Slotted Baffles? (i.e. Graham Audio)

[Slick Minkus]

Curious about the use of slotted baffle driver loading to control (widen) dispersion of large drivers in the mid frequencies, allowing otherwise impossible horizontal matching with tweeters.
Graham Audio famously use the technique originally popularised by the BBC monitor era...

• http://www.grahamaudio.co.uk/technology/slotted-baffles/

But great care must be taken to optimise these slots. If they are too narrow, there is a risk of resonances occuring between the rear of the front panel and the diaphragm. The ideal spacing between the baffle and the diaphragm (and the surround) must be carefully determined with detailed measurements. The thickness of the timber left after the machining operations is also critical. But having researched and built on the original work that went into the design of the LS5/5, we are left wondering why this approach isn't more commonly used today. Aside from the obvious aesthetic concerns, of course.

Does this excerpt truly cover all the caveats/challenges?
Are aesthetics really the only reason we don't see this technique more often?

 

[Soniclife]

I'd love to see some quality measurements of those, that might answer the question. My money is on waveguides being a much better solution.

 

[ctrl]

Does this excerpt truly cover all the caveats/challenges?
Are aesthetics really the only reason we don't see this technique more often?

You missed the main reason in the quoted text why this technique is not used more often:

If they are too narrow, there is a risk of resonances occuring between the rear of the front panel and the diaphragm.

All LS that I know and use this technique have some decay problems. Each of those LS will show some kind of decay problem in the CSD - more about CSD and how to interpret it here.
How much this is perceptible is up to you to decide.

Genelec use this technique with two woofer behind the mid/high baffle to minimize cabinet size. Woofer XO is 320Hz - more details about the decay problem here using 8341a as example.

Genelec 8361A Review (Powered Monitor)

This is a review and detailed measurements of the Genelc 8361A studio monitor (powered active speaker). It is on kind loan from a member and costs US $4,995. The 8361A looks identical to the rest of the series except massively scaled up: It doesn't look it but it is extremely heavy. It has a...

www.audiosciencereview.com

It is even worse with the resonances of the Ocean Way HR5 LS. Due to the higher crossover frequency (presumably >1kHz), much more resonances are excited by the woofer behind the slot.

Ocean Way HR5 Studio Monitor Review

This is a review and detailed measurements of the Ocean Way HR5 Powered Monitor (speaker). It was purchased and kindly sent to me directly by a member. The HR5 costs US $1,400 each or $2,800 for a pair. The HR5 looks unusual with dual horns in a small, "bookshelf" package: Nice to see both...

www.audiosciencereview.com

Another example are synergy horns that use the same principle. - more details here.

Danley Sound Labs SH-50 Professional Audio Speaker Review

Danley Sound Labs SH-50 Speaker Review

www.erinsaudiocorner.com

I'm pretty sure this will be true for the Graham LS as well. A simple simulation how such a resonance looks like without XO can be found here.


Another disadvantage is that in the range of the slot resonance harmonic distortion is can be increased (because the slot resonance will increase SPL for example 15-25 dB in the range of the resonance. If the resonance is not dampend, HD will rise too).

Based on the simulation of a "diffraction slot" you can see that even though the slot resonance is at 1kHz, you still have to expect significantly increased harmonic distortion (in our example HD3) at the crossover frequency even at a crossover frequency of 300Hz.

The slot resonance will rise HD3 of the driver, in our example, around 300Hz (despite the XO at 300Hz), because the slot resonance is around 900Hz and this will increase SPL about 20dB. So HD3 around 300Hz will be 20dB higher than you would expect from the driver and in the example, despite the XO roll-off HD3 will still about 10dB higher then expected.
Hope you can understand my thought process

Update:
Electronic EQ can be used to smooth the slot resonance in the frequency response, but it will not reduce the increased harmonic distortion.
A 300Hz signal causes, in our example, in the driver HD3 at 900Hz after/independent of the electronic filtering (filtering will not reduce HD around the slot resonance). So you need a driver that has very low harmonic distortion in the slot resonance region.

