Ground Plane Measurements
Ground plane measurements are very nearly identical to anechoic measurements. Because the measurement includes both the direct sound from the enclosure, plus the mirrored image from the ground plane, the SPL for the same microphone placement distance will be 6dB higher than the anechoic SPL level.
The only requirement for a ground-plane measurement is a large reflective surface, such as a parking lot or drive-way, with no obstructions for a radius of 20-30 feet or more. The procedure is to place the loudspeaker on its side on the ground, and then place the microphone on the ground in front of the speaker.
Ground plane measurements, like anechoic measurements, will have a rising character in the response caused by diffraction off the loudspeaker baffle board. This occurs in the mid range region. The increase in SPL is about 6dB, the frequency being determined by the total area of the baffle.
If you are measuring a multi-way enclosure with low, mid, and high frequency drivers, the enclosure should be positioned on its side with the high frequency driver nearest the ground. The microphone should be aligned on axis with the high frequency driver as well. The off-axis angle relative to the wavelengths of the mid and low frequency drivers will typically not be significant.
Many times it is a good idea to slightly tilt the enclosure downward toward the microphone. The important goal here is to try to keep the microphone on-axis with the high frequency driver.
The ground plane method is a very good choice and does not require any expensive or elaborate set up. All that is required is a flat surface with some open area around it. See the figure below.
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Post By：2006-2-9 15:48:56 [显示全部帖子]
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