Bookshelf Speakers

Usher Be-718 Bookshelf Speakers with Beryllium Tweeters

ARTICLE INDEX

On the Bench

Like small speakers in general, the Be-718 has significant distortion at very low frequencies. Here, at 40 Hz and 100 dB output, THD+N was nearly 12%. This is actually pretty good performance though, as I expected it to be higher. For the THD measurements, I used a bandwidth of 80 kHz.

At 1 kHz, the woofer was still the output device, but THD+N was now down to 0.25%, which is excellent.

At 10 kHz, the tweeter was outputting the sound, and distortion was still low, at 0.22%.

With 10 kHz and 11 kHz sine waves, the A+B peak at 21 kHz was 47 dB below the fundamental, while B-A at 1 kHz was 65 dB below the fundamental.

This, to me, is the most important graph, as it shows distortion at all the audible frequencies. I first set 1 kHz to output at 100 dB with the microphone at 1 foot, then measured the spectrum. THD+N is off scale below 33 Hz, and comes down rapidly to stabilize around 3% from 50 Hz to 150 Hz, then goes down again to be about 0.2% - 0.3% from 500 Hz up to 20 kHz. Notice that it goes up to nearly 0.5% just above 2 kHz, which is the crossover frequency. The fact that it stays around 0.2% from 500 Hz up to the limits of audibility gives the speaker a very neutral sound. If there were more variability, the speaker might sound harsh, tinny, overly sibilant, and otherwise some sort of skewed sound quality that would take away from being neutral. Like just about any speaker, the Be-718 will benefit from having a good subwoofer, using a crossover (judging from this graph) of about 50 Hz.

This graph shows the room response at 1 foot and 1 meter. Look how flat it is (1 meter) from 50 Hz to 20 kHz. This is terrific!

The impedance appears to be 8 ohms nominal, and dips to 4 ohms at about 3 kHz. Electrical phase stays within + 400 and - 600. The impedance peaks at 20 Hz and 65 Hz are from the woofer interacting with the slot and the enclosure. The upper peak at 65 Hz occurs when the woofer suspension and enclosure air spring are combined with the woofer's moving mass (tighter spring). The lower peak at 20 Hz is created when the woofer and air mass in the slot form one moving mass acting against the woofer suspension (heavier mass). The peak at about 1.8 kHz is due to the crossover. The free air resonance of the woofer is probably about 40 Hz.

Conclusions

Usher's new Be-718 bookshelf monitors are spectacular, and I am sold on beryllium tweeters! Even though $2,795/pair is no chump change, you could spend a lot more and get a lot less.