Wharfedale Super 8 Wood Cone

This project started and ended with the idea of using a wood cone in the Super 8. The purpose was NOT to improve the Super 8; the intention was to study the effect of a wood cone versus paper. The Super 8 units available are easy to disassemble without damage to the cone and spider.

In the beginning, it was noticed that a pair of papyrus cone 6.5" midrange speakers recently purchased had a noticeably different sound. Tests were performed and frequency response was negated as the whole reason. A few other similar speakers exhibited much the same frequency response within less than 2dB variation. However, a spectrum analysis told a much different story. Impulse response tests are shown at the end of this diatribe.

A Tektronix harmonic distortion analyzer was used but stability of the instrument was difficult due to reflections in the room and any motion that would upset the standing wave pattern/reverberant field above about 1khz. The speaker was then compared with another of closely similar parameters using a spectrum analyzer and the result was astounding. The spectrum analyzer was then applied to the Super 8, actually a few of them. Two are completely original, one being a CS, cloth surround (annulus) and the other an FS, foam surround (annulus). These two units also have the older phenolic spider which made disassembly easy without the need of solvents; the adhesive around the annulus is over 60 years old. The foam surround speaker was used only as a reference as disassembly would certainly damage that 6 decades old foam.

After running tests on both the FS and CS, the CS was disassembled and a new paper cone installed. This cone has a sealant treated corrugated cloth annulus, shown in Photo 1. The next 8 photos briefly show that process with short comments.

PHOTO 2: Double spider speaker

The long thing is probably a wood chip but the reason for this piccy is the little fella who was on the dust cap. It may be a baby brown recluse whose bite is worse than that of the black widow. It's about the size of a grain of salt. After taking the photo, the speaker was taken outside and a puff of breath sent the little fella into its natural habitat.

Actually, the part that centers the voice coil was and may still be called a centering device. The term spider came from the appearance of the early centering devices, many being more elaborate than the one shown on this speaker. Photo 3.

Left: The cone and coil assembly removed without the use of solvents. Right: The label in remarkable condition. This and the other original unit were probably never used as the paint around the mounting holes isn't scratched. Way back then, these units were installed behind the baffle.
PHOTO 3	PHOTO 4

PHOTO 6: A Slight Digression

Ahhh, duct tape, aka duck tape, the latter probably a misnomer until the Frog tape was seen. I'm sure there's a Gorilla tape. Since there are two tapes on the right named after animals, can we expect lizard tape, alligator tape, dinosaur tape or pterodactyl tape?

While the above may sound funny or even ludicrous, such tape, regardless of the name, can be used to temporarily fix just about anything. Remember, that stuff saved the Apollo 13 crew.

Q.E.D.

PHOTO 8

This quarter inch thick gasket clamped to the speaker helped but wasn't sufficient. The taped annulus had no effect when running impedance as the voltage applied is very low and the speaker was facing up. The weight of the cone was sufficient to keep the annulus edge from vibrating. Had it done so, that would have showed in the impedance curve.

Description of the speakers involved

Figs 1 & 2 below show the impedance and response curves, resp. The colours for each speaker are the same in both figs.

EB2 yellow; EB3 green; EB4 blue; EB4 red (with new cone & coil); EB4 black (with wood cone)

The EB2 has a cloth annulus and a canvas spider so it was used as a reference. The cone and coil (copper) are original. This unit was not spectrum analyzed.

The EB3 is a Super 8 FS/AL with aluminum voice coil and the original foam annulus. It has a phenolic spider. Also used as a reference.

The EB4 is a Super 8 CS/AL with aluminum voice coil and the original cloth annulus. It has a phenolic spider. The one converted.

The EB4 is the one used for the new paper cone (photos 7&8) and the wood cone replacements.

The EB4 has a hand wound copper coil with a dcr of 11.11 ohms and a Z of 13 ohms.

