diff --git a/crates/wzp-relay/src/audio_scorer.rs b/crates/wzp-relay/src/audio_scorer.rs
new file mode 100644
index 0000000..8f46b5c
--- /dev/null
+++ b/crates/wzp-relay/src/audio_scorer.rs
@@ -0,0 +1,468 @@
+//! Tier F audio scorer — behavioural entropy detection for abuse mitigation.
+//!
+//! Computes a `legitimacy ∈ [0, 1]` score over a 10–30 s observation window.
+//! Features: IAT CoV, payload-size bimodality, silence fraction, bitrate
+//! deviation, and Q-flag cadence.
+
+use std::collections::VecDeque;
+use std::time::{Duration, Instant};
+
+use wzp_proto::{CodecId, MediaHeader, MediaType};
+
+/// Maximum samples kept in rolling windows.
+const MAX_IAT_SAMPLES: usize = 200;
+const MAX_SIZE_SAMPLES: usize = 200;
+const MAX_Q_INTERVALS: usize = 32;
+
+/// Silence threshold: payload below this many bytes is treated as silence / CN.
+const SILENCE_SIZE_THRESHOLD: usize = 16;
+
+/// Observation window for bitrate tracking.
+const BITRATE_WINDOW_SECS: u64 = 30;
+
+/// Number of payload-size histogram bins.
+// (SIZE_BINS reserved for future histogram-based bimodality)
+
+/// Verdict produced by the scorer after sufficient observation.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum Verdict {
+    /// No suspicion. Score ≥ 0.7.
+    Legitimate,
+    /// Tightened monitoring. 0.3 ≤ score < 0.7.
+    Suspect,
+    /// High confidence of abuse. Score < 0.3.
+    Abusive,
+}
+
+/// Audio-specific behavioural scorer (Tier F).
+pub struct AudioScorer {
+    /// Rolling inter-arrival times.
+    iat_samples: VecDeque<Duration>,
+    last_arrival: Option<Instant>,
+
+    /// Rolling payload sizes.
+    size_samples: VecDeque<usize>,
+
+    /// Count of packets below silence threshold.
+    silence_packets: u32,
+    /// Total packets observed in current window.
+    total_packets: u32,
+
+    /// Bitrate window.
+    window_start: Instant,
+    window_bytes: u64,
+
+    /// Q-flag arrival intervals.
+    q_intervals: VecDeque<Duration>,
+    last_q_flag: Option<Instant>,
+
+    /// Codec declared at first packet (used for nominal bitrate baseline).
+    declared_codec: Option<CodecId>,
+}
+
+impl AudioScorer {
+    pub fn new() -> Self {
+        Self {
+            iat_samples: VecDeque::with_capacity(MAX_IAT_SAMPLES),
+            last_arrival: None,
+            size_samples: VecDeque::with_capacity(MAX_SIZE_SAMPLES),
+            silence_packets: 0,
+            total_packets: 0,
+            window_start: Instant::now(),
+            window_bytes: 0,
+            q_intervals: VecDeque::with_capacity(MAX_Q_INTERVALS),
+            last_q_flag: None,
+            declared_codec: None,
+        }
+    }
+
+    /// Feed one packet into the scorer.
+    pub fn observe(&mut self, header: &MediaHeader, payload_len: usize, now: Instant) {
+        // Ignore non-audio traffic.
+        if header.media_type != MediaType::Audio {
+            return;
+        }
+
+        if self.declared_codec.is_none() {
+            self.declared_codec = Some(header.codec_id);
+        }
+
+        // IAT
+        if let Some(last) = self.last_arrival {
+            let iat = now.saturating_duration_since(last);
+            self.iat_samples.push_back(iat);
+            if self.iat_samples.len() > MAX_IAT_SAMPLES {
+                self.iat_samples.pop_front();
+            }
+        }
+        self.last_arrival = Some(now);
+
+        // Payload size
+        self.size_samples.push_back(payload_len);
+        if self.size_samples.len() > MAX_SIZE_SAMPLES {
+            self.size_samples.pop_front();
+        }
+
+        // Silence fraction
+        self.total_packets += 1;
+        if payload_len <= SILENCE_SIZE_THRESHOLD {
+            self.silence_packets += 1;
+        }
+
+        // Bitrate window
+        if now.duration_since(self.window_start) >= Duration::from_secs(BITRATE_WINDOW_SECS) {
+            self.window_start = now;
+            self.window_bytes = 0;
+        }
+        self.window_bytes += (MediaHeader::WIRE_SIZE + payload_len) as u64;
+
+        // Q-flag cadence
+        if header.has_quality() {
+            if let Some(last) = self.last_q_flag {
+                let interval = now.saturating_duration_since(last);
+                self.q_intervals.push_back(interval);
+                if self.q_intervals.len() > MAX_Q_INTERVALS {
+                    self.q_intervals.pop_front();
+                }
+            }
+            self.last_q_flag = Some(now);
+        }
+    }
+
+    /// Compute legitimacy score ∈ [0, 1].
