I shouldn't think the reason nor the result of these two situations in this case would be readily apparent. Interesting nonetheless.
Nor would have I. We were all gobsmacked at the difference it made. As soon as I removed the bass driver and saw the void I wondered if could there be a relationship between the void and the sound. The clue was the void volume - it matched the size for the signature of the resonance of the bass defect.
I can only assume that the boundary between the free air and the tightly stuffed wadding was a great enough discontinuity in mechanical impedance to cause significant reflections.
If you are trying to kill standing wave resonances, absorbent in the center of the box works better than on the walls, ime.
Absolutely, especially if it is constrained in some way so oscillating air molecules are forced to push through it, rather than push and displace it. In large cabinets I use thermally bonded polyester fibre absorbent such as Martini Absorb which can be wedged in place and provide excellent damping of side panels coincidently.
Martini Absorb thermally-bonded polyester fibre insulation
Don’t think he meant that. Porous absorbing material does virtually nothing in damping panel vibration.
Yes I did mean that. The thermally bonded fibers in Martini Absorb give it a second extremely effective absorption mechanism to mechanical flexing unique from other porous absorbing materials, which is specifically why I mentioned it.
The two engineers behind Martini Absorb (made from ~80% recycled PET drink bottles) previously worked for Tontine Insulation, but that company declined them the opportunity to develop the bonded fiber technology. My understanding is that they resigned from Tontine Insulation and either formed or joined Martini Industries to develop it. I specified their product in a small project (a 'glass box' conference room, constructed with all four walls of floor to ceiling glass and a polished concrete floor, for an advertising agency) and was one of the first acoustic consultants to provide independent acoustic performance measurements for the developers from an external trial, and verification of the model parameters for Absorb in CATT-Acoustic software.
A few years after its appearance in the market, Martini Industries was acquired by CSR Bradford Insulation. Ironically Tontine Insulation now make a copy-cat product called Acoustisorb.
The two engineers behind Martini Absorb (made from ~80% recycled PET drink bottles) previously worked for Tontine Insulation, but that company declined them the opportunity to develop the bonded fiber technology. My understanding is that they resigned from Tontine Insulation and either formed or joined Martini Industries to develop it. I specified their product in a small project (a 'glass box' conference room, constructed with all four walls of floor to ceiling glass and a polished concrete floor, for an advertising agency) and was one of the first acoustic consultants to provide independent acoustic performance measurements for the developers from an external trial, and verification of the model parameters for Absorb in CATT-Acoustic software.
A few years after its appearance in the market, Martini Industries was acquired by CSR Bradford Insulation. Ironically Tontine Insulation now make a copy-cat product called Acoustisorb.
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If true, it would be a ground-breaking product.
Over the decades many substances have been tried as interior applications to dampen structural resonances in speaker cabinets. The only method (till now) which seems to have a meaningful damping effect is a constrained layer damping construction.
We have not (till now) found a substance or material which can be simply applied to the interior cabinet wall surface which significantly increases damping without simultaneously decreasing structural resonance frequency.
Do you have a link ?
j.
Over the decades many substances have been tried as interior applications to dampen structural resonances in speaker cabinets. The only method (till now) which seems to have a meaningful damping effect is a constrained layer damping construction.
We have not (till now) found a substance or material which can be simply applied to the interior cabinet wall surface which significantly increases damping without simultaneously decreasing structural resonance frequency.
Do you have a link ?
j.
Martini - Acoustic Polyester - Martini Absorb
Unfortunately you'd never know from the website data about it use in this application.
Speaking about resonances between the panels of a cabinet (standing waves) you have :
- 2 "pressure knots", one on each panel.
- 1 "speed belly", at equidistance from the panles, for the main resonance.
Placing the Rock wool or Cotton wool on the panels will have less effect in damping the standing waves than if placed in the volume (center) : this kind of low density wool (circa 20kg/m3) is better for damping speed than pressure.
I read this in an old book dealing with speaker enclosures that I have, and Martin Colloms (Monitor Audio / High Performance Loudspeakers: Optimising High Fidelity Loudspeaker Systems) also recommends filling the whole volume of the cabinet with soft damping material instead of only treating the panels.
By the way, this has a sense regarding the laws of Physics of vibrations...
A+!
- 2 "pressure knots", one on each panel.
- 1 "speed belly", at equidistance from the panles, for the main resonance.
Placing the Rock wool or Cotton wool on the panels will have less effect in damping the standing waves than if placed in the volume (center) : this kind of low density wool (circa 20kg/m3) is better for damping speed than pressure.
I read this in an old book dealing with speaker enclosures that I have, and Martin Colloms (Monitor Audio / High Performance Loudspeakers: Optimising High Fidelity Loudspeaker Systems) also recommends filling the whole volume of the cabinet with soft damping material instead of only treating the panels.
By the way, this has a sense regarding the laws of Physics of vibrations...
A+!
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Yes, what you say is true. However, with vented box systems there are other considerations. Many people feel the quality of the bass response is best when the central volume is kept free from damping, and only wall-mounted damping is used.
The quality, Jim, or the quantity? My take is that some see the extra bass as hard won. There has long been DIY knowledge of box losses, Vance Dickason for one explained these.
In any case, too little stuffing in a vented box can allow modes to do their thing.
In any case, too little stuffing in a vented box can allow modes to do their thing.
By "quality" I meant subjective quality, and to answer your question, I am not sure, quantity vs quality. If reasonable variations in stuffing alter the bass response near Fb by +/- 1 dB (which I believe is possible), then I suppose the quantity of bass is tied to the perceived quality of the bass. If I recall, response variations near Fb imply a change in total Qt of the system, yes? So maybe in this context quality and quantity are closely interrelated.
My vented box designs have required experimentation to get the level of stuffing just right. My 2-way vented box systems have all been challenging to get optimally stuffed. The level of stuffing which made the midrange sound natural was too much stuffing for the bass to sound its best. In my opinion, a small vented box 2-way seems a simple design, but it takes a lot of finesse to get it right. This is an area where designers with more experience than me will excel. I have given up and gone with sealed boxes.
j.
My vented box designs have required experimentation to get the level of stuffing just right. My 2-way vented box systems have all been challenging to get optimally stuffed. The level of stuffing which made the midrange sound natural was too much stuffing for the bass to sound its best. In my opinion, a small vented box 2-way seems a simple design, but it takes a lot of finesse to get it right. This is an area where designers with more experience than me will excel. I have given up and gone with sealed boxes.
j.