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Re: [Phys-L] A beautiful (short) demo for sound and waves...



Ken,

If the shutter speed is high enough you can eliminate any motion blur.
(This is something different than the frame rate.)

The motion of a vibrating string on an instrument is surprisingly large.
If played really loud it's not difficult to drive a string to vibrate
against the finger board or against another string. It's slowest at the
extreme position, so the collision can be soft. But I'll agree that this
video was created with the string tuned about as flat as possible to reduce
the speed of the motion. It also helps to exaggerate and dramatize this
driven oscillation mode.

I really want to know more about the making of this video. The depth of
focus is amazing. The choice of camera angle really captures everything
very well. And the timing of the video shows the development of the sound
envelope in fantastic detail. You can see the onset of the bow attack, and
the development of the tone as the string rings alone and the sound fades
away. Now I want to see pizzicato, spiccato, and other techniques.

Paul

On Fri, Aug 26, 2016 at 3:39 PM, Ken Caviness <caviness@southern.edu> wrote:

Thanks, Paul, for the additional "footnotes" explaining what we see on the
video, http://physicsfootnotes.com/physl-vibrating-string. Very
helpful! I was not previously familiar with the term "Helmholz resonance"
(or if I learned it in PDE had since forgotten it), but I immediately
noticed the phenomenon in the video and it gives credibility to it. I
agree that we're _not_ seeing anti-aliasing artifacts here.

But I'm troubled by the high amplitude of the fundamental mode vibration,
and wonder whether this string really was tuned to the expected G3 (~ 196
Hz). Perhaps it was under reduced tension, resulting in an abnormally
large amplitude, almost hitting the neighboring string! But perhaps it is
my expectation which is incorrect here, I must confess that I've never
looked particularly closely at a violin string from close range while it
was being played. I think I'll go home now and make the observation on the
instrument I took lessons on for 1 year back in the dark ages....

Yes, I expect the maximum amplitude to be approximately identifiable by
the visible blur of the string. The string is moving slowest near max
amplitude, and therefore spends a lot of its time in those regions, so the
blur should be darkest there. Let's see, it would be 1/sqrt(2) ~~ 71% of
the time at greater than 1/2 amplitude, 38% of the time at greater than 3/4
amplitude, and 15% of the time at greater than 90% amplitude, if I'm
setting the problem up right.

Ken

Kenneth E. Caviness, Ph.D.
Professor of Physics, Southern Adventist University

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@www.phys-l.org] On Behalf Of Paul Nord
Sent: Friday, 26 August, 2016 15:10
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] A beautiful (short) demo for sound and waves...

Doug,

That's a very good question. There are not good references with the video
to describe the camera that was used. But it's pretty clearly a high speed
camera which, most likely, uses a global shutter. With the rolling shutter
problems, parts of the string will show motion blur. Everything here stays
in sharp focus. This seems to indicate that the frame rate is high enough
to properly capture the motion.

The lowest note in the violin (G) is about 200 Hz. To capture individual
vibrations of the string, the frame rate of the camera needs to be many
times this frequency. The video shows dozens of images for each
oscillation of the string. If I had to guess, I'd say that this is taken at
about 5000 frames per second and slowed to 30 frames per second. But I may
be off by an order of magnitude.

You'll also notice at the end of the video the shape of the string changes
to be mostly the fundamental node. The Helmholtz resonance, with the sharp
corner that zips from one end of the string to the other, is the expected
mode from a string being driven by a bow. After the driving motion of the
bow stops, the higher frequency overtones damp out quickly (or are coupled
to drive the fundamental) and the shape of the string changes to simple
jump-rope type modes. Also notice that only the horizontal transverse
waves are excited while the bow is in contact. Once the bow moves, these
also couple to identical vertical modes.

Documentation on the camera setup would be more convincing. But there is
just too much detail there to be explained by rolling shutter aliasing.

Paul





On Fri, Aug 26, 2016 at 12:22 PM, Doug Peltz <dougpeltz@gmail.com> wrote:

Is this video actually showing the phenomenon at hand? I've seen other
videos like this, of guitar strings, and have heard that it's a camera
artifact. Something to do with what's called "rolling shutter." I'd
love clarity here, if any of you can help! Here's an article I had
read which made me wonder about this:

https://www.washingtonpost.com/news/speaking-of-science/
wp/2015/01/20/why-do-guitar-strings-look-so-wibbly-wobbly-
in-smartphone-videos/





On Fri, Aug 26, 2016 at 9:23 AM Paul Nord <paul.nord@valpo.edu> wrote:

That's a beautiful video. You clearly see the Helmholtz resonance
mode.
Notice how most overtones damp out quickly after the bow is lifted
off
the
string. Just the fundamental and a couple of harmonics clearly ring.

Paul

On Fri, Aug 26, 2016 at 11:17 AM, Dan Beeker <debeeker@comcast.net>
wrote:

Slip - stick - Definitely not a sine wave! Nice video.

Dan

Date: Thu, 25 Aug 2016 18:11:16 -0700
From: "Derek McKenzie" <derek@physicsfootnotes.com>
To: phys-l@phys-l.org
Subject: [Phys-L] A beautiful (short) demo for sound and waves...
Message-ID:
<20160825181116.941725d6a6fe87b632a3484f7e5c2b
cf.6305af42b4.wbe@email19.godaddy.com>

Content-Type: text/plain; charset="utf-8"

When my 9 year old daughter didn't believe that the musical
instruments
she practices so diligently all work by vibrations, I showed her
this superb slow motion footage of a vibrating violin string...

http://physicsfootnotes.com/physl-vibrating-string

It seemed to do the trick, as she then played the whole video
through several times over ;-)

It is definitely worth showing your class the whole mesmerizing
video (it's only a few minutes long).

Cheers,

Derek

Derek McKenzie
PhysicsFootnotes.com

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