Noise

The discussion around ATV volume/noise is often presented as subjective.  However, decibels (dB) can be very simply and objectively measured and compared to health and safety thresholds already established.  ATVs generally operate in the 85 to 100 decibel range, which is universally regarded as hazardous.

"The decibel scale is logarithmic: a small increase in the decibel level is, in reality, a big increase in the noise level. For example, an increase of only 3dB doubles the noise level at the human ear and halves the time a person should be exposed to it once harmful levels are reached." - Institution of Occupational Safety and Health

"ATVs are so loud because their engines rev at very high RPMs, they have shortened exhaust systems, have poorly insulated or non existent engine compartments, and are generally ridden very fast wherever they go." - DirtWheelRider (a Pro-ATV website) explains why ATVs are so loud.

"Loud ATV exhaust noise has become an issue both medically and environmentally. The threshold of permanent hearing damage, as set by the OSHA, is 85 dB. A gas-powered lawn mower produces 85 dB. The snowmobile industry cannot, under U.S. law, exceed 78 dBs at full throttle, and since 1979 motorcycle manufacturers must stay between 78 and 84 dB. However, most ATVs are between 85 and 100 dBs, which has led to concerns by both riders and non-riders about excessive noise.”

from "How to Quiet ATV Exhaust Noise (A Practical Guide)" (bettersoundproofing.com)

Engineering Standards specify that the nominal sound level of ATVs and other vehicles are to be measured 20" from the tailpipe.
Taking 95 db as the sound level at this distance:
* at a distance of 15 feet, the sound pressure level is 75 db
* at a distance of 25 feet, the sound pressure level is 70 db
* at a distance of 50 feet, the sound pressure level is 65 db
  
Five ATVs would produce a total sound level of at 101.9 db at the source and 72.41 db at an ear 50 feet away.
For comparison:
* 10 dB Normal breathing
* 20 dB Rustling leaves, mosquito
* 30 dB Whisper
* 40 dB Stream, refrigerator humming
* 50-60 dB Quiet office
* 50-65 dB Normal conversation
* 60-65 dB Laughter, loud conversation 70 dB Vacuum cleaner, hair dryer
* 75 dB Dishwasher
* 78 dB Washing machine
* 80 dB Garbage disposal, city traffic noise

For decibels (deci meaning ten), each 10 decibels means the sound intensity has increased ten times.
* 20 dB is 10 times louder than 10 dB
* 30 dB is 10 times louder than 20 dB (and thus 100x louder than 10 dB),
and so on. ...

Two 50 dB sources together will yield a sound intensity of 53 dB.

Adding 10 sources at 90db each, 10 ATVs at 50 feet would have a loudness of 99.997 db.

references:
http://www.sengpielaudio.com/calculator-spl.htm 
https://www.omnicalculator.com/physics/distance-attenuation
https://www.horsefarmsforever.com/lets-have-a-sound-test-threats/ 

"The Science of Sound" provides an overview of ATV Engine Noise especially as it relates to a specific residential and Farmland Preservation Area in Ocala, Florida.