When Automotive Acoustic Insulation materials in Alpha Cabin Random testing incidence sound absorption. 

Why the Absorption Coefficient will greater than 1.0 in some Frequency. 

When we testing Automotive Acoustic Insulation materials in Alpha Cabin. In theory, the sound absorption coefficient is close to 1.0. But in actual testing, the Absorption Coefficient will greater than 1.0 in some Frequency. What causes the sound absorption coefficient to be more than 1.0?

There are 2 reasons for the Alpha Cabin sound absorption coefficient to be greater than 1.0

Reason 1. The actual samples size is big than 1.2 m², It makes T1 (Reverberation time after samples in Alpha Cabin) reduce. This caused (1/T1 - 1/T0) increased. The S and V is always the same. So it makes the Absorption Coefficient is greater than 1.0 

Reason 2. Border effect, we put the testing samples in Alpha Cabin, All four sizes of the sample will be sealed in Alpha Cabin. But after sealing process, the sound in Alpha Cabin will still enter the sample from the samples four sides. It also make T1 (Reverberation time after samples in Alpha Cabin) reduce.

So for some materials with good sound absorption, the sound absorption coefficient in Alpha Cabin testing will be greater than 1.0 at some frequencies.

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