FIGURE 1

The lopsided black curve (14) is that of the EB4 unit with the wood cone and hand cut cardboard spider. That spider will have to be replaced with a more stable one as this one seems too weak to hold the cone and coil assembly in a vertically centered position. In short, the cone and coil assembly drops about 1mm to 2mm. While this has little effect on low level frequency response as the cone barely moves enough to displace the windings in the magnetic field, it quite possibly can when the cone is driven to an excursion of >1mm. A spectrum analysis should show that. However, a speaker playing music reproduces a multiplicity of frequencies simultaneously. the lower frequencies can easily drive the cone to >1mm. The uneven position of the coil in the field can then cause non-linear distortion at any frequencies being reproduced at that moment.

The cone in the Super 8 has an effective piston area of 0.01887 square meters. Taking a frequency of 500hz which is at the lower practical limit for this speaker (Wharfedale's common crossover point was usually 800hz), at 0.1mm excursion, this will produce 96dB at 1m. At half that excursion, the SPL is 90dB. But, at 0.25mm and 0.5mm, the SPL becomes 104dB and 110dB, resp. Taking this further, at 1mm excursion, which is just about Xmax, the SPL goes to 116dB.

The conclusion is that anyone listening to music at 104dB or higher isn't concerned about non-linear distortion in a loudspeaker.

What's odd here is that the EB3 & EB4 are labeled as 10 ohms and 15 ohms, resp., yet both measure as slightly less that 8 ohms. Probably the effect of 60 years.

FIGURE 2

The green and blue traces are those of units EB3 and EB4, resp. These are both all original units, meaning that the cones are around 60 years old and brittle. The red trace is the EB4 with a new cone. The probable reason for this radical difference will be shown in the spectrum analyses of these units.

The prpple trace will be explained later.

Spectrum Analyses

This can get lengthy so it'll be taken in sections. First, the all original EB3 and EB4 units will be compared. Then the EB4 original will be compared to the EB4 with the new paper cone. Following that, the EB4 with the new paper cone will be compared with the EB4 with the wood cone. Then, the EB4 with the wood cone will be compared to a recently purchased new speaker with a wood cone. (The prpple trace of fig.2)

In these images, the fundamental is the tall trace on the left. The harmonics are the shorter traces above the noise floor. The thin traces below the noise floor are part of that noise. The noise floor is 80dB below below the top of the scale which is auto adjusted. These traces are measured with about 1W applied to the speaker.

The sum of all these spikes other than the fundamental is referred to as THD+N (Total harmonic Distortion plus Noise) The top left image would be about 1% or less THD+N while the last two on the left can be as high as 10% THD+N.

EB3 All original Super 8 FS/AL	EB4 All original Super 8 CS/AL
Figure 3 400hz	Figure 4 400hz
Figure 5 700hz	Figure 6 700hz
Figure 7 1200hz	Figure 8 1200hz
Figure 9 2000hz	Figure 10 2000hz
Figure 11 3000hz	Figure 12 3000hz
Figure 13 4000hz	Figure 14 4000hz
Figure 15 5000hz	Figure 16 5000hz

The spectra of both the EB3 & EB4 are quite similar up to 4khz, where the EB3 goes to hell in a handcart. This may be due to the possibility of the EB4 being a much later manufactured speaker. This assumption is based on its having a dome paper dustcap with a hole in the center. Also, the label on the back of the magnet assembly is white, whereas that on the EB3 is the older gold label. Also, as unscientific as the following may seem, the cone of the EB3 feels stiffer to the touch than that in the EB4, possibly due to becoming brittle with age. This would result in serious cone breakup at higher frequencies.

Looking art the green(EB3) and blue(EB4) traces in fig.2, a bump of about 4dB can be seen in the green trace between about 3800hz and 4400hz. That may be the result of the plethora of harmonics above and below 4khz as seen in fig.13. The LMS does have a 4th order bandpass filter which will remove some but not all these nearby harmonics. However, that logic doesn't explain the difference between between figs 15 & 16, despite the response traces in fig.2 being identical between 4500hz and 5500hz.

