"The frequency range of swiftlet echolocating calls is between 1-6 kHz, with most energy focused between 2-5 kHz (Fullard et al,. 1993). These frequencies fall into the range of normal human hearing. THERE IS NO ULTRASONIC COMPONENT (i.e., 20-160 kHz), which would be inaudible to people. (Cranbrook & Medway, 1965)"
Most tweeters claimed they can transmit 2-20 khz, somes are sold at 4 khz onwards.
If many birds flied into our house but refused to stay, one out of many reasons might be the wrong speaker!
We need to check the frequency of the sounds that come out from our speakers.
Even the tweeters claimed that it can produces sound at 2-20 khz, they do not!
Most of them produced from 4- 20 khz. Then the birds will only hear the sound from 4 khz onwards , which will not be a natural bird sound.
The right speakers should produce sound with frequency from 1 khz onwards.
But not many manufactures will produce speaker at this specification.
Because normal frequency range of tweeter is 2-20 khz, mid range will be 80 - 2000 hz., then the lowest frequency ones called woofer and sub woofer.
Shop carefully for the right tweeters.
Recently, I noticed through a CCTV observing my friend's new bird house that birds totally ignored the internal sound tweeter. Birds were more interested to all the external sound tweeters.
So we run a test to check the frequency of sound from both tweeters.
The picture below shows the frequency of sound recorded from his bird house internal sound speakers.
It showed that this tweeter delivered sound at frequency between 4000-9000 hz. Sound at below 4000 hz and higher than 9000 hz are very weak compared to the 4000-9000 hz range.
But for the below external sound tweeter that birds are crazy about it can deliver sound with the required frequency of 1400 hz upwards.
When compared the above graph with a graph obtained from a famous bird sound from a computer file, they are very similar that most sound are at at 1500-9000 Hz range. This means that this external tweeter can produce sound very similar to the original sound file from a computer.
After seeing the above and few more tests to confirm it, my friend decided to change all his speakers right away.
His bird house now has 360 birds staying within 85 days after operating.