diff --git a/lib/synth-audio.js b/lib/synth-audio.js
index 514cb72..53b0eb4 100644
--- a/lib/synth-audio.js
+++ b/lib/synth-audio.js
@@ -331,8 +331,9 @@ async function _synthOnPremMicrosoft(logger, {
   }
 
   try {
+    const trimSilence = filePath.endsWith('.r8');
     const post = bent('POST', 'buffer', {
-      'X-Microsoft-OutputFormat': 'audio-16khz-128kbitrate-mono-mp3',
+      'X-Microsoft-OutputFormat': trimSilence ? 'raw-8khz-16bit-mono-pcm' : 'audio-16khz-32kbitrate-mono-mp3',
       'Content-Type': 'application/ssml+xml',
       'User-Agent': 'Jambonz'
     });