diff --git a/src/operators/rms_op.ml b/src/operators/rms_op.ml
index a0e9e519de..2418a47c0b 100644
--- a/src/operators/rms_op.ml
+++ b/src/operators/rms_op.ml
@@ -22,7 +22,8 @@
 
 open Source
 
-class rms ~kind source =
+class rms ~kind duration source =
+  let channels = (Frame.type_of_kind kind).Frame.audio in
 object (self)
   inherit operator kind [source] ~name:"rms" as super
 
@@ -31,55 +32,83 @@ object (self)
   method remaining = source#remaining
   method abort_track = source#abort_track
 
-  val mutable volume = 0.
+  (** Sum of squares. *)
+  val sq = Array.create channels 0.
+  (** Duration of the sum of squares in samples. *)
+  val mutable sq_dur = 0
+  (** Last computed rms. *)
+  val mutable rms = Array.create channels 0.
 
   val m = Mutex.create ()
 
-  method rms =
-    Tutils.mutexify m
-      (fun () -> volume) ()
+  method rms = Tutils.mutexify m (fun () -> rms) ()
 
   method private get_frame buf =
     let offset = AFrame.position buf in
     source#get buf;
-    let rms = AFrame.rms buf offset (AFrame.position buf - offset) in
-    let channels = Array.length rms in
-    Tutils.mutexify m
-      (fun () ->
-        volume <- 0.;
-        for i = 0 to channels - 1 do
-          volume <- volume +. rms.(i)
+    let duration = duration () in
+    if duration > 0. then
+      let duration = Frame.audio_of_seconds duration in
+      let position = AFrame.position buf in
+      let buf = AFrame.content buf offset in
+      for i = offset to position - 1 do
+        for c = 0 to channels - 1 do
+          let x = buf.(c).(i) in
+          sq.(c) <- sq.(c) +. x *. x
         done;
-        volume <- volume /. (float channels)) ()
+        sq_dur <- sq_dur + 1;
+        if sq_dur >= duration then
+          let dur = float sq_dur in
+          let rms' = Array.init channels
+            (fun i ->
+              let r = sqrt (sq.(i) /. dur) in
+              sq.(i) <- 0.;
+              r)
+          in
+          sq_dur <- 0;
+          Tutils.mutexify m (fun () -> rms <- rms') ()
+      done
 end
 
-let () =
-  let format = Lang.any_fixed_with ~audio:1 () in
-  let k = Lang.kind_type_of_kind_format ~fresh:1 format in
+let declare suffix format fun_ret_t f_rms =
+  let k = Lang.kind_type_of_kind_format ~fresh:3 format in
   let return_t =
     Lang.product_t
-      (Lang.fun_t [] Lang.float_t)
+      (Lang.fun_t [] fun_ret_t)
       (Lang.source_t k)
   in
-  Lang.add_builtin "rms"
+  Lang.add_builtin ("rms"^suffix)
     ~category:(Lang.string_of_category Lang.Visualization)
     ~descr:"Get current audio RMS volume of the source. \
             Returns a pair @(f,s)@ where s is a new source and \
             @f@ is a function of type @() -> float@ and \
             returns the current RMS of the source."
-    [ "id",Lang.string_t,Some (Lang.string ""),
-      Some "Force the value of the source ID.";
-      "", Lang.source_t k, None, None ] return_t
+    [
+      "id", Lang.string_t,Some (Lang.string ""), Some "Force the value of the source ID.";
+      "duration", Lang.float_getter_t 2, Some (Lang.float 0.5), Some "Duration of the RMS window (in seconds). A value <= 0, means that RMS computation should not be performed.";
+      "", Lang.source_t k, None, None
+    ]
+    return_t
     (fun p t ->
       let f v = List.assoc v p in
       let src = Lang.to_source (f "") in
       let id = Lang.to_string (f "id") in
+      let duration = Lang.to_float_getter (f "duration") in
       let (_,t) = Lang.of_product_t t in
       let kind = Lang.frame_kind_of_kind_type (Lang.of_source_t t) in
-      let s = new rms ~kind src in
+      let s = new rms ~kind duration src in
       if id <> "" then s#set_id id;
-      let f =
-        Lang.val_fun [] ~ret_t:Lang.float_t
-          (fun p t -> Lang.float s#rms)
-      in
+      let f = Lang.val_fun [] ~ret_t:fun_ret_t (fun p t -> f_rms s#rms) in
       Lang.product f (Lang.source (s :> Source.source)))
+
+let () =
+  let f rms =
+    let r = Array.fold_left (+.) 0. rms in
+    let r = r /. float (Array.length rms) in
+    Lang.float r
+  in
+  let f_stereo rms =
+    Lang.product (Lang.float rms.(0)) (Lang.float rms.(1))
+  in
+  declare "" Lang.any_fixed Lang.float_t f;
+  declare ".stereo" (Lang.any_fixed_with ~audio:2 ()) (Lang.product_t Lang.float_t Lang.float_t) f_stereo