Best of luck with your preparations, hope Irma looses some steam before it hits the Bahamas!
I have a slightly stupid question: We built two cabs following the plans posted on the first page (Will post pictures at a later date once grills etc. are finished). When calculating the amount of cone correction needed, do we only correct for the cone volume (B&C 18SW115 in our case), or do we include the volume of the driver cutout into the baffle board as well?
To be honest, I sheepishly assumed I needed to include the baffle cutout as well, and now I am afraid we put too much cone correction volume in. If this is the case, we would have increased the compression ratio far beyond what should be healthy. Our cone correction is made of XPS foam, so we would be able to remove some of it.
Any opinions from you experts?
To be honest, I sheepishly assumed I needed to include the baffle cutout as well, and now I am afraid we put too much cone correction volume in. If this is the case, we would have increased the compression ratio far beyond what should be healthy. Our cone correction is made of XPS foam, so we would be able to remove some of it.
Any opinions from you experts?
Thanks you Xoc1 and USRFobiwan!
Seems I missed the posts on page 162 and 164 when originally reading through this thread before we started. I will remeasure the volume of XPS plates we put in and compare with what is proposed.
Another more general question: What are the actual pros and cons of an increased compression ratio? I understand a higher ratio increases the "backpressure" on the cone and thus better controls driver excursion, while of course stressing the cone more at the same time. Since the 18SW115 cone should be one of the sturdiest around and we are not even close to driving it with the 3kW+ it should be able to handle, do I understand correctly that we are "safe" with a compression ratio that is higher than the default design? Do you have an intuition what happens to the response of the whole system? I will try to simulate it in HornResp myself if I find some time, but I'm sure you have some helpful intuition.
Seems I missed the posts on page 162 and 164 when originally reading through this thread before we started. I will remeasure the volume of XPS plates we put in and compare with what is proposed.
Another more general question: What are the actual pros and cons of an increased compression ratio? I understand a higher ratio increases the "backpressure" on the cone and thus better controls driver excursion, while of course stressing the cone more at the same time. Since the 18SW115 cone should be one of the sturdiest around and we are not even close to driving it with the 3kW+ it should be able to handle, do I understand correctly that we are "safe" with a compression ratio that is higher than the default design? Do you have an intuition what happens to the response of the whole system? I will try to simulate it in HornResp myself if I find some time, but I'm sure you have some helpful intuition.
You can get close to replicating the effect by using the VTC and ATC settings to add the volume for the cone and baffle. In the offset horn geometry this obviously reduces the compression ratio at S2 but Hornresp still will give the same compression ratio when you hover over the S2 input box.
The 'cone correction' is a correction The idea is the the correction gets the box performance closer to the sim data.
The 'cone correction' is a correction The idea is the the correction gets the box performance closer to the sim data.
Hi oss1x,
Mechanical stiffness is something else than acoustical stiffness.The acoustical stiffness of building materials is indicated by the 'sound transmission class'. Mass plays an important role in that factor. For instance XPS foam manufacturers advice to use drywall on both sides to lower its 'sound transmission' property, which makes it questionable as material for cone volume correction.
Regards,
Djim
Mechanical stiffness is something else than acoustical stiffness.The acoustical stiffness of building materials is indicated by the 'sound transmission class'. Mass plays an important role in that factor. For instance XPS foam manufacturers advice to use drywall on both sides to lower its 'sound transmission' property, which makes it questionable as material for cone volume correction.
Regards,
Djim
You need to look at the xmax of the sim. The 18SW115 can do 14mm xmax in the TH18. That is 1300w / 102v at peak level for this design/driver combo. This leaves a few mm (mechanical xmax) extra for short bursts peaks in sound. You can get some more out if it if you highpass a little bit higher (around 36/hz -24db (1500w / 109v) or from 40hz.
But do you really want to push it all the way to the limit for extended time? I advice not to. Build more is my advice.
From my level of questions you can tell that I am by far not an expert on any of this, but I tend to wonder: Should it not primarily be all about reducing the volume of the horn throat, so basically any "stiff" material that is (largely) impermeable to airflow should do it? There might be some additional higher order effects (e.g. the acoustic permeability of the foam will indeed be higher than that of more plywood causing some dispersion/reflection effects) of course, for which I lack the experience to estimate.
