Estimating blade stiffness through sound recording

thejnal wrote: Im am physics teacher. The measuring makes no sence. The frequency depends eg on where you hold the bat or where you hit it. The stiffness has nothing to do with frequency. Sorry.

thejnal wrote: Im am physics teacher. The measuring makes no sence. The frequency depends eg on where you hold the bat or where you hit it. The stiffness has nothing to do with frequency. Sorry.

thejnal wrote: Im am physics teacher. The measuring makes no sence. The frequency depends eg on where you hold the bat or where you hit it. The stiffness has nothing to do with frequency. Sorry.

Too bad wayback saved the thread only once between 2010 and 2017.

Edited by stiltt - 03/20/2021 at 2:02pm

Maybe it's an instance of convergence thinking? 

Edited by stiltt - 03/20/2021 at 1:15pm

stiltt wrote: There is that study linked in many places: https://www.researchgate.net/publication/271637905_Vibro-acoustic_of_table_tennis_rackets_at_ball_impact_Influence_of_the_blade_plywood_composition#pf4 https://www.sciencedirect.com/science/article/pii/S187770581201716X https://www.sciencedirect.com/science/article/pii/S1877705814005840 https://pubmed.ncbi.nlm.nih.gov/29638189/ https://www.tandfonline.com/doi/abs/10.1080/02640414.2018.1462578?journalCode=rjsp20 https://reader.elsevier.com/reader/sd/pii/S1877705814005840?token=9C9FB522D112115F2C8D28DD429B26EBE3AA7B02C50D5DDD32B3915D922FEFCC6AD17EBCA5097CAC39D6918B803CDD3A

Edited by stiltt - 03/20/2021 at 1:07pm

here's slevin's thread

Hi Simone I took the freedom to edit your links here and in your original post in 2018. They work now. I am not sure what happened.

Now there is a new question, for the more experienced than me.

The units are different. dB ≠ dBFS(decibels relative to full scale).

https://www.audiomasterclass.com/newsletter/what-is-the-difference-between-0-db-and-0-dbfs

Decibels are used to describe differences or changes in level. 0 dB means 'no change'. Values in dBFS are used to describe signal levels in comparison with the highest level a WAV file can handle.

Edited by zeio - 11/22/2018 at 5:45am

Just wanted to cross-link this with another thread, dealing with the same subject.

arg0 wrote: Do you have any estimate on how much dwell time is in table tennis shots?

1 millisecond seems a like a reasonable number. If the ball spends half of its time decelerating and have accelerating again then the deceleration time would be about 0.5 seconds.   If we assume the paddle to be stopped and the ball is hitting the paddle at 10 m/s the average speed during decal will be 5 m/s.   Multiply this by 0.5 milliseconds and the result is 2.5mm deceleration distance.   This means the ball deforms, the rubber deforms and the blade deforms about 2.5 mm during the impact and this is not a fast impact. If the dwell time were longer the deformation would need to be proportionately greater.

It seems to me that a blade that vibrates too fast would vibrate out of phase.
I should do a simulation with two springs and make one spring much stiffer than the other just to see what would happen.

I know a TT ball will deform a lot.   I wish I had a good high speed camera.   Not the 500 Casio toys.   I mean a good 10,000 FPS camera. It is only $100K. If any body chips in I may buy one just to answer these questions.

BTW, what do you think the average force between the paddle and the ball during impact if the relative speed is 10 m/s? It isn't hard to calculate but you have to assume a dwell time that makes sense. Assume 1 millisecond.

I am amused at those that talk about extra dwell time on the paddle. The differences can probably be measured in microseconds.

I am bothered about the assertion that blade hardness is determined by frequency.
I like to do my absurdity tests and if the assertion can't pass that then the assertion is wrong or there is more to it than what is stated.
So why do marimba keys sound different if they are made of the same kind of wood?   The answer is size not hardness. Now I do believe that if one were to try to bend a larger marimba key it would flex more than a shorter one.

So why was stiffness replaced by hardness?

Arg0, an object will vibrate at its natural frequency but that can be changed by damping. Rubbers are good dampers and will reduce the frequency of oscillation
http://en.wikipedia.org/wiki/Damping

Anther thing I find interesting is that the period of oscillation is much shorter than the dwell time. This would mean the blade would oscillate a cycle to two times while the ball is in contact with the rubber. That would be like push a child on a swing at a rate faster than the natural frequency.   I would test this out on a hard bat or even a bare wood slow paddle.