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I'd like to create a 3d printed system to damp the vibrations on the phone when we are on the road. The idea is to be able to hold the phone for my little kid when we travel

First I tried a spring system that connects the fixed part to the phone holding part, with no success, the vibrations were not only eliminated but increased.

Also tried inserting sponge/rubber material in between the phone and the phone holder, but seemed useless.

At this point I don't know what else to try, and I didn't find any commercial product.

How would you do it?

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  • $\begingroup$ What do you mean by "damp"? Reduce the amplitude? The peak force? $\endgroup$ Feb 13, 2021 at 22:18
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    $\begingroup$ Adding damping to a system has a specific engineering meaning, which I suspect is not what you need. For a screen to be readable in a vehicle, it's best to be rigidly attached so that it does not move relative to the other pseudostatic reference features in your field of view - the inside of the car. Or, hold the screen in your hand. It's easy reading your own phone screen, or watching TV on a fixed screen in the seat in front, but really annoying reading a screen held by someone else - because your brain can't compensate for that motion. $\endgroup$ Feb 13, 2021 at 22:22
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    $\begingroup$ If that was a good idea your dashboard console and sat-nav would be mounted on a suspension system. Rigid seems to work for those. $\endgroup$
    – Transistor
    Feb 13, 2021 at 22:40
  • $\begingroup$ +1 for everything @JonathanRSwift said. Either make the connection completely stiff or connect it to the kid instead of the car. Probably some kind of stand to put on the kid's lap could work, for example a custom shaped pillow you can attach the phone to. $\endgroup$
    – MaxD
    Feb 14, 2021 at 0:38

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I agree totally with Jonathan R Swift's answer but I will try to give a insight as to why it is not appropriate to use a shock absorber.

The main issue is that the excitation frequency of the vibration is not constant. The vibrations frequency will change depending on how fast the car will travel and on the roughness of the road. All things being equal if you increase the speed of the car the excitation frequency increases, and vice versa.

As the excitation frequency changes the amplitude of the vibration will change. To get a rough idea of the change, below is a graph of the transmissibility ratio for different coefficients of the damping ratio.

enter image description here

The transmissibility ratio shows how much forces and displacements are amplified to a mass if the foundation is excited. The value on the y-axis shows the amplification factor, so for example if Transmissability ratio (T for short) is 2, then if the excitation at the foundation has an amplitude of 1.2 [cm] then the excitation on the examined mass would be 1.2 *2 =2.4[cm]. (Small note - for those who are more knowledgeable-, T is applicable to the steady state response). So to have proper damping we need lower values of T.

One thing to note is the dependence on $\zeta$. $\zeta$ is the damping ratio and indicates how fast the vibrational energy is converted to other forms of energy (usually heat). The higher the value of $\zeta$, the higher the damping. However you will see that for high values of $\zeta$, you will see that the transmissability ratio remains high (while the lower values have pronounced peak and then reduce significantly. So the bottom line is that by increasing $\zeta$ you will not always have less vibration.

Additional issues

The problem gets worse, because, what I showed you is for a simple system with one dof. Real structures have many more degrees of freedom. The end result is that you will get a graph more like:

enter image description here

In that graph you will notice that the amplification drops and increases, and doesn't really drop of below unity.

transient response

I've mentioned above that T applies to the steady state response. However, there is another response, transient response, which in engineering problems is usually neglected (because machines in many cases run at specific rpms for very long periods of time). In this case the transient response is a significant portion of the vibrational response and, cannot be safely removed.

bottom line.

As others mentioned, the best way would be to rigidly connect (as much as possible) the pad to the frame/seat.

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Usually if you can get rid of extra vibration of car seat and be satisfied with the vibration of the car itself you should be happy.

For examplev the instrument panel of an airplane is the absolute most critical interface between the pilot and the plane. it is directly mounted on the frame. They would have mounted it on a shock absorber base if it would help. Even though in choppy air it shakes violently.

Same way in a car the frequency of vibrations and their magnitude are not preditable. one pot hole too big will throw off all the damping.

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