Hello,
What are the major differences between Overton Low D and Alba Low D?
The backpressure?
Is it easy to play the second octave with Alba Low D? Need air flow or air pressure? Both?
Thanks a lot for your informations,
This thread has an eery quality of déjà vu… 
Main differences:
Overton: Backpressure, need not so much air, sixth hole easy to reach
Alba: no backpressure, needs much air, sixth hole harder to reach (not as bad as other whistles)
Tone/sound:
Overton: unparalleled; Alba: very beautiful
Overton takes more getting used to and more control. If you don’t play the Overton much, the bottom end is a bit tricky to fill out. The Alba is easier right from the beginning, but harder to control, esp. between the octaves and in the upper reaches.
What is the problem with 6th hole of Alba, the distance is too long between 5 and 6 holes? or the diameter hole is too big
I didn’t say it was a problem. I find the Alba comfortable to play.
I was refering to the distance between the fifth & sixth hole. I’ve not paid attention to hole diameter that much, it’s not an issue for me. I think the Alba sixth hole is smaller than the Overton sixth hole (it probably has to be, being further from the blade, if I remember my whistle physics).
There are some whistles where I do find the spacing of the 5th & sixth holes a problem: The Susato comes to mind.
I don’t think that between the Alba & Overton hole spacings etc are significant. If you are going to decide between the two, base your decision on the sound (very close call, I prefer the Overton, but like the Alba sound better than all the other low Ds I’ve plaid) and the breath requirements. Overton: little breath, high back pressure. Alba: a lot of breath, no back pressure.
I bought an Overton few weeks ago, you know, see my review
http://chiffboard.mati.ca/viewtopic.php?t=15097&highlight=review
I am very very glad.
I hesitate to buy an Alba, but if the difference is to close I don’t want spend money for that, do you understand…
My view on that would be: learn the play the Overton before you get another whistle. But realize that this the kind of good advice I give but don’t follow myself. 
2 months and it’s Chritmas !!!