95 dB Open Baffle Loudspeaker
Copyright 2011 © Troels Gravesen

Drivers     Crossover     Crossover layout     Baffle    Measurements     Kit Parts     The Sound    

This is my homage to Mr. Gilbert Briggs, founder of Wharfedale loudspeakers and writer of numerous books on hifi. My first book on loudspeaker building was Briggs' "Loudspeakers", edition 5, 1958, reprint 1970, and I still have it on the shelf. Much of what Briggs wrote back in the Fifties and Sixties is still valid today, thus highly recommended.
In particular one construction caught my attention, the SFB, Sand Filled Baffle, and maybe it was due to the simplicity of cabinet construction for a young man with limited funds, or maybe I just liked the physical appearance of the baffle. The latter prevails to this very day.

Wharfedale SFBs were realised from special 12" and 10" drivers supplemented by an up-firing 3" cone tweeter. The crossover was simplicity itself, consisting of only one 4 uF capacitor to the 16 ohms tweeter. Now, making a 10 inch and 12 inch driver running in parallel with no low-pass filter is quite a challenge and I would love to hear how they performed back then, but due to change of drivers' surround in most surviving examples, this may be quite a futile wish. The real SFB sound of the Fifties is probably long gone, as changing the surround of any driver is likely to change the frequency response considerably. A surround is not only a device keeping the cone in place and preventing acoustic shorting, but due to mass, elasticity and resilience, also plays a major role in the driver's overall performance.

The SFBs offered a "dynamic" alternative to the QUAD ESL57 of the same decade. The ESL57s were electrostatic speakers and are still regarded by owners to deliver the best midrange from any speaker at all times. What Briggs could possibly offer was a similar looking speaker with the same benefits of a non-enclosure speaker, but much cheaper, higher sensitivity, deeper bass and better dynamics due to the use of conventional "dynamic" drivers, i.e. standard drivers made from paper, fabric/foam/felt and alnico magnets. Quite a few of the SFBs were produced and a few are still in use today according to response I had on this speaker. Read more about Gilbert Briggs' Sand Filled Baffles here and here.

The SFBs 12 and 10 inch drivers must have had an overall smooth response and not least a smooth roll-off in order not to sound harsh or peaky. The graphs from Briggs' book suggest this was indeed the case. We also know that post World War II manufacturing was quite a challenge in terms of getting materials and components and Wharfedale would from time to time replace surrounds with whatever material was available, from synthetic felt, polyester foam and fabrics, coated or non-coated, as can be seen from surviving drivers.

I'll take my chances and postulate we can make a much better "SFB" today from modern drivers. Should you have the room and curiosity to build and experience an open baffle speaker, read on below. Be prepared it will set you back some 1200-1400 EUR depending on overall fit'n finish. A low-cost version should be available in the 1,000 EUR range.

Many experiments went into this OBL11 open baffle speaker based on the JA8008/TW034 drivers. My experimental OB baffles have seen a lot of drivers and reported are Supravox/JBL/Goodmans drivers (almost vintage) in the OB7 article and in 2009 the JA8008/TW034 drivers took over, supplemented by Eminence DeltaLite II 2515 bass drivers. The experiments can be studied in detail here: OB7 and OB9.

The OB9 experiments were so promising that I've only been waiting to find the right time for constructing the final baffles and implement a passive solution to the crossover between bass and mid/tweeter. The OB9 was based on an electronic crossover solution between bass and mid, but I like simplicity and a passive solution offers simplicity compared to the complexity of electronic crossover where the signal will pass numerous integrated circuits - or being processed by digital means. I have to say that the OB9's dbx 223XL electronic crossover did much better than expected as can be read in the file.

The OBL-11 Open Baffle Loudspeaker

Click image to view large.


The first question I usually have when launching a new constructions is this: "Looks interesting, can I change the cabinet design?" That's diy. We may feel inspired and want to add our own touch to the design. Three things: Maintain front panel dimensions, driver placement and distance from the bass driver's back to the front, i.e. keep the rear support baffle depth. If you reduce or eliminate these you will shorten the distance from the rear of the bass driver to the front and reduce low-end extension. People wan't to use very narrow baffles and don't realise the lack of bass response from doing so. There's no free lunch here. An open baffle needs to be wide - or deep - to get low bass. You can use a narrow baffle like Jamo, use two bass drivers and you're into serious equalisation to make it work - and usually overall system sensitivity is going down, which is the price to pay for this approach.

Yes, you don't necessarily need all the fillet work done here. Make a single plane baffle and flush mount the drivers and off you go. This was how my prototypes were running.
So, please do not ask for whatever design you may have in mind with regard to its performance. I can't tell and I won't guess.

Can the OBL-11 be driven from your 8 wpc 300B amp? Yes, if you don't play particularly loud. Does the Audio Mirror 20 wpc amps sound more dynamic? Yes. Does the Jungson amp - 80 wpc - really kick butt? Yes!
Generally speaker manufacturers are way too optimistic about how little power is needed for driving their speakers and generally amp manufacturers are way too optimistic about how ineefficient speakers can be driven from their amps.

