Lower Frequency Specker System For Swallow Bird (Walet)

Hi :
firstly i would like to say sorry about post something other than mobile stuff .
now i am seaching some divice that can play bird sound in low frequency so that human can not heard the sound.
accually we are playing the sound to attract the bird to house, because of the sound are too loud and cause noisy environment, so i am planning to search or build some device can play low frequency sound so that only swallow can hear it .

thank for the help

For people who have walet or bird house please private message me :D

we are using bread/seed
this solution is 10000%
:)

pico @ THX for the reply
but the swallow bird are not eating breed or seed for food. they eat insert.:)

have you tried reasoning with them ? perhaps they are unhappy with the menu !


Kind Regards to the twitchers ;)

:D