Microphone Feedback With a Bluetooth Speaker
Microphone feedback with a Bluetooth speaker is your own amplified voice coming out of the speaker, back into the phone's microphone, and round again. Stop it with distance, by keeping the phone behind the speaker, and by cutting gain until the ring dies. Nothing else matters as much as those three. That rising howl is not a fault in the app, the phone or the speaker: it is a loop, and loops obey arithmetic you can win.
Why microphone feedback with a Bluetooth speaker happens
The effect is named after the Danish physicist Søren Absalon Larsen. The chain is simple: the mic picks up sound, the app makes it louder, the speaker plays it into the room, and some of that arrives back at the mic. If what comes back is as loud as what went in, the loop feeds itself and climbs until something clips.
Two conditions have to be met at once for a frequency to take off:
- Loop gain of 1 or more. The sound returning to the mic is as strong as what it originally heard.
- Phase alignment. The trip round the loop takes a whole number of cycles at that frequency, so each pass reinforces the last instead of cancelling it.
That is why it picks a pitch rather than a broadband roar, and it answers the usual question of why does my microphone squeal on one note. It lands wherever the room and the speaker are loudest, normally a response peak or a room mode. A small plastic Bluetooth speaker on a table usually squeals in the upper midrange, the region that carries speech intelligibility and presence. That is also where a small cabinet is loudest, because it has almost no output down in the range where the voice actually puts most of its energy.
You can watch the mechanism at any level you like in the browser mic monitor. Raise the gain with the phone near a speaker and the loop announces itself in a second or two. Pull it back and it collapses just as fast. Feedback is not a threshold you cross once, it is a balance you hold.
The four dials that decide whether it squeals
Everything you can do about feedback moves one of four things, ranked by leverage in a normal room.
| Dial | What it does to the loop | How much it buys you |
|---|---|---|
| Distance between speaker and mic | Direct sound reaching the mic falls with distance | Most. Doubling the distance drops direct sound by about 6 dB |
| Gain | Sets loop gain directly | A lot, but it is the one that costs you volume |
| Placement and aim | Keeps the mic out of the speaker's beam | Real, and free |
| Tone shaping | Lowers gain only where the loop is strongest | A few dB. Useful last, never first |
The 6 dB figure is the inverse square law for a source radiating into free space. Indoors, reflections fill some of it back in, but the direction always holds, and moving the phone two metres further away beats any setting in any app.
If your first move when the mic is too quiet is to raise the gain, you will meet the howl every time. Raise gain last. Close the distance to your mouth first: that raises the wanted signal without raising the loop at all.
Placement and distance rules that work
- Get behind the speaker. A speaker beams its higher frequencies forward, so standing behind it, or beside and slightly back, keeps the mic out of the range that feeds back. Low frequencies wrap around a small cabinet almost equally in every direction, but those are rarely what howls.
- Aim the speaker at the audience, not at you.
- Hold the phone close to your mouth. Ten centimetres instead of arm's length is a large increase in wanted signal, so you need far less gain.
- Keep the phone off the same hard surface as the speaker. A shared table couples vibration into the phone's body, giving the loop a second path that has nothing to do with the air.
- Do not cup your hand round the bottom of the phone. Cupping reflects sound into the mic and lifts the frequencies that are about to take off.
An iPhone's microphones are broadly omnidirectional for this purpose. iOS does apply some beamforming in certain modes, but there is no cardioid null you can deliberately aim away from the speaker the way a stage vocalist aims a handheld mic, so distance is your substitute for that trick. There is more on setup in the guide to routing an iPhone microphone into a Bluetooth speaker.
The built-in speaker is the worst possible case
If an app squeals the instant you press start and nothing is connected, the audio is going to the phone's own speaker. Mic and speaker are then centimetres apart in one shell, sharing an air path and a mechanical one, and loop gain is enormous at any usable setting. That is not a setup to tune, it is the physics of putting a microphone on top of a loudspeaker.
