diff --git a/src/mp4box/mp4a.rs b/src/mp4box/mp4a.rs
index a80c6c4..3bb5a00 100644
--- a/src/mp4box/mp4a.rs
+++ b/src/mp4box/mp4a.rs
@@ -181,11 +181,12 @@ impl Mp4Box for EsdsBox {
     }
 
     fn box_size(&self) -> u64 {
+        let es_desc_size = self.es_desc.desc_size();
         HEADER_SIZE
             + HEADER_EXT_SIZE
             + 1
-            + size_of_length(ESDescriptor::desc_size()) as u64
-            + ESDescriptor::desc_size() as u64
+            + size_of_length(es_desc_size) as u64
+            + es_desc_size as u64
     }
 
     fn to_json(&self) -> Result<String> {
@@ -247,7 +248,7 @@ impl<W: Write> WriteBox<&mut W> for EsdsBox {
 
 trait Descriptor: Sized {
     fn desc_tag() -> u8;
-    fn desc_size() -> u32;
+    fn desc_size(&self) -> u32;
 }
 
 trait ReadDesc<T>: Sized {
@@ -325,13 +326,15 @@ impl Descriptor for ESDescriptor {
         0x03
     }
 
-    fn desc_size() -> u32 {
+    fn desc_size(&self) -> u32 {
+        let dec_config_size = self.dec_config.desc_size();
+        let sl_config_size = self.sl_config.desc_size();
         3 + 1
-            + size_of_length(DecoderConfigDescriptor::desc_size())
-            + DecoderConfigDescriptor::desc_size()
+            + size_of_length(dec_config_size)
+            + dec_config_size
             + 1
-            + size_of_length(SLConfigDescriptor::desc_size())
-            + SLConfigDescriptor::desc_size()
+            + size_of_length(sl_config_size)
+            + sl_config_size
     }
 }
 
@@ -373,7 +376,7 @@ impl<R: Read + Seek> ReadDesc<&mut R> for ESDescriptor {
 
 impl<W: Write> WriteDesc<&mut W> for ESDescriptor {
     fn write_desc(&self, writer: &mut W) -> Result<u32> {
-        let size = Self::desc_size();
+        let size = self.desc_size();
         write_desc(writer, Self::desc_tag(), size)?;
 
         writer.write_u16::<BigEndian>(self.es_id)?;
@@ -417,10 +420,9 @@ impl Descriptor for DecoderConfigDescriptor {
         0x04
     }
 
-    fn desc_size() -> u32 {
-        13 + 1
-            + size_of_length(DecoderSpecificDescriptor::desc_size())
-            + DecoderSpecificDescriptor::desc_size()
+    fn desc_size(&self) -> u32 {
+        let dec_specific_size = self.dec_specific.desc_size();
+        13 + 1 + size_of_length(dec_specific_size) + dec_specific_size
     }
 }
 
@@ -467,7 +469,7 @@ impl<R: Read + Seek> ReadDesc<&mut R> for DecoderConfigDescriptor {
 
 impl<W: Write> WriteDesc<&mut W> for DecoderConfigDescriptor {
     fn write_desc(&self, writer: &mut W) -> Result<u32> {
-        let size = Self::desc_size();
+        let size = self.desc_size();
         write_desc(writer, Self::desc_tag(), size)?;
 
         writer.write_u8(self.object_type_indication)?;
@@ -504,7 +506,7 @@ impl Descriptor for DecoderSpecificDescriptor {
         0x05
     }
 
-    fn desc_size() -> u32 {
+    fn desc_size(&self) -> u32 {
         2
     }
 }
@@ -564,7 +566,7 @@ impl<R: Read + Seek> ReadDesc<&mut R> for DecoderSpecificDescriptor {
 
 impl<W: Write> WriteDesc<&mut W> for DecoderSpecificDescriptor {
     fn write_desc(&self, writer: &mut W) -> Result<u32> {
-        let size = Self::desc_size();
+        let size = self.desc_size();
         write_desc(writer, Self::desc_tag(), size)?;
 
         writer.write_u8((self.profile << 3) + (self.freq_index >> 1))?;
@@ -588,7 +590,7 @@ impl Descriptor for SLConfigDescriptor {
         0x06
     }
 
-    fn desc_size() -> u32 {
+    fn desc_size(&self) -> u32 {
         1
     }
 }
@@ -603,7 +605,7 @@ impl<R: Read + Seek> ReadDesc<&mut R> for SLConfigDescriptor {
 
 impl<W: Write> WriteDesc<&mut W> for SLConfigDescriptor {
     fn write_desc(&self, writer: &mut W) -> Result<u32> {
-        let size = Self::desc_size();
+        let size = self.desc_size();
         write_desc(writer, Self::desc_tag(), size)?;
 
         writer.write_u8(2)?; // pre-defined