SpectraSense

SpectraSense listens to a microphone or any audio interface input and draws the spectrum from 20 Hz to 20 kHz. Its job is feedback hunting: during a line check, a ring-out or soundcheck it watches for the narrow, steady tone that feedback is, and flags it before it runs away — with the frequency, the note and the nearest graphic-EQ band. Everything runs in the browser; no audio is recorded or sent anywhere.

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.

Start

1. Tap Start listening and allow the microphone.
2. Pick the input under Input: the device's own microphone, a measurement mic or a USB interface fed from the desk.
3. Bring the system up. When a tone starts to ring, a red marker appears on the spectrum and a card below it shows the frequency, the note (e.g. D7 −7¢), the nearest GEQ band, how far it stands over its surroundings, and whether it is growing, ringing or already gone.
4. Cut that frequency on a parametric EQ — or the GEQ fader the card names.

The phone, the tablet and the laptop all work; the screen stays awake while SpectraSense is listening.

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.

A marker stays on screen for four seconds after the ring stops, fading, so you have time to read it.

The rest of the screen

• Peak 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 hold resets it.
• Freeze (or the space bar) — stops the display so you can read it; tap or drag on the spectrum for the exact frequency, level and note at any point.
• Waterfall — the same frequencies across, time downwards, to watch a ring build up as a fader goes up.
• How fast it reacts (in Settings) — four controls for how fast things come and go: Smoothing (how fast the live line follows), Peak hold (how long the peak line keeps a level), Peak fall (how fast it drops after that) and Feedback marker stays (how long a red marker lingers after the ring stops).
• Console analyser look (in Settings) — tilts the display by +3 dB per octave so pink noise and most music look flat, the way a third-octave analyser on a console shows them. Without it, a normal mix looks bass-heavy here, because every frequency slice is equally narrow.
• Start at 40 Hz (in Settings) — hides the rumble below 40 Hz that a phone microphone picks up once voice processing is off.
• Settings — also Sensitivity, the note check, the A4 reference (440 Hz by default), FFT size, the dB range and waterfall length. They are saved and follow your account to other devices.

Good to know

• Levels are in dBFS — relative to the input's full scale, not sound pressure. Use RoomSense for SPL and room measurements.
• A held pure tone — a test oscillator, a whistle — looks exactly like steady feedback and will be flagged. That is the nature of the job.
• No signal means nothing above −70 dBFS for three seconds: check the input selection and its gain. Input clipping means turn the gain down; a clipped input creates false peaks.
• 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.