experiment on a pair of Jensen RP-201 driver/horn assemblies that I've
had for about 30 years.
The horns are cast iron and seriously need to be damped. A wood clamp was applied to them during the tests for that reason although the ringing was muted to inaudibility with the horn mouth resting in its flange. When mounted on a baffle, sufficient damping should be achieved.
The horn seems to be exponential according to calculations made from the throat and mouth areas and the length of the horn. The cutoff frequency is 665 hz. derived from the above. However, mounted on a baffle, the useable low frequency limit can be as low as 600 hz. which seems to be supported by the frequency responses below. There, the response at 600 hz is about 5 dB down from the passband between 800 hz and 5500 hz. It can be safe to assume that Jensen would have recommended a high pass filter of 800 hz.
|Two impedance curves for each unit;
curves 2 (green) & 21 (blue) are for drivers only 1 & 2,
Curves 3 (black) & 22 (red) are for the driver and horn assemblies, 1 and 2, respectively. The differences between the pairs is attributed to different materials and aging of the diaphragms, both being phenolic and subject to aging. The units were disassembled but the diaphragm assemblies weren't removed as it wasn't known if they are keyed and shimming them to the pole piece for reassembly can be a chore.
Since no buzzing was heard during the tests as well as none being heard when a very low level (<1w) 10 hz signal was applied, foreign material in the gap was considered negative.
|Two response curves, 7
(green) & 26 (black) in the upper group are all measured with a
gated response to remove room reflections and all are smoothed at
0.25 octave. The red curve (29) between them is a scalar
average of the other two.
The light green curve, 28 is measured at 1w 2m. Note the 6 dB drop in level, indicating the LMS is quite capable of removing room reflections. Typically, in a room, the level will drop by about 3 or 4 dB when the distance from the source is doubled. The 6dB drop only happens in a non reverberant field.
The sudden drop below 550 hz is due to the gating limitations. As the distance from the mic to source is increased, the difference between the incident wave and the first reflection approach equality and the system can't get a good sample. It tried between 550 hz and 475 hz but eventually failed, which was expected.
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