SpectraSense

Free. SpectraSense opens with a free Squaresmile account — on a phone, tablet or laptop.

SpectraSense listens to a microphone or an audio interface and draws the spectrum from 20 Hz to 20 kHz. Its job is ringing out: while you push a wedge, an in-ear mix or the PA up to the edge, it watches for the narrow, steady tone that feedback is and flags it while it is still building — with the frequency, the note, the nearest graphic-EQ band and a suggested cut. Every ring it finds goes into the Rings list you can copy as text. Everything runs in the browser; no audio is recorded or sent anywhere.

SpectraSense is a ring-out assistant, not a feedback suppressor: it never touches your desk, and it is made for the ring-out before the show, not for running under music during it.

Ring out, Level, Tune

SpectraSense has three views: Ring out, Level and Tune. Ring out is the default view and everything on this page. The other two views and the Settings page make SpectraSense a small measurement suite:

• Settings — pick the input and the mic channel your measurement mic is on, load its mic curve (a miniDSP UMIK file, another measurement app's file or plain two-column text), calibrate the level with a calibrator or a reference meter, and play a test tone (pink or white noise, a sine, a log sweep) at a safe level.
• Calibrating the mic — every way to calibrate, step by step: a calibrator, a sound level meter, a mic you already calibrated, another measurement app, or the master matched to the mic the way a dual-channel analyzer does it.
• Level — an SPL meter (LAF, LAeq, LCeq, LCpeak, LAFmax) with the live spectrum, with the audience rules of DIN 15905-5, the WHO and others, a level log whose report you can print on A4 with a SHA-256, and Show mode for FOH.
• Tune — the transfer function: magnitude, phase and coherence of a system against a source (the master or the test tone), with a delay finder and snapshots to compare. It works on a phone too; an audio interface with a master is best.

Switching views never restarts the mic.

On a phone

Held upright (up to 600 px wide), a phone shows a tab bar at the bottom: Ring out · Level · Tune · More. More opens Settings, Saved and Help — Help opens the help page of the view you are on.

Above the tab bar, in reach of your thumb, sits the action bar: ▶ Start / ■ Stop and the view's main action:

• Ring out — Freeze and + Next mix.
• Level — ● Start log / ■ Stop log.
• Tune — ▶ Measure.

The status line on top keeps the listening dot, the mic and master bars (one under the other), the calibration chip, the ring chip and ⚙. A phone held sideways, a tablet and a computer keep the views as tabs on top and Start in the status bar. Show mode hides both bars.

With a phone's built-in mic the levels are indicative; SpectraSense is not a replacement for a sound level meter when a record is required. Calibration, level logs and reports stay in this browser — none of it is synced to your account. Snapshots and My mics sync when you are signed in.

Show mode: the spectrum and the levels at once

For the show itself, open Level: it shows Ring out's spectrum (a ring is still marked and lands in the Rings list) with the meters, and Show mode next to the meters puts it all fullscreen for FOH — the spectrum, the same meters, the clock, the rule's reading and verdict, and the log with its ● Start log button. Exit show mode or Esc leaves it. On an iPhone, which has no fullscreen for a page, Show mode covers the page.

Who it is for

• Monitor engineers ringing out wedges and in-ears.
• FOH engineers bringing a system up in a new room.
• Bands without a tech who want to know which frequency is howling.

Getting started: what are you doing today?

The first time you open SpectraSense it asks one question — What are you doing today? — in a short sheet with a stepper (Task · Mic · Ready). Your answer sets up the screen itself, not a tour:

• Ring out monitors / PA — the Ring out view with rings drawn as Plot + list, and the Rings list open beside the spectrum (on a computer screen; on a phone or tablet it waits behind its button).
• Watch the level — the Level view with the meters of the country rule (chosen by the page's language — German: DIN 15905-5 — otherwise the WHO's 100 dB over 15 minutes; any other one under Rule), and the log schedule in view when you are done. A third step, Calibrate with, asks how you know the level: Calibrator 94 or 114 dB, Reference meter __ dB, Known value __ dB (saved at once as the level offset in Settings → Calibration, for this input) or Skip — not calibrated. Calibrator and reference meter open Settings at the level offset with your choice filled in — you press Measure there.
• Tune the PA — the Tune view, with the master or a test tone. The mic step asks which channel carries the master (the console's LR or a matrix). An input with one channel only has no master: Tune then uses the test tone as its source.
• Just a spectrum — Ring out with the rings off and no lists: a plain spectrum. Skip (or Esc) on the first step means this.

