Out-door low frequency microphone

[Ed Majden]

Anyone know where one could find a low frequency out-door microphone,
CHEAP! I'm an amateur astronomer that specializes in recording bright
fireballs (meteors) with an all-sky camera. Sometimes a delayed sonic boom
is generated and we would like to record the delayed audio along with the
video of the fireball.
Funds are very limited so a donation would be most appreciated. If
successful you would be included in any published papers. Please contact me
off list if you can help.
Thanks.