feat: Add bandpass filter (150-800 Hz) for breathing/snoring isolation

Based on respiratory sound research:
- Normal breathing: 100-1500 Hz (peak 400-800 Hz)
- Snoring: 20-800 Hz (fundamental 40-100 Hz, peaks at 300-520 Hz)
- Heart sounds to filter: <100 Hz

Filter settings (research-backed 'goldilocks' range):
- Low cutoff: 150 Hz (filters heart sounds)
- High cutoff: 800 Hz (captures breathing + snoring harmonics)
- Q factor: 0.7 (wide enough for natural variation)
- Center frequency: ~346 Hz (geometric mean)

Sources: PMC2990233, PMC8448177, Stevens Institute sleep monitoring

Author: jeremymanning

services/sleep.ts

@@ -84,10 +84,15 @@ const MOVEMENT_SPIKE_THRESHOLD = 3.0;
 const HR_MOVEMENT_DELTA_THRESHOLD = 8;
 const HR_MOVEMENT_WINDOW_SEC = 30;
 
+const BANDPASS_LOW_FREQ = 150;
+const BANDPASS_HIGH_FREQ = 800;
+const BANDPASS_Q = 0.7;
+
 let currentSession: SleepSession | null = null;
 let recentHRSamples: { hr: number; timestamp: number }[] = [];
 let hrMovementScore = 0;
 let audioContext: AudioContext | null = null;
+let bandpassFilter: BiquadFilterNode | null = null;
 let analyzer: MeydaAnalyzerInstance | null = null;
 let mediaStream: MediaStream | null = null;
 let isAudioRunning = false;
@@ -378,9 +383,17 @@ async function startWebAudioDetection(): Promise<boolean> {
     audioContext = new AudioContext();
     const source = audioContext.createMediaStreamSource(mediaStream);
 
+    const centerFreq = Math.sqrt(BANDPASS_LOW_FREQ * BANDPASS_HIGH_FREQ);
+    bandpassFilter = audioContext.createBiquadFilter();
+    bandpassFilter.type = 'bandpass';
+    bandpassFilter.frequency.value = centerFreq;
+    bandpassFilter.Q.value = BANDPASS_Q;
+
+    source.connect(bandpassFilter);
+
     analyzer = Meyda.createMeydaAnalyzer({
       audioContext,
-      source,
+      source: bandpassFilter,
       bufferSize: 2048,
       featureExtractors: ['rms', 'zcr', 'spectralCentroid', 'spectralFlatness'],
       callback: processAudioFeatures,
@@ -408,6 +421,11 @@ function stopAudioDetection(): void {
     analyzer = null;
   }
 
+  if (bandpassFilter) {
+    bandpassFilter.disconnect();
+    bandpassFilter = null;
+  }
+
   if (mediaStream) {
     mediaStream.getTracks().forEach((track) => track.stop());
     mediaStream = null;
@@ -992,6 +1010,23 @@ export function getDetectionMode(): 'audio' | 'vitals' | 'fused' | 'none' {
   return 'none';
 }
 
+export function getBandpassFilterStatus(): {
+  enabled: boolean;
+  lowFreq: number;
+  highFreq: number;
+  centerFreq: number;
+  q: number;
+} {
+  const centerFreq = Math.sqrt(BANDPASS_LOW_FREQ * BANDPASS_HIGH_FREQ);
+  return {
+    enabled: bandpassFilter !== null,
+    lowFreq: BANDPASS_LOW_FREQ,
+    highFreq: BANDPASS_HIGH_FREQ,
+    centerFreq: Math.round(centerFreq),
+    q: BANDPASS_Q,
+  };
+}
+
 export const sleepService = {
   startSession: startSleepSession,
   endSession: endSleepSession,