+    ///
+    /// Higher = more legitimate.  Returns `None` when insufficient samples
+    /// have been collected (< 20 packets).
+    pub fn legitimacy(&self) -> Option<f32> {
+        if self.total_packets < 20 {
+            return None;
+        }
+
+        let mut score = 1.0f32;
+
+        // 1. IAT CoV penalty
+        if let Some(cov) = self.iat_cov() {
+            if cov > 0.4 {
+                let penalty = ((cov - 0.4) / 0.6).min(1.0) * 0.25;
+                score -= penalty as f32;
+            }
+        }
+
+        // 2. Silence fraction penalty
+        let silence_fraction = self.silence_fraction();
+        if silence_fraction < 0.02 {
+            let penalty = ((0.02 - silence_fraction) / 0.02).min(1.0) * 0.25;
+            score -= penalty as f32;
+        } else if silence_fraction > 0.60 {
+            // Too much silence can also be suspicious (stuffed payloads)
+            let penalty = ((silence_fraction - 0.60) / 0.40).min(1.0) * 0.15;
+            score -= penalty as f32;
+        }
+
+        // 3. Bitrate deviation penalty
+        if let Some(ratio) = self.bitrate_ratio() {
+            if ratio > 1.20 {
+                let penalty = ((ratio - 1.20) / 0.80).min(1.0) * 0.25;
+                score -= penalty as f32;
+            }
+        }
+
+        // 4. Q-flag cadence penalty
+        if let Some(cv) = self.q_flag_cv() {
+            // High variability in Q-flag spacing = suspicious
+            if cv > 0.5 {
+                let penalty = ((cv - 0.5) / 0.5).min(1.0) * 0.15;
+                score -= penalty as f32;
+            }
+        } else {
+            // No Q flags seen at all — mildly suspicious after many packets
+            if self.total_packets > 100 {
+                score -= 0.10;
+            }
+        }
+
+        // 5. Payload-size bimodality bonus/penalty
+        if let Some(bimodality) = self.size_bimodality() {
+            // Bimodality score: 0 = unimodal, 1 = strongly bimodal
+            // Legitimate audio is bimodal (speech + silence)
+            if bimodality < 0.2 {
+                score -= 0.10;
+            }
+        }
+
+        Some(score.max(0.0).min(1.0))
+    }
+
+    /// Map legitimacy score to a [`Verdict`].
+    pub fn verdict(&self) -> Option<Verdict> {
+        self.legitimacy().map(|s| {
+            if s >= 0.7 {
+                Verdict::Legitimate
+            } else if s >= 0.3 {
+                Verdict::Suspect
+            } else {
+                Verdict::Abusive
+            }
+        })
+    }
+
+    // ------------------------------------------------------------------
+    // Feature extractors
+    // ------------------------------------------------------------------
+
+    /// Coefficient of variation of inter-arrival times.
+    fn iat_cov(&self) -> Option<f64> {
+        if self.iat_samples.len() < 10 {
+            return None;
+        }
+        let mean = self.iat_samples.iter().map(|d| d.as_secs_f64()).sum::<f64>()
+            / self.iat_samples.len() as f64;
+        if mean == 0.0 {
+            return None;
+        }
+        let variance = self
+            .iat_samples
+            .iter()
+            .map(|d| {
+                let diff = d.as_secs_f64() - mean;
+                diff * diff
+            })
+            .sum::<f64>()
+            / self.iat_samples.len() as f64;
+        let std = variance.sqrt();
+        Some(std / mean)
+    }
+
+    /// Fraction of packets that are silence / comfort-noise sized.
