what is grass?
Here's an excerpt from the FAI Sporting Code.

The maximum noise level will be 94 dB(A) measured at 3 m from the centre line of the model aircraft with the
model aircraft placed on the ground over concrete or macadam at the flying site. With the motor running at full
power measurement will be taken 90 degrees to the flight path on the right hand side and downwind from the
model aircraft. The microphone will be placed on a stand 30 cm above the ground in line with the motor. No
noise reflecting objects shall be nearer than 3 m to the model aircraft or microphone. The noise measurement
will be made prior to each flight. If a concrete or macadam surface is not available then the measurement may
be taken over bare earth or very short grass in which case the maximum noise level will be 92 dB(A).
In the event a model aircraft fails the noise test, no indication shall be given to the pilot, and/or his team, or the
judges and both the transmitter and the model aircraft shall be impounded by the flight line official
immediately following the flight. No modification or adjustment to the model aircraft shall be permitted (other
than refueling). The model aircraft shall be retested by a second noise steward using a second noise meter and
in the event that the model aircraft fails the retest, the score for the preceding flight shall be zero.
The flight time will be interrupted while the noise check at the flying site is being made. The competitor shall
not be delayed more than 30 seconds for the noise check.