acoustic wadding for Tannoys

[julesd68]

I'd like some advice about acoustic wadding for my Tannoy Chatsworth cabs that house 12" HPD's …

My questions are -

- What effect does too much or too little wadding have on the sound?
- What is the best material to use and where can I buy it?
- What is the optimum amount of wadding for my cabs?
- Where exactly should the wadding be applied and how should it be attached?

cheers!

[walpurgis]

Hi Julian, the Chatsworths are usually quite well lagged with BAF pillows mounted on each internal face apart from the front panel. I have used teased out wool fibre loose in the lower half of the cabinet in Chatsworths, just to add a bit more damping, which won't hurt in the sealed cabinet. A front to back brace inside below the driver will reduce panel vibration quite a bit too.

[The Black Adder]

Can I ask why you are wanting to change the current wadding?

Adding too much or too less can affect the sound in lots of ways so it's best to just apply the same or similar amount as what's in there originally. Always to follow recommendations like Geoff has stated. The brace from front to back in the lower chamber sounds like a good plan too.

[DSJR]

By modern accelerator-researched standards, those old Tannoy boxes are pretty terrible and re-enforcing these box panels can only be a good idea in all honesty!

[Reffc]

As above

some internal bracing to reduce panel resonance is a good idea but to answer your question Jules:

Too much wadding increases the cabinet damping factor (Q) which can dull the sound and flatten bass response. In a sealed cab if you add too much, it fools the driver into thinking it's in a larger cabinet (to do with the total length that the air has to travel through the fibres which creates a longer path for the air to travel compared with no wadding at all) which can roll bass off early in an already tuned cabinet. The vibration of the wadding fibres is converted to heat energy so converting the acoustic energy, hence the acoustic damping effect.

Too little has the opposite effect. ie it decreases "Q" and can cause a hump in bass response (virtually all little bookshelf speakers today are designed with a hump at between 40 and 60Hz as it makes them sound bigger than they are due to the elevated upper bass SPLs). You'll increase impact of upper bass but also risk increasing internal reflections and with Tannoys, this is a bad idea as the cones themselves are virtually acoustically transparent and any reflected sound will manifest as out of phase distortion.

The design by Tannoy for many of their sealed enclosures was originally 4 inches (100mm) of dense open cell foam. Back in the day this was typically mattress foam, but later, more efficient acoustic foams were used.

These days, 50 to 75mm thickness of acoustic foam to all internal faces (Coeff of 0.7...ie 70% of all acoustic energy at 1KHz absorbed) is sufficient (leave the front baffle without any foam to it at all). An additional 30mm of Polyester/lambswool mix about 35cm square placed behind the driver on the rear panel is also a good idea. No more than this is required.

[julesd68]

Can I ask why you are wanting to change the current wadding?

I've not actually decided to change it yet, just trying to find out if I should ... From memory there is a very thin piece of foam on the back wall and wanted to know if this is adequate. I'm going to open up the cabs and have a better look now ...

[julesd68]

As above

some internal bracing to reduce panel resonance is a good idea but to answer your question Jules:

Too much wadding increases the cabinet damping factor (Q) which can dull the sound and flatten bass response. In a sealed cab if you add too much, it fools the driver into thinking it's in a larger cabinet (to do with the total length that the air has to travel through the fibres which creates a longer path for the air to travel compared with no wadding at all) which can roll bass off early in an already tuned cabinet. The vibration of the wadding fibres is converted to heat energy so converting the acoustic energy, hence the acoustic damping effect.

Too little has the opposite effect. ie it decreases "Q" and can cause a hump in bass response (virtually all little bookshelf speakers today are designed with a hump at between 40 and 60Hz as it makes them sound bigger than they are due to the elevated upper bass SPLs). You'll increase impact of upper bass but also risk increasing internal reflections and with Tannoys, this is a bad idea as the cones themselves are virtually acoustically transparent and any reflected sound will manifest as out of phase distortion.

The design by Tannoy for many of their sealed enclosures was originally 4 inches (100mm) of dense open cell foam. Back in the day this was typically mattress foam, but later, more efficient acoustic foams were used.

These days, 50 to 75mm thickness of acoustic foam to all internal faces (Coeff of 0.7...ie 70% of all acoustic energy at 1KHz absorbed) is sufficient (leave the front baffle without any foam to it at all). An additional 30mm of Polyester/lambswool mix about 35cm square placed behind the driver on the rear panel is also a good idea. No more than this is required.

Fantastic, many thanks for this Paul, most informative ...

I think what foam there is in my cabs is a fair bit thinner than that, so will check it out and I'll probably order some new. I've managed to find some suitable poly/lambswool material, but can you recommend a good source for the acoustic foam? I couldn't find anything with the figures you mention. Do you recommend putting anything in the bottom of the cabs apart from a layer of foam?

[Reffc]

Fantastic, many thanks for this Paul, most informative ...

I think what foam there is in my cabs is a fair bit thinner than that, so will check it out and I'll probably order some new. I've managed to find some suitable poly/lambswool material, but can you recommend a good source for the acoustic foam? I couldn't find anything with the figures you mention. Do you recommend putting anything in the bottom of the cabs apart from a layer of foam?

Try Ace Foam Rubber Ltd from Madeley in Telford. They sell the right stuff. I think they also have an Ebay outlet (don't know the Ebay ID though). I'd place some 12mm wool felt underlay in the base with a 40 or 50mm acoustic foam tile(s) over the top. Use carpet spray adhesive to stick the foam and underlay into place. Don't add any long lambswool to this. The trick with lambswool is to tease it out before placing it into place (again, using the spray adhesive but only at 4 or five points...not all across the back) and use it sparingly. If it sounds over-damped, remove a little at a time. The sound (assuming that crossovers are all good) should be clean with good clarity and upper bass attack. If you lose the bass attack you have over damped. If the sound has audible mid distortion (not caused by the horn resonance) and some bass boom then you don't have enough damping or absorption behind the main driver (and to the sides). It's as much an art as a science and takes a systematic approach to get it right. Tip: Use a reference recording with a mix of bass (something with attack and bass overhand in the form of say reverb from low cello or bass guitar notes or the decay from tympani) along with some female vocals for the mids. Make notes and compare until you're happy you have just the right amount in place.

There should be more than just a thin bit in the back Jules. You should have foam to three sides, the top and the bottom. The only area that should be without is the rear of the front baffle. I'd introduce bracing struts front to back either side of the drive unit top and bottom using 2 by 1 inch timber and tie this to the sides with cross brace struts. Glue and screw will be adequate. This will stiffen up the front and rear panels and redistribute the resonances across all side panels rather than just the front and rear plus lift and split the frequencies that resonance occurs as well as lowering panel resonance amplitude.