0.35) and put the same factor back into multiband_compressor.g_out, then scale al_0–al_7 by that factor so the compressor keeps the same behaviour. If bass feels thinner after the pad, nudge virtualbass.amt up (1.0 → 1.2) rather than raising g_in back.

tlim.control limit (-1) is the tweeter ceiling — rarely needs touching, but keep tlim present even if you disable it (limit high), because it also holds the tweeter/woofer time alignment.

Install

./apply.sh

This copies graph.json to /usr/share/t2-linux-audio/15_1/graph.json (needs sudo) and restarts WirePlumber (systemctl --user restart wireplumber).

Requires the t2-linux-audio / t2-apple-audio-dsp package to already be installed (it provides the FIR .wav files, 51-t2-dsp.conf, and the mic.json graph).

Revert

sudo cp /path/to/t2-apple-audio-dsp/configs/15_1/graph.json \
        /usr/share/t2-linux-audio/15_1/graph.json
systemctl --user restart wireplumber

Note: a t2-linux-audio package update will overwrite the installed file and silently revert these changes — re-run ./apply.sh afterward.