Comfort Noise Rate
Understand Comfort Noise insertion measurement in VoIP calls — how CallMeter tracks CNG events during silence, and what the presence or absence of comfort noise means for call quality.
Comfort Noise Rate counts the number of comfort noise insertions per second during silence periods of an audio stream. When a speaker stops talking, the codec's Comfort Noise Generation (CNG) system fills the silence with low-level background noise that matches the acoustic environment. Each insertion represents one comfort noise frame generated to maintain a natural-sounding call.
Think of it like the ambient hum you hear on a phone line when the other person is not talking. Without it, the line goes completely silent — which makes callers think the call dropped.
How It Works
During a VoIP call, when Voice Activity Detection identifies a silence period, the codec can either:
- Continue sending full packets — Wastes bandwidth during silence
- Stop sending entirely (DTX) — Saves bandwidth but creates unsettling dead silence on the receiver
- Send comfort noise parameters — Sends occasional small packets describing the background noise characteristics, so the receiver can generate matching noise locally
CallMeter counts the comfort noise frames inserted by the receiver's decoder during these silence periods. The rate reflects how actively the CNG system is filling gaps in the audio.
No fixed thresholds
Comfort noise insertion rate varies based on call content and DTX behavior. There are no universal good/bad thresholds. What matters is whether comfort noise is present when expected (during silence in DTX-enabled calls) and absent when not expected.
Why It Matters
Comfort noise serves two critical purposes in VoIP:
- Naturalness — Complete silence between speech segments sounds unnatural and alarming to callers. Comfort noise maintains the perception of a continuous, live connection.
- DTX companion — Discontinuous Transmission (DTX) saves bandwidth by not sending packets during silence, but DTX without CNG creates jarring silence-to-speech transitions.
For enterprise testing, monitoring comfort noise behavior validates that the entire CNG/DTX chain is working correctly across your VoIP infrastructure.
Common Causes of Unexpected Comfort Noise Behavior
| Cause | Explanation |
|---|---|
| No comfort noise during silence | CNG not negotiated in SDP, or DTX disabled on the encoder |
| Comfort noise too loud | CNG level set higher than actual background noise |
| Comfort noise too quiet | CNG level not matching environment, creating volume jumps when speech resumes |
| Absent CNG with DTX enabled | Receiver not generating comfort noise despite DTX, causing dead air |
| CNG during speech | Misclassification by VAD causing CNG to activate during quiet speech |
How to Fix It
- Verify CNG negotiation — Check SDP offer/answer for comfort noise codec (typically CN payload type 13). If not present, CNG will not activate.
- Check DTX configuration — CNG is typically paired with DTX. If DTX is disabled, the encoder sends full packets during silence and CNG is unnecessary.
- Adjust CNG level — If comfort noise sounds too loud or too quiet, review the CNG level parameter. It should approximate the natural background noise of the call.
- Compare with speech activity — Cross-reference with Speech Activity (VAD) to confirm that comfort noise insertions align with non-speech periods.
- Test SBC transparency — Some Session Border Controllers strip CNG from the media path. Test with and without SBCs to isolate the issue.
Related Metrics
- Speech Activity (VAD) — Speech ratio that determines when CNG should activate
- Audio Noise Level — Background noise level; CNG should approximate this during silence
- Audio Signal Level — Speech level; large gap between signal and CNG level is expected
- Send Bitrate — Bandwidth drops during DTX+CNG periods, confirming DTX is working
Speech Activity (VAD)
Understand the Voice Activity Detection speech ratio metric — how CallMeter measures the percentage of active speech in audio streams, and what it reveals about DTX and conversation patterns.
Opus Bandwidth
Understand Opus codec bandwidth measurement — how CallMeter tracks the frequency bandwidth of Opus audio streams, thresholds, and what bandwidth changes reveal about audio richness.