- Written by John E. Johnson, Jr.
- Published on 15 February 2008
On the Bench
THD+N measurements were within an 80 kHz bandwidth. I set the SPL to 97 dB instead of 100 dB that I use for conventional speakers because half the sound is coming out of the rear in an ESL. I used the RCA line-level input for the measurements, rather than a power amplifier into the speaker binding posts. I did not measure the impedance/phase because the speaker binding posts are connected to the built-in power amplifier.
At 1 kHz, THD+N was 1.36%.
At 10 kHz, distortion was lower, at 0.24%.
THD+N vs. Frequency showed that the Purity's have more distortion at the lower frequencies, like all speakers. The crossover is at 450 Hz, and there is a spike just above that, probably due to the crossover network. At 1 kHz and above, distortion stays generally at or below 0.5%. Undoubtedly you are surprised at the amount of distortion in the bass frequencies. We are the first consumer A/V magazine to publish graphs of THD+N vs. Frequency for speakers across the entire audible spectrum, and I believe we are going to find, as we measure more speakers, that there is more distortion than we thought there would be. I suspect also that we may be less sensitive to distortion in speakers than we imagine. We might be more sensitive to distortion in amplifiers because the negative feedback that is often used tends to increase the amount of upper harmonics, whereas in speakers, the upper harmonics tend to have a relatively low value. Upper harmonics are very irritating to the ears.
The measured Frequency Response shows why I felt there was a little too much bass. You can see a bass hump regardless of whether the microphone was at 1 foot or 1 meter (I took the measurement with the bass toggle at the zero position). In general, the FR was pretty flat out to 20 kHz. If you use these with an SSP or receiver, I would suggest setting a low frequency crossover to a nice subwoofer at around 60 Hz. In any case, a subwoofer will help with the lowest octave.
At a price of less than $3K/pair for the MartinLogan Purity hybrid ESL/Cone speakers, you can have the kind of detail you won't usually find with conventional midrange drivers and tweeters. ESLs are not for everyone, but even if you are not the adventurous type, you should at least hear them for yourself and then make up your mind. While you're at it, listen to the full range models too.