Several manufacturers make excellent electrical backup vario's LXNav V3 has a built in speaker is small and and one can purchase the associated UPS battery pack to give you 8 hours of electronic vario with audio, if the electrical bus fails. Nice little unit. I have not had a mechanical vario in a sailplane since 1999 when I purchased a Borgelt vario that would run on a 9 volt battery as a backup. If it truly is a backup, nice to have the audio.