Another matter that the venue of the gym provided the opportunity to
investigate is that of the frequency response at and above the top of the
baffle. The height of the 1S6T baffle used in these measurements is about
76 inches. Below are three different views of overlayed plots of four
responses from the baffle:
1. 1m distance, 1m height for amplitude reference.
2. 3m distance, 1m height for amplitude reference.
3. 3m distance, 76" height.
4. 3m distance, 96" height.
Since my general focus is on measurements that might relate directly to practical use, a 3m measurement distance was choosen. The 76" microphone height is the same height as the 1S6T baffle; and, the 96" microphone height represents the ceiling height in many typical homes in the U.S.A.
As part of the continuing data processing tutorial inherent in this study, three different octave 'smoothings' are shown for the same data. Many individuals have only seen smoothed plots and have a naive view of actual loudspeaker responses as measured. The fuzzy plots in the first overlay are the usual reality of FFT processed impulse data, the fuzziness often being referred to as 'grass'. The 3m/1m plot in the 'unsmoothed' overlay is exceptionally clean!
An estimation of the potential for 'ceiling bounce' can be derived from inspection of the plots. We can see that dipole cancelation and the low dispersion of the line-source above it's end point are significant effects. Please note the compressed amplitude scale of 10dB/division used on these plots!
Compare the extreme low frequency response in the 3m/1m plot with that
in the 3m/76" plot. We see the cessation of line-source behavior as the
wavelengths of those frequencies become significantly larger than the
baffle. Below 200Hz the amplitudes are about the same at the two heights,
but at higher frequencies the amplitude difference grows to roughly -10dB.
At the 96" height all cancellation effects are coming into play, and the
overall level of the signal is substantially lower than measured at the
top of the baffle.
UNSMOOTHED FREQUENCY RESPONSES.
1/12 OCTAVE SMOOTHED FREQUENCY RESPONSES.
1/3 OCTAVE SMOOTHED FREQUENCY RESPONSES.
The Gymnasium Measurements -How Big is the Gym?
The Gymnasium Measurements - On the Edge.
The RD75 Dipole Baffle Study - Table
of Contents
Acoustic Line Source Research - Table of Contents.
