I take little as so-called gospel unless, of course, the source is known to be credible. While i wouldn't place that friend as credible in such a technical matter, I did accept the credibility of the author but my curiosity got the better part of me, beside which, I needed another project to occupy my time.

I did find out that the author was right, however slightly exaggerated that claim may appear. The only way to find out is an A-B blind test but in all my years in audio, I found only one who would consent to such a test. We did agree that he may not like the result and I was right, he didn't. That which he thought would be preferred was opposite to that which he thought.
The test involved claims that a kevlar cone was better than a paper cone. I will state now that that statement is true from a purely technical point of view. However, technical analyses have little to noting to do with personal preferences.

The test was done in the mid eighties and involved a pair of 10 inch woofers, one a $20 paper cone and the other a $110 kevlar cone. Two visually identical enclosures were designed and built. Each had slightly different internal volumes for optimum performance in the bass according to BassBoxPro, v6.Once done, the friend came over for the audition. the speakers were placed against the wall opposite the listening position, some 16 feet distant. The room measures 20ft by 17.5ft. The speakers were placed behind a sheer black sheet and the room very dimly lit. this disallowed seeing the yellow kevlar cone. The speakers were placed side by side to eliminate differences in room reflections. A silent switch was used to switch between the speakers. In was unknown which was played first and whether or not a change was made. All I did was ask him to say if he preferred the sound that was playing and I made note thereof.

After about half an hour and several different types of music, the result was tabulated and much to his surprise, he preferred the sound of the $20 speaker's bass more than 90% of the time. See, we get used to things and a change is usually rejected because we don't like to think we were wrong. In this case, he was influenced by what is called bias expectation. He had never heard the kevlar speaker but was told that it was better and believed that it would sound better. Again, depending on one's preference, that may be true but for the most of us, it isn't. A person familiar with how a live performance sounds,

Regarding the SLOB/SLPP design, no audition was performed as my only desire was to determine how much the second harmonic distortion was reduced. I don't doubt that some can hear it but I doubt that I can at my age as the reduction is subtle, percentage wise.

I remember Paul Klipsch stating in one of his papers that an attendant at one of his demonstrations repeatedly demonstrated his ability to differentiate the sound between a Marantz Model 9 and a McIntosh Mc275.

Anyway, my apologies for the diatribe.

The chamber.  It measures 11"x9.625"x1.5" for a volume of 189 cubic inches.  This includes the added volume (12 in^3) of the slot in the yet to be attached front baffle and the combined volumes of the frusta of the two speaker cones. (18 in^3)

The slot area is 16.5 in^2

The larger chamber including the frusta and the slot volume is 274 in^3

The front baffle attached.  It serves little enhancement of the low frequencies as it's longer dimension, 25" will support frequencies above about 500hz.  This size was chosen because it fits the 3 ft^3 enclosure upon which it sits and can be used for future tests.

The assembly mounted on top of another enclosure to get the center of the slot midway between the floor and the ceiling, 48 inches.  the CLIO is internally calibrated to cancel first reflection from the floor and ceiling with the mic at a distance of 1m from the source.

The LMS allows adjustment of these distances and once entered, it will turn off the signal before the first reflection gets to the mic, thus simulating an anechoic response.  Both units will fail below about 400hz as the wavelength is too long to get a good sample before the reflection arrives.

This was a later experiment to see what happens if the chamber volume were increased by 80in^2 by increasing the chamber width by 0.75 inch.  There were two tests done here.  One with the wider slot and another, shown here, with the slot reduced to the original size and both with the larger chamber.  The results were unworthy of consideration and therefore, not reported here. (see fig 2)

It seems the chosen volume may be close to optimum.  Consideration was given to making the chamber smaller but that has yet to be studied.  The main purpose of this experiment was to determine if second harmonic distortion is reduced, as claimed.

It wasn't.  In fact, it got worse. (see figs 5 and 7)

The speakers in a sealed box of 3 ft^3.  Second harmonic distortion was lower than that of the slot system and, obviously, the lower bass was enhanced due to the blocking of the rear wave. 