The Bluetooth mic: Mic to Speaker app handles it by reading the live output route. When that route is the built-in speaker or the earpiece receiver, it caps gain at 0.6 of full scale and warns you instead of letting you walk into the howl. Once a Bluetooth speaker, AirPods, a wired connection or AirPlay takes the route, the cap lifts and the full 0 to 200% range is available.
The app does not pair anything. No iOS app can, because Apple exposes no public API for it. You pair the speaker in iOS Settings, and the app plays into whatever route iOS has already chosen. If the speaker is not paired and selected, you are amplifying into the phone's own speaker, the one arrangement guaranteed to squeal.
Bluetooth delay changes what feedback sounds like
A wired PA squeals almost instantly because the loop is short. Over Bluetooth the trip is longer. Hollyland quotes A2DP end to end at roughly 100 to 300 ms, and AAC, the codec an iPhone uses, at roughly 100 to 200 ms. That delay does not prevent feedback, it changes its character: instead of an immediate tone you hear a stack of repeats getting louder each time, like a slap echo winding up, before it settles on a pitch.
The long round trip also moves which frequencies satisfy the phase condition, so the same room can howl at a different pitch over Bluetooth than it does with a cable. For the real number in your setup, the acoustic loopback latency test measures the round trip with a chirp and cross-correlation, and the delay guide explains which part of it no app can remove. Anyone working out how to stop karaoke microphone feedback should read the karaoke delay guide too, because the fix for delay is not the fix for feedback and confusing the two wastes an evening.
Tone shaping, and the presets that make it worse
Once distance and placement are settled, tone shaping buys a last few dB. The echo reduction control inserts a high-pass filter at 50 + 110 × level Hz with a Q of 0.8, stripping low end out of the loop, and switches the iOS session to voiceChat mode, which brings in the system's own voice-processing path. That helps with a phone-call-shaped echo. It will not save you from a full howl at high gain.
Voice presets cut both ways. The ones that add high-frequency energy bring feedback on sooner, because they raise gain where the loop is already strongest: Bright is a +10 dB high shelf above 3.5 kHz plus a +4 dB peak at 6 kHz, and Megaphone is a +12 dB peak at 2.5 kHz with a Q of 3. Deep and Warm are the safe ones near a live speaker, since both roll the top off. Compare each against its real frequency response on the voice effects page first.
If a howl starts mid-sentence, do not lunge for the volume. Cover the bottom of the phone with your palm for a second, or point it at your chest. That breaks the loop immediately and buys you time to walk further from the speaker.
If it still squeals, work through this order
- Confirm the audio is going to the Bluetooth speaker, not the phone. Play any music track: the route that plays it is normally the route your mic will use as well, although an app can configure its audio session in a way that lands somewhere else.
- Drop the gain to the lowest level where you are audible, then stop touching it.
- Walk behind the speaker. If the ring stops, the problem was placement, not settings.
- Switch to a flat preset. If Bright or Megaphone was on, that alone can be the difference.
- Raise echo reduction one step at a time and listen for a thinner but stable sound.
- Check the speaker with a sweep and channel test. A box with a sharp resonant peak feeds back there long before a flat one does.
- Still fighting it: monitor on headphones so your ears leave the loop, or accept a lower volume. There is a limit and the room sets it.
Before an event, run the chain once in the real room with the speaker in its real position. The pre-party microphone checklist is built for that, and the guide to announcements covers moving around a venue while the distance changes under you. If you were hoping to use Live Listen instead, read why that route does not do what people expect.
The quickest way to learn where your limit sits is to provoke the loop on purpose. Open the live mic monitor, stand where you plan to stand, and raise the gain until it just begins to ring. That number, minus a margin, is your working level for the room. The rest of all the guides covers the delay you notice once the howling stops.
Hear it for yourself, without installing anything
Hear your own microphone through your speakers or headphones, with the app’s gain range and its eight voice presets.