The IMP/MLS instrument provides for high-resolution low-frequency data acquistion. This feature allows for examination of the low frequency performance of a driver/loudspeaker in much greater detail than does the higher 61,200Hz sampling rate.
Shown below is the low frequency response of four RD75 drivers (BG25,26,28, and 46) installed in the test fixture, but without any addition baffling other than that provided by the mounting method. Apparently, without any baffling, installation of the passive notch filter results in about a 1.8dB attenuation in overall amplitude.
As can be seen, there is good consistency between the drivers. A slight underdamping of the low-end response can be seen at the 'knee' of the low-end dropoff. Additionally, the final low frequency dropoff rate below 100Hz seems well in excess of the expected 18dB/octave rate.
Shown below is the individual frequency response plot of the unbaffled BG25 driver and impulse plot from which it was derived. Even though the impulse plot is only 57 sampled points, it represents 29.6mSec of data (at which point the first sign of reflections appear in the data record). There has been no FFT windowing or other smoothing applied to the data.
The plot below shows the effect of two different sized baffles upon the response of the BG25 driver. Working from this data seemed to indicate that a baffle in the range of 20" to 30" width would be adequate.
A great surprise resulted when such a baffle width was measured indoors, and the problematic low frequency 'bump' was discovered. It seems that with a symmetrical baffle, being in a room enhances the output of the RD75 below 1000Hz.
Microphone at 1 meters distance and a height of 1 meter.
Sampling Rate: 1920 Hz - NO reflections in data.
MLS: 8 averages, 57/4096 point FFT, NO windowing.
NO octave smoothing - Amplitude Axis @ 6dB per division.
A Windy Day Outdoors.
Off-axis Frequency Response.
The RD75 Dipole Baffle Study - Table of Contents.
Acoustic Line Source Research - Table of Contents.
