I have also been pondering how far could one extend the calling range of external sounds?
I guess there is no straight answer to this - it is dependent on:
1) Power of the sound source i.e. to the tweeter
2) How the sound energy is projected in one direction
3) How sound energy is dissipated (or absorbed) while travelling in air
4) How sensitive are the birds' hearing
Well, I don't have all the answers.
As for (1), it is simply the raw power one can pump into the tweeter and how much it can handle without too much distortions.
For (2), it can be improved by design with better engineering solutions. The tweeter horn is to project the sound forward. Then there is the 'bazooka' design to extend the range further.
On (3), I did some readings and gained some insights on the subject.
At room temperature & R.H. 65%, a 1KHz sound travels 35Km to loose 6dB SPL (i.e. half amplitude) through absorption loss!
Sounds good but there is the inverse square law that affects the higher frequencies. What this says is that doubling the frequency would mean the same loss occurs at 1/4 the distance; or a 10KHz sound would be 1/2 level a distance 100 times shorter - to about 350m! A 20KHz sound would be 88m at 1/2 level.
I suppose the bird sounds is within the 10KHz - 20KHz range, so this gives an idea what it is like after the sound leaves your tweeters into the air.
What is worthy to note from above is that the medium of sound transmission, i.e. air itself, has a low pass filter characteristic inherently. Higher frequencies are attenuated much more relatively at further distances. In other words, it loose clarity through the losses of its higher frequencies. So no matter how good are your tweeters, sound quality degrades as distance increases is inevitable.
For (4)... totally no clue! If somebody know something, I would love to know.
1 comment:
so, what is the advantage of powerful directional tweeters when it is attenuated by air ! One problem that I have seen from powerful bazookas being used is that the birds are so attracted to the bazookas and miss the in/out hole that we so desperately want them to go into.
Solution is to have many normal tweeters as near as possible to the hole so an approaching bird will not miss it. Also as soon as the new bird enters, the sound must seem to come from the nesting areas and not the outside as it may then turn out.
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