diff --git a/services/audioflinger/TrackMetrics.h b/services/audioflinger/TrackMetrics.h
index 30d69ab..6fc70d6 100644
--- a/services/audioflinger/TrackMetrics.h
+++ b/services/audioflinger/TrackMetrics.h
@@ -64,7 +64,6 @@
                     AMEDIAMETRICS_PROP_EVENT_VALUE_BEGINAUDIOINTERVALGROUP, devices.c_str());
         }
         ++mIntervalCount;
-        mIntervalStartTimeNs = systemTime();
     }
 
     void logConstructor(pid_t creatorPid, uid_t creatorUid, int32_t internalTrackId,
@@ -90,11 +89,9 @@
     // Called when we are removed from the Thread.
     void logEndInterval() {
         std::lock_guard l(mLock);
-        if (mIntervalStartTimeNs != 0) {
-            const int64_t elapsedTimeNs = systemTime() - mIntervalStartTimeNs;
-            mIntervalStartTimeNs = 0;
-            mCumulativeTimeNs += elapsedTimeNs;
-            mDeviceTimeNs += elapsedTimeNs;
+        if (mLastVolumeChangeTimeNs != 0) {
+            logVolume_l(mVolume); // flush out the last volume.
+            mLastVolumeChangeTimeNs = 0;
         }
     }
 
@@ -133,20 +130,8 @@
 
     // may be called multiple times during an interval
     void logVolume(float volume) {
-        const int64_t timeNs = systemTime();
         std::lock_guard l(mLock);
-        if (mStartVolumeTimeNs == 0) {
-            mDeviceVolume = mVolume = volume;
-            mLastVolumeChangeTimeNs = mStartVolumeTimeNs = timeNs;
-            updateMinMaxVolume(0, mVolume);
-            return;
-        }
-        const int64_t durationNs = timeNs - mLastVolumeChangeTimeNs;
-        updateMinMaxVolume(durationNs, mVolume);
-        mDeviceVolume = (mDeviceVolume * (mLastVolumeChangeTimeNs - mStartVolumeTimeNs) +
-            mVolume * durationNs) / (timeNs - mStartVolumeTimeNs);
-        mVolume = volume;
-        mLastVolumeChangeTimeNs = timeNs;
+        logVolume_l(volume);
     }
 
     // Use absolute numbers returned by AudioTrackShared.
@@ -158,6 +143,7 @@
     }
 
 private:
+
     // no lock required - all arguments and constants.
     void deliverDeviceMetrics(const char *eventName, const char *devices) const {
         mediametrics::LogItem(mMetricsId)
@@ -167,6 +153,23 @@
            .record();
     }
 
+    void logVolume_l(float volume) REQUIRES(mLock) {
+        const int64_t timeNs = systemTime();
+        const int64_t durationNs = mLastVolumeChangeTimeNs == 0
+                ? 0 : timeNs - mLastVolumeChangeTimeNs;
+        if (durationNs > 0) {
+            // See West's algorithm for weighted averages
+            // https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance
+            mDeviceVolume += (mVolume - mDeviceVolume) * durationNs
+                      / (durationNs + mDeviceTimeNs);
+            mDeviceTimeNs += durationNs;
+            mCumulativeTimeNs += durationNs;
+        }
+        updateMinMaxVolume(durationNs, mVolume); // always update.
+        mVolume = volume;
+        mLastVolumeChangeTimeNs = timeNs;
+    }
+
     void deliverCumulativeMetrics(const char *eventName) const REQUIRES(mLock) {
         if (mIntervalCount > 0) {
             mediametrics::LogItem item(mMetricsId);
@@ -199,14 +202,12 @@
         // mDevices is not reset by resetIntervalGroupMetrics.
 
         mIntervalCount = 0;
-        mIntervalStartTimeNs = 0;
         // mCumulativeTimeNs is not reset by resetIntervalGroupMetrics.
         mDeviceTimeNs = 0;
 
         mVolume = 0.f;
         mDeviceVolume = 0.f;
-        mStartVolumeTimeNs = 0;
-        mLastVolumeChangeTimeNs = 0;
+        mLastVolumeChangeTimeNs = 0;  // last time volume logged, cleared on endInterval
         mMinVolume = AMEDIAMETRICS_INITIAL_MIN_VOLUME;
         mMaxVolume = AMEDIAMETRICS_INITIAL_MAX_VOLUME;
         mMinVolumeDurationNs = 0;
@@ -230,14 +231,12 @@
 
     // Number of intervals and playing time
     int32_t           mIntervalCount GUARDED_BY(mLock) = 0;
-    int64_t           mIntervalStartTimeNs GUARDED_BY(mLock) = 0;
-    int64_t           mCumulativeTimeNs GUARDED_BY(mLock) = 0;
-    int64_t           mDeviceTimeNs GUARDED_BY(mLock) = 0;
+    int64_t           mCumulativeTimeNs GUARDED_BY(mLock) = 0; // total time.
+    int64_t           mDeviceTimeNs GUARDED_BY(mLock) = 0;     // time on device.
 
     // Average volume
-    double            mVolume GUARDED_BY(mLock) = 0.f;
-    double            mDeviceVolume GUARDED_BY(mLock) = 0.f;
-    int64_t           mStartVolumeTimeNs GUARDED_BY(mLock) = 0;
+    double            mVolume GUARDED_BY(mLock) = 0.f;       // last set volume.
+    double            mDeviceVolume GUARDED_BY(mLock) = 0.f; // running average volume.
     int64_t           mLastVolumeChangeTimeNs GUARDED_BY(mLock) = 0;
 
     // Min/Max volume