I might sound crazy but this is what I strongly believed in:
"Every good external sound should have a compatible internal sound."
You might think that I must be mad or crazy but from what I have tested and doing research on this particular issue I think I am not.
Imagine you are the bird that passed by a BH with this unique sound that you cannot resist.
You came towards the BH and start searching for the source of this particular sound.
You found it and followed into the BH nesting room.
You stayed there to enjoy the soothing or listening to that beautiful mating call sound and you are carried away till after 8:00pm.
At 8:00 pm the sound suddenly stopped and the new sound on those tweeters are now a new sound that is not familiar to your ear.
I am sure you will get a bit pissed off and will find your way out of the BH to rush home.
However if the internal sound is almost the same as the external sound except it has more of those baby sound I am sure you will not be rushing out at 8:00 pm.
This is why I think the internal sound of your BH must be closed or compatible to your external sound just before the external sound is switched off.
I have carried out some research on this matter and what I did was to develop an internal sound to every good external sound.
What you need is to use a suitable sound editor software and you download the selected external sound that you are fond of.
Listen to the sound and find out if there are any where along the line there are any baby sound.
Once you have located this portion copy and start pasting onto the sound so that these baby sound are longer than it was.
If you practise doing this for some time you will eventually be able to make the sound to be your internal sound.
Once that is done you can try to run the external sound together with the new internal sound you have generated.
Try it and I am sure you will find something interesting which you have never given any thought before.
Only Pak Harry will tell you to learn something new.