Understanding Soffits

[gareth.h.rees]

Hi there.

I'm in the middle of constructing my soffits but I'm a bit stuck on the next phase - the box for the speaker to sit in.

I understand the basic concepts of a soffit enclosure, but what purpose does the "speaker box" serve?

Most diagrams show the box to have an air gap of ~2mm around the speaker. Wont the speaker vibrate on the surface it sits on and touch the sides of the box? If one of the aims of soffit mounting is to minimise the wasted energy of a speaker cabinet resonating, wouldnt it make more sense to make the structure around them (ie, the soffit) a very tight fit so that they would reinforce it. Though thinking out loud this would make the structure resonate, which is bad?

Paul Woodlocks design shows the speakers mounted on Sylomer (rubber pads) with a concrete filled box on top, with the speaker on top of the box. I intend my design to be similar, but have been advised that a steel plate would work just as well as the concrete filled box for my purposes.

However, this still confuses me, as the speaker will be sat on a hard surface - surely it will vibrate on the hard surface regardless of any anti-vibration pads underneath?

Should I mount the speakers directly onto the neoprene I have, or use some form of weight between the pads and the speaker?

Here are some pictures illustrating my questions:


Thanks,
Gareth

[gareth.h.rees]

In response to my own questions, and mainly thinking out loud;

The speaker "box" SHOULD be connected to the rest of the structure. However the speaker itself *SHOULD NOT* touch the box, as this would take the sound through the soffit structure.

The speaker in effect should "float" in the box, where the only contact between it and the soffit structure is through the neoprene pads?

However, this still leaves the questions:

> Why is the box surrounding the speaker necessary? What would happen if it wasn't there?

> Should the speaker be mounted on something heavy before the anti-vibration pads, as per Pauls design?

> How do you minimise the amount of movement from the speaker cabinet if it is not physically touching anything?

Also, is the material I have suitable for decoupling speakers. Its "Armacush" - I've linked the data sheet as its slightly too big for upload here [596k], along with a picture of my soffits so far.

Armacush: Armacush PDF

Thanks,
Gareth

[gullfo]

its hard to tell if the Armacush will be correct for decoupling as it may be too dense for just supporting a speaker (although one of its uses appears to be decoupling "Steel working machinery, i.e. presses or guillotines" so if you hve a number of beheadings going on... )

Sorbothane is a decent product http://www.sorbothane.com/

the mass of Paul's setup was designed to put the resonant frequency of the stand well below that of his speaker.

[Spyrow]

Do not take my answers like statements, I'm just answering what I think it is logical from the acoustic theory.

> Why is the box surrounding the speaker necessary? What would happen if it wasn't there?

There wouldn't be reinforcement, the back reflections would travel uncontrolled inside the wall.

> Should the speaker be mounted on something heavy before the anti-vibration pads, as per Pauls design?

Maybe Paul is trying to create an mass-spring-mass system with a low resonance. The more heavy are the leaves, the lower resonance of the system.

> How do you minimise the amount of movement from the speaker cabinet if it is not physically touching anything?

I don't understand this question fully. There is two types of sound transmission between structures: airborne & impact sound. If the speaker doesn't touch anything the transmission is airborne (thru the air) which is easier to control. Let the speaker vibrate free, it was designed for that.

I think the soffit mount of speakers is just to avoid combing filtering from the rear wall, please someone correct me if I'm wrong.

[gareth.h.rees]

Hey guys. Sorry for the delayed response, been away for a while to move back to University for this term. Thanks for the input so far, but I still have a few questions

"Steel working machinery, i.e. presses or guillotines" so if you hve a number of beheadings going on...

Maybe I'll sneak one into the bottom section for use with "rawk stars"

the mass of Paul's setup was designed to put the resonant frequency of the stand well below that of his speaker.

Yeah, I remember reading something like that. How do I meausre the resonence of the speaker vs stand?


Gullfo, in another post I made you suggested the following:

the rubber pad should be ok assuming its firm enough to support the speaker but resilient enough to decouple it. you could tap on it with a mallet to see how well it decouples.

How can you tell if a speaker is decoupled???

There wouldn't be reinforcement, the back reflections would travel uncontrolled inside the wall.

Okay that makes sense.

Maybe Paul is trying to create an mass-spring-mass system with a low resonance. The more heavy are the leaves, the lower resonance of the system.

Is what I have capable of supporting a mass-spring-mass system? Thats what I was aiming to achieve, though I'm not quite sure I understand the exact details of it.


I suppose I just want to be sure of what the next step should be.

Should I attempt to create some sort of Mass-Spring-Mass system, or should I just put the speakers on rubber pads straight onto the soffit shelf?



Thanks for the help guys - just a little unsure of where to go next!
Gareth

[gullfo]

if the speaker isn't causing the cabinet/soffit to vibrate due to mechanical contact, then its decoupled. it will potentially vibrate due to airborne sound but this is normal and damped by the insulation. the tap the base with the hammer will give you the test, if you get little or no mechanical transfer then you're probably in good shape. the stuff the monitor sits on has to compress a bit to enable it to be effective but not too soft or it will compress completely or too much and thus not be effective at all. the neoprene, sylomer, sorbathane, polyurethane foam, etc should all work.