Acoustic resonance - Absorption vs Transmissibility

[PSuran]

Hi everyone.

Firstly, I'm not a physicist, so relatively "intuitive" explanations would be greatly appreciated :)

I'm confused by the following:

Let's say we have a partition / wall, which resonates at a specific natural frequency. If the resonance is relatively high Q (low damping), then Transmissibility is much larger than "1" (LINK) . I read this as sound "amplification". But even if the partition resonance is Low Q, Transmissibility never goes below 1 (https://www.digikey.com/-/media/Images/Blogs/2018/December/Minimize%20Mechanical%20Vibrations%20with%20Highly%20Damped%20Elastomers/minimize-mechanical-vibrations-with-highly-damped-elastomers-img2.jpg?ts=82dc11df-6b94-444b-a9ea-eb8d140d0a80&la=en-US). I interpret this like so - At resonant frequency, the level (sound) will always be amplified (compared to no partition). NOTE: Ignore flanking sound, and other possible sound transmission paths. The above is a theoretical, free field vs. half space example.

But on the other hand, when building sound absorption devices, we use the same device - a resonating panel, optimally low Q (no ringing).

So my confusion is this - How can the same resonating system both amplify and absorb sound?

I'm certain I'm getting something wrong. I would really appreciate an answer, this is really bugging me :)

 

[sophiecentaur]

How can the same resonating system both amplify and absorb sound?

I find this difficult too and it is confusing. The idea that a partition with some loss can transmit more sound than if the partition were not there can't be right as there is a loss mechanism in the partition. Link 2 probably needs some more explanation. I am not sure of the context of your post and there may be a bit of the crossed wires effect.
To be accurate, in normal Engineering use, the word "amplify" means an increase in the Power of a signal. This always requires some external source of extra power from a power supply.
When you have a resonance, it is quite possible for a sound to seem Louder but that will involve energy being better 'matched' to your hearing. The gain in amplitude of vibrations is due to energy building up in a resonating structure / circuit.
We're coming in half way along the resonance story, which makes it difficult to give a simple answer but, in your example, a High Q resonator attached to a wall can match the sound power, arriving at the other side so that More Power Gets Through. The sound level in the source room may even drop a bit because the party wall is reflecting less back into the room.
Also, the idea of a 'room' as source is a bit vague. Is there a constant level of sound in the source room? That would imply that the source loudspeaker would need different power input for different levels of transmissibility, if the detected level is to be the same. On the face of it, the links imply that there's no point in trying to sound-proof a room but that can't be right. It's got to be down to the details of definitions involved.

 

[A.T.]

How can the same resonating system both amplify and absorb sound?

Besides resonance there is also dissipation:
https://en.wikipedia.org/wiki/Dissipation_factor