USRFobiwan: Good point, thanks! I never even checked the simulated excursion. We are currently rarely putting in >500W/driver, but that might change soon ;-). Will be extra careful!
Hi oss1x,
The 'volume' taken by XPS is just our visual obsession for objects and soundwaves don't have eyes ;-)
XPS holds quite some air inside the cells which makes it lightweight. Sound waves usually like to move trough air so you could say they move easily trough materials that hold a lot of air. Sound waves do not 'like' materials of heavy mass and they will be reflected, which is the main property of your cone-volume-correction. XPS only needs very small amount of sound energy to make it vibrate. At mid and low frequencies it does not reflect sound waves at all.
Just remember that 'stiffness' in acoustics relates to mass and not necessary to strength. For instance drywall has a higher acoustical stiffness than your plywood sheets.
For reference;
- EPS: 15 to 40 Kg/m^3
- XPS: 35 to 40 Kg/m^3
- Plywood (Popular): 420 to 450 Kg/m^3
- Plywood (Radiata Pine): 530 to 560 Kg/m^3
- OSB (Sterling): 580 to 620 Kg/m^3
- Plywood (Birch): 675 to 725 Kg/m^3
- Drywall (gypsum): 800 to 1200 Kg/m^3
- Concrete: 1800 to 2400 Kg/m^3
In short, use concrete if you have any doubts ;-)
Regards,
Djim
The 'volume' taken by XPS is just our visual obsession for objects and soundwaves don't have eyes ;-)
XPS holds quite some air inside the cells which makes it lightweight. Sound waves usually like to move trough air so you could say they move easily trough materials that hold a lot of air. Sound waves do not 'like' materials of heavy mass and they will be reflected, which is the main property of your cone-volume-correction. XPS only needs very small amount of sound energy to make it vibrate. At mid and low frequencies it does not reflect sound waves at all.
Just remember that 'stiffness' in acoustics relates to mass and not necessary to strength. For instance drywall has a higher acoustical stiffness than your plywood sheets.
For reference;
- EPS: 15 to 40 Kg/m^3
- XPS: 35 to 40 Kg/m^3
- Plywood (Popular): 420 to 450 Kg/m^3
- Plywood (Radiata Pine): 530 to 560 Kg/m^3
- OSB (Sterling): 580 to 620 Kg/m^3
- Plywood (Birch): 675 to 725 Kg/m^3
- Drywall (gypsum): 800 to 1200 Kg/m^3
- Concrete: 1800 to 2400 Kg/m^3
In short, use concrete if you have any doubts ;-)
Regards,
Djim
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I see the point. I read somewhere (cannot seem to find it right now) that XPS was a reasonable choice.
Well taking out the XPS and putting in some more wood is not a huge deal, so we might just rework it. While at it we can do a direct XPS/nothing/wood comparison as well. I will make sure to post about the results.
Thanks for the input!
Well taking out the XPS and putting in some more wood is not a huge deal, so we might just rework it. While at it we can do a direct XPS/nothing/wood comparison as well. I will make sure to post about the results.
Thanks for the input!
It just needs to be airtight, incompressible, and mounted rigidly to a panel. If in doubt, just paint or tape it over.
I've used Styrofoam blocks to "downsize" a test enclosure for t/s param measurement (Vas, by the delta compliance method).
Hi Brian Steele,
An effective sound barrier has a high mass (weight and density) and a low resonance in order to block and reflect sound waves. The lower the mass and density the higher the resonance and the more 'leaky' the sound barrier becomes as frequency goes down, even if it is airtight ( 'Leaky' here means less reflective).
Besides that tape is not a sound barrier for low frequencies and even if we skip all involved acoustic laws, you could question if a micro watts powered 'Vas-test-box' of extremely low pressure can represent the linear behaviour of a cone-volume-correction at extremely high pressure as result of a compressed 18SW115 driver, that is powered 20.000 times more...
Regards,
Djim
An effective sound barrier has a high mass (weight and density) and a low resonance in order to block and reflect sound waves. The lower the mass and density the higher the resonance and the more 'leaky' the sound barrier becomes as frequency goes down, even if it is airtight ( 'Leaky' here means less reflective).
