Signal path, setup:
Alan Parsons Sound Check 2 CD pink noise 1/3 octave bands - Sony Walkman line out bridged - Pleiades 2N3055 one transistor power amp - linoleum Pleiades Auratone 5C type with Tesla drive unit - reverberant room - Bruel & Kjaer 2209 at 1 foot from speaker axis
The 4133 condenser capsule was connected to the impulse precision sound level meter which is free field equalized. If knowledge is correct it can be pointed towards the source rather than at 90 degrees grazing incidence. Ie presure doubling effect is compensated.
Almost a surprise...
Reference level was chosen 70dB by approaching meter at 1 foot from speaker.
It turned out to be + or - 3dB from 100Hz to 14KHz but...
There was a +8dB resonance at 8KHz. Repeating frequency response at about 3.5ft showed +4dB above reference 1KHz of 60dB? (at such distance).
And something like +4dB at 1.6KHz.
Where is this 8KHz resonance coming from?
(Later addition: it must be coming from the drive unit, see its frequency response curve).
http://www.angelicaaudio.cz/wp-content/uploads/2014/03/arx13020_2.png
Later addition: with a 17KΩ collector to base bias resistor response seems more flat than with 10KΩ.
If driving the speaker directly from Sony Walkman headphone out, response may be even flatter, this was measured at a distance of less than 12in. About 10 to 20 degrees of axis reduces the 5+dB at 8KHz to almost flat 0dB. This may be good news as listener is rarely spot on axis. Measurements should be repeated at free field conditions.
Perhaps a passive Pleiades filter CR or LR making response flat to 50Hz instead of 100Hz may make an even better speaker at expense of efficiency.
Frequency bands on the test CD showed balance, with mid band naturally scoring a few more dB. Also mid bass, this may be from room resonamce as this test was tried at more feet away from speaker.
Experiment should be repeated at free acoustic field conditions eg speaker hung from a tree etc.
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