resonant frequency of a human body

shpresa

Does anyone know why the resonant frequency of a grounded human is smaller when ungrounded?

Bobbywhy

shpresa, Welcome to Physics Forums!

Will you please give more information? What is the driving frequency causing the human body to resonate? Can you give some example of your experiment? What is the source of your question?

I once consulted on a project to kill insects some engineers were working on. Their idea was to bombard flies with high power acoustic frequencies that would cause their bodies to resonate so strongly they would literally explode. Our experiments showed it did not matter if the flies were in flight or resting on a leafy surface. Their little bodies exploded when irradiated with our sound beam in either case. We may infer from this experiment that the resonant frequency of the human body remains the same, whether grounded or not.

Cheers,
Bobbywhy

rollingstein

Maybe he's talking of EM resonance? That'll be different from acoustic right?

shpresa

Well there is no experiment. This was a question my professor asked me.
All I know is that one can find the resonant frequency of a body based on its height using the following formula:

f=114/h (MHz)
where f is the resonant frequency, h is the height of the person in meters. Now this resonant frequency has a higher value when the person is not standing with his feet on the ground, that means the resonant frequency is smaller when the person is grounded or standing with his feet on the ground.
Sorry for my English, hope you could understand my question.

shpresa

Yes I guess its about EM resonance since we learned this from Bioelectromagnetics course...

AlephZero

Not necessarily. If you had been trying to break their legs by resonance rather than explode their body, it would have made a huge difference if they were standing on something (i.e. the body mass supported by 6 springs), or in flight.

FWIW there is a huge literature on the effect of mechanical vibration on humans - but we know know that's not what the OP was asking about.

the_emi_guy

You are not sure? Seems odd.

Anyway, EM resonance of the human body is important in the field of radio frequency dosimetry.

According to my handbook, the average man is 1.75 meters tall, and has a resonant frequency of 80MHz. This is close to, but not exactly the same as, what your formula predicts.

This 80MHz peak causes the average absorption (watts per kg) for a given incident power density to be 10 times higher than at 1MHz or 200MHz.

Why would resonant freq be smaller for grounded human?
Here is a clue, see if you can figure it out:

The 80MHz resonance from dosimetry handbook (and your formula) are based roughly on the human body resonating as a dipole.

What kind of resonator are you if you ground your feet?

rollingstein

Interesting. That ought to be VHF transmissions, right?

Khashishi

Ah, you mean grounded in the sense "standing on the ground". Consider a standing wave along the length of a human body. If you are standing on the ground, and your shoes are sufficiently damping, then your feet are a node. If you are hovering in the air, then your feet don't need to be a node.

Compare a blowing a pipe with one end covered versus an open pipe.