MANUFACTURER'S SPECIFICATIONS
System Type: Three way, bass reflex. Components: One 12-in. woofer, one 5-in. midrange, and four cone tweeters (two each 2 and 2Y2 in.). Frequency Response: 30 to 20,000 Hz. +4 dB. Nominal Impedance: 8 ohms.
Dimensions: 28 in. H. by 22 in. W. by 12 1/4 in. D.
Weight: 75 lbs.
Price: $299.00
Fig. 1--A, Arrangement of the Lowboy speaker units; B, How the speakers are
mounted in the older-design Model III.
The Rectilinear III Lowboy is a recently introduced version of the Model III which was released some four years ago. It uses a similar speaker arrangement in a different style cabinet, so the change only involves styling. The original model is still […] but there is no doubt that many people will find that […] with its attractive wooden grille and low profile will harmonize with their furniture better than the more severe styling of the Model III. Just a matter of taste. But changing the cabinet form factor is not a simple matter and even slight modifications can seriously affect the overall sound quality. However, we can report that the differences between the two models are quite small and are probably less than those caused by room acoustics. Figure la shows the speaker arrangement of the Lowboy; the 12-in. bass unit has a corrugated plasticized surround and the enclosure is a reflex type with the ducted port opening at the lower right. The 5-in. midrange driver is mounted near the top and the cross-over frequency is 500 Hz. It uses a twin-cone and functions up to about 3,000 Hz at which point the four cone tweeters take over. These four units are two each of 2-in, and 2 1/2-in. sizes and are spaced around the top half of the cabinet to give a large source effect. Figure lb shows how the speakers are mounted in the older Model III. At the rear are level controls for midrange and treble, and I was pleased to see that the input terminals were spaced well apart.
Measurements
The frequency response, taken with one-third octave band pink noise, is shown in Fig. 2. Note that the response is easily within specifications, and excepting the small "bump" centered on 70 Hz, is within +5 dB from 40 up to 20,000 Hz. The top curve, A, shows the response on-axis, B is taken at 45 degrees off-axis, and C shows an average of five positions. The speaker level controls were set to the best listening position which was about central for the midrange and a slight reduction in the treble. Figures 3 and 4 show the ranges of the level controls, and the harmonic distortion at low frequency is shown in Fig. 5. Frequency doubling commenced at 75 Hz but a useful output was obtained down to 30 Hz. The tube port is filled with fiberglass and we removed it and checked the low frequency response again. As might be expected, power below 75 Hz fell by half-in other words, the fiberglass reduced the "Q." White noise tests showed up some coloration (some frequencies were increased) when the midrange control was set to minimum, but in the normal or center position, coloration was very low. Tone burst response can be seen from the photographs in Fig. 6. The 10,000 Hz response shows interference effects from the multiple units. Figure 7 shows the impedance characteristics and it will be noted that the lowest point is over 9 ohms, thus extension speakers can be used as well as the main speakers without reducing the load to an undesirable figure. System resonance is at 50 Hz and above this point impedance is remarkably smooth. Sensitivity is quite high-a one-watt signal produces an average output of 95 dB at one meter, so a moderate power amplifier would be quite adequate for small to medium size rooms.
All-in-all, this is a fine sounding system with good frequency response and above-average dispersion.
Fig. 2--Frequency response taken with one-third octave band pink noise; A,
on-axis; B, 45 degrees, and C, average of five positions.
Fig. 3--Showing range of mid-band control. Fig. 4--Range of treble control.
Fig. 5--THD at low frequencies.
Fig. 6--Tone burst characteristics at A, 100 Hz; B, 500 Hz, and C, 10,000
Hz. Note that the 10,000 Hz response shows interference effects because of
the multiple units.
Fig. 7--Impedance characteristics.
-T.W.
(adapted from Audio magazine, Jun. 1972)
Also see:
Rectilinear XIa Loudspeaker System (Equip. Profile, Aug. 1973)
Rectilinear Research Corp. loudspeakers (ad, Oct. 1973)
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