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PostPosted: Sat Aug 25, 2012 5:33 pm 
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the box around the speaker is supposed to be a "tight fit". But you said earlier in this thread that the point is to make it rigid.
It looks like John is busy on one of his projects right now, so I'll take a shot at that for you, and John can correct things later if I get it wrong.

The way I understand John's "rigid" comment, is that the entire soffit, and especially the front panel (baffle) must be very massive, solid, and rigid. The basic theory behind soffits is that the front panel it acts as an "infinite baffle", or an infinitely large extension of the speaker cabinet itself. Of course, "infinite" only in the sense of "very large with respect to the longest wavelengths". To do that, the panel cannot move, vibrate, oscillate, resonate, absorb, or do anything else but just "be there", in order to direct the low frequency sound in the same direction as the high frequencies: straight forwards, into the room. If the panel were to move, or vibrate, then it would be adding it's own tune to the sound, "coloring" it in some way.

So the way I understand John's comment is that the entire soffit must be built with that purpose: Keep the front panel as rigid as possible.

On the other hand, you also do not want the speaker cabinet vibrating around and banging up against the internal structure that holds it in place: Hence, it should be held tightly by whatever is keeping it there. John uses a box, but I've also seen Barefoot (I think it was him) suggest using broad canvas tie-downs with ratchets to hold the speaker tightly: you do not want it moving either.

BUT! Here's the thing: You also do not want the speaker transmitting its vibrations into the structure of the soffit! So it has to be held tightly, yes, but also decoupled from the soffit panel, and from its own supporting structure. It would be neat if you could just sort of levitate it in mid air, without it touching anything, but also without it being able to move.... But there are no magical incantations that seem to work for that, so the solution is to isolate the speaker and box from the support in some way or other. And also to leave a small gap between the front of the speaker and the soffit itself: Maybe a mm or so. The speaker cabinet itself should not touch the actual front panel of the soffit, if possible, so as to not be able to transmit any energy into the panel. Of course, if the panel is hugely massive, then it probably doesn't matter, but better to be safe than sorry.

So, to summarize, the idea is to decouple the speaker from its support but also hold it firmly in place, and also to have a very massive, very rigid front panel on the soffit. Simple, right? :) (siple on paper: not so simple in practice...)

Quote:
Also, if I am soffit mounting passive speakers, do I need to leave vent space in that box,
Passive speakers still need cooling: you are pumping at least a hundred, maybe a couple of hundred watts of electrical power into the speaker coils, and only getting less than one single watt of acoustical power out the front. So guess where all the rest goes: heat. It is dissipated as heat, mostly in the coils of the drivers. So even though they need less cooling than an active speaker, you probably still need some cooling, if you hope to use your speakers for a long time. I would still provide some ventilation.

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If I screwed the speaker right into the shelf it sits on, is that just as good as building a box around it?
Nope... :)

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I heard before that the speaker was actually supposed to sit on something that decoupled it from the stand it sits on..
Yep... :)

Look around the forum: Somewhere Barefoot posted a neat design with an exploded translucent view, so you can see how all the parts fit together, and the concepts behind his design. His design is different from John's, but achieves the same end purpose, just in a different way.


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PostPosted: Sun Aug 26, 2012 1:56 pm 
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Awesome, that answers my questions

About the concept of leaving ventilation for the speaker.. I decided to do a design where I took the tilt angle of the soffit all the way down to the floor. I still packed the whole thing with insulation, no hangers. So do you think I should cut the one foot tall slot along the base of the soffit face so that I can use the cavity as a bass absorber, or is that too much opening to to have considering that the entire soffit cavity is functioning like the upper section of John's soffit in his design.

One more. You said that the soffit panel shouldn't absorb. I was planning on attaching some insulation the the back side of the face to keep the panel from resonating. Sounds like a good idea..you meant something different right?


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PostPosted: Sat Sep 08, 2012 1:55 pm 
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So do you think I should cut the one foot tall slot along the base of the soffit face so that I can use the cavity as a bass absorber, or is that too much opening to to have considering that the entire soffit cavity is functioning like the upper section of John's soffit in his design.
John's design divides the cavity in two, with the top section being for speaker, and the bottom being for bass trapping. There is a shelf-divider in between, to separate the spaces. So if you have that divider in place, then yes, you sure can use the lower section as bass trapping, and if hangers don't fit, then just stuffing that bottom section it full of absorption with a 1 foot gap at the lower end of the front panel would be a reasonable substitute.

Quote:
You said that the soffit panel shouldn't absorb. I was planning on attaching some insulation the the back side of the face to keep the panel from resonating. Sounds like a good idea..you meant something different right?
Yup! I meant the FRONT side of the panel, the side facing the room: the panel itself should not be absorbent. But you sure do need absorption in the cavity inside the soffit itself, where the speaker is! The purpose is to damp the resonance in there, and since it is mostly low frequencies going on in there, you an throw in as much absorption as will fit! All your absorption off-cuts and waste from the rest of the room can go in there, along with old clothes, dirty washing, discarded carpets, a sofa or two, etc.... :) Seriously, the cavity inside the soffit can be your "trash can" for bits of absorption: fill it to the brim, if you want. The point I was trying to make is that the soffit panel itself (the "baffle") needs to be solid, rigid, massive, hard, thick, etc.

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PostPosted: Wed Sep 02, 2015 10:43 pm 
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John Sayers wrote:

regarding taking the angle all the way down here's Masterphonics.

Attachment:
masterphonics_1.jpg


In this case is the seperate cavity for bass trapping underneath the speaker section still present? I can't see the gap in that picture.

The reason there is no desk reflection absorption underneath the speaker soffit is the fact the whole soffit section is tilted?

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