Understanding frequency calculations of an object (e.g human body)

[Nathan M]

Apologies for the noob question.

The frequency of the human body can be calculated based on weins law as follows:

Wavelength = 0.002898/310 = 0.00000934838 m

Plugging this into the equation λν = c we get

frequency = 299792458 / 0.00000934838 = 32069 Ghz

Elsewhere, I see that the resonant frequencies of the human body (e.g the ocular cavity) are at around 20 Hz. I assume this is physical vibrational frequencies (and I even see papers on the fact that infra sound could cause undesirable effects). I am having some trouble reconciling these two notions of frequencies. The frequency is a result of the vibrations of the atoms (apologies for the oversimplification) so should there not be just one frequency per object. If the atoms of a cell vibrate at a particular frequency in order to generate the propagating field, should there not be just one frequency? How could there be two different frequencies for a particular cell?

Also, am I right in assuming that resonance could occur at both frequencies.

 

[mfb]

There is no "resonant frequency of a human body". You'll only find this in pseudoscience.

What you calculated is the rough frequency where things at room temperature (for this purpose, humans are at room temperature) emit most thermal radiation. This has nothing to do with resonances.

If the atoms of a cell vibrate at a particular frequency in order to generate the propagating field

There is no such thing happening.

As the thread is asking about something that doesn't exist I closed it.

 

[Dale]

Apologies to @mfb. I was working on a reply and still wanted to post it.

The frequency is a result of the vibrations of the atoms (apologies for the oversimplification) so should there not be just one frequency per object.

This is not correct. The tissue vibrational frequency has very little to do with any vibrational frequency of the atoms. Acoustic resonance takes place with wavelengths that are many orders of magnitude larger than atoms. It depends on tissue stiffness, density, and shape at large scales, not atomic properties.

Consider for example water at its triple point. The atoms of the liquid, solid, and gas phases all are the same material and have the same frequency black body radiation corresponding to atomic-level vibrations and other thermal motions. But they all have different mechanical vibrations based on their differing stiffness and density and more importantly their macroscopic shape. The two types of frequencies are largely unrelated.

As another example, consider two strings of the same material and at the same temperature but at different lengths. The thermal vibrations are identical as are all atomic and molecular properties. However, because of the different macroscopic shape (length) the principle resonance frequency is different between the two.

How could there be two different frequencies for a particular cell?

Speaking strictly of mechanical resonance. The idea that there is only one resonant frequency of some mechanical structure. Is a huge oversimplification. Most structures have many resonant frequencies. For example many will have multiple resonances in bending and some resonances in torsion and maybe some in compression. Speaking purely of mechanical waves the idea that there is only one resonance is wrong. One resonance may be the strongest, but there are usually several. Great care must be taken to produce a structure like a tuning fork that has a nice clean single resonance. And even then it probably has other resonances, they are just outside the audible range.