Hi everyone.
As I mentioned in my introduction post, I recently built two wireless speakers. Both sound great overall, but there's one major issue: port noise, especially from the subwoofer.
I built the subwoofer using a GRS 8SW-4HE-8 8" driver. I settled on a box volume of 40L, excluding port and woofer displacement. I chose a tuning frequency of 26Hz. The transfer function magnitude and SPL curve for this particular configuration looked quite good:
For the port, I went with a 12 by 5 centimeter rectangular port. At a tuning frequency of 26Hz, this port has to be 60cm long. To accomplish this, I had to bend the port once inside of the enclosure. I also added an angle to turn the 90° bend into two 45° ones. Here's a rough sketch of how it looks like from the side:
I accounted for all wood thicknesses during construction to make sure the port length was accurate. The internal length of the port came out to be exactly 60cm, just as planned.
In WinISD, the simulated peak port velocity looked perfectly acceptable for this size and power level. At 90W input, the velocity peaks at around 18.5 m/s at 24Hz, which is already below the tuning frequency of 26Hz, so I didn’t expect any major issues with port noise.
Despite the promising results in WinISD, there are two major issues in practice:
Now my questions are:
Where did I go wrong?
Do I have to simply flare the port?
Why else is there so much port noise?
As I mentioned in my introduction post, I recently built two wireless speakers. Both sound great overall, but there's one major issue: port noise, especially from the subwoofer.
I built the subwoofer using a GRS 8SW-4HE-8 8" driver. I settled on a box volume of 40L, excluding port and woofer displacement. I chose a tuning frequency of 26Hz. The transfer function magnitude and SPL curve for this particular configuration looked quite good:
For the port, I went with a 12 by 5 centimeter rectangular port. At a tuning frequency of 26Hz, this port has to be 60cm long. To accomplish this, I had to bend the port once inside of the enclosure. I also added an angle to turn the 90° bend into two 45° ones. Here's a rough sketch of how it looks like from the side:
I accounted for all wood thicknesses during construction to make sure the port length was accurate. The internal length of the port came out to be exactly 60cm, just as planned.
In WinISD, the simulated peak port velocity looked perfectly acceptable for this size and power level. At 90W input, the velocity peaks at around 18.5 m/s at 24Hz, which is already below the tuning frequency of 26Hz, so I didn’t expect any major issues with port noise.
Despite the promising results in WinISD, there are two major issues in practice:
- It doesn't seem to produce much sound at 26Hz. Audible output really starts around 32–34Hz when sitting directly in front of it, not the 26Hz I was aiming for.
- Port chuffing is excessive. From ~25Hz to ~40Hz, the port noise becomes very noticeable. At times, it seems louder than the speaker itself.
I’ve attached a video showing the port noise when playing a 28Hz sine wave for reference.
Now my questions are:
Where did I go wrong?
Do I have to simply flare the port?
Why else is there so much port noise?
The response at Fb should be at least -3dB from the 32Hz response.
You didn't mention if you had tested the Fb frequency is correct.
The cone looked to be moving quite a bit, it should move the least at Fb.
Because of the way our hearing works at low frequencies, as little as 4dB difference may seem like only about half as loud ~25Hz.
Port noise is high frequency, 40dB (around the noise level from a refrigerator) at 1kHz would sound as loud as 100dB at ~30Hz.
You may not have. Room modes can have a huge effect on SPL at any particular position.
Amp clipping or driver distortion may also be responsible for some of the noise, impossible to tell their contribution from the video.
Rounding the port's exit would reduce noise.
Single tone sine wave testing is the worst case for port noise.
Most musical low frequency is impulsive, and it's high frequency content masks a little wind noise.
That said, the port looks a bit on the small side for an 8" driver with 10.5mm Xmax.
Did you look at excursion in your simulation?
Art
I think the port is too small in terms of area, I bet if you were to measure output of the driver and port separately the port level would be significantly lower than the driver.
Although.. it might just be that the port is too narrow(slot like) and if it was more square turbulance would be lower.
Although.. it might just be that the port is too narrow(slot like) and if it was more square turbulance would be lower.
Thanks a lot for both of your responses!
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I didn’t think the port was overly small. I chose the largest port I could fit with only one bend, since I've read that multiple bends increase turbulence. Simulated air velocity also looked acceptable. Is there something I might have overlooked?
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I'm not entirely sure how to accurately test the Fb frequency, but I'll try playing different sine waves and focus on identifying the point where the cone moves the least.
