alternative bracing methods for the statement monitors?

[Coconutout]

hello, i was wondering if there is more effective way of bracing the lower cabinet of the statement monitors, albeit more complicated one. i've already cut out the rectangle pieces that are to be placed behind the dayton woofer as a vertical bracing, but am thinking if it would be more effective if i were to divide the piece into fours and attach them vertically on the side walls of the cabinet continuing down along the lines of midwoofer braces. you know, the four bracing pieces that supports the tunnel for tang-band driver. or how about covering the side walls with concrete in equal amount of volume to the bracing piece? i can claim no knowledge on speaker designs but seems to me that if the bracing was done as per the original cabinet design, there could be an issue of corner resonance( right term?) since the bracing practically creates a pocket chamber on the bottom of the enclosure. please let me know what you think. good day.

[fbov]

Bracing is an open-ended topic, so expect no definitive answers.

The objective of bracing is to damp vibrational modes in the enclosure walls. A brace connecting the centers of opposite sides eliminates any vibrational mode that requires the center to move. A brace placed on a surface increases stiffness in the direction of the brace, pushing all resonance in that direction to higher frequencies, but it does not eliminate any single mode.

Covering surfaces with a structurally-different material moves you into constraint layer damping, an approach like the surface brace in that it also damps a wide range of frequencies. It's most effective when there's a visco-elastic layer between two stiff layers, a sandwich in which bending of the inner panel is damped by the soft layer, reducing energy transmitted to the outer layer. It's most effective when combined with bracing, to further reduce vibration in the inner layer.

Internal resonances are a completely different issue, albeit also resulting from the motion of a speaker. Here, the vibrations are still in air, not in the box. Resonance in the sense of standing waves has a critical geometric constraint: the spacing must be an integer half wavelength (1/2 wave, 1 wave, 3/2 wave). The frequency of interest is the 1/2 wavelength. Assuming a 500Hz W-M crossover, the shortest wavelength you would need to worry about is 13.6" (1/2 wave @ 500Hz). The only dimension that size or greater is box height, and you've got a mid tunnel running through the middle to damp it.

That said, notice that resonance only applies to specific frequencies. You will want to damp non-resonant frequences as well, to prevent that energy from leaking out the port or through the driver itself. All air vibrations can be damped by sonic absorbing treatment on the walls, and by fill in the volume. Curt's build instructions will tell you what internal treatments you need, and where to put them. Look for:
"I used 2" wedge foam to line the inner cabinet walls behind the RS180 but
fiberglass can be used with equal success. However, be sure not to block the
path to the port."

In this design, the mid tunnel is the primary bracing, with an extra brace behind the woofer that also connects the sides. Other spans are short. I think you'll be fine with the published design, as long as you include the internal damping material quoted above.

Have fun,
Frank

[Fdas]

Bracing is an open-ended topic, so expect no definitive answers.

The objective of bracing is to damp vibrational modes in the enclosure walls. A brace connecting the centers of opposite sides eliminates any vibrational mode that requires the center to move. A brace placed on a surface increases stiffness in the direction of the brace, pushing all resonance in that direction to higher frequencies, but it does not eliminate any single mode.

Covering surfaces with a structurally-different material moves you into constraint layer damping, an approach like the surface brace in that it also damps a wide range of frequencies. It's most effective when there's a visco-elastic layer between two stiff layers, a sandwich in which bending of the inner panel is damped by the soft layer, reducing energy transmitted to the outer layer. It's most effective when combined with bracing, to further reduce vibration in the inner layer.

Internal resonances are a completely different issue, albeit also resulting from the motion of a speaker. Here, the vibrations are still in air, not in the box. Resonance in the sense of standing waves has a critical geometric constraint: the spacing must be an integer half wavelength (1/2 wave, 1 wave, 3/2 wave). The frequency of interest is the 1/2 wavelength. Assuming a 500Hz W-M crossover, the shortest wavelength you would need to worry about is 13.6" (1/2 wave @ 500Hz). The only dimension that size or greater is box height, and you've got a mid tunnel running through the middle to damp it.

That said, notice that resonance only applies to specific frequencies. You will want to damp non-resonant frequences as well, to prevent that energy from leaking out the port or through the driver itself. All air vibrations can be damped by sonic absorbing treatment on the walls, and by fill in the volume. Curt's build instructions will tell you what internal treatments you need, and where to put them. Look for:
"I used 2" wedge foam to line the inner cabinet walls behind the RS180 but
fiberglass can be used with equal success. However, be sure not to block the
path to the port."

In this design, the mid tunnel is the primary bracing, with an extra brace behind the woofer that also connects the sides. Other spans are short. I think you'll be fine with the published design, as long as you include the internal damping material quoted above.

Have fun,
Frank

Thats an awesome response

[fbov]

Thank you!

[cnclld]

The common practise seems to be to fathom the enclosure as separate panels, pinned on their edges. I think this is in many cases over simplifying. In modes the enclosure acts as a whole, and the panels affect each other's behavior. Consequently opposite walls in an enclosure can bend to the same direction in a mode, especially if some of the enclosure's dimension is short compared to others. Connecting the walls together from one point in the middle of the panel therefore has little or no effect on the resonance in this case.

The above applies only to enclosures structural resonances. Internal standing waves and coincidence effect may also cause problems but I believe they should be an order of magnitude lower in level if the internal volume is properly damped. One should also take in to account the braces' effect on Helmolz resonance etc. when designing the enclosure.

These are my own conclusions and I recognize that I may disagree with many respected authors in the field. I would still like to point out that most of the literature is written before modern FEM softwares and the modal analysis of an enclosure has been mathematically very difficult and so many simplifications have had to be made. This is still a topic with surprisingly little good research as very few companies have enough resources to study such and the enclosure is still pretty easy to make "good enough" with traditional means.

Best wishes,
Jaakko

[Coconutout]

thanks guys. i feel stuffed with food for thought.