Transmittance calculation in COMSOL

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Transmittance can be expressed in decibels (dB) using the formula 10 × log10(T), where T is the transmittance value. For a transmittance of 0.03, the dB value is approximately -15.2 dB. In engineering contexts, power is often plotted in dB, which relates to sound wave amplitude. If pressure correlates with sound wave amplitude, the power would be represented as 20 × log10(T), resulting in -30.4 dB for T = 0.03. Understanding these conversions is essential for accurate analysis in COMSOL simulations.
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I am emulating a transmittance vs frequency plot for a research project I am apart of. However I am having difficulty understanding something. The plot has the transmittance values as a unit of decibels anywhere from 0 to -40, but the equation they’ve given me is attached below. The equation has the absolute value of the pressures so how exactly are they getting negative numbers out of this? Is there something I’m missing?
Any help would be great thank you.
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Are you familiar with decibels? If ##T=0.03## in normal form (unitless), then in dB it would be ##10\times \log_{10} \left(T\right)\approx -15.2dB##. Note also that in some cases, especially in engineering it is common to plot power in decibels. I am not sure what your setup is, but if pressure is related to the amplitude of the sound waves, I would guess that the power of the sound-waves is the square of it (with some constants), so the dB expression in this case would be ##20\times\log_{10}\left(T\right)=-30.4 dB##
 
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This is very helpful thank you!
 
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