Parliament

I'm going to be spending most of next week in Parliament (the legislative body, not the George Clinton band, more's the pity) as part of the Royal Society's MP-Scientist pairing scheme. I've been paired with Mark Oaten, my local constituency MP. Subject to connectivity and time I'll be blogging about it here.

There's no such thing as a free anything

It's easy to destroy everything in the universe. Every student learning acoustics begins by doing so, and they do so with four simple words: "consider a free field". Actually they don't destroy everything, they leave the air, or some other compressible fluid, in fact they fill the newly denuded universe with it which, for all I know, might even leave the universe ahead on the deal. Electromagnetics students get to destroy everything.

Anyway, there's nothing left, apart from maybe a source at the origin, which is a pity for those long enough in the tooth to remember when the old place really looked like something, but it does make the equations considerably easier. That's because now there are no reflections, so energy leaving the source is guaranteed not to come back, and a host of other simplifications. A free field is so useful that we often go to great lengths to create a physical approximation to part of one by building (or hiring) an anechoic chamber.
This is ISVR's large anechoic chamber and it's a lot more impressive when you're standing in it. But it's still an approximation, albeit a close and useful one. A truly free field does not exist.

Economists also have a useful simplification, called a free market. You don't have to bulldoze the universe to make one, but you do have to ensure that all prices are set by mutual consent with no monopolies or cartels and a few other things. No real market behaves exactly like this, though many are close enough to make it a reasonable starting point.

Here's the thing: I don't know any acousticians who believe that free field solutions are inherently better than others and should be pursued before all alternatives. As it happens, non free field problems can often be solved by using a free field Green's function and the appropriate boundary conditions but that still isn't necessarily the best way to do it. I'm just saying.

I'm aware that I've drawn a far from perfect analogy. Sometime I'll discuss whether a perfect analogy is useful - in acoustics at least.

Forthcoming appearance

OK, lesson learned, don't start a blog just before the start of the academic year. And that goes double if you're getting to grips with a new interface.

Anyway, I'm going to be holding a conversation at Portsmouth Cafe Scientifique entitled 'A Scientist's Guide to the Orchestra' on November 25th. I say 'conversation' rather than 'talk' because the idea with cafes scientifique is that it's as much about the audience as the speaker. I've been asked to give it a mathematical flavour, since the 'evening is organised in conjunction with the Fifth Annual Festival of Mathematics and Art' though ultimately, of course, the direction is up to the audience. The last time I did this, at Southampton Science Cafe was a lot of fun, even though I talked too much and couldn't get my musical saw to sound when I wanted.

Talking of Maths, Stefan Bleeck and I will be giving a talk to Southampton's applied maths group on neural coding in the auditory system on November 11th.

A rift in the lute?

At the beginning of this month (before this blog existed) there was a flurry of press releases about some European researchers who'd recreated the sound of the epigonion, an extinct kind of psaltery (though everyone's been calling it a lute). It's a nice story, but the sounds, to me, are rather disappointing. A clue as to why this might be comes from the project webpage:

"Although physical modeling was not a new concept in acoustics and synthesis, having been implemented using finite difference approximations of the wave equation by Hiller and Ruiz in 1971, it was not until the development of the Karplus-Strong algorithm, the subsequent refinement and generalization of the algorithm into the extremely efficient digital waveguide synthesis by Julius O. Smith III and others, and the increase in DSP power in the late 1980s that commercial implementations became feasible."

The extreme efficiency of digital waveguide modelling comes at a price. With this method it's very hard to capture dispersiveness* or nonlinearity, both of which can make an important contribution to the distinctiveness of the sound of an instrument. Of course, I'm not saying these researchers didn't address these issues, but I'll have to wait until there's a published paper to find out how.

The method's great advantage is that it's probably the only one currently able to compute waveguide response quickly enough to work in real-time, which is vital if you want to be able to change some parameter during a note while playing your synthesizer. But all the computations for this project were done off-line; apparently it took four hours to compute 30 seconds of sound. Other methods (such as finite elements) would take longer but would be better placed to capture the physics.

Even if we assume perfect (or as good as possible) modelling there's another issue endemic to musical instrument acoustics research: how detailed to make the model? Two violins that look very different can sound very different to the people who matter, the players. Wood is complicated stuff, just read chapter 6 of J. E. Gordon's excellent "The New Science of Strong Materials", a masterpiece of science writing. The chapter is headed by Charles Gurney's couplet "Plastics are made by fools like me, /But only God can make a tree". Two pieces of the same wood, the same size and the same shape can behave very differently. So when you model your epigonion how much detail do you put into your model of the wood? And what about the pick with which it was plucked, that can make a big difference too. Again, I'm not saying these researchers haven't thought about these issues, but I'm pretty sure the news outlets who've taken their press release and turned it into a "triumph of science" story haven't. I'll be interested to read the paper when it comes out, I'll post my comments here.

* Not so, see later post.

Welcome

This is the obligatory 'hello world' post to help me get to grips with the technology. It may well get deleted later.