Please see the attachment. From the equation, it is clear that for very low frequencies, there will be some regions where STL will be negative. However, negative STL is a violation of energy conservation. Can someone explain this to me?
Negative dB is not a negative number. dB is an exponent. It’s expressing a value as a power of 10 (except, for no fathomable reason, the power of 10 is multiplied by ten)
So, for example, -30 dB is 10^(30/10) = 0.001 a small positive number.
I think it used to be customary to look at sound insulation down to 100 Hz one third octave bands. However, it is more cutomary to strech this down to 50 Hz or even 20 Hz in special cases nowadays. Most textbooks does not account for the data below 100Hz, as they did not look at these. So use the rules of thumb with a decent amount of criticism.
You seem to look at a single plate. There will be resonances influencing the sound insulation, and they will come less dense the lower the frequencies are and the smaller the plate are.
For double constructions the stiffness dominates the performance below the double wall resonance, and the mass above. There are also whole plate modes that may give different regions for stiffness controlled and mass controlled. Near resonance the damping also have a major effect. And the amount of porous absortion inside the wall play a significant role addin damping to the plates as well as reducing the sound level inside the double wall.
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