How much difference do flared ports make? 
I have scoured the net for empirical data on this subject to no avail.
Finally I decided to make a project of it and do my own testing.
Using 90mm ports with a range of flare sizes, I determined the velocity at which "chuffing" occurs.
Unflared - chuffed at all useable speeds
10mm flare - chuffed at 8 metres per second
25mm flare - chuffed at 13 metres per second
35mm flare - chuffed at 17 metres per second
50mm flare - chuffed at 23 metres per second
Along the way, I had to learn different techniques for producing flares - using a router, using a heat gun over a mold, and making "one off" flares with "bondo" type automotive body filler.
Hopefully these results will be useful to the DIY community.
To put my methods under the spotlight, see the writeup at
http://www.users.bigpond.com/bcolliso/flare-testing.htm
All constructive criticism is welcome
regards
Collo
I have scoured the net for empirical data on this subject to no avail.
Finally I decided to make a project of it and do my own testing.
Using 90mm ports with a range of flare sizes, I determined the velocity at which "chuffing" occurs.
Unflared - chuffed at all useable speeds
10mm flare - chuffed at 8 metres per second
25mm flare - chuffed at 13 metres per second
35mm flare - chuffed at 17 metres per second
50mm flare - chuffed at 23 metres per second
Along the way, I had to learn different techniques for producing flares - using a router, using a heat gun over a mold, and making "one off" flares with "bondo" type automotive body filler.
Hopefully these results will be useful to the DIY community.
To put my methods under the spotlight, see the writeup at
http://www.users.bigpond.com/bcolliso/flare-testing.htm
All constructive criticism is welcome
regards
Collo
Very impressive work so far! I do have some questions though.
It looks like all your ports are the same length and do not take into consideration that the tuning frequency of the box changes slightly with the flare difference. That is, the box is being tuned at a different frequency for each flare radius. If this tuning frequency changes and you are always measuring at the same frequency (30Hz.) then the results may be skewed due to decreased/increased output from the port. Did you verify the port tuning for each of the test ports? An easy way to get close is to looks for the frequency that provides the minimum cone motion for a constant signal in. If this has been already been accounted for...then I missed it in your write-up. Keep up the good work!
GregoryD
It looks like all your ports are the same length and do not take into consideration that the tuning frequency of the box changes slightly with the flare difference. That is, the box is being tuned at a different frequency for each flare radius. If this tuning frequency changes and you are always measuring at the same frequency (30Hz.) then the results may be skewed due to decreased/increased output from the port. Did you verify the port tuning for each of the test ports? An easy way to get close is to looks for the frequency that provides the minimum cone motion for a constant signal in. If this has been already been accounted for...then I missed it in your write-up. Keep up the good work!
GregoryD
Thanks for those comments.
Near the start of the article is a link to "more on construction of test ports" which takes you to another page with pictures and a table of the physical length for each port.
After asking around on the web, I found that the generally accepted approach is to assign half the length of the flare to the port.
All the ports have an acoustic length of 300mm, so, for example, the port with a 50mm flare each end has a physical length of 350mm.
Near the start of the article is a link to "more on construction of test ports" which takes you to another page with pictures and a table of the physical length for each port.
After asking around on the web, I found that the generally accepted approach is to assign half the length of the flare to the port.
All the ports have an acoustic length of 300mm, so, for example, the port with a 50mm flare each end has a physical length of 350mm.
I would suggest you measure the output voltage of the amp at the onset of chuffing, then you could get an estimate of velocity and SPL by plugging in this number into your program.
Measuring within a foot or so of the outlet will allow you to neglect room effects for the most part and more easily compare the results. You could even record the sound for each case, notch out the fundamental and have a more objective measure of chuffing.
Measuring within a foot or so of the outlet will allow you to neglect room effects for the most part and more easily compare the results. You could even record the sound for each case, notch out the fundamental and have a more objective measure of chuffing.
Ron E said:
RonE....Thanks for that suggestion. I had considered this approach, but wasn't sure if picking the impedance from the WinISD graph and applying ohms law was valid.
I've attached the impedence graph, showing 4.099ohms @ 30hz, the test frequency.
Using a multimeter would give me the RMS voltage.
Can I simply use P=(E^2)/R to calculate the power?
The reason I conducted the testing in-room was to combine the outputs of the front mounted port with the side mounted drivers.
If I can use your method, life would be much easier. At least for the single frequency part of the tests.
