95 dB Open Baffle Loudspeaker
Copyright 2011 © Troels Gravesen
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.
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
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.
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
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!
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.
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.
Thanks to Mr. Ranson/AU for these
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:
- back to the real world:
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
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.
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.
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.
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.
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.
Prices here (pdf files):