Interesting. I'll try placing the sub closer to a corner or moving it to a different room to see if the response improves. The room I tested it in might be negatively affecting low-frequency performance, as another sub I tested in this room behaved strangely as well.
I don’t think it’s amp clipping or driver distortion in this case. I did try turning the volume higher, and while distortion did appear, it sounded very different from the chuffing I’m hearing at moderate levels.
I’ll give that a try by sanding down the port flares a bit. Should I do this on both the internal and external ends?
I've noticed this as well, the chuffing is less noticeable during music playback, but it’s still clearly audible in songs with lower bass notes.
Yes, I did take excursion into account. At 90W input, the driver doesn’t exceed its Xmax in WinISD:
I didn’t think the port was overly small. I chose the largest port I could fit with only one bend, since I've read that multiple bends increase turbulence. Simulated air velocity also looked acceptable. Is there something I might have overlooked?
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I'm not sure how I’d measure port output separately. As I mentioned above, the simulated air velocity didn’t seem too high, but maybe there's more to it.
Regarding the shape, I did some research before designing it and saw that a maximum ratio of about 1:5 is generally recommended. My port is 12 by 5 cm, giving a ratio of about 1:2.4, which should be well within that range. I've even seen builds with narrower ports.
From what I can tell, the lower the frequency I play, the more the cone moves, which seems different from what I expected.
I opened up WinISD and looked at the frequency where the cone should move the least (Fb), and compared it to where the cone should move the most.
- At 35Hz, the cone moved noticeably, but not excessively. I could also clearly hear the bass.
- At the tuned Fb of 26Hz, the cone moved much more than at 35Hz, and the audible output was significantly lower.
So now I’m not really sure what’s going on here…
The cone excursion should be minimum at the tuning frequency, and it should be fairly obvious by eye. If it's moving more as you lower the frequency with constant drive voltage, that indicates you are already below Fb. Perhaps the box is tuned higher than expected. Have you checked for air leaks? And the obvious, no bass boost in the amp or other processing affecting the signal during testing?
Your vent area seems reasonable at 9.3 in^2 , a bit better than 3" pipe which would be 7 in^2. I have a Dayton RSS 8" sub tuned Fb 25 Hz using 3" pipe vent, and it will chuff with 70W sinewaves but not noticeably with music.
Your vent area seems reasonable at 9.3 in^2 , a bit better than 3" pipe which would be 7 in^2. I have a Dayton RSS 8" sub tuned Fb 25 Hz using 3" pipe vent, and it will chuff with 70W sinewaves but not noticeably with music.
That would indicate the actual Fb (box tuning) is closer to 35Hz than 26Hz, more than 1/3 octave higher than you planned.
The excursion (and impedance) minima is at Fb.
Your port would need to be longer to drop the tuning frequency to your prediction.
Measuring the excursion curve accurately should give you an idea of the ratio the sim is off by.
At Fb, the output of the port lags the driver output by 90 degrees.
Below Fb, response drops at ~24dB per octave as the port output becomes 180 degrees out of phase with the driver.
The port output is basically useless and destructive less than 1/3 octave below Fb, and the excursion maxima is ~1/3 octave above Fb.
Putting a light colored dot near the cone surround makes it easier to see movement, I usually use a silver Sharpie marker pen. Look across the cone at an angle where you can just see the dot, holding a ruler near the dot.
Xmax is one way excursion, if you see 21mm peak to peak travel, your driver has reached it, which should take ~20 volts.
You don't need to test using that much voltage, around 10 volts should hit ~10mm peak to peak, should be easy to see the difference between 10mm and 4mm at Fb.
Or you can use this technique taping a paper ruler wedge with 2/1 increments marked on it:
If you don't have a sweepable sine wave generator, note the peak to peak excursion in 1Hz increments either side of Fb.
Sine waves contain ~eight times the average power of music reaching the same peak voltage, so keep the duty cycle low, a few seconds to measure, 10 seconds to cool while you chart the excursion.
Art
There should not be any air leaks. I’m pretty sure I’d hear them. There actually was a leak at the beginning that made a very distinctive noise, but I’ve since patched it up. As for signal processing, the only filtering happening is from the ADAU1452 DSP, which is cutting everything below ~22Hz and above ~90Hz.
Okay, that’s actually good to hear. It sounds like the amount of port noise I’m experiencing is normal for this port size.