It should be noted that the figures on the left of table 2 are then same as those on the right of table 1

EB4 All original Super 8 CS/AL	EB4 with flexair© type cone © Jensen Speakers
Figure 17 400hz 1.76%THD+N	Figure 18 400hz
Figure 19 700hz 1.07% THD+N	Figure 20 700hz
Figure 21 1200hz 2.24% THD+N	Figure 22 1200hz
Figure 23 2000hz 2.49% THD+N	Figure 24 2000hz
Figure 25 3000hz 2.49% THD+N	Figure 26 3000hz
Figure 27 4000hz 2.67% THD+N	Figure 28 4000hz
Figure 29 5000hz 3.62% THD+N	Figure 30 5000hz

The comparison between the EB4 with the new paper cone and the EB4 with the wood cone

It should be noted that the figures on the left of table 3 are then same as those on the right of table 2

EB4 with flexair© type cone © Jensen Speakers	EB4 with wood cone
Figure 31 400hz	Figure 32 400hz
Figure 33 700hz	Figure 34 700hz
Figure 35 1200hz	Figure 36 1200hz
Figure 37 2000hz	Figure 38 2000hz
Figure 39 3000hz	Figure 40 3000hz
Figure 41 4000hz	Figure 42 4000hz
Figure 43 5000hz	Figure 44 5000hz

Again, six of one and half a dozen of another. Very little difference between the new paper cone and a wood one. BTW, the wood is red oak veneer. It see3ms worthy to mention that a very high end 7 inch midrange unit costing over $900 has 0.6% second and the same at third harmoni9c between 2khz and 5khz. This sums to around 1% and that's only two harmonics. A spectrum analysis or a complete THD+N analysis will undoubtedly yield higher distortion figures. The point that is being attempted to clarify here is that no speaker is free of distortion, with possible exception of plasma speakers, which have no diaphragm and are totally impractical outside the laboratory.

Someday. Keep in mind that in the sixties, the laser existed only jn a lab and was huge requiring lots of power. Today, one can generate a laser with a AAA battery in a key fob.

The comparison between the EB4 with the wood cone and the newly purchased wood cone 6" drivers.

It should be noted that the figures on the left of table 4 are then same as those on the right of table 3

EB4 with wood cone	New 6" wood cone speakers
Figure 45 400hz	Figure 46 400hz
Figure 47 700hz	Figure 48 700hz
Figure 49 1200hz	Figure 50 1200hz
Figure 51 2000hz	Figure 52 2000hz
Figure 53 3000hz	Figure 54 3000hz
Figure 55 4000hz	Figure 56 4000hz
Figure 57 5000hz	Figure 58 5000hz

Impulse response comparison between the EB4 with the wood cone and the newly purchased wood cone 6" drivers.

These impulses were generated with the CLIO as it will allow a half cycle to be generated at any frequency. The signal was captured by the PicoScope as this system makes it easier to capture the image. The 6" wood cone claims a response to about 75hz which renders the 50hz samples kind of meaningless. At 100hz and 500hz, the EB4 seems to have a better damping. However, the 6" unit does better above 2khz.

This driver is better used as a midrange above 400hz if used in a 3-way system. It may also do quite well with or without a tweeter for low level easy listening as it does well up to about 7khz. This unit will find it's way into a 3-way system as a midrange covering the band from 400hz to 5khz

The generator is a constant voltage output. The power figures associated with each figure are calculated at the impedance of the speaker at the generated frequency.

EB4 with the Wood Cone	Wood Cone 6"
Fig.58 50hz 12w	Fig.59 50hz 3.6w
Fig.60 100hz 6.4w	Fig.61 100hz 17w
Fig.62 500hz 16w	Fig.63 500hz 23w
Fig.64 2000hz 13w	Fig.65 2000hz 18w
Fig.66 3000hz 11w	Fig.67 3000hz 17w
Fig.68 4000hz10w	Fig.69 4000hz 16.5w