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Old 14th November 2004, 06:49 PM   #41
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If you want your box to prevent the rearwave coming back through the speaker cone, make the box anechoic. From my understanding even good anechoic chambers dont absorb the extreme low frequency (which is why stuffing a subwoofer cab doesnt do much), But I'd imagine practically all of the HF is absorbed. With that in mind you should stop worrying so much about the external layer of the box and start figuring out what should be used on the inside of the box. The matter isnt simply material, but the shape of the box, and the internal extrusions and crevaces will effect the absorbtion also. Imagine if an anechoic chamber wasnt made of all those wedges... It probably wouldnt be so anechoic

What does anyone else think about this?
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Old 14th November 2004, 09:17 PM   #42
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Nuuk Said:
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It may be worth pointing out that while a number of theories have been put forward in this thread, my suggestion to use polystyrene is based on (my) experience!
I agree with you Nuuk. Making a cabinet and trying it out is the way to learn. I'd love to find out what happens with the melted plastic approach to cabinet making as long as it's safe.

Three of us mentioned, experience making plastic/composite based speakers mine is moulded polyester based and it sounds like Nerd Mike did one in poly urethane, while you used a light-weight skinned composite.

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If practical, I prefer to 'do' rather than hypothesize. It saves quite a lot of time!
It can if it works, but if it fails it can be expensive/time consuming or some people may not want to take a risk.

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The idea that you must stop sound escaping from a loudspeaker cabinet is almost ludicrous, and is pratically unachievable. Think about it, if you had a cone that sound-proofed the speaker cabinet, it would be too heavy to move!

Agreed, there is a need to prevent the rear wave of the cone meeting the front wave and causing bass cancellation, but I know of no other reason to prevent all the sound from inside the box getting out. In fact, it may even be beneficial to have some 'escape'!
I took the room comment as a thought experiment RDF was using to rationalize the effectiveness of the light-weight material. Not a literal thing.

Perhaps it's more an issue of what are we trying to accomplish in building a cabinet. I would say we are trying to minimize the effect of the cabinet on the sound of the speaker, by any mechanism

Re-radiation is one mechanism, energy storage is another. If you look at an impedance plot of a speaker with a bad cabinet vibration you can actually see it in the electrical impedance characteristics. I've actually seen this in measurements I've done in LMS. The question is: how best to minimize these effects.

Personally I'd love to hear more details of your experiences with the skinned composite, and how you thought your speaker turned out. Any pictures?
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Old 15th November 2004, 08:23 AM   #43
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Personally I'd love to hear more details of your experiences with the skinned composite, and how you thought your speaker turned out. Any pictures?
My main work with polystyrene was done pre Decibel Dungeon, and pre digital camera days, so sadly I don't have pictures. However, the finished result looked no different to any other boxed speaker!

The two inch thick polystyrene was covered with wallpaper and painted. The front baffle and base were made of wood. It was a ported design using a 170 mm Audax woofer and Audax tweeter mounted on top of the box.

The sound however was remarkable. They were active two-way speakers as described on Decibel Dungeon but with the polystyrene cabinets, the timing was the best that I have heard on any speaker, including my current OB's! They were slightly bass light although the speed of the bass was just incredible. The bass light problem was at least partly due to making the volume a little on the small side as I had miscalculated the effect of using two inch thick material.

Of late, I have spent time developing my OB's so have not taken this much further but if I ever went back to a boxed speaker, polystyrene would be my first choice for construction material!
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