Does the front grille fabric influence the sound? From measurements the deviation is <0.5 dB. But psychologically the front grille may take some adjustments as we are so used to being able to view the drivers from our speaker system. All of a sudden we're forced to rely on our hearing alone and our brains don't have any visual information "helping" (e.g. fooling) our minds on what is going on.

OBL-11 Drivers
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The OBL-11 drivers, Eminence DeltaLite II 2515 and JA8008/TW034 + waveguide combo.

The JA8008 and TW034 drivers are described in detail in the TQWT file, follow link here.

Eminence DeltaLite II 2515, 15" bass driver is available from Thomann/Germany here. For the time being at 205 EUR each. Thomann has the best shipping rates I know of, so highly recommended but their prices have risen considerably due to neodym shortage. 108 £ in the UK is more like it.
For the US, look at PartsExpress
here. Alternatively US-Speaker here. Both dealers charge 170 USD/each.
For UK residents, look at Blue Aran
here (108 £).
Do you live in Australia, Essential Audio may be a supplier, look
here (335 AUD/each). Search "deltalite" on page.
These prices are updated from time to time and may vary a lot due to China holding 95+ % of the world marked for rare earth metals.

Thanks to Mr. Ranson/AU for these comments:
"Thought I'd let you know that the deltalite 15 is worth buying from Thomann to send here to Australia. They have a flat rate shipping of 50 euros to 30 kg. You can therefore buy these drivers at $180AUD including shipping each if you buy two. I'm not one to take sales away from local business but that's too big a difference to ignore and thought you may want to mention this in your OB11 write up."

OBL-11 Crossover
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Crossover based on simulation

I can imagine someone saying this is a bit far from Briggs' single cap SFB! Right! In the example above I've tried to make a crossover as if we had ideal drivers, or rather I've tried to model a response as though we had ideal drivers only needing a few components. This is what it takes to make the drivers respond as I would like them to. Please note there are only a few components in series with the drivers. The rest is modest LCR circuits shaping the drivers' roll-off. The result is this:

Nice and smooth bass LR2 roll-off. Perfect LR2 to the midrange. Nice and smooth LR2 to the tweeter.
How about that?
It has to be said that drivers need careful alignment to render this performance, i.e. stepped baffle.

Driver phase integration? Nice and smooth.

Impedance? Flat! 4.2 ohms minimum at 80 Hz and mostly 5-6 ohms above and below.
Not bad for a 3-way made from 8 ohms drivers. Even modest valve amps should be able to cope with this.

- back to the real world:

After some fine-tuning this appears to do the trick. In short:

The bass: The DeltaLite needs a baffle step compensation circuit to decrease sensitivity and equalize the low end. 8.2 mH shunted by 5 ohms does the trick and we're down in the 95 dB range. The DeltaLite 2515 really does some 98-99 dB as stated in driver's spec. Generally I find Eminence data not too far from reality, and even frequency response from this 15" driver is excellent.
After the baffle step compensation circuit we have a standard 2nd order low-pass section at around 150-175 Hz.
I my listening room I have a 1.0 ohm resistor (10 watt) in series with the bass section to better match room acoustics. Two 15" bass drivers have no problem loading a 25 sqm listening space!

The midrange: We need a high-pass section on the midrange. Based on measurements it turned out 80-90 uF could do the job. Next the impedance flattening circuit was tried (R-L-C-2051) and didn't make any noticeable difference, so it was left out. The 8008 driver has a little rise above 1 kHz due to the wide baffle, thus a 1 mH for low-pass section. The rest is very close to the crossover used for TQWT and DTQWT.

The tweeter section: Identical to TQWT and DTQWT crossover.

All in all more manageable compared to the no-compromise #1 crossover.

There is one serious issue here: The quality of the 88 uF to the midrange. Initially I ran 82 uF* standard polyprop caps bypassed by 1 uF Superior Z-cap. After several hours of listening I pooled all my Superior Z-caps to make 84 uF and replaced the standard caps. Sorry to say, it's night and day. Any standard polyprop sounds compressed compared to the Superior Z combo. Eight times 22 uF quality caps don't come for nothing, but once we've heard the super caps vs. standard polyprops, it's very clear there's no free lunch here.

I'm sure I'll have the question on active crossover for bass and mid and with this construction I have no reservations. Maintain the passive baffle step compensation circuit to the bass (8.2 mH bypassed by 5 ohms) and target a 2nd order slope around 175 Hz. Next: Target 1st order slope to 8008/TW034 combo at the same point of crossover and finetune by measurements to flat response. I don't think an electronic solution can be implemented properly without measurements.
Considering the sensitivity of the OBL-11 (95 dB), there is hardly a rationale for active filtering and an additional power amp. The cost will be the same or more compared to 8 x 22 uF Superior Z-caps.

*Anything from 80-90 uF is fine for midrange series capacitor.

Go to crossover layout

The Baffle
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Go to OBL-11 Baffle Construction Page

From outer appearance my baffles may resemble the SFBs, but rather than making a sandwich from thin plywood filled with sand, I'll laminate two sheets of 25 mm Baltic birch and there will be some heavy bracing between the 15" and JA8008 drivers.
The original SFBs came with a stretched metal grille, probably not bad from an acoustic perspective, but hard to realise in a diy context, thus a frame with fabric will replace the metal grille.

