Speaker design discussion

Hi Ian,

The ARs just sat looking unloved but the Dynaco A25 got a good run.
Very similar conceptually to the AR - directly connected bass mid (but 10") and a large diameter tweeter able to cross low and cope with the lower crossover point.
Both work very well, stupidly well when you factor in the current used prices.
IIRC my Dynacos cost £25 and the ARs were about £50 including a diy re-foam.

You are welcome to borrow either but the Epos you have will take some beating IMO.

 

Do you remember those speakers that Russ Andrews did for a while? They used a 10" volt driver and a ribbon tweeter. I'm sure the Volt was run open with no X-over. I'm not sure how that worked with the Ribbon though because usually they don't like crossing over very low and I would be surprised if the Volt could reach very high.

Sorry, is this hijacking Nando's amp thread?

 

DeccaVolts?

Tony.

 

Yes, but don't worry, I plan to split the thread when I get time.

 

Cheers Dev.

Tony, don't know if they were DeccaVolts. I've been having quite a lot of minimal X-over speaker thoughts recently (although, in my case, I would like the drive units to work together )

 

Here's a Decca Volt:



I'm not sure what Decca tweeter that is though, it doesn't look familiar.

I had a pair of big 3 way 1960s Mordaunt Short speakers in my old recording studio as the full range monitors, I think they were MS700s or something. They had the now very collectable gold Decca London ribbon tweeter, which was a lovely sounding unit, but very easy to blow up - I fried them both within a short while, the first by forgetting to mute a mixer channels when plugging a jack in, the second was taken out with an Arp Odyssey. I wish I'd kept them as even broken ones are worth a couple of hundred these days. Can't remember what happened to the speakers at all, I certainly never took them home after the studio closed despite their technically being mine. They weren't good speakers, very muddy and slow.

Tony.

 

Not Decca Volt - just a Volt as far as I am aware. Is therte supposed to be a picture Tony?

 

Yes, should be a picture - it's linked in from elsewhere (a photo hosting site that allows linking) and showing ok here.

Tony.

 

How long is Richard banned for?

I'd like to ask him which unique quality of acrylic allows high frequencies to be dissipated in the cabinet without acoustic wadding.

I know Richard can't reply now, but to lay out my reasons for asking the question, and for others reading:

First we consider the range where sound acts as a pure pressure wave. This is from the lowest frequencies up to where at least a 1/2 cycle can be contained between the boundaries. In 'The Cube' this range is from 0Hz - ~500Hz (guessing a size of 35cm cubed). It doesn't matter about the shape of the cabinet at these low frequencies because the sound does not reflect and bounce around, it just causes instantaneous pressurization of the enclosed space. Cube or not it doesn't matter. The energy can not be dissipated substantially by acoustic wadding and it generally transfers in to the cabinet structure which then transfers to other connected structures and also to the air in the room. It tends not to cause frequency response errors, but does cause 'time smearing' while the energy in the cabinet structure is less than perfectly transferred to the air. Basically in this range the cabinet shape doesn't matter, but construction and material does (and all methods have pros and cons).

Next we can consider the modal range. A little confusion may lay here, because in rooms the modal range is low frequency, but in a small space like a cabinet the modal range is high frequency. So at these high frequencies the sound in the cabinet can bounce back and forward between the boundaries of the cabinet and make standing waves at specific frequencies. Also the sound originates from the centre of the cone, some distance into the cabinet. In this case the source of the sound will not be equidistant from the boundaries in 1 axis. However it is still equidistant from the boundaries in the other 2 axis'. The result will be strong dips and peaks throughout the sound inside the cabinet, but not quite as strong as if the source was flat on the wall of the cube. A non-cube cabinet would spread those peaks and dips over a wider range, and give them less intensity.

This is where my question arises as the sound inside the cabinet through this modal range needs to dissipate in some way. Usually acoustic wadding is used to convert a portion of it to heat. In an un-damped cabinet I see no way for the sound to dissipate other than exit through the driver and into the room, or transfer to the cabinet walls. I believe the sound will transfer through the thin cone material of the driver long before it can transfer to the walls of the cabinet and dissipate as heat in the elasticity of the material.

I believe this because the acoustic impedance of air and that of the cabinet wall is very different, whether they are made of MDF, chipboard, metal or something 'soft' like acrylic - it isn't soft compared with air, and so there is a large impedance mismatch making transfer of energy extremely small upon each reflection. I don't think acrylic will dissipate modal range energy inside the cabinet notably quicker than another common construction material since it will struggle to transfer to the material at all.

 

Eternity.

Good post.