+    fn silence_fraction(&self) -> f64 {
+        if self.total_packets == 0 {
+            return 0.0;
+        }
+        self.silence_packets as f64 / self.total_packets as f64
+    }
+
+    /// Ratio of observed bitrate to nominal bitrate over the 30 s window.
+    fn bitrate_ratio(&self) -> Option<f64> {
+        let codec = self.declared_codec?;
+        let nominal_bps = codec.bitrate_bps() as f64;
+        if nominal_bps == 0.0 {
+            return None;
+        }
+        let observed_bps = self.window_bytes as f64 * 8.0 / BITRATE_WINDOW_SECS as f64;
+        Some(observed_bps / nominal_bps)
+    }
+
+    /// Coefficient of variation of Q-flag intervals.
+    fn q_flag_cv(&self) -> Option<f64> {
+        if self.q_intervals.len() < 3 {
+            return None;
+        }
+        let mean = self.q_intervals.iter().map(|d| d.as_secs_f64()).sum::<f64>()
+            / self.q_intervals.len() as f64;
+        if mean == 0.0 {
+            return None;
+        }
+        let variance = self
+            .q_intervals
+            .iter()
+            .map(|d| {
+                let diff = d.as_secs_f64() - mean;
+                diff * diff
+            })
+            .sum::<f64>()
+            / self.q_intervals.len() as f64;
+        let std = variance.sqrt();
+        Some(std / mean)
+    }
+
+    /// Simple bimodality score based on a 2-bin histogram.
+    ///
+    /// Splits payload sizes into "small" (≤ threshold) and "large" bins.
+    /// Returns a score in [0, 1] where 1 = strongly bimodal.
+    fn size_bimodality(&self) -> Option<f64> {
+        if self.size_samples.len() < 20 {
+            return None;
+        }
+        let small = self
+            .size_samples
+            .iter()
+            .filter(|&&s| s <= SILENCE_SIZE_THRESHOLD)
+            .count();
+        let large = self.size_samples.len() - small;
+        let total = self.size_samples.len() as f64;
+        let p_small = small as f64 / total;
+        let p_large = large as f64 / total;
+        // Max bimodality when both bins are equally populated (~0.5 each)
+        let bimodality = 1.0 - (p_small - 0.5).abs() * 2.0;
+        Some(bimodality)
+    }
+}
+
+impl Default for AudioScorer {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn audio_header(payload_len: usize, has_quality: bool) -> MediaHeader {
+        MediaHeader {
+            version: 2,
+            flags: if has_quality { 0x40 } else { 0 },
+            media_type: MediaType::Audio,
+            codec_id: CodecId::Opus24k,
+            stream_id: 0,
+            fec_ratio: 0,
+            seq: 0,
+            timestamp: 0,
+            fec_block: 0,
+        }
+    }
+
+    #[test]
+    fn audio_scorer_ignores_video() {
+        let mut scorer = AudioScorer::new();
+        let mut h = audio_header(100, false);
+        h.media_type = MediaType::Video;
+        scorer.observe(&h, 100, Instant::now());
+        assert_eq!(scorer.total_packets, 0);
+    }
+
+    #[test]
+    fn audio_scorer_counts_packets() {
+        let mut scorer = AudioScorer::new();
+        for i in 0..25 {
+            let h = audio_header(100, false);
+            scorer.observe(&h, 100, Instant::now() + Duration::from_millis(i * 20));
+        }
+        assert_eq!(scorer.total_packets, 25);
+        assert!(scorer.legitimacy().is_some());
+    }
+
+    #[test]
+    fn audio_scorer_legitimate_traffic() {
+        let mut scorer = AudioScorer::new();
+        let base = Instant::now();
+        // Simulate 200 packets of legitimate audio:
+        // ~20 ms IAT, mixed speech (100 B) and silence (8 B), periodic Q flags.
