About Transmission Loss
Transmission loss expressed in decibel (dB) shows the degree of sound that has reduced as it transmits a glass pane.As for sound quality expressed in hertz (Hz), the most audible range of human hearing is from 1000 to 4000Hz.

As shown in the left figure, the transmission losses at 2000Hz are:
35dB for nomal laminated glass, and
40dB for sound control laminated glass.
The transmission loss of the sound control glass is 5dB higher than the normal glass.

Where a sound level is 83dB (the same level of the inside of a train), the effect that this 5dB of transmission loss has is as in follow.

No transmission loss --> 38dB at a point 90m apart from the sound source (same level of a midnight residential area)
5dB of transmission loss --> 38dB at a point 45m apart from the sound source (same level of a midnight residential area)
The above proves that 5dB of transmission loss is fairly effective for sound reduction.