I need help with this assignment on gain structure. I have attached it for better viewing. Thx

Inverse square law

Assuming a free-field environment, that drop in level can be described as:

NR (noise reduction, or level reduction, over distance) = 20log (Distance1/Distance0)

This translates into a drop of 6 dB for every doubling of distance.

Note that indoors the drop will typically be less, due to room reflections and reverberation, but 6 dB/doubling is still a good general rule of thumb to go by.

Typically, a sound source's acoustic level, such as the output from a speaker, will be given in dB SPL at a measure distance, say 4 feet. If we put our mic 10 feet away, we can expect a drop in level of:

NR = 20log (10/4) = 20log 2.5 = 8 dB

If we started with 90 dB SPL @ 4 feet, we get 90 – 8 = 82 dB SPL at the mic.