The Mic step says first what happens to the sound — Runs on this device · Nothing is uploaded · You start and stop — and only then, with ▶ Start, asks the browser for the microphone. The input picker sits right there once it is allowed. No microphone, or access refused? Use the demo signal: SpectraSense makes a signal in the browser — noise, a few notes and a ring at 2.5 kHz that rises slowly — so you can see how a ring is found. The status bar's level says Demo signal until you start a real input with Start.

Back and Skip work on every step. To change the task later, open Settings → This device — the task is named there — and press Change task. Picking Watch the level again asks before it puts back meters you changed (Also put the Level meters back to the factory set? — Use factory meters). A link to a view (#level, #tune) always opens that view (an old #show link opens Level, an old #transfer link opens Tune); without one, SpectraSense opens on your task's view. The task is a preference and syncs with your account like the other settings. If you used SpectraSense before the question existed, you see a small New: pick your task chip on the status bar once instead.

The status bar

One bar above every view says what is running:

• ▶ Start / ■ Stop — the one control that opens and closes the input — and Listening, Off or Paused (the phone took the mic: ▶ Resume).
• Mic and Master levels, one under the other on the same dB FS scale, each with a peak tick and a red tip while it overloads. Master silent when the desk feed carries nothing while the mic hears sound; your name for the master (Settings → Input); Clocks in sync / drifting when the master comes from a second device. A tap on the levels opens Settings → Input.
• Δ x.x dB · Match — the difference between mic and master; Match sets the master's trim, then it reads ✓ Matched.
• The calibration: ✓ Calibrated 94 dB · 14:02, or Estimated · Calibrate while the levels are estimated dB SPL — a tap opens Settings → Calibration.
• A chip for whatever runs: a ring the detector holds (on every view but Ring out — a tap goes there), the test tone with its ■, the log (● Log 0:12:40 or Log starts 20:00), Screen on.
• ⚙ (Settings) — a page of its own; ← Back returns to the view, still listening. The bar's ▶ Start is the only Start: there is none inside Settings or Level.

On an upright phone the bar is one line: the dot, two thin bars, the calibration word, the ring, ⚙. Start and Stop sit in the action bar at the bottom (see On a phone).

Start

1. Tap ▶ Start and allow the microphone.
2. Pick the input under Settings → Input: the device's own microphone, a measurement mic or a USB interface fed from the desk. The browser names your inputs only once the microphone is allowed — until then the list says Start first to list your inputs.
3. Bring the system up. When a tone starts to ring, it is drawn red on the spectrum, and the ring's readout shows the frequency, the note (e.g. D7 −7¢), the nearest GEQ band, a suggested cut, and whether the ring is growing, ringing or already gone.
4. Cut that frequency on the mix's EQ — and bring the fader up again for the next one.

The phone, the tablet and the laptop all work; the screen stays awake while SpectraSense is listening. On a phone the ring readout sits at the top, above the display (held sideways, it floats over the display), with the frequency in large type, so you can read it at arm's length with the other hand on a fader.

Ringing out a wedge, step by step

1. Music off, unused mics muted. Only the mic and the mix you are ringing out stay open.
2. Hold the phone where the singer's mouth will be — at the mic, not against the wedge — with the bottom of the phone (where its microphone is) pointing at the wedge. A phone microphone overloads long before a wedge does.
3. Type the label for this mix — Wedge 3, IEM 2, Sidefill — so the rings you find carry it.
4. Push the send slowly until it just starts to ring. SpectraSense flags it; pull the fader back.
5. Cut as suggested on that mix's output EQ, then go up again for the next ring. If the same frequency comes back, cut it a little deeper.
6. Stop when several frequencies ring at once, back the fader off a few dB, and move on to the next mix.
7. Copy as text when you are done and paste the list into a message or the advance sheet for the next show in that room.

Low hoots read smaller on a phone. Phone microphones roll off the low end, so a hoot around 100–300 Hz can look weaker on the display than it sounds in the room. Trust your ears there — or use a measurement mic or a USB interface.

The suggested cut

Each ring comes with a starting point, the way most engineers ring out. Tell SpectraSense which EQ you cut with under Options → Cut advice for: a Graphic EQ (31-band) (the default), a Parametric EQ, or Both.

• Start at −3 dB. On a graphic EQ the readout says Pull the fader −3 dB and names the fader — no Q: a graphic's faders have a fixed width (about 1/3 octave), there is nothing to set. On a parametric EQ it says how narrow: about 1/10 octave (Q ≈ 14), the width feedback filters use. Cut only as much as it takes; deep, wide cuts take the voice with them.
• Deeper if it returns. When the same frequency rings again, the suggestion goes a further 3 dB down, to −9 dB at most. If it still rings after that, the mic or the wedge is too loud for the room — turn it down or move it.
• On a graphic EQ the readout names the nearest band. When the ring sits between two bands, it names the neighbor too — Pull both faders a little rather than one a lot. With Parametric the band is left out; with Both you get the band and the notch.

