diff --git a/mp3gain_tracks.sh b/mp3gain_tracks.sh
index b6477b0..6ec2a81 100755
--- a/mp3gain_tracks.sh
+++ b/mp3gain_tracks.sh
@@ -1,2 +1,14 @@
 #!/bin/sh
-mp3gain -k -p -r -s i *.mp3
+if [ "$#" = "0" ]; then
+    echo "Usage: $0 file1 [file2 file3 ... fileN]"
+    exit 1
+fi
+
+if [ -z "$CONCURRENCY_LEVEL" ]; then
+    CONCURRENCY_LEVEL=1
+fi
+
+echo "Processing using $CONCURRENCY_LEVEL processes..."
+find "$@" -print0 | xargs -0 -P $CONCURRENCY_LEVEL -n 1 -I {} mp3gain -q -k -p -r -s i "{}"
+
+echo "All done."