The OBL-11 baffle. As close as possible to the Wharfedale SFB.

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I was seriously considering not showing any measurement of the OBL-11! Measuring an open baffle is difficult and the results prone to misinterpretation. Only reliable graph below is the impedance plot, but I'll show a few SPL plots and some in-room FFT measurements with a few comments.

Left: The reliable reading; system impedance. Due to the mid-driver's high-pass filter (88 uF) we have a minimum impedance of 5.4 ohms. I was please to see that. This is only little below the DTQWT's 6 ohms minimum. The main bass impedance peak came at 27 Hz, which was a bit surprising as the raw bass driver impedance - on baffle - is some 10 Hz higher.
Right: Is this a true representation of the frequency response of the OBL-11? It isn't, but one way of giving you an idea of performance. The graph was made from three separate measurements: Mid and tweeter at 0.5 m distance down to 800 Hz. From 200-800 Hz we have the summed response of the Deltalite and 8008 drivers. From 200 Hz and below we see a nearfield measurement of the Eminence bass driver, sans low-pass filter but with the baffle step compensation circuit. Follow? More below.

Left: Here we have the same mid-tweeter reading above 700 Hz merged with in-room measurements below 700 Hz. All heavily smoothed. To the right you can see the below-700 Hz reading un-smoothed. That's how things look without gating.
The useful information here is low-end extension. This baffle has a suitable response down to 35 Hz. Considering the overall system sensitivity this pleased me a lot.

Left: An FFT in-room reading makes more sense and to the left here we have the left speaker in my workshop at four different microphone locations - somewhere close to my listening position. Again, in some places useful bass response well down below 40 Hz. Note 20 dB scaling.
Right: Despite fairly symmetrical placement of the panels in my workshop, here we have the right panel (blue) compared to left panel. Some out-phasing at 140-150 Hz, which has been shown on previous occasions. The room just is like that with current interior decoration (shelves from floor to ceiling, filled with drivers behind right speaker).

What interests me the most here is the impedance plot and as said, I was happy to see 5.4 ohms minimum, thanks to eliminating the 8008 bass notch filter. Had the notch filter been in place we would have been down to 4.2 ohms as suggested by simulation.
Due to use of low order filtering for the mid and tweeter we have some crossover lobing moving from tweeter height downwards and looking bad on paper, nothing that is readily audible changing listening position - unless you prefer lying on the floor when listening....

The Sound
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Over the years I have reduced my writing on actual sound - as such - as sound really cannot be described, although audio magazines may write page after page trying to give us an impression of what a given speaker sounds like. It's futile.

First of all: From a visitor hearing the prototype OBL-11s, I can report that these 15" Eminence DeltaLites beat the crap out of Eminence Alpha 15A, which is often referred to in OB connections as being an ideal driver for this application. This was my expectation and what I had hoped for by choosing a fairly low-Qt driver and applying the equalisation step.

OBL-11 obviously has a lot in common with the TQWT, DTQWT and QUATTRO featuring the same mid and treble drivers - and almost the same crossover. What make the difference here are two things, the wide baffle and the 15" bass driver.
Regarding the wide baffle I can point to some thoughts and comments here:
http://www.troelsgravesen.dk/Acapella_WB.htm. The two quotes from Paul Messenger and Josť Victor Henriques is very close to narrowing down what this is all about.

As experienced during the set-up of OB7 and OB9, having a 12-15 inch bass driver on an open baffle is special and we immediately realise what it means to have a bass driver in a box. Anything else but an open baffle sounds "boxy". This 15" Eminence bass driver makes bass physical and a drum kit all of a sudden starts sounding like a - drum kit. I've been raving about 15 inch bass drivers on open baffles before and won't repeat myself. It's special and makes most conventional speakers sound dull and overdamped.

Kit Parts
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Complete kit version 3 incl. drivers and waveguide (excl. Eminence bass drivers) available from Jantzen Audio.

Prices here (pdf files):

Download sales presentation version 1

Download sales presentation version 2

Download sales presentation version 3

Crossover Layout
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As we need to increase bass driver's Qt, the coils here are chosen deliberately having the DCRs shown. 
Do not waste money on air coils, trafos or the like. Due to the sensitivity of the 15" driver, only little power is passing here.
2 x 68 uF PP caps can be replaced by electrolytics if budget is low.

OBL-11 mid-tweeter crossover layout, version 3.
Using a 16AWG coil for mid is deliberate. It provides minor attenuation although I cannot justify this by measurements.
This is quite a large board and a crossover made from standard coils and PP caps could reduce size to one third - but not
with the same sonic performance. The midrange is the most important frequency band and this is where we should spend our money.

Crossover pics
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Take care not having the caps' leads touching the aluminium tubes.
The two resisters are for tweeter attenuation. Use 2R2 or 2R7 to suit your preference.

Mid-tweeter section from four angles.

Bass section.

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