# [PD] physical modelling/general pd

Mark Sexton mark.sexton at port.ac.uk
Fri Aug 8 11:24:34 CEST 2008

```Hi Julian
Building a physical model of a wind chime might be easier than you think, if
you use modal or banded waveguide approaches to physical modelling rather
than the brute force approach of pmpd.

If you think of the wind chimes should as stiff bars, banded waveguides
would be ideal and are much more computationally efficient to implement than
brute force approaches: a resonant filter and delay per mode you want to
synthesis.  I'd recommend perhaps starting with a simple modal
implementation using filters and build up from there. This paper gives a
good introduction:
http://www.cs.ubc.ca/~kvdoel/publications/modalpaper.pdf

If you're not familiar with modal synthesis and banded waveguides there's
plenty of information online and Perry Cook's book gives a good overview of
a range of approaches to modelling.
http://www.cs.princeton.edu/~prc/AKPetersBook.htm

Some starting hints if you want to go down this route:

1. Create an impulse: a buffer of noise or single sample impulse
2. Feed this into perhaps 5 band pass IIR filters with a very narrow Q,
these will provide your resonant modes for each chime.
3. The frequencies of these filters will probably be non-integer multiples
of the fundamental, eventually you can get these by analysing an actual wind
chime, but if you wanted to build a proof of concept now then these are
typical modes of an aluminium bar (you can find further modal frequency
ratios in the Csound manual):
[1, 2.756, 5.423, 8.988, 13.448, 18.680]
4. Scale the outputs of each of the resonant filters as appropriate, this
should be straight forward once you've done an audio analysis of your wind
chime.

At this point you have a simple resonating model of a wind chime.

5. Perhaps replace the impulse: you can remove the resonant components of
your wind chime recording and this will leave you with the original noise
impulse. Using this to trigger your model should help improve realism.
6. Create a banded waveguide version, by adding feedback delays for each
mode. (have a read of this paper and a look at Fig. 4):
http://soundlab.cs.princeton.edu/publications/1999_icmc_bar.pdf

There's a few further tweaks and improvements that can be done, but
something along these lines should give a good result, be fairly easy to
implement and run more efficiently than brute force.

Happy to chat more on or off list on the physical model side or algorithmic
composition side, but you may find it easier than you thought once you get
going.

All the best

Mark Sexton
Senior Lecturer
MSc Computational Sound
University of Portsmouth

>
> Message: 2
> Date: Thu, 7 Aug 2008 12:30:51 +0100
> From: "Julian Brooks" <julian.brooks at virgin.net>
> Subject: [PD] physical modelling/general pd - mentor/tuition sought
> (money offered)
> To: <pd-list at iem.at>
> Message-ID: <000001c8f881\$0d514a40\$27f3dec0\$@brooks at virgin.net>
> Content-Type: text/plain; charset="us-ascii"
>
> Hi all,
>
>
>
> I have a 12 month project as part of a masters degree, where I wish to build
> a physical model of a wind chime.  I then want to use the interface to play
> some of my indeterminate compositions.  I was going to attempt it for my
> undergraduate degree but realised that it was far too complex for the
> available time that I had then.
>
>
>
> I have been using pd for a few years now, list lurking, working through
> basic examples, building simple tools, using other peoples patches etc.  But
> this is too complex for me to do on my own.  At my uni there isn't anyone
> with better skills than me and I don't know of any local fellow patchers.
>
>
>
> Now as a musician, when I need to up my skills, I will look to find some
> lessons when I have got as far as I can on my own.
>
>
>
> So here goes...
>
>
>
> Is there anyone with an hour a week to spare who can offer some
> mentoring/tuition for what we can deem to be the 'going rate'.  I am more
> than happy to do this remotely/online, I'm sure there is a way we can work
> it out.  There would be full credit given of course.
>
>
>
> Pmpd seems like the way to go with this.  I have worked through the
> examples, and, although I have my eye on what examples I would presume to be
> the best starting points, I'm struggling to get started.  The physical
> modelling is where I first need to start but there's loads of pd stuff I
> would like to be able to work through with someone, so this could be a (me
> love you)longtime regular small money earner, if anyone's interested.
>
>
>
> I am in Hebden Bridge, West Yorkshire, UK, by the way.  Any pd'ers local,
> give us a shout.
>
>
>
> Best wishes to all,
>
>
>
> Jb
>

```