Besides that tape is not a sound barrier for low frequencies and even if we skip all involved acoustic laws, you could question if a micro watts powered 'Vas-test-box' of extremely low pressure can represent the linear behaviour of a cone-volume-correction at extremely high pressure as result of a compressed 18SW115 driver, that is powered 20.000 times more...
Regards,
Djim
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Thank you for this very interesting discussion!
I lack the experience and specific knowledge in this topic, but I believe just the mass (or density) of the foam by itself can hardly be of prime relevance here, since it is tightly glued to the panel as Brian said. In order to reflect a sound wave you don't need an actual high mass, but a high effective mass of "everything that is connected". Just like you don't necessarily need a wall of high mass to stop (or reflect ;-) ) a moving car, but a rigid wall that is tightly coupled to the (very high mass) earth will do fine.
For my intuition, the fitness of a given material to "eat up" enclosure volume is analogous to the ability of said material to completely block the horn path. I have no doubt that if you filled a given (narrow) section of the TH18 horn path with a 5cm thick block of XPS foam, the rest of the horn would be "dead".
At the same time a square meter sheet of similar thickness XPS in front of the horn mouth would not nearly influece the output as much, as the whole panel would act as a membrane.
So I'm still not entirely convinced that XPS does not work as a volume reduction. It's certainly not a suitable material to build a speaker.
I lack the experience and specific knowledge in this topic, but I believe just the mass (or density) of the foam by itself can hardly be of prime relevance here, since it is tightly glued to the panel as Brian said. In order to reflect a sound wave you don't need an actual high mass, but a high effective mass of "everything that is connected". Just like you don't necessarily need a wall of high mass to stop (or reflect ;-) ) a moving car, but a rigid wall that is tightly coupled to the (very high mass) earth will do fine.
For my intuition, the fitness of a given material to "eat up" enclosure volume is analogous to the ability of said material to completely block the horn path. I have no doubt that if you filled a given (narrow) section of the TH18 horn path with a 5cm thick block of XPS foam, the rest of the horn would be "dead".
At the same time a square meter sheet of similar thickness XPS in front of the horn mouth would not nearly influece the output as much, as the whole panel would act as a membrane.
So I'm still not entirely convinced that XPS does not work as a volume reduction. It's certainly not a suitable material to build a speaker.
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This has nothing to do with being a "sound barrier", so any argument based on that point is invalid. After all, how much of a "sound barrier" do you think the 1mm or less of the driver's paper cone would be, if being a "sound barrier" was what was important here? No TH subwoofer would work - all the sound would escape the box through the cone rather than follow the horn
.All that's required here is to reduce a certain volume with rigid walls to a smaller volume with rigid walls. Styrofoam or similar rigid and airtight material should work fine.
I think the 18SW115 has a lot more that a few mm to go before xmech and you crash a coil. Power requirements go up nonlinearly with excursion, compounded even further when the suspension itself begins to go non-linear. I remember somebody (Art maybe?) doing high power testing, and IIRC the suspension locks up before you hit xmech.
Especially in a tapped horn in its passband, I'd worry about toasting a coil or folding up your cone before xmech becomes an issue.
Hi Brian Steele,This has nothing to do with being a "sound barrier", so any argument based on that point is invalid. After all, how much of a "sound barrier" do you think the 1mm or less of the driver's paper cone would be, if being a "sound barrier" was what was important here? No TH subwoofer would work - all the sound would escape the box through the cone rather than follow the horn
A 'sound barrier' or 'acoustic barrier' are general terms in acoustics for any material that blocks sound waves from flowing freely.
The more than 1mm cone of an 18SW115 is reinforced with glassfibres and specifically cone shaped in order to deliver enough acoustic stiffness. The mass of reinforced paper with glassfibres per cubic meter is relatively high. I assume higher than plywood and for sure many times higher than any foam type. That’s how pro LF-drivers with extreme excursion limits are able to block sound at low frequencies under high sound pressure conditions.
But yes you are right that if you put a 'seal' on top of XPS that has all the acoustic properties needed to behave linear under all pressure differences, it should work. Wood is only much easier (and probably cheaper), I think.
Regards,
Djim