As for detecting the "chuffing", it is quite easy to be consistent. As you approach the "chuff zone", the nature of the sound changes noticably when you increase the level by 1dB
Attachments
Bill, that's great work! A very useful result. It just goes to show that the standards that are used aren't very helpful - ie 17m/s preferable and 34 m/s absolute limit. In most cases, 17m/s will be inadequate, however from your results I suspect that it may be possible to get acceptable performance at 34m/s if the flare is larger, the subwoofer is far enough away with an undisturbed vent exit and the overall SPL level is sufficient to have a masking effect.
My main question is will others find that they can get away with higher vent velocities? Are your figures based on the ideal chuff free port which will not chuff unless you put your ear right up to it?
Another area of investigation would be dimpled ports B&W style. It's a pity you aren't in Melbourne as I have some dimpled vents which I haven't gotten around to testing properly. I'd like to do some similar testing and see what numbers I come up with. My flares are 18mm, 36mm and 72mm radius with a 100mm vent, although I'd probably cheat a little and use the same vent length since my vents tuned lower would get expensive with the required length of PVC!
Again, some very interesting results. It's actually surprising that more diyers don't focus more on getting vents right, rather than just using a passive radiator at extra expense. This is in the spirit of diy ie. put in time and innovation and spend more on drivers rather than throwing money at the problem. This illustrates what I have pointed out many times - vent flares make a huge difference. They are probably even worthwhile going up to 100mm flare radius.
My main question is will others find that they can get away with higher vent velocities? Are your figures based on the ideal chuff free port which will not chuff unless you put your ear right up to it?
Another area of investigation would be dimpled ports B&W style. It's a pity you aren't in Melbourne as I have some dimpled vents which I haven't gotten around to testing properly. I'd like to do some similar testing and see what numbers I come up with. My flares are 18mm, 36mm and 72mm radius with a 100mm vent, although I'd probably cheat a little and use the same vent length since my vents tuned lower would get expensive with the required length of PVC!
Again, some very interesting results. It's actually surprising that more diyers don't focus more on getting vents right, rather than just using a passive radiator at extra expense. This is in the spirit of diy ie. put in time and innovation and spend more on drivers rather than throwing money at the problem. This illustrates what I have pointed out many times - vent flares make a huge difference. They are probably even worthwhile going up to 100mm flare radius.
Thanks for those comments Paul.
re your query on an acceptable level of chuffing
The test results list the level at which chuffing was first detected. At the seating position, you could probably go an extra 3dB, but not more than that. Re-working the figures with 3dB added for the upper limit gives:
for a 90mm diameter port
10mm flare min 8m/s max 11m/s
25mm flare min 14m/s max 20m/s
35mm flare min 17m/s max 24m/s
50mm flare min 24m/s max 35m/s
So yes, you can go to 34m/sec but only with a 50mm flare or larger each end, and if you're sure you won't occasionally exceed this speed on peaks. An even larger flare would give more improvement, although it is a case of diminishing returns and fitting it in could be a challenge.
Dimples obviously make a difference. I have heard Rabbitz's sub with the B&W "flowport" and can confirm that it seems to work quite well. It would be good if you were able to do a comparison of "dimpled" vs "non-dimpled" for the same flare radius.
With a 100mm dia port, you'll need to push 25% more air than with the 90mm dia that I used. I'm sure your setup would have no worries in that department.
The "masking effect" you speak of would depend on the composition of the signal. For example a passage with most of its spectral content well above the tuning frequency would mask out some chuffing. Another passage with most of its content at the tuning frequency would not mask the "chuff". I think if you designed this way, you would get caught out.
Having only tested one diameter of ports, it becomes an educated guess for other sizes. I would say that the required flare size depends on the ratio of the flared / unflared areas. This logic would give:
for 8m/sec a 90mm port needs a 10mm flare, but a 100mm port would need an 11mm flare
for 14m/sec a 90mm port needs a 25mm flare, but a 100mm port would need a 27mm flare
for 17m/sec a 90mm port needs a 35mm flare, but a 100mm port would need a 40mm flare
for 24m/sec a 90mm port needs a 50mm flare, but a 100mm port would need a 61mm flare
If I was in the US and using a 100mm "Precision Port" (which is about 35mm flare), I would know that my sub would be noise free below about 16m/sec and I would design to not exceed about 22 m/sec
re your query on an acceptable level of chuffing
The test results list the level at which chuffing was first detected. At the seating position, you could probably go an extra 3dB, but not more than that. Re-working the figures with 3dB added for the upper limit gives:
for a 90mm diameter port
10mm flare min 8m/s max 11m/s
25mm flare min 14m/s max 20m/s
35mm flare min 17m/s max 24m/s
50mm flare min 24m/s max 35m/s
So yes, you can go to 34m/sec but only with a 50mm flare or larger each end, and if you're sure you won't occasionally exceed this speed on peaks. An even larger flare would give more improvement, although it is a case of diminishing returns and fitting it in could be a challenge.
