diff --git a/README.md b/README.md
index 601a14b..071f964 100644
--- a/README.md
+++ b/README.md
@@ -50,6 +50,7 @@ pi_fm_rds [-freq freq] [-audio file] [-ppm ppm_error] [-pi pi_code] [-ps ps_text
 All arguments are optional:
 
 * `-freq` specifies the carrier frequency (in MHz). Example: `-freq 107.9`.
+* `-vol` specifies the volume. Example: `-vol 4`. Default : 1
 * `-audio` specifies an audio file to play as audio. The sample rate does not matter: Pi-FM-RDS will resample and filter it. If a stereo file is provided, Pi-FM-RDS will produce an FM-Stereo signal. Example: `-audio sound.wav`. The supported formats depend on `libsndfile`. This includes WAV and Ogg/Vorbis (among others) but not MP3. Specify `-` as the file name to read audio data on standard input (useful for piping audio into Pi-FM-RDS, see below).
 * `-pi` specifies the PI-code of the RDS broadcast. 4 hexadecimal digits. Example: `-pi FFFF`.
 * `-ps` specifies the station name (Program Service name, PS) of the RDS broadcast. Limit: 8 characters. Example: `-ps RASP-PI`.