That’s a good point. After reading these two threads:
https://www.diymobileaudio.com/threads/winisd-and-slot-ports.156355/
https://www.diyaudio.com/community/threads/first-diy-ported-sub-need-help-with-port.400541/
…I actually started wondering if my tuning frequency was too low, not too high, especially since WinISD doesn't seem to apply the correct end correction factor for slot ports. This can apparently lead to slot ports being longer than they should be. But that wouldn't quite be in line with the excursion values I saw today.
I'll have to accurately measure the excursion to check the actual tuning frequency of my box.
40 L is pretty big for an 8" driver. PE recommends 1 cu.ft. (28 L) tuned to 27 Hz. Me? If I had access to DSP I'd probably opt for a sealed design and use the DSP to extend the LF response, and / or opt for passively-assisted sealed design (think big capacitor added in series with the driver).
It's important to check to see if your build matches your sim, if you're noticing things that you didn't expect to notice. I usually start my troubleshooting by taking an impedance curve, as that can disclose a LOT of things (e.g. actual Fb, if the box is lossy, etc.). DATS is a good tool to do this, but if you want to spare some $$ you can build your own impedance jig to use with REW. Plans to do so are available via Google Search.
It's important to check to see if your build matches your sim, if you're noticing things that you didn't expect to notice. I usually start my troubleshooting by taking an impedance curve, as that can disclose a LOT of things (e.g. actual Fb, if the box is lossy, etc.). DATS is a good tool to do this, but if you want to spare some $$ you can build your own impedance jig to use with REW. Plans to do so are available via Google Search.
Just to clarify - Is that 90W into 8 ohms (Eg = 26.83 volts), 90W into 4 ohms (Eg = 18.97 volts) or a constant 90W at all frequencies?
This is worth checking. I had to shorten the vent in mine several inches vs the model in order to get the correct Fb, confirmed via impedance sweep.
You just close mic(couple centimeters away) the driver and port separately. This assumes you have a test mic and software like REW.. which is free. That will produce graphs that may look something like this where the contributions of each are clearly visible.
That seems an entirely reasonable tuning to have started with. The driver has a relatively high Qts value, so requires a Vb that is greater than Vas for a fairly traditional alignment. Your transfer functions seem entirely reasonable.
I went and set up a model of the driver in the enclosure with the port that you specified. The port length suggested by VituixCAD matches up quite well with your calculations.
Oh dear! It seems that the right-angle bend is definitely not ideal for this application. Air flowing backwards and forwards around a 90° corner seems likely to have a propensity to generate lots of turbulence via separation vortices, which will lead to considerable amounts of flow-induced noise.
By the way, your port is has an equivalent diameter of only 8.7cm, which is not particularly large for this particular low-frequency application. That's why large enclosures with large drivers and a number of large ports are typically used when tring to reproduce ferquencies in the 20–30Hz range. There is nothing that can be done about that, apart from switching to an equalised closed-box enclosure and sacrificing maximum low-frequency output level for a given driver displacement.
The VituixCAD simulations, performed at a nominal 50W re 8 ohms of input power, are shown below. The vent velocity peaks at 23.4Hz, and is at the suggested "turbulence warning" limit. However, with its 90° turn, your port is far from ideal, and so the port chuffing limit is going to be much, much lower.
It might be useful for your to try to do some measurements using a frequency sweep to see what's happening.
That 90° sharp right-angle "bend" (in name only) is rearing its ugly head. This issue needs to be addressed in a major way, otherwise it's unlikely that progress will happen on the turbulence noise front.
Flaring the port probably won't help that much, as the source of the turbulence-induced noise is likely to be elsewhere (as mentioned above).
Nice of you to model the enclosure, what I was thinking of doing.
End correction is set at .73 which assumes no boundaries or shared walls.
The port he is using shares 2 walls and likely the entrance is close and tight to another wall.
The correction factor would be above the decimal point.
End correction would be 1.72 and could be as high as 1.85 in real life.
Likely realistic end factor would be 1.8 and we can see the alignment then.
port efficiency or Qp of about 30 looks about right.
Leakage or Ql be about 7 and no lining for Qa of 100
You should definitely round over
- port entry (inside)
- port exit (outside)
- the 90 degree edge at the port bend (if possible)
Try to avoid any sharp edges anywhere near the fast moving air.
I made a slow motion video to show port air movement with different port edges here, just in case this may be interesting for you: https://www.diyaudio.com/community/...rbers-and-port-geometries.388264/post-7594556
(See second part of video)
... and my port geometry optimizer in the same thread, just if you plan to make a new port alltogether or want to verify port opening surface.