Dimples obviously make a difference. I have heard Rabbitz's sub with the B&W "flowport" and can confirm that it seems to work quite well. It would be good if you were able to do a comparison of "dimpled" vs "non-dimpled" for the same flare radius.
With a 100mm dia port, you'll need to push 25% more air than with the 90mm dia that I used. I'm sure your setup would have no worries in that department.
The "masking effect" you speak of would depend on the composition of the signal. For example a passage with most of its spectral content well above the tuning frequency would mask out some chuffing. Another passage with most of its content at the tuning frequency would not mask the "chuff". I think if you designed this way, you would get caught out.
Having only tested one diameter of ports, it becomes an educated guess for other sizes. I would say that the required flare size depends on the ratio of the flared / unflared areas. This logic would give:
for 8m/sec a 90mm port needs a 10mm flare, but a 100mm port would need an 11mm flare
for 14m/sec a 90mm port needs a 25mm flare, but a 100mm port would need a 27mm flare
for 17m/sec a 90mm port needs a 35mm flare, but a 100mm port would need a 40mm flare
for 24m/sec a 90mm port needs a 50mm flare, but a 100mm port would need a 61mm flare
If I was in the US and using a 100mm "Precision Port" (which is about 35mm flare), I would know that my sub would be noise free below about 16m/sec and I would design to not exceed about 22 m/sec
Bill, whether or not the radius needs to be scaled up as a ratio remains to be seen. I'm not sure that you really do need to. I think it could go either way. I'm hoping that with my 4" vents with 72mm flare radius I can get away with 35m/s as in some simulations it's been difficult to get the velocity below this with my crazy drivers!
One thing I've looked into is designing a box a bit larger than normally optimal, to allow for the effect of the rumble filter, which turns the F3 into an F6! This increases the vent velocity a little.
One thing I've looked into is designing a box a bit larger than normally optimal, to allow for the effect of the rumble filter, which turns the F3 into an F6! This increases the vent velocity a little.
The only way to know for sure would be more testing.
I can't move enough air to fully test 100mm ports with large flares, but the relationship could just as easily by found using smaller ports.
I might have to consider another set of tests with 65mm pipe. Inducements anyone?
I like the sound of Ron.E's suggestion of measuring voltage across the drivers but need to look into it further.
Doing some quick measurements, I found that the SPL as measured by a sound meter did not agree with the one calculated from the voltage and resistance.
I'm assembling a query to post on the WinISD forum to find what needs to be done to use this method
ps Thanks morbo!
I can't move enough air to fully test 100mm ports with large flares, but the relationship could just as easily by found using smaller ports.
I might have to consider another set of tests with 65mm pipe. Inducements anyone?
I like the sound of Ron.E's suggestion of measuring voltage across the drivers but need to look into it further.
Doing some quick measurements, I found that the SPL as measured by a sound meter did not agree with the one calculated from the voltage and resistance.
I'm assembling a query to post on the WinISD forum to find what needs to be done to use this method
ps Thanks morbo!
IMO 30hz is a rather high frequency to be testing at...
Testing the audibility of chuffing at different frequencys of tuning is really something that needs to be done. From past experience it seems to me that the higher you tune the higher the vent velocity can be before chuffing is audible. It probably is mostly because of the masking effect that the sound output has. If you tune very low, say 10hz-20hz where the human ear cannot actually hear the fundemental chuffing is the only thing to actually hear thus being more easily detected and even more annoying
Mostly the port chuffing is caused by sharp edges. I typically do not flare my ports at all and use a simple roundover bit in the router to make the edges smooth. I always use ports with a large cross section however.
