Hsu Research HB-1 Horn Speaker and VTF-3 MK3 Subwoofer Measurements
At deadline, we were informed that the samples of the HB-1 sent to us by Hsu Research for the measurements presented below have been updated from the ones FM reviewed, with the tweeter in the new version redesigned for a smoother response. FM is sending two of his samples to our LA test facility, where we will measure them and add an update to this report in the near future.
The ported cabinet HB-1 is tuned to approximately 52Hz, with a minimum impedance of 5Ω at 172Hz. I would rate the speaker's nominal impedance at 6Ω. It should be an easy load for any competent amplifier.
Our measurements indicate a sensitivity of about 88dB/2.83V/m. This is a perfectly respectable result, but well short of the specified 92dB. The difference between our measured sensitivity and the specification may be due to the fact that the specification was taken in half space. Our measurements are made in an open studio, 10 or more feet from any nearby walls, with the effect of those walls "time-windowed" out of the results by our measurement software (LMS from Linear X Systems, Inc.). Nevertheless, compared to measurements we have made on other speakers in the same way, the sensitivity of the HB-1, rather than being high enough to tempt lovers of flea-power, single-ended triode amps, is about average.
The horizontal front response of the HB-1 shown in Fig.1 (violet) is the pseudo-anechoic response averaged over a 30° forward horizontal angle, taken at tweeter height, combined with the nearfield responses of the woofer and port. The response, while not particularly noteworthy, is very good for a speaker in this price range. The main aberration is a valley centered at about 3.5kHz.
Fig.1: HB-1, pseudo-anechoic response at 45° (red) and 60° (blue) off-axis in the horizontal plane.
The off-axis response shows a more rapid roll off in the high frequencies than we typically see with dome tweeters, but that is not unusual for a horn design. According to Hsu, it is deliberate, to avoid over emphasizing the treble in the power response.
The effective lower limit of the HB-1's response is about 42Hz (-10dB). This is respectable for such a small speaker, but as FM noted in his review, the HB-1 is clearly designed for use with a subwoofer.
Fig.2 shows the same averaged horizontal front response (purple), this time overlaid with the vertical responses taken at +15° (red) and –15° (blue). These curves suggest that the vertical position of your ear should be as close to the center of the tweeter as possible. If you must deviate from that, the response is smoother above the speaker than below it, though I would not expect the audible differences to be dramatic. But if you must place it high, such as above a big-screen television, you might try positioning it upside down.
Fig.2: HB-1, pseudo-anechoic response at 15° above (red) and 15° below (blue) the tweeter axis.
FM did not audition the Hsu HC-1 center channel, and that speaker is not part of his review. But Hsu did send us a sample, so we ran it through our usual suite of measurements for those who may be interested in using it, rather than a fifth HB-1, for center channel duties.
The HC-1's ported cabinet is tuned to about 54Hz. Its minimum impedance measured 7.8Ω at 213Hz. I would rate its nominal impedance at 12Ω and its sensitivity at approximately 89dB/2.83V/m. As with the HB-1, it should be an easy load to drive. Its effective bass response extends to about 46Hz (-10dB).
The measured front horizontal response of the HC-1, taken on the tweeter axis and averaged in the same manner as described above for the HB-1, is shown in Fig.3 (violet curve). As with the HB-1, the HC-1 is designed for use with a subwoofer for full-range response.
The HC-1's on-axis response is noticeably smoother than that of the HB-1, though the center speaker has a similar suckout in the presence region (narrower in this case, and centered at approximately 2.3kHz). The surprising thing here is the smoothness of the HC-1's treble in comparison to that of the HB-1, considering the fact that both speakers use the same tweeter design. Though the tweeter in the HC-1 may be balanced a bit on the high side, it produced one of the flattest, smoothest curves we have ever measured from 4kHz-20kHz.
The off-axis response of the HC-1, also shown in Fig.3, is a classic example of the sort of comb filtering that results from horizontally configured, woofer-tweeter-woofer, center channel speakers. If you want good results from such a speaker, avoid sitting much more than 15° off-center.
Fig.3: HC-1, pseudo-anechoic response at 45° (red) and 60° (blue) off-axis in the horizontal plane.
Fig.4 shows the vertical response of the HC-1 at + 15° (red) and –15° (blue), again overlaid with the averaged front horizontal response. The vertical off-axis response is actually a bit flatter through the midrange and low treble than the on-axis average. This is actually a plus, as a center channel speaker will most often be mounted above or below the screen (and thus above or below the ear).
Fig.4: HC-1, pseudo-anechoic response at 15° above (red) and 15° below (blue) the tweeter axis.
Apart from a few off-axis concerns, particularly in the HC-1, this is a very impressive set of measurements for such an inexpensive speaker.—TJN
All figures: Violet curve: pseudo-anechoic response on the front axis, averaged across a 30° horizontal window, combined with nearfield responses of the woofers and port. All measurements were taken at one meter.