A noise barrier fence must be designed to protect a residential receptor (house) from the noise generated by a drive-thru speaker at a nearby Tim Hortons restaurant. A preliminary barrier design has suggested a 4 m tall barrier fence be installed along the shared property line which is 5 m from the drive-thru speaker and 20 meters from the representative receptor location. The average sound pressure level produced by the speaker is 57 dBA, measured at the representative receptor location before the noise barrier is constructed. You can assume for calculation purposes that all the sound is within the 500 Hz octave band.
Predict the attenuated sound level at the representative receptor location after the noise barrier wall is installed.