SciLib

Basically you need to calculate the ratio of the sound power at the source and the sound power at 10 m. (sound power at source/surface area of sphere 10m radius)

I wouldn't recommending home testing your ability to smell cyanide. Think of the consequences if you weren't able to smell it, you could end up being poisoned.

Just for the record, I can smell cyanide compounds, as can my mum and as could my dad. My two brothers and my sister cannot. I have worked with hydrogen cyanide and potassium cyanide, when I was employed by a large chemical company.