SRS中如何设置音频filter

vhost __defaultVhost__ {
        ingest livestream {
        enabled      on;
        input {
            type    stream;
            url     rtmp://124.207.19.118/live/jctx;
        }
        ffmpeg      ./objs/ffmpeg;
        engine sd {
            enabled          on;
            vcodec    copy;
            acodec  libfdk_aac;
            abitrate    70;
            asample_rate    44100;
            achannels   2;
            aparams {
                af volume=0.5;   #设置声音音量为原来的一半，如果是2倍，这里就是2
            }
            output          rtmp://127.0.0.1:[port]/live/livestream_sd;
        }
}