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The frequency of the tuning will drop by about 2–3Hz when setting the modelled length to be equal to the physical length. This doesn't produce a large change in the response, as expected.
Indeed.
So with sinewave testing sounds like he is right at the threshold of chuffing at about 80 to 90 watts.
With normal music likely less audible.
No big deal, the difference between 1.8 end factor or .8 end factor would remove 10 cm of length
The port could be slightly bigger and tuning would still be fine from 24 to 26 Hz.
Would knock down that chuff threshold a little with more port area, same length even shorter.
Run against a back wall that is longer, so no L shape or bend to the port, just a straight shot.
Been awhile, guess I remember now the love hate with slot ports, easy to tune low.
But they suck at loud playback levels.
Some of my slot port subs would make noise watching movies with big transients of sound effects.
I got away with small ports for easy tuning using flared round ports, nothing special just flared at one end.
Goes either way, a long tube can rattle a bit without a little bracing if really long too.
Frustrating, just a excuse to build more boxes anyways. The trick was learning mistakes
so if building for friends or charging money. You dont make mistakes with a customer or friends
spending their money.
Think you will be ok with normal music playback, just add a second sub.
I was really happy when I moved to 12" subs and 2 of them.
I played around with no smaller than 10" and never really cared much for them.
Of course didnt really understand Qts much. Most of them were typical .5 Qts 10"
Once you find a .3 Qts sub in 12" the box isnt much bigger.
Maybe ported for a large room, in a car sealed subs really hit clean and fast with normal EQ
So with sinewave testing sounds like he is right at the threshold of chuffing at about 80 to 90 watts.
With normal music likely less audible.
No big deal, the difference between 1.8 end factor or .8 end factor would remove 10 cm of length
The port could be slightly bigger and tuning would still be fine from 24 to 26 Hz.
Would knock down that chuff threshold a little with more port area, same length even shorter.
Run against a back wall that is longer, so no L shape or bend to the port, just a straight shot.
Been awhile, guess I remember now the love hate with slot ports, easy to tune low.
But they suck at loud playback levels.
Some of my slot port subs would make noise watching movies with big transients of sound effects.
I got away with small ports for easy tuning using flared round ports, nothing special just flared at one end.
Goes either way, a long tube can rattle a bit without a little bracing if really long too.
Frustrating, just a excuse to build more boxes anyways. The trick was learning mistakes
so if building for friends or charging money. You dont make mistakes with a customer or friends
spending their money.
Think you will be ok with normal music playback, just add a second sub.
I was really happy when I moved to 12" subs and 2 of them.
I played around with no smaller than 10" and never really cared much for them.
Of course didnt really understand Qts much. Most of them were typical .5 Qts 10"
Once you find a .3 Qts sub in 12" the box isnt much bigger.
Maybe ported for a large room, in a car sealed subs really hit clean and fast with normal EQ
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Heya.
Sorry for late response. I think that other members did great job by helping you investigate.
Currently, I am juggling few 8" ported projects, and have practical experience with exactly this. An 8" ported design with rectangular port bent at the back of the enclosure.
Generally, I can spew about four rules of thumb:
1) Ordinary 8" will not be happy playing under 30Hz. If the room mode saves you, it can, but the outcome is often not there, and I burned myself with this one few times despite the simulation saying it will play there. After mesuring outside with a microphone, it does NOT. It can, it perfectly is possible, but with the troubleshooting approach where you have to fish for issues, I would be less trusting, and I´d say it wont output much. Depending on the room, you might have constructive and destructive interferences of the sound, and while the subwoofer might output something where you want it, the room will eat it and you will hear either less, or nasty boomy modes. So, this kind of build evaluation might be kind of faulty in your approach.
2) WinISD usually overestimates slot port length, because it does not count much with the air drag inside and the bend losses. If you went with WinISD, you are 20-30% longer than you should, probably hanging near 20Hz as of now. I routinely build 25-33% shorter slot ports with WinISD sims in order to get within 1% guess.
3) Slot ports with almost no treatment do get noisy. This was "an oversight". You clearly need to have the inner corner VERY round as well, meaning 3cm radius as an absolute minimum for it to start working. I do it by grinding these port panels to the bone, almost leaving gap in the corner between these two panels, and filling the thin corner from the inside. That is what barely starts to work. The outer corner indeed can be rather straight 45 degrees. Also both port openings deserve 30% more area with another angled piece. Yes it makes the work much more tedious and difficult, but that is what needs to be done.
