Aka "concert A," a musical note that is the A above middle C, having a frequency hertz of 440 Hz. One hears it every time an orchestra tunes up at the start of a rehearsal or performance. Traditionally done by the first oboe, as that instrument produces the "purest" tone of that frequency.
[video=youtube;gPBgWWZhcA4]http://www.youtube.com/watch?v=gPBgWWZhcA4[/video]
http://www.youtube.com/watch?v=gPBgWWZhcA4
[video=youtube;gPBgWWZhcA4]http://www.youtube.com/watch?v=gPBgWWZhcA4[/video]
http://www.youtube.com/watch?v=gPBgWWZhcA4