The Physics of Bagpipes

[Sparky49]

Hi everyone!

 

Firstly, let me say hi to everyone! I hope to participate as much as I can - both asking and answering questions when need be. I'm sure I'll enjoy it - this looks to be a pretty good forum!

 

Now onto my first question. I have very little understanding about fluid and 'gas' mechanics, however I'd like some help when it comes to thinking about a certain instrument - the bagpipes.

 

I've played them for about nine years now, and throughout that time, there has been quiet a 'debate' over wether or not the material of the bag affects the tone of the bagpipe. Please understand that 'proper' playing of the bagpipes results in a nice smooth tone - not a bag of screeching cats!

 

The biggest debate is over the use of synthetic and natural bags. Natural bags made from sheepskin have pores in them from the skin, so we 'season' the skin with a gel like substance. This not only seals the bag, but it also draws moisture, protecting the reeds from excess moisture. However, with synthetic bags (gore tex, for example) this seasoning isn't needed, but the problem of moisture still exists. So we use what is called a moisture control system, where the air is forced through silica gel in a canister before being directed into the pipes via tubes.

 

Here is where I'd like some clarification.

 

I presume that the absence of tubes and unnatural flow of air would cause a difference in the movement of the air. With a sheepskin bag, the air is free to move where it wants, but with a synthetic bag, air is blown into the bag where it is forced throught gel and through tubes before getting to the pipes.

 

But what difference would this make? Would it be enough to audibly affect the sound?

 

Second, presuming we removed the moisture control from the synthetic bag, would the actual material of the bag directly cause a difference to the sound produced? I don't believe it would, as I believe the sound to be created from the reed upwards, but then again, the flow of air before the reed might affect its operation.

 

 

 

I realise there's a lot said here, I'm sorry if I haven't made myself clear. If you need any more information, please ask, and I will be only too happy to help.

 

I also apologise if I've posted this in the wrong place.

 

 

 

Sparky

[Phi for All]

It would seem to me that the bag, natural or synthetic, is providing air to the drones but the reed vibrations are what produces the sound through the drones and chanter. The bag shouldn't greatly affect the timbre of what comes through the drones.

 

To me, the moisture control seems to be a more pressing matter. If the bagpipes are wet, they won't make good kindling for an accordion fire.

[Enthalpy]

The reeds produce a sound that propagates downward in the pipes (which determine the pitch) and upward in the bag.

 

The downward sound is strong because the pipes couple it efficiently into air by resonance. That is, the reed has a high impedance (= pressure oscillations divided by volume throughput oscillations), the air a low one, and the quarter-wave pipe between them converts the high impedance into a small one; this impedance matching makes the power transfer efficient.

 

The upward sound is weaker for having no such impedance adaptation, but for instance the higher harmonics of the note, which are very important for sound quality, aren't amplified on the downward path neither. So the upward path certainly plays a role.

 

The physical properties of the bag certainly influence how these higher hamonics pass to the surround air. The first being the bag's mass per surface unit, but others must have an effect: the humidity at the inner surface (not easy to explain, few people are aware of, but it seems to act in a clarinette), maybe the sound absorption by the material, the roughness of the inner surface, the speed and attenuation of bending waves travelling in the bag's material...

 

As compared with direct radiation through the bag, I suppose the path further through a moisture absorber and through the player's mouth, throat and chest is less important to the public - but is all important to the player, who perceives the tone via his teeth and skull conducting to the ears, and through his chest for the lowest notes. For instance, a saxophone sounds completely different for the player and for the public; the difference is less important with a bassoon (no hard mouthpiece).

 

So my position is:

- Can be explained if needed - but less important than the downward path

- Try if the public feels a difference, not just the player

- Play a harmonica within bags of said materials, and listeners outside will hear a difference. But this difference contributes less than the strong sound through the downward path of the bagpipe.