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How digital room correction works in Clearroom

By Rupendar Venkatesh

I mix for a living in a room in Chennai, and I corrected it by hand in REW. Clearroom is that process turned into an app. This post explains the decisions behind it: why the correction is a FIR filter, why there is an intensity fader, what stops it from doing damage, and why it asks for three mic positions and not twenty-four. I will not publish every constant. I will tell you what each part does and why.

One filter that draws the whole curve

Most room EQ is a set of parametric bands, an IIR filter. A handful of bells, each with a frequency, a gain and a Q. It works and it is cheap. But a room's response is not a handful of bells, so the software fits bands to the curve, and whatever the bands cannot draw is left behind.

Clearroom builds a FIR filter instead. Thousands of taps draw the correction the measurement asks for, point by point. There is no fitting step and no budget of bands.

I want to be straight about phase, because this is where FIR gets oversold. Our filter is minimum phase. It has the same kind of phase behaviour as an analogue EQ or a parametric drawing the same curve. It is not linear phase and it does not fix timing. What it buys is the shape. It adds no latency at 48 kHz, so you can leave it on while you track. Its length is picked from how low your speakers actually go, between 2,048 and 8,192 taps.

Narrow where the room is narrow, broad where it is not

A room is two different problems. In the bass it is modes: tall, narrow peaks at fixed frequencies that ring. Up top it is reflections: fine ripple that changes when you move your head a few centimetres. Correct both at one resolution and you lose either way. Too broad and the modes survive. Too fine and you are correcting ripple that only exists at the mic tip.

Clearroom reads the measurement through a frequency-dependent window, the idea REW users know as FDW. The window is long in the bass, so the engine sees a mode at its true width and can cut it narrowly. It is short in the treble, so the fine ripple averages out and the correction there stays broad and smooth. The filter itself gets the same treatment: its tail is tapered so the bass correction keeps its length while the high-frequency detail does not ring. Above 16 kHz the correction fades out, and by 19 kHz it is gone.

We checked the engine against the corrections I used to build by hand in REW. Across 40 Hz to 16 kHz it lands within about 1.6 dB of them on average.

Why there is an intensity fader

No single setting is right for every room or every engineer. A room with one bad mode needs one cut. A room with a lumpy midrange needs much more. Even in the same room, how much correction sounds right is taste: some people want the room gone, others want the worst mode tamed and their monitors left alone. With a filter of thousands of taps there is no Q or depth for you to find by hand, so a fixed setting would force somebody to guess.

So Clearroom builds ten levels, and you pick by ear. A few of them, with the most each can cut and boost:

  • Level 1, Subtle: cuts up to 2 dB, boosts up to 1 dB.
  • Level 3, Light: cuts up to 4 dB, boosts up to 2 dB.
  • Level 5, Balanced (the default): cuts up to 8 dB, boosts up to 4 dB.
  • Level 7, Strong: cuts up to 12 dB, boosts up to 5 dB.
  • Level 10, Full: cuts up to 20 dB, boosts up to 6 dB.

The part that matters is how the levels relate. The engine decides once, per channel, what in your room deserves correcting, and that decision knows nothing about the fader. The ten levels are that one correction with different limits. So turning up never removes correction anywhere; it only lets more of the same shape through. Every level is loudness matched, so a higher level cannot win just by being louder, and switching between them is seamless.

Why not a dry/wet knob

The obvious way to make correction adjustable is to blend the corrected signal with the dry one. It does not do what it promises. Any EQ moves phase along with level, and the correction does too. Blend it with the dry signal and the two partly cancel wherever that phase has moved. What comes out is not a smaller copy of the designed curve: cuts and boosts shrink by different amounts, and the result is a filter nobody designed or checked. If the correction has latency and the dry path does not, the blend is plain comb filtering.

Clearroom switches between ten complete filters instead. Each is designed and checked to stand on its own, so every position of the fader is a real filter.

With great power: the guardrails

A filter that can draw any curve can also draw a harmful one. Most of the engineering in Clearroom is about what it refuses to do.

  • It never boosts a null. A deep dip in the bass is usually a cancellation: sound from the speaker meeting its own reflection out of phase. Boosting it feeds both waves, so the dip stays and your amplifier and woofer pay for it. The engine tells a cancellation apart from a broad, honest lack of bass, and leaves the cancellation alone at every level. It also treats the edges of each null as untrusted, because those edges move when the mic moves.
  • Bass boost is capped hardest. Below 200 Hz the most it will ever add is 4.5 dB, at full intensity, rising to 6 dB by 400 Hz. Lower levels scale that down. Cuts in the bass are where the real work is, and cuts are safe.
  • It respects your speakers. The engine finds where your speakers roll off in the bass and rolls the target off there too, so it never asks a small woofer for bass it cannot make. Below 30 Hz the correction fades out altogether.
  • It leaves the top alone. Above 16 kHz the correction fades, and by 19 kHz it is gone. That band is air, taste and the mic's own limits, not your room.
  • It cannot clip you. The filter pair is scaled so its loudest frequency stays under 0 dBFS, and left and right are level matched to each other.
  • The comparison is fair. Bypass is loudness matched, so switching the correction off does not make things quieter or louder. You hear the tone change, nothing else.
  • It refuses a bad measurement. More on that below.

Three positions, not twenty-four

The app first centres the mic. It pings left and right and tells you which way to slide it until it sits within 2 cm of the exact middle between your speakers. Then it measures three spots: that centre, 10 cm to the left, and 10 cm to the right. Each speaker is swept at each spot, and the three are averaged into one response per speaker.

Some systems ask for a dozen or two dozen positions spread over a sofa. That makes sense for a living room with several listeners. A mix position has one head. Below 200 Hz a wavelength is 1.7 m or longer, so the bass barely changes across the 20 cm your head moves in, and three spots across that span sample it well. Spread the mic over a metre or two and you average in places you never sit. The modal peaks that are real at your chair get diluted, and the correction under-corrects exactly where you listen. In the treble, where things do change over centimetres, the frequency-dependent window already keeps the correction broad, so more points buy little.

What the three spots do give you: a peak or dip that only one spot sees is diluted by the other two, and the correction holds across the chair instead of at one mic tip.

The score comes before the correction

A correction is only as good as the measurement under it. So before Clearroom corrects anything, it scores the session: how far the sweep stood above the room's noise, whether left and right agree, whether the timing is clean, whether every sweep is there, and whether the mic has a calibration file. It checks the room is quiet before each sweep, and it stops a take that clips.

The score comes out as one of four words. Full is a clean measurement. Tone is good enough to fix the tone, which is what most good rooms score and what 1.0 corrects. Limited is usable, but something was noisy or inconsistent. Redo means the app will not continue: retake one position, and the other two are kept. I would rather tell you your measurement is bad than hand you a confident filter built on it.

What 1.0 does not do

Clearroom 1.0 corrects magnitude, the tonal balance of your room. It does not correct phase, manage a subwoofer, or fix a room that needs treatment. If one speaker's timing or polarity is off, it offers to line them up. Treatment first, then correction, is still the right order.

Clearroom is room and headphone calibration for macOS. The user manual walks through a measurement step by step, and there is a 14-day trial, no card needed.