It is a suggestion, not a setting: SpectraSense never changes anything on your desk.

The Rings list

Every ring SpectraSense confirms is kept in the Rings list: the time, the frequency, the note, the GEQ band, the suggested cut and the label of the mix you were ringing out (Wedge or mix). The live ring fades a few seconds after it stops, and the readout then keeps showing the last ring ("Last ring") until another one comes; the list keeps every ring, so you can pull the fader first and read afterward.

• Every confirmed ring stays — through Stop, a pause (the phone took the mic), Freeze and a reload — until you clear it (Clear all).
• Copy as text puts the whole list on the clipboard, one ring per line.
• Rings in the view bar opens the list; the number next to it is how many rings it holds. On a computer it opens over the right edge of the spectrum; on a phone or tablet it slides up from the bottom. Either way the spectrum and the ring readout stay exactly where they are. ✕, Esc (or, on a phone, a tap above the sheet) closes it. On a computer SpectraSense remembers whether it was open.
• Saved opens the same way, next to it — one of the two at a time.
• The list is kept on this device only, in the browser's storage. It is not synced to your account and never uploaded.

Was it feedback? Yes / No

Under each ring the list asks Feedback? — tap Yes or No if you know. Nothing changes in the list; the answer is kept together with the detector's numbers for that ring (how far it stood over the floor, how it rose and held, how steady its pitch was — no sound, no recording) so that a later version can be checked against real rings, not just ours. A second tap changes the answer.

The answers stay on this device, are never synced or sent, and at most 2,000 are kept (the oldest go first). Settings → This device → Your answers: was it feedback? shows how many there are, Export answers (JSON) saves them as a file you can send us if you want to, and Delete answers (tap twice) removes them all.

Draw rings: plot and list, or neither

Draw rings in Spectrum options (under Detection) decides what you see of a ring:

• Plot + list (the default) — the ring drawn red on the spectrum and its readout, the answer cards, beside or under it (on a phone, above it).
• Plot only — the ring on the spectrum, no readout. The page ends under the spectrum.
• Off — nothing drawn: a plain spectrum. The rings are gone from the waterfall and Level's spectrum too.

It only changes what is drawn. SpectraSense keeps listening for rings in every setting, so each ring still lands in the Rings list — open it to see what rang while drawing was off. The choice follows your account to your other devices.

Rings: Below | Sidebar in Ring out's view bar puts the readout under the spectrum (the default, so the spectrum keeps its full width) or in a sidebar to its right, on a wide screen. SpectraSense remembers the choice and syncs it with your account. On a phone or tablet the two are always stacked.

How it decides something is feedback

Research on howling detection (van Waterschoot & Moonen, Fifty years of acoustic feedback control, 2011) showed that no single test is reliable: "it lasts" alone is wrong more than half the time. SpectraSense flags a frequency only when three things are true at once for 0.3 seconds:

• It is a narrow peak standing clearly above the spectrum around it (the Sensitivity setting, 14 dB by default).
• It is a lone tone. A musical note has overtones at 2×, 3×, 4× its frequency; feedback does not. Notes are ignored (you can switch this off).
• It is steady or growing, not decaying like a piano note or wobbling like a vibrato.

And it looks at how the peak appeared. Feedback builds up: it leaves the noise as a straight line in dB, rising by about the same amount every fraction of a second. A played or sung note appears with an attack — in the analyzer it jumps up within about a sixth of a second and then holds or dies away. A peak that appeared like that must stay a lone, steady tone for 1.5 seconds instead of 0.3 before it is flagged, unless it then starts to grow (a note that set off a ring). In our tests with synthetic piano, singing and a synth lead this cut the false rings by about nine in ten, and every ring that builds up was flagged as fast as before.

Two things it cannot tell apart: a ring that jumps to full level within that sixth of a second (it is then flagged after 1.5 s instead of 0.3 s — you will have heard it anyway), and a pure tone held longer than 1.5 s.

Ignore musical notes in Spectrum options is on by default. Switch it off to see every narrow peak: both note tests — the overtones and the attack — are then skipped.

Pre-console playback as the master. If the master — another channel of your interface — carries the playback itself — a laptop or player patched straight in, before the console — tick Master is pre-console playback in Settings. A peak the playback holds just as clearly is then music, not a ring, unless it grows in the room while it does not grow in the playback. Leave it off when the master is the console's output: feedback runs through the console, so it is in there too, and SpectraSense could not tell them apart.

Once a ring is confirmed it stays flagged for as long as it is there. After it stops, it fades for a few seconds (Ring stays, four by default), so you have time to read it.