This kind of HD3 from our example, you might can decrease passively using series notch filter, as described here in Purifi blog.

 

[thewas]

By the way here is the 1967(!) BBC paper about the development of the LS5/5 and LS5/6 including directivity, HD and IMD measurements!

https://downloads.bbc.co.uk/rd/pubs/reports/1967-57.pdf

Here also some harmonic and multitone distortion measurements of the new Graham version:

https://www.audio-offensive.de/wp-content/uploads/2021/03/210-TT-Lautsprecher-Graham-LS-5_5-stereoplay-2021-04_lowres_pdf.pdf

https://www.lowbeats.de/test-graham-ls-5-5-die-koenigin-der-bbc-monitore/

 

[fineMen]

By the way here is the 1967(!) BBC paper about the development of the LS5/5 and LS5/6 including directivity, HD and IMD measurements!

https://downloads.bbc.co.uk/rd/pubs/reports/1967-57.pdf

Here also some harmonic and multitone distortion measurements of the new Graham version:

https://www.audio-offensive.de/wp-content/uploads/2021/03/210-TT-Lautsprecher-Graham-LS-5_5-stereoplay-2021-04_lowres_pdf.pdf

https://www.lowbeats.de/test-graham-ls-5-5-die-koenigin-der-bbc-monitore/

Thanks a lot for sharing. The slot makes dips, not peaks, nice!

However, such a slot is probably used with compression drivers to the very best success. So it is already possible, with effort you get it.

 

[GXAlan]

It works, but there are newer geometries

JBL Professional EON615

15" Two-Way Multipurpose Self-Powered Sound Reinforcement More than just another great JBL sound...

jblpro.com

 

[Slick Minkus]

I'd love to see some quality measurements of those, that might answer the question. My money is on waveguides being a much better solution.

Agreed, approaching the issue up top with modern computer-modeled-shallow-waveguides seems to be the better alternative - especially considering the advancements in tweeter manufacturing.

thewas delivered on measurements... It's nice to be reminded how principled this whole game is - 1967!

By the way here is the 1967(!) BBC paper about the development of the LS5/5 and LS5/6 including directivity, HD and IMD measurements!

https://downloads.bbc.co.uk/rd/pubs/reports/1967-57.pdf

Here also some harmonic and multitone distortion measurements of the new Graham version:

https://www.audio-offensive.de/wp-content/uploads/2021/03/210-TT-Lautsprecher-Graham-LS-5_5-stereoplay-2021-04_lowres_pdf.pdf

https://www.lowbeats.de/test-graham-ls-5-5-die-koenigin-der-bbc-monitore/

The BBC paper and current review measurements cover distortion and impact on FR but understandably do not feature a high resolution CSD, as thoroughly referenced by ctrl.
I've never had a good grasp on the importance of cumulative spectral decays, but i've also never had better context to apply.
Big thanks to ctrl and everyone else's responses, the rabbit hole I was seeking has been revealed and I believe my original question has been satisfied...

Using a slot to control (widen) directivity requires a careful balance of the following three factors in order to control (i.e. mitigate not prevent) inevitable resonances and corresponding distortion...

• If the slot is too narrow, there is a risk of resonances occurring between the rear of the front panel and the diaphragm.
• The ideal spacing between the baffle and the diaphragm (and the surround) must be carefully determined with detailed measurements (CSD, HD, IMD).
• The thickness of the baffle material is also critical.

 

[ctrl]

By the way here is the 1967(!) BBC paper about the development of the LS5/5 and LS5/6 including directivity, HD and IMD measurements!

Using the information from the BBC paper @thewas provided, a simple simulation was created for a 12'' woofer.
For this, a round slot of 120mm diameter, with a baffle thickness of only 10mm, was created for the woofer in infinite baffle (forgive me, but I am too lazy to create a full simulation for LS5/5 speaker).



