# Wall thickness vs frequency... sound isolation

#### Trontron

When sound hits a wall,part of it gets reflected and part of it gets transmitted due to acoustic impedance difference between solid and air.

My problem,and point of this question is that having material or gas impedance number is completly useless unless we know the frequency and wall thickness.Let me give you an example.

Acoustic impedance of aluminum is 35000 times bigger than impedance of air,if all we looked at was impedance difference,we would assume every time sound in air hits anything made of aluminum,then it will reflect back 99.9965% sound energy,and 0.0035% gets inside.

If you tried to isolate subwoofer playing 40 Hz tone with aluminum foil,you will see it doesnt work like that,becose sound reflection and transmission depend on two aditional factors,frequency and wall thickness.

Thicker walls are more reflective than thin walls made of same material with same acoustic impedance.Low fequencies reflect less and more easily penetrate walls than higher frequencies.That means effective impedance depends on frequency and thickness,so if the wall is thin and frequency low,the effective impedance difference is much lower than intrinsic impedance of the two materials or gases.

How can I calculate how much sound will get transmitted through wall at certain frequency and certain wall thickness? I would like to be able to calculate graph where one axis is frequency,and other axis sound energy transmitted to other side with wall of certain material and thickness.

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