Dispersion Foam Used Around Royd Minstrel Tweeters?

[Minstrel SE]

Hi

Im thinking of a full veneering project for some Minstrel cabinets. I will keep mine as is and bring in some tatty cabinets to be refurbished before swapping out drivers and crossovers

I dont think I can shave off the original foam cleanly or even whether it will be in good enough cosmetic condition to save

I will therefore want to get some of the dispersion foam Joe Ackroyd used around the vifa tweeters.

Its about 2mm thick and grey coloured with a smoothish surface. Ive been looking at sheet foam PVC... EVA? but havent a clue.

I havent asked people such as Wilmslow audio yet( nothing on the web page) so I will badger you folks until someone has the answer :)

I would like to get them looking as near original as I can. I really wish Joe Ackroyd was still with us cos he would be making some amazing speakers now and could tell me what that foam is made from :)

Cheers
Martin

[fatmarley]

I've never really thought about it until now but that stuff is too thin to have much, if any affect on the sound, and the small step it creates can only make things worse.

Would love to know the reason why Joe put it there. Looks? Or to hide access to the crossover/ modified tweeter? (crossover is actually behind the tweeter). The tweeter is a modified Vifa 19 with a cardboard tube glued to the back to create a bigger rear chamber.

If it were me, i'd carefully remove the foam and not worry about it.

[Minstrel SE]

I've never really thought about it until now but that stuff is too thin to have much, if any affect on the sound, and the small step it creates can only make things worse.

Would love to know the reason why Joe put it there. Looks? Or to hide access to the crossover/ modified tweeter? (crossover is actually behind the tweeter). The tweeter is a modified Vifa 19 with a cardboard tube glued to the back to create a bigger rear chamber.

If it were me, i'd carefully remove the foam and not worry about it.

Thanks..... yes I was thinking along those lines but dare not say it. Im not sure what actual effect it could have but it does make the speaker look more boffiny :)

I am going to be putting a nice real oak veneer on them. As you say I will probably leave the foam off and find some fancy allen bolt or screw heads to enhance the looks.

Yes the speakers are not anywhere near the last word in sound...I realise that but its a project I would like to do....one day ie it may never happen :D

[Lawrence001]

I always assumed it would stop the treble traveling along the cabinet and diffracting on the cabinet corners causing smearing of the image.

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[Minstrel SE]

I always assumed it would stop the treble traveling along the cabinet and diffracting on the cabinet corners causing smearing of the image.

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yeah I dont think he would have stuck it on for no real reason. I will probably get some of those foam pieces if I can find it. I will have to cut nice circles so its all extra work to be done properly.

What I dont understand then is why you dont really see it on other speaker designs if it is important

[Lawrence001]

A few do, LS3/5As for example. It can deaden the sound a little, a bit like excess wadding in a speaker or stiffening a lossy cabinet. Or more similarly, absorbing the rear radiation from a dipole tweeter. I guess you need to tune the treble level to account for the expected dispersion when designing a speaker, so adding it to a speaker voiced "in room" without it can make them sound dull. I'd try them without for a while and see if you prefer the sound.

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[Yomanze]

yeah I dont think he would have stuck it on for no real reason. I will probably get some of those foam pieces if I can find it. I will have to cut nice circles so its all extra work to be done properly.

What I dont understand then is why you dont really see it on other speaker designs if it is important

Joe Akroyd had quite a unique approach to speaker design, involving a lot of computer modelling, hand-doping of bespoke drivers on a specialised measurement jig etc. The foam is there to take the edge off the already quite toppy and raw traditional Royd designs. Also, the grilles are designed to be left on (despite my profile pic)... ;)

[fatmarley]

I always assumed it would stop the treble traveling along the cabinet and diffracting on the cabinet corners causing smearing of the image.

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I can say with 100% confidence that it will make no difference. It's only about 2mm thick and closed cell foam. LS3/5a foam would be open cell and a lot thicker.

[Lawrence001]

I can say with 100% confidence that it will make no difference. It's only about 2mm thick and closed cell foam. LS3/5a foam would be open cell and a lot thicker.How would closed cell foam be less effective at reducing diffraction effects than open cell? Genuine question.

