diff --git a/src/audio/realtime-segmenter.ts b/src/audio/realtime-segmenter.ts
index 028d045..f753748 100644
--- a/src/audio/realtime-segmenter.ts
+++ b/src/audio/realtime-segmenter.ts
@@ -1,5 +1,9 @@
 interface RealtimeSegmenterOptions {
   onSegment: (pcm16: Buffer) => void;
+  onLevel?: (peak: number) => void;
+  onSpeechStart?: (peak: number) => void;
+  onSpeechDiscarded?: (samples: number) => void;
+  onSpeechReady?: (samples: number) => void;
 }
 
 export class RealtimeSegmenter {
@@ -44,6 +48,8 @@ export class RealtimeSegmenter {
       }
     }
 
+    this.options.onLevel?.(peak);
+
     if (!this.speechActive) {
       appendWithCap(this.preRoll, frame, this.preRollSamples);
       if (peak >= this.speechStartThreshold) {
@@ -60,6 +66,7 @@ export class RealtimeSegmenter {
       this.silenceFrames = 0;
       this.speech.splice(0, this.speech.length, ...this.preRoll);
       this.preRoll.splice(0, this.preRoll.length);
+      this.options.onSpeechStart?.(peak);
     }
 
     this.speech.push(...frame);
@@ -81,9 +88,11 @@ export class RealtimeSegmenter {
     this.speechCandidateFrames = 0;
 
     if (speechPcm.length < this.minSpeechSamples * 2) {
+      this.options.onSpeechDiscarded?.(speechPcm.length / 2);
       return;
     }
 
+    this.options.onSpeechReady?.(speechPcm.length / 2);
     this.options.onSegment(speechPcm);
   }
 }
diff --git a/src/index.ts b/src/index.ts
index f021d9b..d3246ea 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -14,6 +14,13 @@ async function runLoopback(): Promise<void> {
   const stt = new FasterWhisperSttService(config, logger);
   let capture = null as ReturnType<typeof spawnLoopbackCapture> | null;
   let shuttingDown: Promise<void> | null = null;
+  let receivedChunks = 0;
+  let receivedBytes = 0;
+  let maxPeak = 0;
+  let lastChunkAt = 0;
+  let lastLevelLogAt = 0;
+  let sawSpeechStart = false;
+  let emittedSegmentCount = 0;
 
   const shutdown = async (exitCode: number, reason: string, error?: unknown): Promise<void> => {
     if (shuttingDown) {
@@ -60,6 +67,7 @@ async function runLoopback(): Promise<void> {
   });
 
   await stt.warmup();
+  logger.info("STT warmup finished");
 
   const transcriptionQueue: Buffer[] = [];
   let transcribing = false;
@@ -93,14 +101,50 @@ async function runLoopback(): Promise<void> {
   };
 
   const segmenter = new RealtimeSegmenter({
+    onLevel: (peak) => {
+      if (peak > maxPeak) {
+        maxPeak = peak;
+      }
+
+      const now = Date.now();
+      if (now - lastLevelLogAt >= 3000) {
+        lastLevelLogAt = now;
+        logger.info("Audio input heartbeat", {
+          chunks: receivedChunks,
+          bytes: receivedBytes,
+          peak: maxPeak,
+          speech_started: sawSpeechStart,
+          emitted_segments: emittedSegmentCount,
+        });
+        maxPeak = 0;
+      }
+    },
+    onSpeechStart: (peak) => {
+      sawSpeechStart = true;
+      logger.info("Speech start detected", { peak });
+    },
+    onSpeechDiscarded: (samples) => {
+      logger.info("Discarded short speech segment", { samples });
+    },
+    onSpeechReady: (samples) => {
+      emittedSegmentCount += 1;
+      logger.info("Speech segment ready", { index: emittedSegmentCount, samples });
+    },
     onSegment: (pcm16) => {
       transcriptionQueue.push(pcm16);
+      logger.info("Queued segment for STT", {
+        queue: transcriptionQueue.length,
+        bytes: pcm16.length,
+      });
       void runNext();
     },
   });
 
   capture = spawnLoopbackCapture(config, logger);
   capture.stdout.on("data", (chunk: Buffer) => {
+    receivedChunks += 1;
+    receivedBytes += chunk.length;
+    lastChunkAt = Date.now();
     segmenter.pushChunk(chunk);
   });
   capture.stderr.on("data", (chunk: Buffer) => {
@@ -123,6 +167,18 @@ async function runLoopback(): Promise<void> {
   console.log(`source: ${config.AUDIO_SOURCE ?? "unset"}`);
   console.log(`model: ${config.WHISPER_MODEL}`);
   console.log(`language: ${config.WHISPER_LANGUAGE}`);
+
+  setInterval(() => {
+    const now = Date.now();
+    if (lastChunkAt === 0 && !shuttingDown) {
+      logger.warn("아직 캡처 PCM 데이터가 들어오지 않았습니다. AUDIO_SOURCE 가 잘못됐거나 loopback 입력이 아닌 장치일 수 있습니다.");
+      return;
+    }
+
+    if (lastChunkAt > 0 && now - lastChunkAt >= 5000 && !shuttingDown) {
+      logger.warn("최근 5초 동안 새 PCM chunk 가 들어오지 않았습니다.");
+    }
+  }, 5000).unref();
 }
 
 async function main(): Promise<void> {