I intentionally cut the M-T baffle 5 mm short on the sides and top. I intend to fill this in with a trim piece later on. It would not be possible for me to machine the M-T baffle to exactly fit the assembled cabinet. I can hand plane the thin strip of wood trim for an exact fit.
I decided to paint the exposed edges black, to see if I would like the look. This had to be done before the veneering. Now that I see it, I don't like it, so I will stick with the original plan of a trim piece.
I decided to paint the exposed edges black, to see if I would like the look. This had to be done before the veneering. Now that I see it, I don't like it, so I will stick with the original plan of a trim piece.
@DualTriode Your findings with the chassis dampening affecting the HD curve are similar to what I discovered with midbass chassis mounting. I started using strips of higher density neoprene foam under the midbass and LF drivers when I got some weird results from chassis measurements in an unreinforced flat baffle.
I found that when I measured the TSPs of a driver in free air, there were some odd looking peaks in the impedance curve. After measuring with drivers clamped in a larger vise (protecting the chassis with a thin layer of neoprene foam, there was a noticeable improvement in the impedance curve and some changes in TSPs. Mounting the driver in a large (non- IEC standard) test baffle, there were further differences. This started to frustrate me and I decided to check again with the driver in a smaller sealed cab with and without 1/8" neoprene foam between the chassis and enclosure, varying the fastener torque. Again, there were considerable differences. There was such a jumble of results I was drained from the process and slammed the lid shut on my computer (without saving or labeling the results!!!).
I did learn something very valuable from the whole process, mainly that it REALLY MATTERS how you mount the raw driver when measuring TSP data FR and THD. I started paying close attention to midbass and LF chassis dampening.
The simplest and most effective way to deaden and couple the driver chassis to the enclosure is with a low to medium density elastomer foam and grommets between the fasteners to FULLY ISOLATE the chassis from the enclosure. Its obviously also critical to try different fastener torque settings and measure the results. If you see anything in the impedance curve that wasn't there prior to mounting the driver, check whether its being caused by compromised coupling from chassis to enclosure. I find the best torque settings are the loosest you can get away with to provide a good air seal.
I use pass through EPDM or nitrile rubber grommets under the screws or bolts and orings under the fastener heads. This allows for keeping a constant and consistent preload on the chassis. I learned this method from KEF when I restored my Reference 102/3 crossovers. They used a steel dowel rod with a rubber coupler to pull in the entire LF chassis. Its mounted this way to the enclosure without using any flange screws. Very clever and already using this mounting method back in the late 60s.
I found that when I measured the TSPs of a driver in free air, there were some odd looking peaks in the impedance curve. After measuring with drivers clamped in a larger vise (protecting the chassis with a thin layer of neoprene foam, there was a noticeable improvement in the impedance curve and some changes in TSPs. Mounting the driver in a large (non- IEC standard) test baffle, there were further differences. This started to frustrate me and I decided to check again with the driver in a smaller sealed cab with and without 1/8" neoprene foam between the chassis and enclosure, varying the fastener torque. Again, there were considerable differences. There was such a jumble of results I was drained from the process and slammed the lid shut on my computer (without saving or labeling the results!!!).
I did learn something very valuable from the whole process, mainly that it REALLY MATTERS how you mount the raw driver when measuring TSP data FR and THD. I started paying close attention to midbass and LF chassis dampening.
The simplest and most effective way to deaden and couple the driver chassis to the enclosure is with a low to medium density elastomer foam and grommets between the fasteners to FULLY ISOLATE the chassis from the enclosure. Its obviously also critical to try different fastener torque settings and measure the results. If you see anything in the impedance curve that wasn't there prior to mounting the driver, check whether its being caused by compromised coupling from chassis to enclosure. I find the best torque settings are the loosest you can get away with to provide a good air seal.
I use pass through EPDM or nitrile rubber grommets under the screws or bolts and orings under the fastener heads. This allows for keeping a constant and consistent preload on the chassis. I learned this method from KEF when I restored my Reference 102/3 crossovers. They used a steel dowel rod with a rubber coupler to pull in the entire LF chassis. Its mounted this way to the enclosure without using any flange screws. Very clever and already using this mounting method back in the late 60s.
@profiguy I never had problems with my last projects with driver mounting - but I use front baffles 25-40mmm thick and stiffen them. So my concept is to tighten it to this stiff and heavy piece, not decoupling. Will give it a try with different force when I find time in the next project.
But putting them in a sheet of IEC dimension ... I really can imagine there is plenty of stuff going on.
But putting them in a sheet of IEC dimension ... I really can imagine there is plenty of stuff going on.
It's really "eye opening" to knock your box with one hand while tightening driver mounting screws with another. At least on my last boxes, it literally tuned the box like a guitar. As I tightened screws the knock sound got higher and higher pitched. Or, basically making it tight first and then loosing the screws dropped the pitch quite fast.
The main conclusion I'd draw from chassis born vibration mitigation and enclosure construction is basically trying to do one of two things - reinforce the enclosure walls to the point of moving the vibrational modes higher up out of the chassis FR, or use thinner panels to lower the modes and apply viscoelastic dampening media to them. Using both thicker panels with extensive reinforcement/bracing and applying surface dampening treatment is the fool proof, expensive way.