Testing the audibility of chuffing at different frequencys of tuning is really something that needs to be done. From past experience it seems to me that the higher you tune the higher the vent velocity can be before chuffing is audible. It probably is mostly because of the masking effect that the sound output has. If you tune very low, say 10hz-20hz where the human ear cannot actually hear the fundemental chuffing is the only thing to actually hear thus being more easily detected and even more annoying
Mostly the port chuffing is caused by sharp edges. I typically do not flare my ports at all and use a simple roundover bit in the router to make the edges smooth. I always use ports with a large cross section however.
For determining airflow at onset of turbulence, I feel that the frequency is arbitrary.
I chose 30hz because it gave a workable port length, for the box and drivers I had available.
As to the matter of how much "chuffing" can be tolerated at different frequencies, that is a fish of an entirely different color, which I leave to others to fight over.
I suspect that what both you, and Paul, say about turbulence being "masked" when the driver is contributing significant output, has some merit. Just one more piece of the jigsaw to be aware of....
Like you, I have gone for large ports with the ends cleaned up with a router and rollover bit.
This works fine for me, although the resulting long ports can have problems with "pipe mode" resonance. For a single 10inch driver I can squeeze a pair of 90mm ports inside the box. Go for more output and you are looking at external ports.
Personally I like 'em, but if the sub is for a friend they usually request something that doesn't look like it came out of a research facility.
I used to design for 10m/sec with a 10mm routed flare. I now have the option of choosing a higher velocity, secure in the knowledge that I can select the correct flare that will give me the same performance
I'm certainly no expert on fluid dynamics, but my take on it goes as follows:
Chuffing is the result of a column of air moving at speed, trying unsuccessfuly to integrate smoothly with the surrounding air, which is at rest.
If the speed is low, the air is able to maintain laminar flow and all is well. Above a certain speed, eddies occur, which is what we call the "chuffing". Flared ports lower the velocity at the "port / ouside world boundary", and "offer the airstream up" to a larger area of still air.
Additionally, some air follows the flare, travelling outwards from the port at right angles. When the velocity gets too high, the air tears away from the surface much like a car going into a skid. All of a sudden you have chuffing!
I chose 30hz because it gave a workable port length, for the box and drivers I had available.
As to the matter of how much "chuffing" can be tolerated at different frequencies, that is a fish of an entirely different color, which I leave to others to fight over.
I suspect that what both you, and Paul, say about turbulence being "masked" when the driver is contributing significant output, has some merit. Just one more piece of the jigsaw to be aware of....
Like you, I have gone for large ports with the ends cleaned up with a router and rollover bit.
This works fine for me, although the resulting long ports can have problems with "pipe mode" resonance. For a single 10inch driver I can squeeze a pair of 90mm ports inside the box. Go for more output and you are looking at external ports.
Personally I like 'em, but if the sub is for a friend they usually request something that doesn't look like it came out of a research facility.
I used to design for 10m/sec with a 10mm routed flare. I now have the option of choosing a higher velocity, secure in the knowledge that I can select the correct flare that will give me the same performance
I'm certainly no expert on fluid dynamics, but my take on it goes as follows:
Chuffing is the result of a column of air moving at speed, trying unsuccessfuly to integrate smoothly with the surrounding air, which is at rest.
If the speed is low, the air is able to maintain laminar flow and all is well. Above a certain speed, eddies occur, which is what we call the "chuffing". Flared ports lower the velocity at the "port / ouside world boundary", and "offer the airstream up" to a larger area of still air.
Additionally, some air follows the flare, travelling outwards from the port at right angles. When the velocity gets too high, the air tears away from the surface much like a car going into a skid. All of a sudden you have chuffing!
Collo
There is supposed to be a good study done by some Harman International guys, 1998 105th AES Convention (preprint No. 4855) and could give you some hints. You have to buy it from their site but could be very useful as you're a port nut.
They did conclude that extreme port flares showed more compression and distortion than moderate flares at higher SPL's and that assymetrical ports with a smaller radius at the inlet and a larger radius at the outlet provided the best performance..... and the crunch was that no particular profile was optimal.
Put that in your PVC pipe and smoke it
Cheers
There is supposed to be a good study done by some Harman International guys, 1998 105th AES Convention (preprint No. 4855) and could give you some hints. You have to buy it from their site but could be very useful as you're a port nut.
They did conclude that extreme port flares showed more compression and distortion than moderate flares at higher SPL's and that assymetrical ports with a smaller radius at the inlet and a larger radius at the outlet provided the best performance..... and the crunch was that no particular profile was optimal.
