Dumb question of the year…take two whistles - aluminium alloy with delrin or tufnol fipple for the sake of argument. In simple terms, why is one pure sounding and the other chiffy and is it possible to change these characteristics in an individual whistle to some extent? 
Trisha
Not a dumb question at all, but also not easy to answer simply.
It’s all in the voicing. There are many, many little dimensions to the head section of a whistle: width, height and length of windway, flat vs curved windway, angle of blade, distance from windway to blade, sharpness of blade, etc. Each one of these has an effect of the sound of a whistle: it’s volume, tone, chiffiness, purity, ease of jumping octaves… In a handmade whistle, this is where the magic comes in-- no two handmade whistles ever sound EXACTLY alike ( at least mine don’t..). Even whistles of the same wood, with the same metal fittings will sound subtly different.
Yes, it is indeed possible to change these characteristics to some degree to produce different sounds. For example, it is pretty easy to change the sharpness of the blade which is one factor that seems to affect chiffiness. Width of windway and blade seems to affect total loudness, and this is easy to vary.
Yep, everything Paul said and also if the plug comes out ever so slightly past the windows opening will add a little chiff, how far the fipple is above or below the blade will affect how solid or chiffy the sound, difficult or easy to jump octaves… man theres a lot of things that just the tiniest little varience will make sometimes a very noticceable difference… and we’re talking just a couple of thousandths of an inch or less even in some cases.
Take care,
John
Thanks both - wow there’s a lot of fun to be had tweaking whistles 
Trisha
Seriously - thanks…3 hours hard whistling at rehearsal tonight and I can’t type straight - let alone think..
Trisha
OOHHH! You were talking whistles! I thought you were talking about some of the posters…
I agree with what Paul has Stated and most of what John says. There are a great many things that can produce “chiff”. I will give an “Extremes” answer to your question.
A deep small voicing hole with a steep labium ramp will give a reedy or pure mellow tone with little “chiff”, like a Recorder. A thin voicing hole with a shallow flat labium ramp will give a flutey tone with a lot of “chiff”, like a Tabor pipe.
I say the Major factor in chiff production is the angle of the labium ramp and the sharpness of its edge. This feature of the voicing determins the stability of the oscillations.
“Chiff” is high frequency harmonics and a momentary “unstableness” or fluttering oscillation caused by a shallow/thin labium with a sharp edge(for the most part). This is enhanced by an undercut “reflector”, like a Generation Whistle has and by a large bore, toneholes and voicing window. High frequencies are weak in energy and need more free space to survive, thus Large = Chiff
Many C&F members already know that filling in the “undercut reflector” on a Generation will mellow the “chiff” by eliminating the high harmonic reflections produced by the labium ramp. Most know that rounding the edge of the labium ramp will help to stabilize “chiffy/squealing” whistles by creating more stable oscillations.
You can improve on this further by adding “Blu-Tack” or similar putty to the top face of the labium ramp to increase the “angle of attack”. You can add a little more on the underside of the ramp near the labium edge to cause a “spiral wind eddy”(like a wood shaving) that will enhance stable oscillations.
Another “tweak” would be to round off the sharp edges of the windway exit. Inspection of a Recorder voicing will show this. This tweak reduces any negative eddys caused by the sharp edges affecting the smooth Laminar flow airtream as it oscillates above and below the labium ramp edge.
(sorry for the long post, I was inspired ) 
As they say in these parts, I’m gobsmacked (by this inspired rendition).
As for Stewy…I wonder in which camp he’d feel most comfortable 
Trisha, calling it a day at 00:23 local - getting too old for filling up haybarns…yawn.
Chiff, ? Labium ramp..? Sorry did I get onto an obscure porn site by mistake!?
I just blow the things and try to get a perfect note with my own pathetic attempts at manufacture. What in heavens name is a labium ramp? What in actual fact is chiff?
The “labium ramp” is more commonly referred to here as the “blade”. Chiff is defined as any sound coming out of the whistle that ain’t music. It might be harmonics, it might be blowby air above, below, or to the sides of the vibrating column, or any combination. Might even be a vibration from a loose head.
Another “discovery” I made, in trying to make a louder whistle, is that it’s possible to make the window too wide - you can reach a point where your whistle is quite loud, but will only play reliably in the lowest octave. I found that on the way to the Dreadnought, which will blast past multiple fiddles, and overpower the bodhran! 
The comments about blade angle, sharpness, hidden “feechurs” in the castings, etc., are all very good, and explain much.
And chamfering the top inside edge of the fipple plug a bit will help with eliminating some chiff, as well as making the whistle a tad louder. Thanks to Stacy O’Gorman of Alba Whistles, for that bit of knowledge! 
Have fun!
serpent