BBC Thin Wall Enclosure - Ported?

[Tenson]

I've been thinking about thin-wall enclosures that the BBC pioneered and I wonder - does it need to be ported to work right?

Most of the BBC designs were ported, and at first thought it seems it would be necessary to prevent the thin wall enclosure from being internally pressurized and flexed excessively. However, a port actually only operates as an open 'hole' to sound below its tuning frequency. At or above its tuned frequency it has a high acoustic impedance and is seen by the woofer pretty much as a closed box. So in such a speaker with a 50Hz tuned port, at 100Hz the enclosure will still be pressurized regardless. Does that seem right?

If this is indeed the case then it makes one wonder if it really matters that the cabinet is ported since it would only reduce flexing of the walls at very low frequencies. Maybe that matters though?

Comments and thoughts?

[Ian J]

Actually at resonance it's much worse for the ported box - here the output of the port is in phase with the front of the cone so the box is being (de/)pressurised roughly twice as much as a sealed one of the same dimensions. Yes, it's better for the ported box below resonance but which is better overall from the viewpoint of wall flexing? I guess it depends which frequencies you're optimising for.

[bbggg]

Originally Posted by Tenson

does it need to be ported to work right?

No. The LS3/5A was sealed whereas larger models were ported -as was the Spendor BC1.

[Tenson]

Ahh but the LS3/5 is a pretender, its too small for the thin wall theory to work right IMO. At that size it's pretty much ridged.

Ian I think you have a good point. The idea of the BBC design was to optimise for midrange frequencies, so make the cab resonance low, and I think in this case it doesn't matter is it is closed.

[bbggg]

Originally Posted by Tenson

it's pretty much ridged

That too.

[Charles Darwin]

Originally Posted by bbggg

That too.

For her pleasure?

[saschagabor]

I'm currently building a thin walled cabinet (9 mm) for my SEAS full range drivers. Net volume will be about 45 liters, and I'll use an aperiodic vent.

In Norwegian:
Tynne høyttalerkabinetter, BBC-style

[dabbler]

Originally Posted by Tenson

Ahh but the LS3/5 is a pretender, its too small for the thin wall theory to work right IMO. At that size it's pretty much ridged. .

Tenson, what exactly is the "thin wall theory"?

I have some old celestion speakers with very thin walls. They perform surprisingly well with a bit of EQ.

[bbggg]

Originally Posted by dabbler

Tenson, what exactly is the "thin wall theory"?

This:
http://diy-audio.narod.ru/litr/1977-03.pdf
and other seminal papers are available at the BBC online archives.

[speaker dave]

We debated this quite a bit recently here:

What are the characteristics of a better material for enclosure?

Follow the links to the excellent paper by Harwood at the BBC. In general he shows (proves) that you can achieve better cabinet performance with a thinner cabinet but with a greater thickness of damping material (more of the cabinet mass is from the damping material rather than the wooden structure).

This approach works just as well for sealed cabinets as vented boxes. Much confusion here. Note that the pressure in the box is related to the pressure outside the box. It is in fact the 2pi response curve integrated twice (given a 12dB per Octave bass rise EQ). So a sealed box and a vented box with the same response (same bandwidth, say) will have equal pressure inside the box. Presure at the vent frequency is no higher than for other frequencies.

David S,