Okay, I've finally managed to get my active studio monitor project finished (pictures to follow, before you ask SkinnyBoy 🙂 ). They've turned out pretty well - I did some A/B tests against Genelec 1029s, the JBL copies (can't remember the model) and Spirit Absolute 4Ps that we had around the office and they compared very well.
However, the only problem I've come across that I can't quite figure out the reason for is the port noise which is much worse than I expected. I've used double flanged ports specifically to try and eliminate the problem, but when I play particularly bassy tunes, the chuffing is audible. I checked that I'm not hearing things by running a sine wave through it and sure enough, below about 60Hz there is audible port noise. The port is tuned at 45Hz and the -3dB point is about 48Hz. I've put in a 2nd order high pass filter at 40Hz but this really just helps to reduce the LF cone excursion and reduce distortion a little, and not have any effect on the port.
Has anybody got any ideas at all?
However, the only problem I've come across that I can't quite figure out the reason for is the port noise which is much worse than I expected. I've used double flanged ports specifically to try and eliminate the problem, but when I play particularly bassy tunes, the chuffing is audible. I checked that I'm not hearing things by running a sine wave through it and sure enough, below about 60Hz there is audible port noise. The port is tuned at 45Hz and the -3dB point is about 48Hz. I've put in a 2nd order high pass filter at 40Hz but this really just helps to reduce the LF cone excursion and reduce distortion a little, and not have any effect on the port.
Has anybody got any ideas at all?
Have you tried lightly stuffing the port (dacron wool or real wool). This will affect your tuning thou... should lower the Q and maybe improve the bass (you are pushing the enclosure towards aperiodic).
The other thing you could try are dimples in the port exit (ala B&W). This idea originates with sharks and reduces pot noise.
dave
The other thing you could try are dimples in the port exit (ala B&W). This idea originates with sharks and reduces pot noise.
dave
Yeah, i did think about that but i haven't got any suitable fluff to hand at the moment. I'll see what I can find......
I was trying to work out where the turbulence is coming from - it sounds like it's inside the port rather than at the exit flare (kind of hard to tell though) so I guess it might be where the 2 sections of the port join. The only other obvious place is the internal flare, although I'd be a bit surprised if that was the source of the problem.
I was trying to work out where the turbulence is coming from - it sounds like it's inside the port rather than at the exit flare (kind of hard to tell though) so I guess it might be where the 2 sections of the port join. The only other obvious place is the internal flare, although I'd be a bit surprised if that was the source of the problem.
How close is the internal end of the port to a wall? A port width space is recommended minimum.
Just a thought....
Pete
Just a thought....
Pete
how about stuffing the port with plastic soda straws...Northcreek Music Systems recommends this, lowers the base a little, might stop the chuffing.
audionut said:
This is one of those tricks that makes a BR more aperiodic. I 1st saw it done in 1973, wool is easier, easier to tune and more effective, IME.
dave
Interesting question,
I think we’d need to know a little more about the box to get a real accurate answer. You didn’t mention at what levels you observed the noise. It is however interesting that the 40 Hz hp filter doesn’t seem to affect the noise.
Can you sweep the box with a sine wave and find the center of the affected band?
My guess is that 45hz is likely not the optimum frequency for the port.
I’ve wanted to build a set of active monitors for some time. I got really hooked on Genelec 1031 and Meyer HD-1’s. Can’t wait to see the details on your project. It’ll be particularly interesting to see the amps and crossovers.
I think we’d need to know a little more about the box to get a real accurate answer. You didn’t mention at what levels you observed the noise. It is however interesting that the 40 Hz hp filter doesn’t seem to affect the noise.
Can you sweep the box with a sine wave and find the center of the affected band?
My guess is that 45hz is likely not the optimum frequency for the port.
I’ve wanted to build a set of active monitors for some time. I got really hooked on Genelec 1031 and Meyer HD-1’s. Can’t wait to see the details on your project. It’ll be particularly interesting to see the amps and crossovers.
The volume of the box and the diameter of the tubes would be helpful.
Flared tubes are supposed to be have the same noise level as unflared tubes twice their area. Not twice the diameter-twice the area. So a flared 2" is approximately equal, port noisewise, to a 3" unflared. And a 3" flared is approximately equal to a 4" unflared. However, the length of the flared tubes is just a bit longer than the length of unflared tubes the same diameter.