+        for i in 0..200 {
+            let payload = if i % 3 == 0 { 8 } else { 100 };
+            let has_q = i % 10 == 0;
+            let h = audio_header(payload, has_q);
+            scorer.observe(&h, payload, base + Duration::from_millis(i * 20));
+        }
+        let leg = scorer.legitimacy().unwrap();
+        assert!(
+            leg >= 0.7,
+            "legitimate traffic should score ≥ 0.7, got {leg}"
+        );
+        assert_eq!(scorer.verdict(), Some(Verdict::Legitimate));
+    }
+
+    #[test]
+    fn audio_scorer_abusive_uniform_iat() {
+        let mut scorer = AudioScorer::new();
+        let base = Instant::now();
+        // Uniform IAT (no jitter), all same size, no Q flags — tunnel-like
+        for i in 0..200 {
+            let h = audio_header(200, false);
+            scorer.observe(&h, 200, base + Duration::from_millis(i * 20));
+        }
+        let leg = scorer.legitimacy().unwrap();
+        assert!(
+            leg < 0.6,
+            "uniform tunnel-like traffic should score < 0.6, got {leg}"
+        );
+    }
+
+    #[test]
+    fn audio_scorer_abusive_no_silence() {
+        let mut scorer = AudioScorer::new();
+        let base = Instant::now();
+        // No silence packets at all, very regular IAT
+        for i in 0..200 {
+            let h = audio_header(150, false);
+            scorer.observe(&h, 150, base + Duration::from_millis(i * 20));
+        }
+        let leg = scorer.legitimacy().unwrap();
+        assert!(
+            leg < 0.6,
+            "no-silence traffic should score < 0.6, got {leg}"
+        );
+    }
+
+    #[test]
+    fn audio_scorer_insufficient_samples() {
+        let scorer = AudioScorer::new();
+        assert_eq!(scorer.legitimacy(), None);
+        assert_eq!(scorer.verdict(), None);
+    }
+
+    #[test]
+    fn silence_fraction_computed_correctly() {
+        let mut scorer = AudioScorer::new();
+        let base = Instant::now();
+        for i in 0..100 {
+            let payload = if i < 30 { 8 } else { 100 };
+            let h = audio_header(payload, false);
+            scorer.observe(&h, payload, base + Duration::from_millis(i * 20));
+        }
+        assert!((scorer.silence_fraction() - 0.30).abs() < 0.01);
+    }
+
+    #[test]
+    fn bitrate_ratio_saturates_when_no_codec() {
+        let scorer = AudioScorer::new();
+        assert_eq!(scorer.bitrate_ratio(), None);
+    }
+
+    #[test]
+    fn q_flag_cv_regular_spacing() {
+        let mut scorer = AudioScorer::new();
+        let base = Instant::now();
+        for i in 0..50 {
+            let has_q = i % 5 == 0;
+            let h = audio_header(100, has_q);
+            scorer.observe(&h, 100, base + Duration::from_millis(i * 20));
+        }
+        let cv = scorer.q_flag_cv().unwrap();
+        assert!(
+            cv < 0.1,
+            "regular Q-flag spacing should have CV < 0.1, got {cv}"
+        );
+    }
+
+    #[test]
+    fn size_bimodality_for_mixed_traffic() {
+        let mut scorer = AudioScorer::new();
+        let base = Instant::now();
+        for i in 0..100 {
+            let payload = if i % 2 == 0 { 8 } else { 120 };
+            let h = audio_header(payload, false);
+            scorer.observe(&h, payload, base + Duration::from_millis(i * 20));
+        }
+        let bim = scorer.size_bimodality().unwrap();
+        assert!(
+            bim > 0.8,
+            "perfectly mixed small/large should be highly bimodal, got {bim}"
+        );
+    }
+
+    #[test]
+    fn size_bimodality_for_uniform_traffic() {
+        let mut scorer = AudioScorer::new();
+        let base = Instant::now();
+        for i in 0..100 {
+            let h = audio_header(100, false);
+            scorer.observe(&h, 100, base + Duration::from_millis(i * 20));
+        }
+        let bim = scorer.size_bimodality().unwrap();
+        assert!(
+            bim < 0.3,
+            "uniform size traffic should be unimodal, got {bim}"
+        );
+    }
+}
diff --git a/crates/wzp-relay/src/lib.rs b/crates/wzp-relay/src/lib.rs
index 674e14a..0c01794 100644
--- a/crates/wzp-relay/src/lib.rs
+++ b/crates/wzp-relay/src/lib.rs
@@ -10,6 +10,7 @@
 pub mod auth;
 pub mod call_registry;
 pub mod config;
+pub mod audio_scorer;
 pub mod conformance;
 pub mod event_log;
 pub mod federation;