recreating a sealed loudspeaker enclosure

[skodajag]

I have a 1951 Philips FX804A radiogram with a scruffy cabinet. I found another cabinet recently: sadly it was minus the chassis and deck because someone had converted it to a drinks cabinet circa 1970. On the plus side this meant it had been looked after.

Now the two cabinets are indentical apart from the speaker enclosure (I assume they had different chassis originally. On my original cabinet the rear of the enclosure is VERY permanently sealed with glue, nails and screws. The back itself is made of half inch ply. Also, from about half way up the back panel slopes slopes inwards by about 45 degrees, also in half inch ply. Internally it has thick wadding on all sides. (Incidentally, in case you were wondering, the speakers are accessed and removed from the front). Now, the “new” cabinet simply has a conventional thin cardboard back with a thinner wadding in the inside walls. I can replicate the sealed back of the original cabinet on the new one without too much difficulty, but am wondering if it’s important to get it air tight. And is there anything else I should be aware of in recreating the original sealed chamber? Like – is it worth the trouble!?

[dave walsh]

Sounds like an infinite baffle structure as there is no 'port' opening to 'tune it'. There is a lot of psychology involved in enclosures. I would think you need something stronger than cardboard but a try blocking the back with a piece of wood on a temporary basis to see how it sounds. I've had RG's in the past that sounded no less good with the back panel off altogether but it 's down to perception in the end. If you cut a panel to size and tape it you it will not as final as glue and nails if you neeed to get in again! I would personally go with a wood panel but not be desperate to get an air tight seal. No doubt others will disagree but by definition, the RG as a piece of furniture is an acoustic compromise!
Dave

[skodajag]

Thanks Dave. I've heard the concept of an infinite baffle structure many times but never understood what it is. Can anyone enlighten me?
Jonathan

[Nickthedentist]

As far as I understand it, an "infinite baffle" is just a totally sealed enclosure, of precisely-calculated volume to match the speaker's characteristics.

I think a good seal is fairly important. If it were not, I can't see why the manufacturers of such enclosures would put in such a great deal of effort to make them airtight (e.g. foam gaskets around the speaker's edge, hot-melt glue where the wires enter the cabinet etc.). The speaker relies on the "cushion" of air behind it to stop the cone flapping around uncontrollably, so speakers that are designed to be used in sealed enclosures can be damaged if driven hard in unsealed ones (as well as not sounding right).

P.S. Alternatives to infinite baffle include a normal baffle (just a big board with the speaker mounted over a hole, often with some rudimentry sides to make it look nice and stay upright, i.e. what you'd find in most TVs, table radios and cheap radiograms) and bass reflex (a box with a small opening, usually with a short inwards-pointing tube attached to it - very popular because they're quite efficient and give good bass response... I think ).

N.

[skodajag]

Thanks Nick - that's helpful. As I mentioned in the original posting, the original cabinet was not only sealed with thick ply, but had the top half of the rear panel sloping inwards by 45 degrees. The new cabinet had the same (twin) speaker set up but with a perforated board back and no slope. However the back was covered with a fine material which may have affected the sound, or may just have been to prevent the ingress of dust.

I decided to replicate the original sealed enclosure and it's turning out to be rather more intricate than I thought. Philips certainly went to some trouble with this, especially the sloping back, so they must have thought it worthwhile back in 1950. I propose getting the cabinet as sealed as possible with close fitting joints, and then running some seam sealer along them.

Jonathan

[dave walsh]

Should work fine but remember the original units had brand new speakers that weren't expected to fail just yet! Although there is no opening [port]
perhaps the sloping panel is to tune out an unwanted resonance and get the bass response lower [just guessing].
Re what Nick said, you can see illustrations from the thirties of speakers just mounted on a simple baffle board [sometimes across the corner of a workshop about 3'x3']. As I remember though, to reproduce a full range of frequencies, it was said that an 8" speaker needed a baffle board 5' 6" square. Hence all the work on enclosures as your mono [and especially stereo] system would be a touch anti social-even if you lived alone. Dave

[chipp1968]

I wonder why two versions were made ? one more expensive or an alteration to original design?

[skodajag]

Originally Posted by chipp1968

I wonder why two versions were made ? one more expensive or an alteration to original design?

Almost certainly the former - the cabinet was used for several years on a range of different sets, at least up to 1954/5.

Jonathan

[skodajag]

Well, I've completed the "recreation" and assembled the whole thing and it sounds good. However, I have a further query: both speaker chambers originally had wadding on all sides. In the new arrangement I've been trying it with and without this wadding and I must say it seems to sound as good if not better without it. So the question is this: would that original wadding have been there to generally improve the sound quality, or was there there specifically to prevent resonances at certain frequencies? Thankyou again all for your previous advice.
Jonathan