Polk has invented the Powerport which they claim cuts down port turbulence so much that you get 3 dB extra output in the bass. I'll believe it when I see it, LOL.
I cannot vouch for it or how well it lives up to it's billing, but here is the Polk page for it:
http://www.polkaudio.com/home/technology/pwrport.php?category=3&speaker=2
Thomas W, of KloneAudio and a member here, has given us a spreadsheet on these things. I have never checked it out much, though the spreadsheet is on my hard drive. However, another member, Master P, has found the link to the spreadsheet. Here is the link to that thread that contains the spreadsheet link. The link is in Post # 22:
http://www.diyaudio.com/forums/show...2&perpage=15&highlight=PowerPort&pagenumber=2
Good luck!! 🙂
PS: I can't help but note that an extra 3 dB of bass, which is claimed, translates into an enclosure that gives the same bass but has only 75% of the volume, or an enclosure the same size whose F3 is a quarter octave lower. So a box with an F3 of 40, if fitted with these Powerports, theoretically should have an F3 of 34.6. Maybe, maybe not. I'll leave that decision to you to research.
Flared tubes are supposed to be have the same noise level as unflared tubes twice their area. Not twice the diameter-twice the area. So a flared 2" is approximately equal, port noisewise, to a 3" unflared. And a 3" flared is approximately equal to a 4" unflared. However, the length of the flared tubes is just a bit longer than the length of unflared tubes the same diameter.
Polk has invented the Powerport which they claim cuts down port turbulence so much that you get 3 dB extra output in the bass. I'll believe it when I see it, LOL.
I cannot vouch for it or how well it lives up to it's billing, but here is the Polk page for it:
http://www.polkaudio.com/home/technology/pwrport.php?category=3&speaker=2
Thomas W, of KloneAudio and a member here, has given us a spreadsheet on these things. I have never checked it out much, though the spreadsheet is on my hard drive. However, another member, Master P, has found the link to the spreadsheet. Here is the link to that thread that contains the spreadsheet link. The link is in Post # 22:
http://www.diyaudio.com/forums/show...2&perpage=15&highlight=PowerPort&pagenumber=2
Good luck!! 🙂
PS: I can't help but note that an extra 3 dB of bass, which is claimed, translates into an enclosure that gives the same bass but has only 75% of the volume, or an enclosure the same size whose F3 is a quarter octave lower. So a box with an F3 of 40, if fitted with these Powerports, theoretically should have an F3 of 34.6. Maybe, maybe not. I'll leave that decision to you to research.
If you have Excel, run the numbers through Unibox and see if you can fit a larger port in your enclosure. If not, post the woofer specs and box/port dimensions and perhaps someone can help you. A misaligned box can have port noise problems (like if you made incorrect calculations). Port velocity is highest just below tuning. You should not typically have noise above tuning unless the port is way undersized, or you have made an error in calcs.
Damping the port with stuffing or straws will reduce your bass output and should only be done as a last resort.
With a pro cabinet tuned to 45Hz, I would think you should make at least a 6" diameter port (or 2x4" ports). When trying for a lot of SPL, more port area is better. When designing, keep increasing the area as long as the length to make 45Hz fits in the box.
Damping the port with stuffing or straws will reduce your bass output and should only be done as a last resort.
With a pro cabinet tuned to 45Hz, I would think you should make at least a 6" diameter port (or 2x4" ports). When trying for a lot of SPL, more port area is better. When designing, keep increasing the area as long as the length to make 45Hz fits in the box.
Ron E said:
The F3 may well go a bit higher, but the F10 will probably be lower... with less group delay and a flatter impedance curve
dave
Thanks for the suggestions and comments folks. I'm afraid I've not got around to trying the wool method yet. Hopefully I'll get a chance tonight.
I should have given a bit more detail about the speakers, so here goes. They're Vifa P17WJ and D27TG in a 17 litre box. There's a separate cavity at the back of the box below the bracing, about 50mm deep in which the electronics goes.
The electronics consists of a linear power supply with a 24dB active crossover and 2 LM3886 amps with a balanced input. It also has an anti power-on thump circuit which is very simple but took bloomin ages to get right (and damaged a P17WJ in the process). Oh, and purple power LEDs - I'm a sucker for a gimmick 🙂 The electronics are mounted on the back of a recessed panel mount heatsink, which keeps it all looking neat and easy to get at (for me to mess with).
