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Daft idea?

 
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terry
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PostPosted: Sun May 25, 2008 1:53 pm    Post subject: Daft idea? Reply with quote

Has anyone any experience of using wall cupboard space to accommodate floor standing speakers? What did you do with the wall cupboard environment to minimise unwanted effects?
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proffski
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PostPosted: Wed May 28, 2008 11:40 am    Post subject: Re: Daft idea? Reply with quote

Gosh Terry are you serious?

I have seen 'professional' AVA bods doing so in magazine articles for custom Home Cinema.
But unless they know something that you and I do not... Shocked
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speakerguru
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PostPosted: Wed May 28, 2008 12:20 pm    Post subject: Reply with quote

The official KEF advice used to be 1m away from side walls and .5m away from the back wall. The next best thing is half space or 2pi mounting aka "soffit" mounting with some bass eq. Try to avoid gaps and cavities between the speakers and the recess as they will have audible resonances and produce unwanted reflections and refractions. Totally removeable doors would be a good idea too.

p.s. Actually, if your speakers are on the floor and flush to the wall/cupboard you will be quarter space or pi mounted as far as the bass is concerned - so, even more bass eq called for.
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proffski
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PostPosted: Wed May 28, 2008 12:27 pm    Post subject: Reply with quote

Shocked That is what I found most interesting.
The system in the article was more a year’s salary for me, yet the speakers were mounted in alcoves with bags of air space around them!
And yes to keep the Mrs happy they had doors or maybe panels which were caustically 'transparent'. If I can find the offending magazine I’ll publish full details…
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I contend that for a nation to try to tax itself into prosperity is like a
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terry
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PostPosted: Wed May 28, 2008 4:44 pm    Post subject: Reply with quote

Thanks for comments proffski and speakerguru.

Speakers are basically 1976 KEFKIT 3 (not T27s) but heavier due to the double thickness construction.

Having done a bit of experimentation (no doors, no Science, just fiddling) I was surprised that the only real degradation was in the bass. In fact there may have been gains elsewhere. I am sure, however, that it lost some bass definition (as expected) and perhaps became a bit bloated.

The fronts of the speakers and the walls are flush. There was clear improvement in sound quality when I pulled the speakers flush.

There is a little space all round which I suspected I should fill with some damping material. Any thoughts? I initially thought about concrete to get a great deal of extra mass but that will couple the speakers to the walls and I expect that that would not be a good idea.

I have also experimented with the floors in the cupboards and I have currently settled on the following arrangement. Each speaker has 4 downward pointing spikes, sits on 4 inches of concrete slab, which sits on a 22mm layer of MDF, which sits on a matrix of rubber door stoppers, which are screwed onto 22mm of MDF which is screwed to the floor.

The insides of the cupboards are constructed from double thicknesses of 22mm MDF to add mass.

Unfortunately I can't aspire to anything like what professional recording studios have in relation to soffit mounting. I believe that soffit mounting 'amplifies' the bass and has other benefits, but I think that in an ideal installation the soffit is acoustically decoupled from the room and the room walls are double thickness plasterboard or brick. Positioning of the speakers is also critical in relation to the size of the soffit and of course the shape of the room. There is I believe quite a fiddling with acoustic hangers to create bass traps and all that requires space under or to the side of the speakers.

My application is dictated by pragmatic considerations and my intention is to minimise the problems rather than to maximise the gains from having a wall built to act as an 'amplifer'. The speakers are in cupboards because they cannot go elsewhere unfortunately.
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proffski
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PostPosted: Wed May 28, 2008 6:45 pm    Post subject: Reply with quote

Guess that you seem to have made the best of a tricky situation.
Surprised that the listening tests were so positive.
I'd feel inclined to fill the remaining space with wool socks.. Wink
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speakerguru
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PostPosted: Thu May 29, 2008 8:15 am    Post subject: Reply with quote

terry wrote:
.....and perhaps became a bit bloated.


No perhaps about it. Each reflecting surface will add 6dB, yes 6dB, to the bass. This is because at large wavelengths, each reflecting surface adds an "image" speaker by mirror action.

So, if the speaker was designed for freestanding and you mount it in the corner of a room you will need to shelve down (filter shelf, not furniture) the bass by -18dB. Floor and wall; -12dB. This is not subtle stuff. A deaf man could hear the difference Wink . You will need to put in bass cut eq.
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terry
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PostPosted: Thu May 29, 2008 10:09 pm    Post subject: My wife says I am deaf Reply with quote

Yes Proffski, I was very surprised too that there wasn't significant degradation. I was expecting the worst and so perhaps I am more positive as a result.

Thanks Speakerguru - I know there is an issue and the re-jigging of the filter can be a retirement project, or perhaps a new set of speakers designed and built for the job might be more appropriate. A return to a former interest perhaps! My first qualification was in Electronics at the University of Aberdeen, and I was interviewed and given a job for the then British Aircraft Corporation working on automatic landing systems. Two weeks before starting I did however turn down the job in favour of staying in Scotland with my then girlfriend (then subsequently my wife). As a result I didn't ever become a professional engineer and I drifted into the education business educating others about computing and computers.

ps My wife does sometime say that I am deaf - it could, however, be selective deafness Very Happy
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proffski
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PostPosted: Thu May 29, 2008 10:34 pm    Post subject: Re: My wife says I am deaf Reply with quote

Ah well, finished building an amplifier to drive a large subwoofer for a friend today.

I have done this so many times that the usual checks were dispensed as I felt so damned confident, and I admired the lovely board, the ever so tidy wiring looms and I was so proud...

Rather than measuring for DC offset as I normally do, and measuring voltages across the power supply capacitors as I normally do, I decided to plug it in and go, I was that confident...

Something at the back of my mind stopped me using the twin 12” subwoofer and instead we dug out one of the many 8” woofers I have from the original NAIM SBL loudspeakers. It was connected up and we switched on, a loud hum followed by a pop and smoke poring from the loudspeaker…

I stared with disbelief, whilst the smoke cleared.

Checking the voltages indeed proved interesting, lots and lots of DC offset at the LS terminals and silly voltage reading at the power supply, oi, oi, oi.

It took several minutes for the obvious to become apparent to the shaken brain and the stupidity of my actions to sink in.

There staring back at me was the unconnected centre tap on the huge toroidal transformer, oh ****er!

I’d have expected the o/p to have swung one way or another as nothing is symmetrical by 100% and all should have been well once the oversight was corrected, sadly a few of the output devices failed.

Half an hour later we were up and running again, and a lesson learnt good ‘n proper!

Sorry to go off at a tangent, just had to tell somebody…
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I contend that for a nation to try to tax itself into prosperity is like a
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-Winston Churchill
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terry
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Joined: 22 Apr 2003
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PostPosted: Sat May 31, 2008 1:48 pm    Post subject: more than me daft! Reply with quote

proffski

Glad to hear that there are others out there who do silly things sometimes Embarassed Embarassed
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