Largely, the frustration I have is with the manufacturer data not being consistent with self measured data. The impedance curves are often embellished or smoothed by pretty much all of the driver brands. My findings are that in the critical lower midrange some of the most important driver behavior is being covered up. The spider and surround both contribute to chassis born vibration, but the spider alone is responsible for the majority of vibrational transmission in the critical 250 hz - 1k lower midrange. The spider has the most stored energy issues in this range and much of that bounces back and forth between the cone and chassis to mounting flange.
The other significant cause of vibration from the whole chassis is the counteracting forces from the accelerating and decelerating moving mass. This one isn't so hard to deal with if the chassis is designed properly in regards to stiffness, but a decoupled chassis counteracts this if the driver bandwidth exceeds the reactive decoupling frequency. The enclosure mass will effectively cancel most of the forces if its significant enough and the enclosure itself is well coupled to the ground using spikes or other means of increasing the coupling pressure through a very small surface area exerting concentrated force to the surface it rests on.
The spider is the most influential part of the chassis to deal with. Unless you have some sort of idea where the main resonant chassis modes(s) are based on accurate impedance curves, there isnt much you can do to minimize the spider's stored energy, keeping it out of the enclosure walls.
Largely, the frustration I have is with the manufacturer data not being consistent with self measured data. The impedance curves are often embellished or smoothed by pretty much all of the driver brands. My findings are that in the critical lower midrange some of the most important driver behavior is being covered up. The spider and surround both contribute to chassis born vibration, but the spider alone is responsible for the majority of vibrational transmission in the critical 250 hz - 1k lower midrange. The spider has the most stored energy issues in this range and much of that bounces back and forth between the cone and chassis to mounting flange.
The other significant cause of vibration from the whole chassis is the counteracting forces from the accelerating and decelerating moving mass. This one isn't so hard to deal with if the chassis is designed properly in regards to stiffness, but a decoupled chassis counteracts this if the driver bandwidth exceeds the reactive decoupling frequency. The enclosure mass will effectively cancel most of the forces if its significant enough and the enclosure itself is well coupled to the ground using spikes or other means of increasing the coupling pressure through a very small surface area exerting concentrated force to the surface it rests on.
The spider is the most influential part of the chassis to deal with. Unless you have some sort of idea where the main resonant chassis modes(s) are based on accurate impedance curves, there isnt much you can do to minimize the spider's stored energy, keeping it out of the enclosure walls.
I forgot to mention, sometimes a blip in the impedance won't necessarily show up in the FR and may not make it into the cone. It will however affect a passive filter, as the fluctuation in impedance will load the filter differently and possible cause a swing in FR vs no filter.
I think one should not underestimate the reaction forces of the driver motor. These are a magnitude bigger than the reaction forces the spider transmits to the chassis. While spider resonances of course are unwanted, the mayhem of baffle vibrations is caused by the magnet trying to move.
This is an interesting approach from KEF minimizing vibrations caused by the magnet force
I had similar experience, the first time I ever tried to measure the impedance of a woofer, years ago - I clamped the driver to a portable workbench and when I measured the impedance, there was a weird double peak at the fundamental resonance. I tightened up the clamps, and suddenly got a nice clean single peak. An early lesson in the interactions between the mechanical and electrical behaviour of drivers. The experience made me wary of compliant mountings - the effect might not always be desirable.
Yes, it's just Newton's third law. A woofer motor is incredibly strong, and the full force that is exerted on the voice coil is also exerted directly on the magnet, trying to move it in the opposite direction.
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That Newton guy is really annoying. He always makes things so difficult, but alot of people keep saying he's so predictable.
All of us engineers make a lot of money off of Newton. Same with Watt, Maxwell, Pascal, Hooke, Poisson, Tesla, Reynolds, Euler, Dirac, Laplace, and many others... 😉
Oh boy, this one really took a turn... must be all that anti-matter being pulled in by the nearest black hole. Thats where all my money goes after buying all those speaker drive units that end up sitting on the shelf.
Keep in mind we aren't all engineers. Some of us have no idea why Wayne Newton is so important to speaker building or how to make a lot of money off of him!
I have made some progress over the last several weeks. I finished all the veneer work and finished profiling and smoothing the Mid-Tweeter baffle and the transition block. I then applied 2 coats of polyurethane varnish to everything. I expected that the next assembly steps would produce a lot of adhesive squeeze-out, and the varnish would protect the raw wood surface from glue stains.
I bonded the transition block first, and after the adhesive cured, I attached the M-T baffle. In this photo, the transition block has been attached, and the M-T baffle is fully taped and ready for adhesive.
I am now in the process of applying trim pieces to cover the stepped joint where the M-T baffle meets the main cabinet. After all trim is sanded smooth, I will re-sand the entire box, and apply the final finish.
As much as I enjoy the woodworking aspect, I am looking forward to getting the drivers mounted so I can start with the measurements phase...
j.
I bonded the transition block first, and after the adhesive cured, I attached the M-T baffle. In this photo, the transition block has been attached, and the M-T baffle is fully taped and ready for adhesive.
I am now in the process of applying trim pieces to cover the stepped joint where the M-T baffle meets the main cabinet. After all trim is sanded smooth, I will re-sand the entire box, and apply the final finish.
As much as I enjoy the woodworking aspect, I am looking forward to getting the drivers mounted so I can start with the measurements phase...
j.