I've mounted the port on the front of the box, which i know isn't everybody's cup of tea, but seemed to make more sense as I'll be using them mostly as nearfield monitors. The port is a 2" diameter dual flared, which seems about right to me for a 6 1/2" driver. So, the port length is about 130mm which leaves about 80mm or so to the back of the box which I reckon should be fine. One of the side effects of having the port on the front is that I've had to move the tweeter to the side slightly (kind of proac style) and move the port the other way slightly off centre to fit them in.
I've been running some tests with a Soundweb to put some tones through and play with crossover settings. The noise only starts at higher SPL, and like i said earlier it's only really below about 60Hz or so. To be honest I'm just a little confused - I've not come across this problem to such an extent before and I'm just intrigued to know what it is.
I should have given a bit more detail about the speakers, so here goes. They're Vifa P17WJ and D27TG in a 17 litre box. There's a separate cavity at the back of the box below the bracing, about 50mm deep in which the electronics goes.
The electronics consists of a linear power supply with a 24dB active crossover and 2 LM3886 amps with a balanced input. It also has an anti power-on thump circuit which is very simple but took bloomin ages to get right (and damaged a P17WJ in the process). Oh, and purple power LEDs - I'm a sucker for a gimmick 🙂 The electronics are mounted on the back of a recessed panel mount heatsink, which keeps it all looking neat and easy to get at (for me to mess with).
I've mounted the port on the front of the box, which i know isn't everybody's cup of tea, but seemed to make more sense as I'll be using them mostly as nearfield monitors. The port is a 2" diameter dual flared, which seems about right to me for a 6 1/2" driver. So, the port length is about 130mm which leaves about 80mm or so to the back of the box which I reckon should be fine. One of the side effects of having the port on the front is that I've had to move the tweeter to the side slightly (kind of proac style) and move the port the other way slightly off centre to fit them in.
I've been running some tests with a Soundweb to put some tones through and play with crossover settings. The noise only starts at higher SPL, and like i said earlier it's only really below about 60Hz or so. To be honest I'm just a little confused - I've not come across this problem to such an extent before and I'm just intrigued to know what it is.
The port is on the smallish side for full range, but nonetheless, you may be choiced in that cabinet.
Have you checked the tuning? What does the impedance curve look like? Is the saddle where you would predict from your model alignment? Did you calculate the alignment for a QL of 7, and if so, is your cabinet indeed showing a QL of 7?
I'd make sure that the system is working the way you intended it at small signal levels before starting to do things like stuffing the cabinet or port or putting in resistance in the vent.
And one more thing- given the wavelengths involved, there's no penalty, even for near field, in putting the vent on the back of the cabinet.
Have you checked the tuning? What does the impedance curve look like? Is the saddle where you would predict from your model alignment? Did you calculate the alignment for a QL of 7, and if so, is your cabinet indeed showing a QL of 7?
I'd make sure that the system is working the way you intended it at small signal levels before starting to do things like stuffing the cabinet or port or putting in resistance in the vent.
And one more thing- given the wavelengths involved, there's no penalty, even for near field, in putting the vent on the back of the cabinet.
Ports
There is an article in the July 02 issue of AudioXpress re the superiority of flared ports especially with noise. Apparently reduces the turbulance at high power levels thus reducing noise.
I have not done tried it out myself.
Cheers
Craig Ryder
There is an article in the July 02 issue of AudioXpress re the superiority of flared ports especially with noise. Apparently reduces the turbulance at high power levels thus reducing noise.
I have not done tried it out myself.
Cheers
Craig Ryder
All I can add right now is that the box is tuned to approx 46 Hz with a 130 mm 2 inch port. The flares might add or subtract a little from that, but not to make any real difference in tuning or performance.
According to WinISD, there is a miniscule "bump" around 95Hz of 0.47 dB, which of course should be ignored. A double flared 2 inch port, (I haven't tried them), is supposed to have no more turbulence than an unflared 3 inch port, and an unflared 3 inch port is more than enough, or should be, for a 6.5" woofer.
Also, the port noise commences about half an octave above tuning frequency, where the port really starts to work. In short, the design is sound, the port should be adequate.
According to WinISD, there is a miniscule "bump" around 95Hz of 0.47 dB, which of course should be ignored. A double flared 2 inch port, (I haven't tried them), is supposed to have no more turbulence than an unflared 3 inch port, and an unflared 3 inch port is more than enough, or should be, for a 6.5" woofer.