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[fatmarley]

How would closed cell foam be less effective at reducing diffraction effects than open cell? Genuine question.

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It's quite a complicated question to answer (properly). Closed cell foam would be no different to a flat baffle made of something solid. The sound would still diffract off it in the same way. Open cell foam absorbs sound but if the open cell foam was only 2mm thick it would hardly make any difference. The sound energy absorbed by acoustic foam can be measured and the official term is NRC (Noise reduction coefficient). In an ideal situation, the whole baffle would be covered in a thick layer of it.

[struth]

I read once it should be open then medium then closed foam. Dunno if there is anything In it

[Lawrence001]

Thanks makes sense but I expected closed cell foam to absorb the treble and open cell foam to let it pass through (like some speaker grilles that are made of it do). I've seen tweeters covered in foam (eg. Proac I think).

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[fatmarley]

Thanks makes sense but I expected closed cell foam to absorb the treble and open cell foam to let it pass through (like some speaker grilles that are made of it do). I've seen tweeters covered in foam (eg. Proac I think).

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Acoustic foam is a much more dense open-cell-foam than the stuff used for speaker grills.

[fatmarley]

I read once it should be open then medium then closed foam. Dunno if there is anything In it

yes, that's what I always imagined would work best. Very open on the outside and getting more and more dense as you get further in.

[Minstrel SE]

So what are the physics of it then in Peter and Jane terms?

The sound radiates out from the tweeter at all angles so what is that foam supposed to do...focus it.... tame it...break it up from travelling along the cabinet? I cant see it making a significant difference.

I see Wharfedale (well wang tung industries) recessing the tweeter now so it looks like a mini driver...is that the same aim?

Ive just done a computer test with 16k on the screen equalizer and I can hear any change between min or max levels. I can hear the effects of the 8k slider :).........probably puts my treble hearing into perspective regarding speakers.

Dont get me wrong...I never said Minstrels were anywhere near the last word in sound. The top end can be unruly and they tend towards a tonal brightness...still put a smile on when bouncing along with fine material and a good amp...I just wanted to hear the quirky looking things. I swap speakers out regularly..... SL6... celestion 3s, Conistons and any 80s or 90s speaker I get my hands on.....I must get up to date with speakers :)

[Lawrence001]

So what are the physics of it then in Peter and Jane terms?

The sound radiates out from the tweeter at all angles so what is that foam supposed to do...focus it.... tame it...break it up from travelling along the cabinet? I cant see it making a significant difference.

I see Wharfedale (well wang tung industries) recessing the tweeter now so it looks like a mini driver...is that the same aim?

Ive just done a computer test with 16k on the screen equalizer and I can hear any change between min or max levels. I can hear the effects of the 8k slider :).........probably puts my treble hearing into perspective regarding speakers.

Dont get me wrong...I never said Minstrels were anywhere near the last word in sound. The top end can be unruly and they tend towards a tonal brightness...still put a smile on when bouncing along with fine material and a good amp...I just wanted to hear the quirky looking things. I swap speakers out regularly..... SL6... celestion 3s, Conistons and any 80s or 90s speaker I get my hands on.....I must get up to date with speakers :)I think most recessed tweeters are essentially gently horn loading them to increase efficiency. I don't know the effect on the signal along the edge but I imagine if diffracts a bit at the edge of the horn and again on the cabinet edge (but weaker having already diffracted once on the way). Does anyone technically minded have any computer models to run it through?

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[YNWaN]

The problem is that whenever the tweeter radiation strikes an edge it is diffracted again and can, in effect, act like a secondary echo - this is an issue with high frequencies because their wavelength is so much shorter than bass frequencies. This is the reason you sometimes see tweeters offset to one side of the baffle - this doesn't reduce the diffraction effects of the cabinet edges but it does mean that the two edges do not radiate at the same time and so their impact is minimised. The foam has two (or it can do, depending on the implementation) purposes - it can absorb some of the energy passing over the face of the cabinet and it can randomise that diffraction which does take place. Having said all that, the foam that surrounds the Minstrel tweeter is too thin to do a great deal (I think someone said this earlier in the thread)