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Romy the Cat wrote:

RF at Ona wrote: Personally, I think it debatable whether the mouth edge should be free or attached as there are desirable aspects to both approaches. If attached, it need not be attached with great pressure.  Robert, can you elaborate why if attached, it need not be attached with great pressure?  I was under opposite impression. In fact, we consider binding the front for to all joists and to all beams across the entire horn. The leading edge of the mouth is 2x4 and 1.5” of plywood – it is a lot of thickens to bind through. In our view the leading edge of the mouth is what all horn mass will be “grounded” Take a look in the picture - the horn tail all the way up and the horn is like a large snow plow  that is lowered down and lies on the payment – in my case the payment are the locations where the roof joists and the frame’s beams come together.

Further thoughts:

To a large extent supporting the weight of the horn and mechanically decoupling it to prevent the transmission of mechanical vibration are mutually opposed. In decoupling you try to isolate the horn and leave it to vibrate on its own - with those internal vibrations more or less suppressed and damped by the construction materials and design of the horn itself. In tightly affixing the horn to the structure supporting it you are trying to control the vibration by using the external structural stiffness, mass, vibration transmission and damping. Sometimes you can use both elements in a frequency selective way.

I was under the impression that you wanted to tightly couple the horn mouth to the attic framing into order to control the vibration of the horn mouth – these vibrations are a genuine issue as to the basic acoustical operation of the horn. But you also realized that tightly coupling the mouth without sufficient anti-vibration padding could easily allow it to transmit its vibration to the house structure.

It is these conflicting approaches which require some balancing.

Had you wanted 1 or 2 inches of anti-vibration padding around the horn mouth, your original design to support the horn at the very end would have been fine, although you might have had a smaller mouth area as a consequence.

If you wish to add additional decoupling at this late stage of construction then you can obtain it in a fairly simple way by moving the support points for the mouth end back several inches or perhaps or a foot or more to where the horn geometry gives you enough space to put an inch or two of anti-vibration padding with some appropriately shaped wood supports.

The very end of the horn mouth could be free standing as it might have been had you extended the horn a few inches past the wall into the listening room or it could be lightly loaded. You can attach it to the frame in a number of different ways including the shims you have mentioned. If the fit is tight you would only need a thin layer of padding. If there is a gap, you could use a series of screws or bolts with the horn end slightly off the framing, or you could stuff padding tightly between it and the frame – the trick being to prevent horn edge vibration without a high-pressure constraint lacking anti-vibration material.

However, I do not know what level of stabilization the extreme mouth end needs if any.

But as you know bass frequencies are extremely penetrating in ordinary house materials. And I would also like to point out that your horn is made of the same material as the house framing  - wood. Identical materials maximize the vibration transmission potential to and through the framing. One technique for controlling vibration transmission is to use dissimilar materials another is to use materials that dissipate the vibration as frictional heat. So I have suggested anti-vibration pads which sandwich dissimilar materials and even layering two pads with dissimilar cores.

If you were supporting the horns with concrete or steel posts sunk in the ground or foundation the concept of mechanical grounding would make more sense. With wood attached to wood the tighter the attachment the greater the vibration transmission as well as the stiffness control.

The other day I was stopped at a red light when another car pulled up. The airborne bass from this true boom box on wheels had my car trembling like a vibrating lounge chair and my rear view mirrors flapping to the point that they were unusable. I could only laugh and I thought of your experience with the 2 EV 30” woofers. I suspect the vibration you felt with those woofers was principally the effect of airborne vibration shaking the floor and not the mechanical vibration of the speaker mounting. Airborne bass can be very powerful so bear in mind that mechanical vibration control and sound isolation are two different but sometimes related things.

In the end, I don’t think the mechanical vibration of your horns will be a significant problem. Although decoupling is a worthwhile precaution I think it also reasonable to stay with your original installation plan. If your horn vibration is significant than the potential for the exterior of the horns to radiate sound into the attic, vibrate the wallboard and largely pass through into the listening room would be a problem decoupling wouldn’t influence.

Ever the optimist,
Robert

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