1) 70dbA is only half the data point with which you have to comply. At what distance must it be measured? Much as said above, 70 dbA cannot be met if measured at 10 ft. (a common specified measuring point). If measured at the field edge and that is 300 ft away, you might have a reasonable chance.
2) By what authority is the HOA making this requirement? Are you flying from property within the HOA's domain? Or, is the 70 dbA requirement at the edge of their authority and some distance from your flight area.
2) Does the HOA have like requirements for lawn care tools? At least in Texas, laws governing HOA rules are clear and strict. HOA rules are very difficult to change, and even more difficult to apply unevenly. I don't think any electric or gas powered lawn care tools would comply with a 70 dbA restriction.
Good luck, and don't give up.
Bedford