diff --git a/av/audio/plane.pxd b/av/audio/plane.pxd
index 316c84031..de912ac22 100644
--- a/av/audio/plane.pxd
+++ b/av/audio/plane.pxd
@@ -2,7 +2,5 @@ from av.plane cimport Plane
 
 
 cdef class AudioPlane(Plane):
-
     cdef readonly size_t buffer_size
-
     cdef size_t _buffer_size(self)
diff --git a/av/audio/plane.py b/av/audio/plane.py
new file mode 100644
index 000000000..bdaf15708
--- /dev/null
+++ b/av/audio/plane.py
@@ -0,0 +1,13 @@
+import cython
+from cython.cimports.av.audio.frame import AudioFrame
+
+
+@cython.cclass
+class AudioPlane(Plane):
+    def __cinit__(self, frame: AudioFrame, index: cython.int):
+        # Only the first linesize is ever populated, but it applies to every plane.
+        self.buffer_size = self.frame.ptr.linesize[0]
+
+    @cython.cfunc
+    def _buffer_size(self) -> cython.size_t:
+        return self.buffer_size
diff --git a/av/audio/plane.pyx b/av/audio/plane.pyx
deleted file mode 100644
index 92c508cbd..000000000
--- a/av/audio/plane.pyx
+++ /dev/null
@@ -1,11 +0,0 @@
-from av.audio.frame cimport AudioFrame
-
-
-cdef class AudioPlane(Plane):
-
-    def __cinit__(self, AudioFrame frame, int index):
-        # Only the first linesize is ever populated, but it applies to every plane.
-        self.buffer_size = self.frame.ptr.linesize[0]
-
-    cdef size_t _buffer_size(self):
-        return self.buffer_size