And 4) - The flow does not tell you anything much at all about noise SPL of that flow. It counts on fair use, psychoacoustics and acceptable tolerance. I.E. Normal program content and sometimes closing one if not both eyes, here ear/s. If you want real hi-fi experience, you need to have very different metrics for such qualities.
I have been there many times, I feel your pain, but this to me looks like "DOH". You messed up by following these generic rules, and today you matured by learning these are not dogmas. These are starting point recommendations, that unfortunately failed you. Do not despair, "Today/that day", a knowledgeable skilled hi-fi box builder was born. Congratulations. Next time, you´ll know your schtick.
Sorry for late response. I think that other members did great job by helping you investigate.
Currently, I am juggling few 8" ported projects, and have practical experience with exactly this. An 8" ported design with rectangular port bent at the back of the enclosure.
Generally, I can spew about four rules of thumb:
1) Ordinary 8" will not be happy playing under 30Hz. If the room mode saves you, it can, but the outcome is often not there, and I burned myself with this one few times despite the simulation saying it will play there. After mesuring outside with a microphone, it does NOT. It can, it perfectly is possible, but with the troubleshooting approach where you have to fish for issues, I would be less trusting, and I´d say it wont output much. Depending on the room, you might have constructive and destructive interferences of the sound, and while the subwoofer might output something where you want it, the room will eat it and you will hear either less, or nasty boomy modes. So, this kind of build evaluation might be kind of faulty in your approach.
2) WinISD usually overestimates slot port length, because it does not count much with the air drag inside and the bend losses. If you went with WinISD, you are 20-30% longer than you should, probably hanging near 20Hz as of now. I routinely build 25-33% shorter slot ports with WinISD sims in order to get within 1% guess.
3) Slot ports with almost no treatment do get noisy. This was "an oversight". You clearly need to have the inner corner VERY round as well, meaning 3cm radius as an absolute minimum for it to start working. I do it by grinding these port panels to the bone, almost leaving gap in the corner between these two panels, and filling the thin corner from the inside. That is what barely starts to work. The outer corner indeed can be rather straight 45 degrees. Also both port openings deserve 30% more area with another angled piece. Yes it makes the work much more tedious and difficult, but that is what needs to be done.
And 4) - The flow does not tell you anything much at all about noise SPL of that flow. It counts on fair use, psychoacoustics and acceptable tolerance. I.E. Normal program content and sometimes closing one if not both eyes, here ear/s. If you want real hi-fi experience, you need to have very different metrics for such qualities.
I have been there many times, I feel your pain, but this to me looks like "DOH". You messed up by following these generic rules, and today you matured by learning these are not dogmas. These are starting point recommendations, that unfortunately failed you. Do not despair, "Today/that day", a knowledgeable skilled hi-fi box builder was born. Congratulations. Next time, you´ll know your schtick.
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First off, I just want to thank everyone for being really helpful so far!
Several of you suggested doing a frequency sweep while observing cone excursion, or measuring the impedance curve using REW. I haven’t gotten around to doing those two things yet, but I plan to when I have some more spare time.
Several of you suggested doing a frequency sweep while observing cone excursion, or measuring the impedance curve using REW. I haven’t gotten around to doing those two things yet, but I plan to when I have some more spare time.
It should be 90W into 4Ω across the frequency range. I’m using a TPA3221 amp board in PBTL mode, which delivers around 95W into 4Ω at 1% THD.
That’s a good point, it does seem like the bend might be adding to the port noise. Just to clarify, though, it’s not a sharp 90° bend. It’s actually made up of two 45° angles. Here’s a sketch showing how it looks when viewed from the side:
This sub is actually part of a portable speaker project, so I went with a ported design to get as much low-end output as possible in a somewhat compact form.
Interesting. I was actually a bit skeptical myself when I saw WinISD predicting that kind of low end extension from an 8", but I figured I’d try it anyway and see how it performed.
I've seen this quite a few times now, and I think you are right. My current tuning frequency probably isn’t ideal for this driver. Unfortunately, the enclosure is mostly glued together already. Only the front baffle is removable (it's sealed with foam rubber), so modifying the port would probably require rebuilding the whole box.
Yeah, I can definitely see that now. The inner corner wasnt really rounded, except for the 45 degree piece as shown in my sketch above. Angling the outer edge of the port is still doable though, so I might give that a try to help reduce the turbulence a bit.
Thank you for the kind words! Lessons learned I guess...