The results are pretty good. The resonance turns out to be reasonable with about 7dB SPL increase at 700Hz. If we use the slot only in the horizontal plane instead of a round slot, the resonance should decrease further.
Since the resonance sound pressure increase is small, we can now focus on the advantage of the slot's acoustic low-pass filtering effect.
The acoustic filtering has the effect of reducing harmonic distortion above the resonant frequency.

In our example a real 12'' driver will show first break-up resonances above 1.5kHz (possibly with increasing HD). The acoustic filter behavior of the slot ensures that the HD above 900Hz is significantly reduced. For example HD2 at >450Hz will be decreased (HD3 >300Hz, ...) - if the slot affects only the horizontal plane, the acoustic filter will of course not act in the vertical direction.

We can look at the sound pressure distribution in the model at the resonance around 700Hz and at the maximum cancellation around 2kHz (slot, cone and air act as a Helmholtz resonator):


The radiation pattern is very wide for a 12'' woofer, due to the slot. Second image is same woofer without slot (keep in mind that it simulates an ideal woofer, so no break-up) - both in infinite baffle - FR deg0 to deg80:




The LS5/5 seems, according to the BBC paper, to get the best possible out of slots.
If the baffle is as thin as possible, and the air volume between slot and cone is as small as possible (for example, by a flat membrane cone), then good results can be achieved with slots.

Perhaps the LS5/5 is the speaker where the advantages of slots clearly outweigh the disadvantages.

 

[Da cynics]

(mid)Bass horns and partially covered drivers

Hi, After auditioning the Blumenhofer Grand Goia at an audio show, I've been researching these kind of speakers. Speakers with some kind of horn/waveguide on the bass section. I hope somebody can answer some questions I have about these. What is the effect of these kind of 'horns'? Are they...

www.diyaudio.com

What about horn...

 

[fineMen]

The ideal spacing between the baffle and the diaphragm (and the surround) must be carefully determined with detailed measurements (CSD, HD, IMD).

Just revisit how compression drivers are designed.

 

[Topper2k]

I own a couple of JBL Eons with their "waveguide" slotted baffles, like the one pictured above.

This is a total noob question, but am I understanding correctly that a slotted baffle (or waveguide) essentially disperses the sound more broadly? At least for certain frequencies? So it makes those frequencies less directional?

If that's correct, does that mean that a slotted baffle will decrease how far the speaker will throw sound directly in front of it?

I ask because I've noticed my speakers with the waveguide are very loud when up close (like on the dance floor) but that the perceived volume quickly drops as you walk away from the speaker. I know that's how sound works, but I mean this effect seems more pronounced on these speakers than others. It's particularly noticeable when outside where there aren't any walls reflecting the sound. Could the waveguide contribute to this effect?

 

[fineMen]

I ask because I've noticed my speakers with the waveguide are very loud when up close (like on the dance floor) but ...

The 'throw' is an undefined term used by (desperate) professionals to relate to problems with a particular set-up. And how this or another magic trick would solve it. You're utterly right in assuming that it has to do with more or less dominance of reflections in-room. The so called slot is there to provide more dispersion, and that is it. If may support the transition from the bass/mid-driver to the horn.

 

[Topper2k]

The 'throw' is an undefined term used by (desperate) professionals to relate to problems with a particular set-up. And how this or another magic trick would solve it. You're utterly right in assuming that it has to do with more or less dominance of reflections in-room. The so called slot is there to provide more dispersion, and that is it. If may support the transition from the bass/mid-driver to the horn.

So you don't think the slot would reduce the perceived "throw" in any significant way?

 

[fineMen]

So you don't think the slot would reduce the perceived "throw" in any significant way?

To me the 'throw' is simply an undefined term. It surely makes sense in the context of professional chit-chat. But me ain't familiar with its sensible use--professionals tend to develop an own language for good reasons.