Also, the port noise commences about half an octave above tuning frequency, where the port really starts to work. In short, the design is sound, the port should be adequate.
noise
The noise should be strongest when the mach number of the air in the port is highest, which normally is a high output levels (which you already noted) and when the port works at its maximum which is the port tuning frequency. I don't think you can do anything about these two points other than retune the port, but then it would appear again somewhere else.
Most times this results from the port internal orifice being too close to a back or side panel, or a port too long for its width, neither of which seems out of the ordinary here. What you could try is to put in a wider but shorter flared tube, that normally reduces noise. How much is hard to tell, but you could do that in the existing box probably.
Jan Didden
The noise should be strongest when the mach number of the air in the port is highest, which normally is a high output levels (which you already noted) and when the port works at its maximum which is the port tuning frequency. I don't think you can do anything about these two points other than retune the port, but then it would appear again somewhere else.
Most times this results from the port internal orifice being too close to a back or side panel, or a port too long for its width, neither of which seems out of the ordinary here. What you could try is to put in a wider but shorter flared tube, that normally reduces noise. How much is hard to tell, but you could do that in the existing box probably.
Jan Didden
kelticwizard - your words of wisdom are appreciated. i'm more reassured that i haven't overlooked something in the design now. i would have been a bit embarassed if this was a real clanger 'cos i've put quite a lot of work into this now.
i've just been testing a few things out in WinISD, just to see if the box model throws any light on it. the only thing i've really got out of it is that next time i might try putting the port on the rear, and maybe go for a 3" port. as the port surface area is more than double, the port air velocity more than halves which would at the very least reduce the problem. the thing is, a 3" port seems darned big for a 6 1/2" driver. or is it just me?
i've just been testing a few things out in WinISD, just to see if the box model throws any light on it. the only thing i've really got out of it is that next time i might try putting the port on the rear, and maybe go for a 3" port. as the port surface area is more than double, the port air velocity more than halves which would at the very least reduce the problem. the thing is, a 3" port seems darned big for a 6 1/2" driver. or is it just me?
I agree with the Wizard, the box you have seems normal, as does the port diameter for the size of the box.
Off on a tangent, I know, but have you tried damping the basket of the driver, there could be turbulence around that! Is there any loose wiring or crossover components that could be excited into vibration at that frequency?
Other than that, you could add another 2" rear firing port if you have the room in the box, but with such a small box you might find the internal volume compromised, as any port volume should be subtracted from the calculated box volume.
Off on a tangent, I know, but have you tried damping the basket of the driver, there could be turbulence around that! Is there any loose wiring or crossover components that could be excited into vibration at that frequency?
Other than that, you could add another 2" rear firing port if you have the room in the box, but with such a small box you might find the internal volume compromised, as any port volume should be subtracted from the calculated box volume.
For a quick test, you might try a pair of socks.
I’ve a friend who enjoys telling the story of having a pair of his GI socks getting sucked inside his cabinets while listening to a Queen album.
I’ve a friend who enjoys telling the story of having a pair of his GI socks getting sucked inside his cabinets while listening to a Queen album.
What is the voltage measurement where you start to get noise?
Attached is the port velocity graph for 10W RMS (12.65Volts RMS). Velocity is proportional to Voltage, so you need twice the voltage or four times the power to double velocity. Supposedly we want to keep out port velocity below 15m/s to avoid problems.
Chances are you are expecting too much and running the woofers louder than they can handle. The predicted displacement limited power handling of that box is only ~10WRMS (12.65 Volts RMS) at 65Hz, but since the suspension will not be perfectly linear, it might handle twice that power.
Any more than that and it isn't hifi, it's distortion.
P.S. I know the rated thermal power handling is 80+ Watts, but that has nothing to do with driver mechanical limits.
Attached is the port velocity graph for 10W RMS (12.65Volts RMS). Velocity is proportional to Voltage, so you need twice the voltage or four times the power to double velocity. Supposedly we want to keep out port velocity below 15m/s to avoid problems.
Chances are you are expecting too much and running the woofers louder than they can handle. The predicted displacement limited power handling of that box is only ~10WRMS (12.65 Volts RMS) at 65Hz, but since the suspension will not be perfectly linear, it might handle twice that power.
Any more than that and it isn't hifi, it's distortion.
P.S. I know the rated thermal power handling is 80+ Watts, but that has nothing to do with driver mechanical limits.
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