The two eights on an open back baffle,

Z   BLK=2 in parallel in SLOT Chamber; RED=2 in parallel in 3 cu ft sealed

At first glance, it may seem counterintuitive that f_s would be lower with the pair facing each other in a small chamber. The rear of the speakers is radiating into open space, unlike the pair in the closed box.  Despite the fronts of the speakers radiating into a small space, that space is open at the slot and while it may impose a heavier load on the speakers, the speakers may see it as additional mass, hence the lower resonant point.  The open back imposes a far less load than the 3 ft^3 sealed box thus resembling an open air condition.

The open air resonant point of these speakers is 32hz and in the SLOB, that resonant point has lowered to about 29hz, very likely due to the effect of the air load in the slotted chamber being pushed by one speaker against the other

The "blips" at about 170hz and 340hz are nor floor bounces as both speakers were measured in the same location and under the same conditions. They may be due to reflections from the back wall as they only show during the open back sweep.

See the note at the bottom of this page.

1w1m

big box is 79 in^3 larger and its slot is area is 24.75 in^2  Its slot volume adds 18 in^3 to the chamber.

RED=small box small slot  ORN=2^nd harmonic

GRN=big box big slot  BLU=2^nd harmonic

BLK=big box small slot  GREY=2^nd harmonic

It seems that there is little effect with the above combinations although the small box with the small slot has less 2^nd harmonic distortion below 230hz

1w1m small slot and chamber RED=THD

 GREEN=2^nd harmonic; ORANGE=3^rd harmonic

This shows that the total harmonic distortion is mostly 2^nd harmonic

Fig 4 shows the same comparison at 10 watts

10w1m

GREY=small slot and chamber RED=THD

 GREEN=2^nd harmonic; ORANGE=3^rd harmonic

The major result of the 10 watts is an increase in response level by 10dB, which was expected..  What wasn't expected was the distortion components not increasing more than 10dB.

10w1m

GREY=3 cu ft sealed YEL=2^nd harmonic

RED small slot and chamber ORN=2^nd harmonic

Comparison between the two eights in a closed box(GREY) with its 2^nd harmonic(YELLOW) and the SLOB(RED) with its 2^nd harmonic(ORANGE)

In their passbands below 500hz, the closed box has much less 2^nd harmonic distortion than the SLOB, contrary to claims of the opposite.

Keep in mind that the response below 150hz or so is affected by the room hence the higher bass output of the closed box. 

FIGURE 6

OPEN BACK GRN=1w1m Blue=2^nd harmonic BLK=10w1m GREY=2^nd harmonic

Usually, the 2^nd harmonic curves will be very similar but spaced 10dB apart.  Here, the two curves differ in shape, between about 70hz and 200hz.  Also, the difference in SPL far exceeds the expected 10dB; it's more like 20dB. This is attributed to the speakers' cones exceeding Xmax.  This was not only observed but also heard.

For obvious reasons, the sweep was not repeated a second time.

FIGURE 7

1w1m DARKBLUE=OB LTBLUE=2^nd harmonic BLK=SMLCHAMBERslot GREY=2^nd harmonic ORN=SEALEDBOX YEL=2^nd harmonic

The dark blue(OB) and orange(SB) upper traces show the increased bass with a sealed box.  The lower traces show that the OB has more 2^nd harmonic distortion.

The upper black trace is that of the SLOB.  It shows less distortion below about 150hz and 3 peaks at about 170hz, 340hz and another just past 500hz.

In short, 2^nd harmonic distortion is reduced with the SLOB/SLPP configuration but this comes at the cost of reduced bass output, typical of an open back system.  As can be seen by the dark blue and black traces, the SLOP/SLPP does have an improved bass output over the traditional open back.

It can be humorously said that to further reduce the distortion, reduce the bass.  Taken to an extreme, low frequency distortion can be brought to zero by eliminating the bass altogether.