Put that in your PVC pipe and smoke it
Cheers
Collo, You are right about chuffing being eddies. With sharp edges the eddies are very easily formed.
Compression is one disadvantage of large flares, but how much distortion is caused by compression is what really needs to be looked at. Say the port suffers 100% compression(which it never would) it would only turn the box into a sealed box as far as SPL, which isnt that bad anyhow, but how much distortion would it induce?
I normally dont use straight or round ports. Slot type ports are much easier to "fit in the box" with extremely long lengths and are easy to make bends with. PVC can become very costly when you start putting elbows in it and use larger diameter pipes. I'm not entirely certian, but I think putting bends in the port somewhat dampenens pipe resonances.\
Using slot ports also makes "invisible" or "magic" ports, as I like to call them, highly possible and easy to make.
Compression is one disadvantage of large flares, but how much distortion is caused by compression is what really needs to be looked at. Say the port suffers 100% compression(which it never would) it would only turn the box into a sealed box as far as SPL, which isnt that bad anyhow, but how much distortion would it induce?
I normally dont use straight or round ports. Slot type ports are much easier to "fit in the box" with extremely long lengths and are easy to make bends with. PVC can become very costly when you start putting elbows in it and use larger diameter pipes. I'm not entirely certian, but I think putting bends in the port somewhat dampenens pipe resonances.\
Using slot ports also makes "invisible" or "magic" ports, as I like to call them, highly possible and easy to make.
Bill, your description of what causes chuffing pretty well describes my understanding as well. I did some quick tests some time ago with different flares, and I was surprised at what I noticed. Without a flare, the air tends to shoot out the vent further. Put your hand out 300mm beyond the flare when the air is moving at high velocity and you can feel it! Then put a 72mm flare and the change is dramatic - suddenly much quieter and the air gently spreads out and you have to almost block the vent to feel air movement - it appears as if it is moving much slower.
BA makes an interesting point about port compression. Unfortunately it means that when making a vented box where you need to rely on a larger flare, you won't reach predicted performance due to compression. This will be the case with any sub. At high output, there will be compression due to VC heating, and non linearities related to high excursion. I suppose that exposing the vent to a larger surface area adds surface friction which would lead to compression. Fortunately the ear is becoming more sensitive to bass as the SPL increases to counteract this effect to some extent.
What do you mean by "invisible" or "magic ports?"
I'm not very keen on slot ports. You can't give them any kind of decent flare, hence they require a much larger cross section to get a lower vent velocity. Being flat sided, they must have thick walls. For these two reasons, they take up much more space. So they don't actually fit inside the box easier, they in fact make the box quite a bit bigger to do the same job. Let's take an example:
100L box tuned @ 20 Hz
90mm circular vent with 50mm radius flare >>> 414mm long and 2.6L total volume
to get similar performance out of a shelf vent with say a 10mm radius flare on the edges ...
it needs to be something like 68 x 210 mm cross section and 970mm long with a net volume of 13.8L NOT including the extra volume occupied by MDF required to be added to the box which would probably take up about another 4L at least (more than the total volume of the circular vent!)
When you design a shelf vent for equal "chuffing" performance, the idea that they are easier to fit in doesn't seem logical to me. They don't "fit in" more easily, they just make your box a lot bigger! In this example, about 18L bigger! The circular vent by contrast, displaces so little volume, that in a 100L box, you could omit making the box bigger without audible consequences.
I can see how some may prefer to make PA boxes with a shelf vent, where the tuning is not very low and where chuffing is not going to be an issue. It may be easier from a construction point of view. But for a sub where vent performance is critical, the only advanage I can see is that you can avoid the awkwardness of building a vent with lots of bends, which might making bracing also difficult.
BA makes an interesting point about port compression. Unfortunately it means that when making a vented box where you need to rely on a larger flare, you won't reach predicted performance due to compression. This will be the case with any sub. At high output, there will be compression due to VC heating, and non linearities related to high excursion. I suppose that exposing the vent to a larger surface area adds surface friction which would lead to compression. Fortunately the ear is becoming more sensitive to bass as the SPL increases to counteract this effect to some extent.
What do you mean by "invisible" or "magic ports?"
