Fig. 1
Advent, a new name among hi fi products, has a long history of well. known audio designs in the person of Henry Kloss behind it. Among his list of credits are the KLH Model 6 loudspeaker and the KLH Model 8 FM radio, the first home product to use active equalization to compensate for speaker shortcomings.
The new Advent loudspeaker does not represent any "breakthroughs," but is rather the highly refined product coming from years of accumulated experience.
The enclosure houses a 10" woofer working on the acoustic-suspension principle, crossing over at 1000 Hz to the 2" tweeter. The woofer cone is a 7.5" piston formed by a new low-vacuum process, with high internal damping which goes a long way to eliminate "cone cry." This, incidentally, is the second low-frequency speaker to employ this new molding process-one that will probably attract more attention in the future. The annulus is of heat-formed polyurethane, permitting maximum linear cone travel at the lowest frequencies.
The 7.5" diameter of the woofer cone ensures piston action as well as good dispersion up to the crossover frequency.
The treble portion of the spectrum is handled by the 2" high-frequency direct radiator. The diaphragm looks something like a 2" doughnut with a 0.75" dome in its center; the surface is coated with a lacquer-like substance to increase its radiating efficiency.
The tweeter assembly mounting is forward of the front panel. This is to prevent unwanted reflections from the front of the enclosure so as to minimize response irregularities. The grill cloth, held in place by six Velcro pads, incorporates a fine wire screen in front of the tweeter to protect it from prying fingers.
The enclosure dimensions are 25.5” X 14 1/4" X 11.5” and the unit weighs 35 pounds, still supportable by a sturdy shelf. While this is a large bookshelf speaker, its front edge molding is slightly beveled, giving the impression of smaller size.
Amplifier connection is via knurled nuts in a recessed rectangle on the speaker's rear panel, thus making it possible to push the speaker flat against the wall. A three-position switch permits tweeter level adjustment, whose effect is shown in Fig. 2. All four sides of the enclosure are finished in oiled walnut, permitting vertical or horizontal use.
We measured frequency response, placing the speaker upright on a 2-foot high stand, flat against the wall, tweeter control in increase position. We ran two response curves, one with the condenser microphone aimed at the geometric center of the front panel, and the other with the microphone 45 degrees off axis, both from a 3-foot distance. The test signals were 25 bands of 1/3 octave pink noise, fed to the speaker at a 2-volt input level. This level produced 90 dB, SPL, 3 feet on axis. We have come to prefer this method, since it is less prone to produce the misleading peaks and dips that result with swept sinewave testing, which reflect mostly room effects and not the true loudspeaker response. The results thus obtained agreed closely with the subjective impression formed during the listening part of the test.
Fig. 2--The upper curve shows the on-axis frequency response with effect of
tweeter level control in its three positions. Middle curve shows the 45-deg.
off-axis response to the same 1/3-octave band pink noise. The lowest curve
shows third harmonic distortion at low frequencies.
As evident in Fig. 1, the frequency response was flat, with only ±3 dB variations over the major portion of its range. What is more important is that the 45 degree off-axis response follows the on axis response to a remarkable degree, denoting excellent high-frequency power response. While the desirability of good high-frequency dispersion is not a new idea, (Ed Villchur's article in Audio, Oct. 1958), it is a goal that has, for the most part, eluded the majority of loudspeaker designers until very recently.
The low-frequency aberrations shown in the response, between 100 and 320 Hz are due to room effects. When measuring another highly regarded bass reproducer in the same position we found virtually identical response in that range.
Harmonic distortion measurements were made using a 4-volt input and the same microphone position. See Fig. 2. Above 40 Hz, third-harmonic distortion ran under 1%, with the exception of 1000 Hz, the crossover frequency, at which point distortion rose to 2.5%. With 6.5-volt input, TDH at 32 Hz was 6%. We consider this frequency to be the low limit of the speaker. 40 Hz distortion remained under 1%.
Efficiency of the Advent speaker is average for an acoustic suspension unit. 6.5 volts into a nominal impedance of 8 ohms, representing 5 watts, was enough to rattle windows with this speaker. While a good 25-watt-per-channel amplifier should be sufficient, 50 to 75 watts is not excessive.
Oscilloscope photos of tone bursts are shown in Fig. 3. They reflect, favorably, the transient response of the speaker. Since the foregoing data indicated this to be an excellent reproducer, we looked forward to the listening session.
Fig. 3--Tone-burst response at 12,000 Hz and 5000 Hz.
We played excerpts from Copland's Rodeo, Turn. 34169, Stravinsky's Petrouchka, Col. MS-6332, and Handel's Messiah, PHS-3-992, among others. It was gratifying to note that our ears confirmed the measured result. Particularly noteworthy is the high-frequency dispersion, resulting in excellent definition and adding spaciousness to the sound.
One could say that at twice the price the Advent speaker would be a good value, but at $112.00 it is a bargain. An auspicious beginning indeed, for a new company.
A. R.
(Audio magazine, Jul. 1970)
Also see:
Advent "Smaller" Loudspeaker (Equip. Profile, Dec. 1972)
Advent loudspeakers (ad, Dec. 1972)
Advent Model 300 Stereo FM Receiver (Equip. Profile, June 1977)
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