The Tempest

By Scott C. Blaier
31 Mar 1996

System Measurements

Upon completion, and after listening tests, an audio generator, frequency counter, and Radio Shack SPL meter were used to generate the "nearfield" amplitude response curve. The graph was compensated for the SPL meter's high frequency rolloff beyond 15 kHz. A graph of the frequency response curve is included with the article Fig. 5. Frequency Response.

The measurements were performed with the Tempest positioned on the floor (no stand), 8.0 inches from the rear wall, no furniture within 3 feet, and the grill removed. The listening room was a 36 x 24 x 7.5 foot finished cellar, with a carpeted concrete floor, drywall walls, and acoustic ceiling tile. I consider the room to be absorptive ("dead"), as opposed to reflective ("live"). The measurements were taken at low acoustic output, with the microphone close to the drivers (nearfield), to minimize the effect of the room on frequency response. I believe this curve gives a good idea of how this loudspeaker will preform, with respect to frequency response, in a similar room under similar conditions. One amplifier watt produced an 89 dB sound pressure level, 1.0 meter away, on axis, in this room. In a real listening environment (not anechoic chamber) the Tempest's overall response is within +/- 3 dB of the 89 dB reference mark, from 28 to 20 kHz., with two exceptions (explained below).

Please note that loudspeaker response below approximately 300Hz. is largely dependent on the listening room. Also, Vifa's response graph shows that tweeter will respond to 30 kHz. However, I can not confirm this because that is well beyond the range of the Radio Shack SPL meter's microphone.

One of the two exceptions to the +/- 3 dB tolerance is a peak in low bass centered around 36 Hz. When I first saw the acoustic response curve for the NHT 3.3 posted on the Internet, I noticed it had a low bass peak also. I am not sure whether this phenomenon is related to room interaction, my measurement techniques, the loudspeaker itself, or all of the above. In any event, we heard no audible evidence of this peak during music.

I believe the second anomaly is a manifestation of Roy Allison's "Wall Dip". I have read Allison's published work on this phenomenon and it seems to explain this dip in response. In fact, years ago, Acoustic Research and Allison Acoustics used a "side-firing woofer" loudspeaker configuration to compensate for this "dip" that plagues almost all conventional loudspeakers. Probably not coincidentally, the NHT's 3.3 also uses the "side-firing" configuration. I did manipulate the 300 Hz. crossover to minimize the "wall dip" effect. Fortunately, in the Tempest, the dip is represented by a modest trough in response between 200 and 300 Hz., and would likely change depending on the loudspeakers position in the room. We did not find it audible.

In addition to a frequency response sweep, we used a compact disc test recording to evaluate the Tempest's imaging, phase, presence of crossover "suckout", dynamic range, etc. The disc contains pink and white noise, individual instruments, etc., and does a fairly rigorous job at evaluating different aspects of a loudspeaker system. The Tempest faired extremely well on all tests.

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