Very Interesting

[Sock Puppet]

Stumbled across this article on Jay-Mitchell:

Directivity Modifier for Guitar Speakers

Rob.

[pdf64]

See the TGP thread
Speaker Directivity - Page 41 - The Gear Page
I've tried the 12, 15 and 18mm material from efoam, see post 608 by hywleg.
The 12mm works great, though yet to evaluate it fully.

[pdf64]

Finally got chance to try out the donut equiped amp in a band situation - absolutely brilliant, almost the exact same tone all around the room, the dreaded beam virtually gone. It was actually a bit weird how even the tone was, having gotten so used over the years to having it change as you move around.
This is with a Fender 75 1x12 amp, with a Fane AXA12.
Foam was 12mm from
eFoam, all sorts of foam: Acoustic Sound Proofing Foam
fitted inside the (removeable) grille frame - the speaker is front mounting on the baffle.
This is probably the first real improvement in guitar speaker cab technology, with regard to how they actually sound, since, well, ever!
Everyone with a guitar or bass cab should check this out.
The only application in which I can think this wouldn't help is when experimenting with close miking the speaker cone. Pete.

[txstrat]

I built a cab with beam blockers right in the baffle board - now I know, the beam blocker thing is not too big an improvement - OK.
Would it make a difference, if I just use a sheet of appropriate foam and cover both cutouts completely? I mean, when I cut a hole in the foam to reveal the dust cap of the speaker, it would (in my case) reveal the beam blocker only.
The frame in front of the baffle board is for the grille cloth and to be attached to the baffle with wood screws from inside the cab.

[Old Tele man]

...sounds suspiciously like the re-invention of the "Tone-Rings" that Leo Fender used in his 1960's white-Tolex amps!

[pdf64]

txstrat - you'd be best putting this query on the TGP thread that I linked to above. Jay Mitchell, the guy who's designed this breakthrough (no, it really is!) and put it into the public domain, monitors the thread and will respond. With an ac30 cab (they have a vertical strip across the cut out, for mechanical speaker protection), he advised cutting the donut around it, ie so it's in 2 halves. That might be tricky with your design, might be easier to cut the blocker out?

Old Tele Man - the tone ring was effectively a bass port, as I understand it?
http://www.schematicheaven.com/faq/t...g_cabinets.pdf
As such, it's a chalk and cheese thing compared to the Mitchell 'donut', see
Speaker Directivity Modifier, Part II :: TGP Webzine
which modifies the directivity of the speaker above about 1k2Hz.

[txstrat]

might be easier to cut the blocker out?

Yeah, that's what I thought. I only haven't heard an amp with a "tone ring".
You got soundclips?
How does the sound change, compared to with no "tone ring".
And above all, does the headroom change? (we got a strong drummer and the amp has 35W only )

[pdf64]

Best not to call the foam donuts a tone ring - it's just going to cause confusion?
No soundclip recorded that session unfortunately, I'll try to get one done in the next month. I was sure that someone on the TGP thread had put one up, but I can't find reference to it now.
The sound changes are that the on-axis tone gets less trebly, the off axis tone gets more trebly. No more sweet spot / bad spot / dead zone, just the tone you want all around.
Jay advises that there's a max overall loss of 1 - 1.5dB. But that's with 18mm foam - when I tried that in the house, I did notice a slight deadening of the tone with the 18mm, which wasn't there with the 12mm.
As you know, 1dB is the smallest change that an average human can perceive.
I couldn't notice any overall level change when comparing the grille (containing the foam donut) on and off, although the changeover wasn't an instant A/B thing.
So I doubt that you will notice a change in headroom with 12mm donuts.
But a powerful drummer can make it very difficult to get a nice balance, especially as the snare can get much louder than the kick drum, and everyone's volume has to spiral up, so the kick drum has to get miked, and so on.
I think that 35w with 2x12 would put out the loudest stage level that it's sensible to use - if that's not enough then it's best to look to get miked up, rather than move up to crazy high sound levels on stage.

[benito_red]

So far I did not try a Directivity Modifier, but as I understand, the theory of Beam-Blocker conflicts with the theory of the Mitchell Foam Donut. In fact the first is based on the assumption that the high frequencies are originated from the center of the speaker, while the second states that the high frequencies do not come from center of the speaker but from its peripheral part. This means that one of the two is wrong and must be uneffective.
But as everibody knows if you want to mic a cabinet, the position of the mic will yield different results: placing mic in front of center of the speaker will give more highs, the more distant from the center, the less highs. This seems to confirm the theory of beam blocker.
Can somebody explain/clarify that?

[Steve Conner]

Mitchell claims that the Beam Blocker doesn't work, but his design does.

The Beam Blocker is really the same thing as a "phase plug" in a hi-fi midrange driver. It disrupts the wavefront coming from the centre part of the speaker.

The Mitchell donut lets the wavefront from the centre of the cone through, and diffuses/attenuates the rest.

But in both cases, the effect is that the phase relationship between the middle of the cone and the edges is broken up, hence so is the "beam".

Because the hole in the middle of the Mitchell donut approximates a point source better than the cone area not covered by a beam blocker, I think the Mitchell donut has the potential to work better.

[benito_red]

Mitchell claims that the Beam Blocker doesn't work, but his design does.

The Beam Blocker is really the same thing as a "phase plug" in a hi-fi midrange driver. It disrupts the wavefront coming from the centre part of the speaker.

The Mitchell donut lets the wavefront from the centre of the cone through, and diffuses/attenuates the rest.

But in both cases, the effect is that the phase relationship between the middle of the cone and the edges is broken up, hence so is the "beam".

Because the hole in the middle of the Mitchell donut approximates a point source better than the cone area not covered by a beam blocker, I think the Mitchell donut has the potential to work better.

thank you very much Steve !
In this way I understand that the center of the cone is a point source that spreads the high frequencies, while part of the the rest of the cone beams them.
I must give a try to the donut.