On the Bench
Because of their (relatively) small size for an ESL,
low frequencies are not reproduced all that well. Here for example, at 100
Hz (microphone at 10" from the center of the panel), THD+N was almost 6%.
Even just an 8" cone woofer would do better than this, and it's why a good
subwoofer is always helpful when using ESLs. I would recommend crossing
these speakers over at 120 Hz when used with an SSP.
At the higher frequencies, however, ESLs do great. At 1
kHz, about 0.4% THD+N resulted.
At 1 kHz and 1.5 kHz, the A+B IM peak at 2.5 kHz was 45
dB below the fundamental, and the B-A peak at 500 Hz was 40 dB below. If you
look at the IM peaks we have gathered for amplifiers, you will see that
speakers don't do as well. This is because electrons move faster than cones
and membranes which are physical objects. Even though ESL membranes are very
light, they still have inertia.
At 5 kHz, THD+N was 0.74%.
And, at 10 kHz, THD+N was 0.43%
The room response showed a roll-off below about 700 Hz.
This is because ESLs are dipoles. The sound from the rear comes around to
the front and interacts with them, attenuating some of the frequencies. So,
again, here is the reason why I suggested crossing them over at 120 Hz. The
peak at 90 Hz is a room mode.
The impedance of the 400is is low at about 10 kHz and
above.
Even though that is only part of the audio spectrum, it is still a good
reason to use an amplifier rated into 4 ohms. Electrical phase is mostly
capacitive (the part of the red line below 00 is due to
capacitive influence, and the part above 00 is due to inductive
influence). That is because an ESL acts like a big capacitor. The phase peak
at 100 Hz - 200 Hz is due to the transformer.
Conclusions
My little experiment with small ESLs was a big success.
Final Sound has managed to design small ones that deliver where it counts:
SPL and detail. Even though the panel is flat, dispersion is still
acceptable because the panel is narrow.
ESLs are not for everyone, but Final Sound has just
enlarged the market, as the 400i will fit in any home theater or music room.
- John E. Johnson, Jr. -