I have always emphasized on the importance of sound frequency below 20Hz for Internal sound when doing Swiftlet sound recording. I believe the frequency of Swiftlet chicks (baby birds) are much lower compared to the External sound which plays more of Adult birds sound. After many years of experiment and experience of witnessing young Swiftlets hanging on the Internal tweeters for hours after an Internal sound was played. I knew both Internal and the External Sound frequencies are entirely different. They both attract different Swiftlets (Different listeners). Both have their own set of frequencies. With such knowledge, I remixed and recorded them according to their respective frequency. I have been doing this since 2007.
Yesterday while searching for information on sound frequency, I stumbled on an article which strengthen my belief. All this while I was right about the sound of Swiftlet chicks which produce lower sound frequency compared to Adult Swiftlets. Though the article was about human hearing capability, but I believe it shares some similarities.
“Audio equipment must be able to reproduce frequencies high enough and low enough to be realistic. The human hearing range, for healthy young persons, is 20 Hz to 20,000 Hz.  Most adults can't hear higher than 15 kHz. CDs are capable of reproducing frequencies as low as 10 Hz and as high as 22.05 kHz, making their frequency range adequate for reproducing all the frequencies that people can hear.
The equipment must also provide no noticeable distortion of the signal or emphasis or de-emphasis of any frequency in this frequency range”.
Notably it shows that we need a different set of frequencies for different Internal and external sound. Tuning the Amplifier “Bass, Middle and Treble” can effectively alter the sound. The Bass and Middle control knobs are vital for the effectiveness of Internal sound while External sound relies on the Treble knob. The same applies to Amplifiers that have multiple control knobs “equalizer”.
There are many types of Amplifier on the market, but not all are specially made for Swiftlet farming. Most Amplifiers in the Market are factory tuned to a frequency towards 20,000Hz. This makes the sound more crisp and sharp which is good for Instrumental and Vocal quality. But Swiftlet chirping sounds response is different. It may be good for our hearing, but the sounds came out from these Amplifiers may not attract many birds.
To Maximize on this, 2 most important things we need to have are:
1) The original recording of the Swiftlet sound has to be recorded to the frequency based on its application. ( Internal or External use).
2) The Amplifier that plays these sounds (Internal or External) have to be properly tuned according to its frequency. Bear in mind the frequency of both Internal and External are set differently.
Hope the above information would help you to tune your Amplifiers correctly.
Good Swiftlet farming All!.