Flex-8
8" 2-Way with Multiple Woofer Options
Design Summary:
An 8" 2-Way horn loaded bookshelf style design with multiple woofer options each with their own custom tuned crossover.
Excellent price/performance ratio, the sum of the parts deliver sound quality that punches well above its class. I was shooting for a fairly neutral uncolored sound from these allowing them to be suited to a wide range of playback material either music or multichannel use. Several of the woofers give very decent bass extension so they could even be used without a sub, though subs always help. In general the Flex-8 designs have much better bass response then my VBS-6.2 / 10.2 so they are more suited then those for 2 channel or music playback with or without subs.
The horn design used for the high frequencies does have a slightly tighter coverage pattern then normal designs so while you gain intelligibility and directness from the sound and imaging you may loose some spaciousness or ambiance in the sound presentation in normal rooms. This may be well suited for use in more reflective rooms improving clarity by limiting off axis energy in the top octaves or in multichannel systems where the additional speakers channels provide the sound field while limiting high frequency spill.
HF Selection / Design Inception:
What kicked this design off was a single measurement and it snowballed from there. I had just picked up a few new compression drivers one of which was the very reasonably priced LaVoce DF10.101LS. I usually grab a couple horns and test the compression drivers on them to get a rough idea on the frequency response and distortion performance. One of the horns I grabbed was the Celestion H1SC-8050. When I ran sweep I was blown away, extremely linear response, a smooth low frequency roll-off and very low distortion that bests most other drivers anywhere near this price or even those more then double the price. The only downside of the compression driver is the roll off above 15-16kHz which I feel is only a minor negative since that high up you loose very little actual musical content and it does everything else so well. Anyways I knew then I had to use this combo in some kind of design and thus the Flex-8 project began.
Original measurement I took of the LaVoce DF10.101LS on the Celestion H1SC-8050
No Smoothing
The H1SC-8050 horn does have decent directivity behavior over most of it's range, it is a tad more narrow then I would have liked with the dispersion as tested in this cabinet around 70 degrees horizontal above 4000Hz . Below 3000Hz it begins to widen some reaching 120 degrees at the crossover point. This does result in a smooth directivity match to the woofer at the crossover. While perhaps not a horn that will best something like the SEOS waveguide designs in terms of directivity performance the general trend is still fairly decent when taking a look at the overall power response and directivity index compared to many other horns.
Woofer Selection:
With the knowledge that I wanted to use that driver/horn for the high frequencies I moved on to determine what woofer I wanted to pair it with. Knowing the limitation of the horn/CD combo I was looking for 8" woofers which I knew should provide a good directivity match to this size horn. The driver would also have to play relatively cleanly up past 2kHz as I had a feeling the crossover would likely end up in the 1500-2000Hz range. There was much back and forth between several woofers some of which I had on hand, others I was already interested in trying for a similar style design. At some point I just decided that I could design a cabinet with a usable internal volume for all the 8" woofers I'm considering and just test each one in box to see how they perform. This then evolved into testing all of the 8" woofers I had on had, at least those that would fit in the cutouts of the two cabinets I built and designing a crossover variant for each suitable woofer.
While I wasn't looking specifically for a certain type of woofer (pro style or conventional Hi-Fi), the main considerations were smooth response up to the crossover and bass extension to at least 80hz when tested 4pi. All the woofers I've designed variants for so far have been of the more conventional hi-fi style and as such the efficiency of these designs has ended up in the mid/low 80's, with the Silver Flute W20RC38-04 at the high end (88.5dB / 2.83v or 85.5dB 1w/1m), and the Dayton SD215A-88 at the low end (85dB / 2.83v or 82dB 1w/1m). Those two are 4 ohm models so the voltage sensitivity is higher which is why I give SPL at both the 2.83v and 1w ratings.
I am glad I tested all the woofers I had as it resulted in one making the cut that I didn't even consider initially, the GRS 8SW-4. The 8SW-4 has smooth frequency response up past 2 kHz which was totally unexpected since it is labeled as a subwoofer. Funny how it's actually better suited for 2-way designs and has a smoother frequency response then any of GRS's other 8" woofers.
The woofers I tested that did not make the cut:
Celestion:
TF0818 - These just don't have the bass extension I was looking for.
