Suspended one-sided RD-75 dipole baffle showing microphone stand.

As can be seen in the photo at left and in the preceeding photos the free air measurement condition involved a 1 5/8" thick diameter tall microphone stand created from closet rods. As can be seen in the photo, the Mitey Mic capsule is only the length of the probe, and slightly offset, from the microphone stand.

Despite the small pieces of foam over the microphone probe, it will be shown that there was a significant reflection from the stand recorded in the measured data.

Shown below is a screen capture of the IMP/MLS display (640 x 350 pixels) that was created for this correction. A decision to use the 3m free air impulse data instead of the 1m data was made on the basis of practical listening distance - the reflection problem appears in all the free air 1m, 2m, and 3m impulse data.

The 3m impulse of the bare RD-75 driver suspended in free air is shown in the bottom window of the display at a "normal" screen gain of x1. The duration of the plot is 66.65mS. A windowing marker is seen at the beginning of the impulse. One can clearly see a least three areas of 'reflections' following the impulse.

In the upper window is the 57.9mS windowed portion of the impulse multiplied x5. The seriousness of the reflection problem becomes manifest at this screen gain. Reflections overlooked in haste, now are seen to be major problems. Remember the approximate equivelance of 1mS duration for a flight distance of one foot in air. The upper trace then shows about 58 feet of soundwave travel.

RD-75 3m free air suspended impulse at a Screen Gain of x5, and 58mS duration.

A sharp postive and negative going spike is seen immediately following the main impulse - that is the reflection from the microphone stand. Later comes a lower level extended sequence, which is unclear in origin, but the rear wall of the stage and/or the hanging stage curtains are possible origins. At about 2/3rds of the plot trace is seen the main reflections from the rear wall of the auditorium. Following will come several further dissections of the impulse as the screen gain is increased more-and-more to reveal the actual state of the potential semi- or quasi-anechoic status of the data.

RD75 1m free air suspended CSD plot 3mS duration & 1/12 octave smoothing.

The CSD plot at left was originally displayed here, and is in ERROR!