Live Monitor

IAudioMonitor sends the microphone straight to the current audio output in near real time, like a PA system or megaphone. A typical use is talking into the phone and hearing it from a Bluetooth speaker. IAudioSource gives you PCM to process yourself. The monitor instead connects input to output inside the platform’s own audio engine, so nothing goes through your code.

Available on iOS, Mac Catalyst, macOS, Android and Linux. IAudio.Monitor throws PlatformNotSupportedException on Windows and in the browser.

Starting and stopping

using Shiny;
using Shiny.Audio;

public partial class MicViewModel(IAudio audio) : ObservableObject
{
    [ObservableProperty] double level;   // bind to a level meter

    public async Task Toggle()
    {
        var monitor = audio.Monitor;
        if (monitor.IsMonitoring)
        {
            await monitor.Stop();
            return;
        }

        if (await monitor.RequestAccess() != AccessState.Available)
            return;

        monitor.InputLevelChanged += (_, l) =>
            MainThread.BeginInvokeOnMainThread(() => Level = l);

        // No Processing, so it can reach a Bluetooth A2DP speaker (see below)
        await monitor.Start(new AudioMonitorOptions { Gain = 1.0 });
    }
}

Output goes to the current route: the built-in speaker, a wired output, or a connected Bluetooth speaker (A2DP) while the phone’s own mic captures. You can change Gain (0.0–1.0) while it runs. InputLevelChanged reports a normalized level for a meter.

AudioMonitorOptions	Default
Gain	1.0	Initial output level, from 0.0 (mute) to 1.0 (unity)
Processing	null	Echo cancellation, noise suppression and gain control. Read the iOS caution below first
DuckOtherAudio	false	Keep other audio, such as music, playing at a lower volume instead of interrupting it, so you can talk over it. iOS adds DuckOthers to the session and Android takes transient MayDuck audio focus
InputDevice	null	Preferred mic, from IAudioDevices.GetInputs()
OutputDevice	null	Preferred output, from IAudioDevices.GetOutputs()

Member
RequestAccess()	Asks for the mic permission
Start(options, ct) / Stop()	Starts the monitor, or stops it and restores the audio session
IsMonitoring	true while running
Gain	Output level, adjustable live
SetInputDevice(device)	Switches the mic, live. null uses the OS default
SetOutputDevice(device)	Switches the output, live, where the platform allows. See Devices & Routes
InputLevelChanged	Input level, 0.0–1.0

On iOS you get a Bluetooth speaker or echo cancellation, not both

Turning on AudioProcessingOptions (echo cancellation) engages iOS’s voice-processing unit, which forces Bluetooth onto the low-quality HFP call profile. A Bluetooth speaker (A2DP, no mic) can’t use HFP, so the audio falls back to the phone. Leave processing off to reach a Bluetooth speaker. Only turn it on when the output is the phone’s own speaker and feedback is a problem.

Feedback

When the output is the phone speaker, the mic can hear it and howl. Keep some distance, lower Gain, or turn on echo cancellation. Always stop the monitor when the page is dismissed and when the app goes to the background.

AirPlay (HomePod, Apple TV) can’t carry a live mic. iOS only allows AirPlay for playback, not while recording. Use Bluetooth for a wireless live PA.

How it works

Platform	Implementation
iOS / Mac Catalyst	AVAudioEngine (input node → main mixer → output) in a PlayAndRecord session. It doesn’t use DefaultToSpeaker, so it follows a Bluetooth or wired route, and it uses Default mode unless processing is requested. The engine rebuilds on route changes, so monitoring moves to a newly connected output. The audio session is saved at start and restored on Stop
macOS	The same engine, without AVAudioSession
Android	AudioRecord → AudioTrack
Linux	A PulseAudio/ALSA capture stream fed into a playback stream. See Linux

Effects aren’t applied to the monitor

IAudioMonitor doesn’t run an AudioEffectChain. Effects apply to IAudioSource capture and to IAudioRecorder.