AMR

AMR is a patented audio file format used primarily for voice recording.

Overview

AMR stands for Adaptive Multi-Rate audio. It features high compression by limiting bandwidth and varying the sample rate. An AMR codec was adopted for speech use by 3GPP. The narrow frequency range of 200–3400 Hz is recorded at variable bit rates ranging from 4.75 to 12.2 Kbit/s. Good quality speech starts at 7.4 Kbit/s. It will not provide good results on audio other than speech.

For more information

http://en.wikipedia.org/wiki/Adaptive_Multi-Rate_audio_codec