GRS:
8PF-8 - Unsuitable frequency response, high Qts resulted in large bass peak.
8PT-8 - Had high hopes for this model since the 10" worked so well in my VBS-10.2 design but the frequency response on this 8" was very rough through the midrange.
Dayton:
DC200-8 - Very large dip centered around 1kHz proved too difficult to work around.
I'm also considering the B&C 8PS21 and HiVi M8N-1B as well as a few others but they require either a different baffle cutout or I don't have one to test yet.
Cabinet Design:
As I had mentioned above I was looking to use a cabinet that would work with a variety of woofers, it appeared something around 0.75cuft internal would be well suited to most of the 8" woofers. Using that figure for internal volume I settled on a cabinet that was 10" wide x 17.5" high and 12" deep using 3/4" material for the cabinet walls and baffle. This keeps the baffle size fairly compact compared to the drivers.
Since most require a ported enclosure a 2-1/2" flared port was added to the rear of the enclosure which keeps port velocity in check. Since it's used at full length (8.5") for some of the woofers placement has to be fairly precise to clear the magnet structure of the woofer and compression driver. The GRS 8SW-8 is the only woofer which is best used sealed and thus does not require the port.
I used a pair of window style braces and a singular brace the runs side to side behind the baffle between the cutouts of the horn and woofer.
Damping:
Acoustic foam, denim, poly batting or other batt style damping material can be used if around 1-1.5" thick. The cabinet side walls, top and bottom can be covered but leave a spot for the crossover on the bottom behind the woofer.
Loose Polyfil could also be used instead of sheet style damping material by lightly stuffing the area above the top cabinet brace and the back half of the middle portion of the cabinet behind the port tube exit. Some material can be placed in the bottom behind the woofer as well but don't completely cover the crossover as you don't want the resistors to be insulated by the damping material since they can get warm with spirited listening sessions.
Cutlist:
Baffle/Rear: 10" x 17.5"
Sides: 10.5" x 17.5"
Top/Bottom: 8.5" x 10.5"
Window Braces: 8.5" x 10.5"
Baffle Brace 8.5" x 1.5"
The woofer cutout is centered 5" up from the bottom of the cabinet, horn cutout is centered 4.5" down from the top of the cabinet, port cutout is centered 9.5" up from the bottom.
The woofer and horn do not need to be flush mounted allowing for easier cabinet construction or if you lack the tools to accurately do so.
I did add a large 5/8" round-over to my cabinets however this too is not a firm requirement.
Crossover Design:
Each variant in this design has its own crossover with values optimized for the specific woofer used more in depth details of which can be found in their specific pages but all of the designs share the same basic topology with only some minor differences.
The very smooth response from the LaVoce DF10.101LS compression driver paired with the Celestion H1SC-8050 allows the use of very simple 2nd order highpass + resistive L-pad, just four parts resulting in +-1dB linearity from the crossover point (~1800Hz) up past 10kHz.
The woofer side requires a bit more complexity of the lowpass network, a 4th order filter is used for more adjustment ability in the shape of the rolloff, added correction ability of the frequency response from woofer and to give the required time/phase alignment at the crossover. Then a LCR notch is placed in parallel with the driver to handle the baffle step / diffraction ripple, the LCR notch not only gives further control over the frequency response but I find it prevents a scoop in the lower midrange in this design if the baffle step compensation was done through the 4th order lowpass filter alone.
Crossover sim of Silver Flute variant.
Flex-8 Variants:
Crossover info, detailed measurements & additional subjective impressions for each design listed in their respective pages.
4 Ohm design, sealed cabinet / no port makes for easier cabinet construction, lowest cost and potentially the best value of the bunch.
4 Ohm design, very nice looking woofer with a smooth frequency response, good mix between sensitivity and bass response.
4 Ohm design, deepest bass extension, can be used without a sub, comes at the cost of the lowest overall efficiency.
8 Ohm design, beefy motor gives the lowest bass and low midrange distortion with minimal compression as well.
8 Ohm design, neat looking aluminum cone has a very smooth frequency response, excellent bass and midrange sound quality.
8 Ohm Design, high output design most suited for home theater use, tradeoff is much less bass extension then others.