diff --git a/.gitignore b/.gitignore
index 566a3c3f..25bd6d0e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -14,6 +14,7 @@ linuxbuild/
 test/songs/
 test/delta/
 test/result/
+test/assert_delta
 android/.gradle/
 android/app/build/
 android/app/.cxx/
diff --git a/test/assert_delta.c b/test/assert_delta.c
new file mode 100644
index 00000000..6dad6992
--- /dev/null
+++ b/test/assert_delta.c
@@ -0,0 +1,49 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sndfile.h>
+
+#define BUF_SIZE 8192
+
+// usage: assert_delta file
+// return values:
+// - 0: pass (file is silence)
+// - 1: fail (noise found)
+// - 2: command line error
+// - 3: file open error
+int main(int argc, char** argv) {
+  if (argc<2) return 2;
+
+  SF_INFO si;
+  memset(&si,0,sizeof(SF_INFO));
+  SNDFILE* sf=sf_open(argv[1],SFM_READ,&si);
+  if (sf==NULL) {
+    fprintf(stderr,"open: %s\n",sf_strerror(NULL));
+    return 3;
+  }
+
+  if (si.channels<1) {
+    fprintf(stderr,"invalid channel count\n");
+    return 3;
+  }
+
+  float* buf=malloc(BUF_SIZE*si.channels*sizeof(float));
+
+  sf_count_t totalRead=0;
+  size_t seekPos=0;
+  while ((totalRead=sf_readf_float(sf,buf,BUF_SIZE))!=0) {
+    for (int i=0; i<totalRead*si.channels; i++) {
+      if (buf[i]!=0.0f) {
+        printf("%ld\n",seekPos+(i/si.channels));
+        sf_close(sf);
+        free(buf);
+        return 1;
+      }
+    }
+    seekPos+=BUF_SIZE;
+  }
+
+  sf_close(sf);
+  free(buf);
+  return 0;
+}
diff --git a/test/furnace-test.sh b/test/furnace-test.sh
index d2bab55c..0b1568a2 100755
--- a/test/furnace-test.sh
+++ b/test/furnace-test.sh
@@ -12,6 +12,14 @@ fi
 
 echo "lastTest is $lastTest"
 
+if [ -e "test/assert_delta" ]; then
+  echo "assert_delta present."
+else
+  echo "compiling assert_delta..."
+  gcc -Wall -Wextra -Werror -o "test/assert_delta" "test/assert_delta.c" -lsndfile || exit 1
+fi
+  
+
 echo "furnace test suite begin..."
 echo "--- STEP 1: render test files"
 mkdir -p "test/result/$testDir" || exit 1
@@ -23,9 +31,17 @@ else
   mkdir -p "test/delta/$testDir" || exit 1
   ls "test/result/$testDir/" | parallel --verbose -j4 ffmpeg -i "test/result/$lastTest/{0}" -i "test/result/$testDir/{0}" -filter_complex stereotools=phasel=1:phaser=1,amix=inputs=2:duration=longest -c:a pcm_s16le -y "test/delta/$testDir/{0}"
 fi
-echo "--- STEP 3: make delta images"
+echo "--- STEP 3: check deltas"
 if [ -z $lastTest ]; then
   echo "skipping since this apparently is your first run."
 else
-  ls "test/result/$testDir/" | parallel --verbose -j4 ffmpeg -i "test/delta/$testDir/{0}" -lavfi showspectrumpic "test/delta/$testDir/{0}.png"
+  for i in `ls "test/result/$testDir"`; do
+    echo -n "$i... "
+    if ./test/assert_delta "test/delta/$testDir/$i"; then
+      echo "OK"
+    else
+      echo "FAIL FAIL FAIL"
+      ffmpeg -loglevel quiet -i "test/delta/$testDir/$i" -lavfi showspectrumpic "test/delta/$testDir/$i.png"
+    fi
+  done
 fi