The rest of the screen

• Hold — a thin line that keeps the loudest level each frequency reached for a few seconds, so a short ring between two glances still shows. Clear resets it.
• Save snapshot — a 5-second average of the spectrum, kept in Saved: compare before and after a cut, or wedge A with wedge B (see Snapshots).
• Freeze (or the space bar) — stops the display so you can read it; tap or drag on the spectrum to read the frequency, level and note under your finger.
• Waterfall — the same frequencies across, time downward, to watch a ring build up as a fader goes up.

Spectrum options

Options in Ring out's view bar (and in Level's toolbar) opens Spectrum options: one side panel, with the spectrum still in view beside it (on a phone a full-height sheet). Close or Esc closes it. Ring out and Level share one look; each view keeps only its own bands/line choice — Ring out the FFT line by default, Level 1/3-octave bars. Everything here is saved and follows your account to other devices; Reset to defaults puts it all back.

• FFT size — larger is finer, especially low down (up to 32768).
• Bands — FFT (every bin) shows every frequency bin, the finest view and the best for finding a ring: it is the line for the view you opened the panel from. Or sum the bins into fractional-octave bands per IEC 61260 — 1/1, 1/3, 1/6, 1/12, 1/24 or 1/48 octave, the band width both views share — the way a console's RTA shows them: pink noise reads flat, and once the input is calibrated (and its mic channel picked in Settings) the bands read dB SPL. As bars, low bands narrower than the FFT can resolve are drawn dimmer: a tone that narrow leaks into the neighboring bands, so a single tone there is not to be trusted (broadband music and pink noise still read right). A note over the spectrum says where it ends — "Faint bars below 280 Hz" — and a tap on it opens FFT size: each doubling moves that edge down an octave, wider bands move it down too.
• Draw as — Bars or Line.
• Weighting — Z (flat), A or C on the spectrum, the same weighting as the level meters.
• Averaging — Off (every frame), Fast (125 ms) and Slow (1 s) as on a sound level meter, Long (2 or 4 s) for a calm show RTA, or Until Clear — it averages everything since you last pressed Clear. Averaging is of power, never of dB.
• Hold time (how long the Hold line keeps a level — or Until Clear) and Hold fall (how fast it drops after that).
• dB floor / dB ceiling — the display's range. For the floor, typing 90 is enough: it means −90 dB. The floor runs from −140 to −20 dB, the ceiling from −60 to +20 dB, at least 20 dB apart; a value outside is kept at the nearest allowed one.
• Console tilt (+3 dB/oct) — tilts the FFT view by +3 dB per octave so pink noise and most music look flat, the way a third-octave analyzer on a console shows them. Without it, a normal mix looks bass-heavy here, because every frequency slice is equally narrow. The tap readout keeps the true level, and detection is not affected. Bands do not need it — they read flat by themselves.
• Start at 40 Hz — hides the rumble below 40 Hz that a phone microphone picks up once voice processing is off.
• Waterfall length and Waterfall range — the display's dB range, or 40, 60 or 80 dB under the loudest level, following it.
• Master (only when a master is set in Settings) — the master's spectrum as a dashed line under the mic's, on Ring out and Level.
• Detection: Draw rings (see above), Sensitivity, Ignore musical notes, A4 pitch (Hz) (440 Hz by default), Cut advice for and Ring stays (how long a ring stays drawn after it stops). Detection always reads its own, raw analyzer — none of the display settings change what is flagged.

The input (in Settings) is the exception: each device remembers its own choice, so picking the measurement mic on the laptop does not change what your phone listens to.

Good to know

• Feedback is detected from 60 Hz to 16 kHz; the display shows more.
• On an interface with several inputs, SpectraSense hears the first one until you pick another mic channel in Settings. How many channels a browser passes differs: Chrome and Edge pass them all, Safari and Firefox often only one or two.
• The FFT view is in dB FS — relative to the input's full scale; a single FFT bin is no sound level. In bands, calibrated, the axis reads dB SPL; for the meters and the log use the Level view.
• A held pure tone — a test oscillator, a whistle, a flute or a synth pad with few overtones — looks exactly like steady feedback and will be flagged once it is held longer than 1.5 seconds. That is the nature of the job: rings are for the ring-out — during the show (on Level or in Show mode) a held note can be flagged too.
• No signal means nothing above −70 dBFS for three seconds: check the input selection and its gain. Input clipping means the input is overloaded: move the phone further from the wedge or turn the gain down — a clipped input adds overtones and false peaks of its own. The line says which channel (Mic (ch 1), Master (ch 2)), and Check level opens Settings → Input level: the meter, the trim and the way to your system's input volume. Low input (under −60 dB FS for three seconds) gets the same button.
• If the browser insists on voice processing (echo cancellation or automatic gain), SpectraSense says so — readings can be off; a USB interface usually avoids it.