Something that will improve the number of swiftlet to your entrance hole by at least three times the normal arrangements.
My recommendation is the installation of hexagonal tweeters just above your main entrance holes.
At my recently revamped BH at Bukit Selambau Kedah I proved to the BH owner how fantastic the tweeters helps to draw more birds to the top of his entrance holes.
The front side entrance was enlarged and the hexagonal was positioned just above the entrance hole in the middle part about 1.5 feet above the roof floors.
Similarly at the back part of the same BH with another entrance with top entry system was also equipped with a hexagonal tweeter too.
For the back entrance hole I insisted that the tweeter must be slightly above the peak of the roof highest position. My main reason being to get the sound to 360 direction with no blockages from the roof.
There were five 2"x 3" tweeters on each hexagonal tweeter.
The sound generated will usually travelled a couple of meters to attract any potential tenants into the BH.
Most failed BHs are not equip with any of these kind of gadget.
To those who are new or having problem to attract more birds to their new BH try this technique.
It won't cost you more than 250 ringgit to get one unit install.
The result will be much better as compared to without it.