Parabolic reflector for detecting whispers at distance

[skinnyl]

Hi there all !

i have been given a question for my Advanced Acoustics assignment as follows:

An MI6 agent uses a large parabolic reflector of aperture diameter 1.20meters to focus a sound into a tube of diameter 10mm at 20% acoustic efficiency which then goes to the ear.

how far away can a whispered conversation be heard from the system?

assuming the intersity level of a whisper is 20dB at 1.0m from the source, which maybe assumed to be a point source.

i have no idea where to start with this. I am not asking for it to be done for me but a shove in the right direction would be appreciated. what methods should i be looking to use or research?

Cheers a lot

Jack

 

[OlderDan]

Hi there all !

i have been given a question for my Advanced Acoustics assignment as follows:

An MI6 agent uses a large parabolic reflector of aperture diameter 1.20meters to focus a sound into a tube of diameter 10mm at 20% acoustic efficiency which then goes to the ear.

how far away can a whispered conversation be heard from the system?

assuming the intersity level of a whisper is 20dB at 1.0m from the source, which maybe assumed to be a point source.

i have no idea where to start with this. I am not asking for it to be done for me but a shove in the right direction would be appreciated. what methods should i be looking to use or research?

Cheers a lot

Jack

To start, you need to know the minimum sound level that can be heard by the ear. Then you need to use the efficiency information to figure out how much sound power has to be captured by the reflector to deliver the minimum needed to the collector tube. Then you need to figure out how far away the reflector can be from the source and still capture that much power. How does the intensity level vary with distance?

 

[skinnyl]

Hi, that great. helped a lot!

Cheers