# The Naked Scientists Forum

### Author Topic: How does one find the resonant frequency of a round bar of steel?  (Read 4919 times)

#### stanm

• First timers
• Posts: 1
##### How does one find the resonant frequency of a round bar of steel?
« on: 03/01/2015 17:34:26 »
how to find the resonant frequency of a round bar of steel and the tools needed
« Last Edit: 03/01/2015 21:10:05 by CliffordK »

#### David Cooper

• Neilep Level Member
• Posts: 1505
##### Re: science
« Reply #1 on: 03/01/2015 19:26:47 »
A piano and a stick?

#### evan_au

• Neilep Level Member
• Posts: 4116
• Thanked: 245 times
##### Re: science
« Reply #2 on: 03/01/2015 19:40:01 »
Physics textbooks often solve a simplified version of this problem to find the resonant frequency of a guitar string. They assume zero diameter for the string, and a finite mass per unit length (immediate contradiction visible here!).

If you want to solve it on a computer, use the Finite Element method. This can be used to solve many kinds of mechanical problems.

There are a variety of software packages (some of them free).

#### Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1915
• Thanked: 123 times
##### Re: science
« Reply #3 on: 24/01/2015 22:38:15 »
A piano and a stick?
You also need a piece of thin string, cotton if bar is small, to suspend from centre.

#### David Cooper

• Neilep Level Member
• Posts: 1505
##### Re: How does one find the resonant frequency of a round bar of steel?
« Reply #4 on: 25/01/2015 21:10:46 »
Gravity acts slowly enough for to manage without the string, and that also means the note produced won't be distorted by it at all.

#### PmbPhy

• Neilep Level Member
• Posts: 2762
• Thanked: 38 times
##### Re: How does one find the resonant frequency of a round bar of steel?
« Reply #5 on: 26/01/2015 07:22:38 »
Quote from: stanm
how to find the resonant frequency of a round bar of steel and the tools needed
It depends on the dimensions of the rod. If its length is much longer than its radius then you can use an approximation. What dimensions are you thinking about and what accuracy do you require in the answer?

#### RD

• Neilep Level Member
• Posts: 8126
• Thanked: 53 times
##### Re: How does one find the resonant frequency of a round bar of steel?
« Reply #6 on: 26/01/2015 10:05:05 »
A piano and a stick?

Free spectrogram software is available which will show the frequency content of sound , e.g. Audacity or SoX. If you have a computer such programs are cheaper,  more convenient and more accurate than using a piano to estimate the frequency.
« Last Edit: 26/01/2015 10:12:46 by RD »

#### David Cooper

• Neilep Level Member
• Posts: 1505
##### Re: How does one find the resonant frequency of a round bar of steel?
« Reply #7 on: 26/01/2015 19:18:08 »
I've hunted for a spectrogram function in Audacity but can't find one. Can it show sound in the same way as in this photo? The photo is of a spectrogram made by my own software (not using FFT) and I'd like to see if FFT gives a cleaner result. The sounds shown in the photo are the 8 vowels: oo, oh, aw, ah, a (as in "cat"), e (as in "met"), ay and ee. (The exposure compensation on the camera isn't strong enough to make it look as clear as it does to the eye - the noise at lower frequencies is less bright when seen directly by eye.) The lowest yellow lines are the base note, while the next yellow line above them are a harmonic an octave above. More harmonics appear as we go through the vowels before they start to deminish while more white noise comes in at high pitch.
« Last Edit: 27/01/2015 21:51:50 by David Cooper »

#### David Cooper

• Neilep Level Member
• Posts: 1505
##### Re: How does one find the resonant frequency of a round bar of steel?
« Reply #8 on: 26/01/2015 19:49:38 »
Ah, I've found out where it's hidden (in Audacity). Hard to describe, but there's a guide to it here:-

http://pretzellogic.net/2012/03/12/make-a-spectrogram/

Edit: and I've now attached a picture of the same 8 vowels as they are displayed by Audacity (though not from the same recording). Note that it uses different colours for loudness whereas mine uses brightness - the colours in mine indicate pitch, repeating for each octave.
« Last Edit: 27/01/2015 21:49:40 by David Cooper »

#### syhprum

• Neilep Level Member
• Posts: 3818
• Thanked: 19 times
##### Re: How does one find the resonant frequency of a round bar of steel?
« Reply #9 on: 26/01/2015 21:05:06 »
I will cancel my order to Harrods for a piano!

#### Colin2B

• Global Moderator
• Neilep Level Member
• Posts: 1915
• Thanked: 123 times
##### Re: How does one find the resonant frequency of a round bar of steel?
« Reply #10 on: 30/01/2015 18:02:12 »
how to find the resonant frequency of a round bar of steel and the tools needed
Are you looking at a specific application? I can probably help if it is musical acoustics.

#### The Naked Scientists Forum

##### Re: How does one find the resonant frequency of a round bar of steel?
« Reply #10 on: 30/01/2015 18:02:12 »