Very interesting, I had never considered this. Ivan, if I understand you correctly, what you are saying implies that if the sub is flown and the listener or mic is close to the ground, there is an additive reinforcement of SPL due to reflection of the wave off the ground near the listener. This makes physical sense for a sub if the phase of the reflected wave is less than 90 degrees (1/4 wavelength) behind the phase of the original pre-reflection wave. That means the listener's ear or mic would need to be less than 1/8 wavelength from the reflecting surface so the round trip distance from ear to floor to ear is less than 1/4 wavelength. 1/8 wavelength would be about 1.4 feet for 100 Hz or 2.8 feet for 50 Hz, assuming the wavefront is perpendicular to the floor but would be even less if the sub were not directly overhead. At 1/4 wavelength from the floor, there would be near complete cancellation of the sound (for that particular frequency) as the reflected wave would be 180 degrees out of phase with the original and only slightly less amplitude. So it is not clear to me how all this factors into the measurement and reporting of SPL levels of subs. What has been your real world experience doing these measurements at Danley?
THe distance from either the sub or the ear (mic) to the floor ONLY applies if all of the devices are in line (or stacked up vertically. to get the "simple distances".
THe argument that putting the subs a couple of feet off of the floor and causing cancellations ONLY applies IF the listener is ABOVE the subs.
They are not
The difference in distances between the direct path of the subs and the reflection off of the floor is what causes the combfiltering. NOT the distance up off of the floor.
We only measure the subs one way. The sub is on the ground and the mic is also on the ground.
This eliminates any reflections which would cause cancellations at some freq based to the different path distances.
We also put the mic 10M away (a 20dB loss) and drive the subs with 28.3V (a 20dB gain) so that the "back calculation is equal to a 1M distance.
In order to get a true "free space" measurement requires you to place the sub and the mic VERY HIGH in the air. Or have a HUGE anechoic chamber.