I'm not very keen on slot ports. You can't give them any kind of decent flare, hence they require a much larger cross section to get a lower vent velocity. Being flat sided, they must have thick walls. For these two reasons, they take up much more space. So they don't actually fit inside the box easier, they in fact make the box quite a bit bigger to do the same job. Let's take an example:
100L box tuned @ 20 Hz
90mm circular vent with 50mm radius flare >>> 414mm long and 2.6L total volume
to get similar performance out of a shelf vent with say a 10mm radius flare on the edges ...
it needs to be something like 68 x 210 mm cross section and 970mm long with a net volume of 13.8L NOT including the extra volume occupied by MDF required to be added to the box which would probably take up about another 4L at least (more than the total volume of the circular vent!)
When you design a shelf vent for equal "chuffing" performance, the idea that they are easier to fit in doesn't seem logical to me. They don't "fit in" more easily, they just make your box a lot bigger! In this example, about 18L bigger! The circular vent by contrast, displaces so little volume, that in a 100L box, you could omit making the box bigger without audible consequences.
I can see how some may prefer to make PA boxes with a shelf vent, where the tuning is not very low and where chuffing is not going to be an issue. It may be easier from a construction point of view. But for a sub where vent performance is critical, the only advanage I can see is that you can avoid the awkwardness of building a vent with lots of bends, which might making bracing also difficult.
Paul, I can confirm that a port with a smaller flare ejects a column of air for quite a distance. With the large flare at full volume, I couldn't feel any air moving with my hand 2 feet away.
For fun, I fitted the 10mm flare and played the "Constantine" bath scene as loud as I was game. At the seating position (about 2.5m away) the air swirled around enough to be noticable. Apart from the extreme chuffing, the effect was quite atmospheric!
I don't know a lot about Port Compression, but what I suspect is that a port can only flow so much air before it becomes a bottleneck. The use of large flares moves the normal operating velocity that much closer to this limit. Once the port starts to "push back", the apparent length changes which changes the tuning frequency leading to a non-linear response.
BA, I did some experiments a while ago hoping to achieve some damping of pipe resonance. I build a 3-layer concenrtic port where the air went down the port, turned around, went back the way it came, turned around again, finally exiting at the front. I measured the output of the port at the "pipe mode" frequency. The port was then dismantled and foam damping fitted to the endcaps. Apon re-test I found that the damping made no difference.
Rabbitz.... thanks for the pointer to that article. I suspect the main reason that commercial ports have a smaller flare on the inside end is that it allows the port to be fitted after the box is built.
You could use the same size flare each end with a larger flange on the outside. This looks wrong, so they increase the outer flare for cosmetic reasons. I suspect the performance would be limited by the smaller flare.
I've added a new item to my "to do" list. Cut the test ports in half and using a sleeve, do some "mix and match" tests!
For fun, I fitted the 10mm flare and played the "Constantine" bath scene as loud as I was game. At the seating position (about 2.5m away) the air swirled around enough to be noticable. Apart from the extreme chuffing, the effect was quite atmospheric!
I don't know a lot about Port Compression, but what I suspect is that a port can only flow so much air before it becomes a bottleneck. The use of large flares moves the normal operating velocity that much closer to this limit. Once the port starts to "push back", the apparent length changes which changes the tuning frequency leading to a non-linear response.
BA, I did some experiments a while ago hoping to achieve some damping of pipe resonance. I build a 3-layer concenrtic port where the air went down the port, turned around, went back the way it came, turned around again, finally exiting at the front. I measured the output of the port at the "pipe mode" frequency. The port was then dismantled and foam damping fitted to the endcaps. Apon re-test I found that the damping made no difference.
Rabbitz.... thanks for the pointer to that article. I suspect the main reason that commercial ports have a smaller flare on the inside end is that it allows the port to be fitted after the box is built.
You could use the same size flare each end with a larger flange on the outside. This looks wrong, so they increase the outer flare for cosmetic reasons. I suspect the performance would be limited by the smaller flare.
I've added a new item to my "to do" list. Cut the test ports in half and using a sleeve, do some "mix and match" tests!
I think the inside flare is not as critical, since any chuffing sounds are fairly high frequency and must go through the vent to be heard, hence they are attenuated. If larger flares increase compression, then it would make sense to have a large outer flare, and a smaller inside one, when it comes to a compromise of port compression and reducing chuffing. Also there is less space inside, especially when you have bracing as well.
For me, a good combination seems to be 36mm internal, 72mm external, corresponding to 2 of 4 sheets of 18mm MDF.
For me, a good combination seems to be 36mm internal, 72mm external, corresponding to 2 of 4 sheets of 18mm MDF.
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