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

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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