How to Set Up Two-Channel Transfer Function Measurement

TL;DR

Two-channel transfer function is the professional standard: one channel carries the electrical reference signal, the other carries the acoustic measurement. SonaVyx achieves this with two devices paired via WebSocket — one acts as the source (generating and sending the reference), the other as the listener (capturing the microphone). This guide covers the full setup.

Why Two-Channel?

Single-device loopback uses the same device for both playback and capture, which means the reference signal passes through the device's DAC and speaker. Any coloration from these components appears in the transfer function. Two-channel measurement sends the reference signal directly from the source device to SonaVyx, bypassing the acoustic path. The result is the true acoustic transfer function of the room + speaker, without contamination from the measurement speaker.

Step-by-Step Procedure

Open SonaVyx on two devices. Navigate to /measure?mode=tf on both. Both devices need internet access for the WebSocket connection (even on the same LAN, the pairing goes through the SonaVyx server).
Create a session on the source device. The source device is the one connected to the sound system (typically via a 3.5mm to mixer cable, Bluetooth, or USB audio interface). Tap "Create Session" to get a 6-digit room code.
Join the session on the listener device. The listener device holds the measurement microphone. Enter the 6-digit room code and tap "Join." Both devices show a connected status indicator.
Set device roles. The source device plays the test signal (pink noise, sweep, or MLS) through the sound system. The listener device captures the microphone signal. SonaVyx pairs these signals using NTP-synchronized timestamps.
Start the measurement. Tap "Start" on either device. The source generates pink noise and sends the reference signal data over WebSocket. The listener captures the acoustic signal from the microphone. SonaVyx computes the transfer function H(f) = Y(f)/X(f) where X is the reference and Y is the measurement.
Verify coherence. Coherence should be above 0.8 across the measurement band. If coherence is low, check: signal level (increase if needed), ambient noise (reduce if possible), and device synchronization (the NTP sync should show less than 5 ms offset).
Move the listener to additional positions. The source keeps playing. Move the listener device to the next measurement position and capture another trace. This is much faster than single-device measurement because you do not need to reconfigure the source each time.

Hardware Connection Options

Connection	Quality	Notes
3.5mm aux to mixer	Best	Analog direct input, no latency
USB audio interface	Excellent	Low latency, clean signal
Bluetooth to speaker	Acceptable	50-200ms latency (SonaVyx compensates)
Built-in speaker	Single-device only	Use loopback mode instead

Common Mistakes

Swapping source and listener roles. The source must be connected to the sound system output. The listener must hold the measurement microphone. Reversing them gives you the inverse transfer function.
Both devices on cellular. WebSocket latency spikes on cellular can cause synchronization issues. Use Wi-Fi for both devices if possible.
Forgetting to set the reference level. The source device's output level affects the transfer function gain. Set a consistent level and note it in your report.

Tool Bridge

Open SonaVyx Transfer Function mode on two devices and pair them with a room code. The source generates the reference signal; the listener captures the acoustic response.