I have 1.2 cu ft ported enclosures tuned to about 34 Hz. The port in each is 3 inches diameter by 10 inches long. The internal box dimensions are 20x8x13 inches. There is 4 inches of free space between the end of the port and the interior of the front baffle (1" MDF). The box is rear ported with the port 1" below vertical center, to allow for a brace.
I measured the port output and found a substantial midrange resonance centered at approximately 540 Hz, with high Q. The peak was only about 5dB below the max port output at 40 Hz. Are these midrange resonances avoidable? Also, are there any mathmatical formulas to model this behavior?
I also measured the port output of a pair of Paradigm Studio 40 V2s that I have. These also have a midrange resonance, at around 850 Hz, but not quite as high in amplitude.
Thanks for any info.
Jim
I measured the port output and found a substantial midrange resonance centered at approximately 540 Hz, with high Q. The peak was only about 5dB below the max port output at 40 Hz. Are these midrange resonances avoidable? Also, are there any mathmatical formulas to model this behavior?
I also measured the port output of a pair of Paradigm Studio 40 V2s that I have. These also have a midrange resonance, at around 850 Hz, but not quite as high in amplitude.
Thanks for any info.
Jim
The only way to deal with such noises are:
a) Move the port opening inside the enclosure, to hopefully find a "dead node" for the frequency in question. Sometimes, this requires right-angle ports... once you do that, you have more options on "aiming" the opening.
b) Damp the heck out of the inside of the cabinet, to ABSORB the midrange "noise" before it gets to the port. This can require damping material "curtains", cabinet wall-damping materials such as Black Hole, and the like.
c) The ultimate solution? IMHO, a passive radiator. Almost impermeable at midrange frequencies...
Regards,
Gordon.
a) Move the port opening inside the enclosure, to hopefully find a "dead node" for the frequency in question. Sometimes, this requires right-angle ports... once you do that, you have more options on "aiming" the opening.
b) Damp the heck out of the inside of the cabinet, to ABSORB the midrange "noise" before it gets to the port. This can require damping material "curtains", cabinet wall-damping materials such as Black Hole, and the like.
c) The ultimate solution? IMHO, a passive radiator. Almost impermeable at midrange frequencies...
Regards,
Gordon.
wonder how much port leakage I had from this 18" reflex?
An externally hosted image should be here but it was not working when we last tested it.
I tried stuffing the port with some polyfill just as an experiment. That weakened the deeper bass only slightly, but the midrange resonance was still present. I also jammed an old cotton shirt in each port. It killed the port resonance and unfortunately the bass response too. This is a solution if I add a sub.
The addition of a right angle attachment to move the interior port entrance is a possibility, as well as improving the internal damping. I thought I read to avoid the port angle attachments, but I can't find any comments about that now.
Since the speakers are rear ported, it's not too big of an issue. Thanks for the responses thus far.
The addition of a right angle attachment to move the interior port entrance is a possibility, as well as improving the internal damping. I thought I read to avoid the port angle attachments, but I can't find any comments about that now.
Since the speakers are rear ported, it's not too big of an issue. Thanks for the responses thus far.
4fun said:
Dirty ones may have better absorbing qualities.
jimangie1973,
Do you have direct sight between cone and inner end of port tube?
If so and at low angle will be troublesome. Some bracing or simular in the direct path will damp mid leakage.
I'll see if I can add some bracing in the path. It's an MTM so the tweeter is directly in front of the port. The angle is maybe 30 to 45 degrees to the lower woofer. The upper woofer is slightly farther away, and behind bracing.
jimangie1973 said:
Most bassreflex enclosures have:
1) midrange leakage through port from back of speaker cone
2) pipe resonance in the reflex tube
These are two different problems that must be addressed in different ways. Midrange leakage is always present to some degree, but can be minimized (as have been suggested) by placing ports on rear (not front) of cabinet, stuffing more damping inside of cabinet, damping inside of port (old socks work fine
) or placing the port in less proximity (maybe angled) to the driver. Your question (as I read it) concerns pipe resonance in the reflex tube. All reflex tubes have this behaviour. The resonance frequency is (mostly) a function of pipe length and width. I do not know of any formula to excactly describe this. The longer the pipe, the lower the resonance. Short or wide pipes exhibit resonance of less intensity ("Q") compared to long or thin pipes.
Unibox (http://home20.inet.tele.dk/kou/ubmodel.html) does a good job in simulating this resonance.
Pipe resonance can be dampened by stuffing the tube (as suggested by many here), but the resonance frequency and Q of the enclosure will change somewhat.
Pit Hinder said:...Make the port 1.4 times as long. Drill half a dozen 8-10mm holes at half length. Problem gone. A bassreflex port behaves like a TL, and right where you don't need the standing wave, so kill it.
Good idea! Never tried it, but sounds like an excellent way of dampening (or killing?)pipe resonance. I'll try it in my next project.
Espen
Pit Hinder said:Jim,
easy solution, and I've been the lone preacher in the desert since times immerorable.
Make the port 1.4 times as long. Drill half a dozen 8-10mm holes at half length. Problem gone. A bassreflex port behaves like a TL, and right where you don't need the standing wave, so kill it.
Pit
That's a very interesting idea. I will try that if I can get the port out of the cabinet. Unfortunately it is glued in. I also have the issue that 1.4 times as long would be into the front baffle. I can get away with a slight rise in tuning frequency though. Thanks for the idea.
Also, thank you EspenE for the descriptions and link, very helpful.
The position of a vent in the enclosure can affect the coupling to the low frequency standing wave within the box which can actually affect the ports response at the system's resonance frequency.
One way of varying the ports position without moving it! is to have a right angle bend (usually available if it PVC pipe you are using for the port) and then try twisting the bend on the pipe within the box.
One way of varying the ports position without moving it! is to have a right angle bend (usually available if it PVC pipe you are using for the port) and then try twisting the bend on the pipe within the box.
Consort,
enclosure resonances and port resonances - that means two different kettles of fish we have to deal with.
And talking about problems - your profile says you're into 3wheelers...if you have more Moggies than garage space...would you awfully mind...not begging, mindya, just offering square meters in Germany...
Pit
enclosure resonances and port resonances - that means two different kettles of fish we have to deal with.
And talking about problems - your profile says you're into 3wheelers...if you have more Moggies than garage space...would you awfully mind...not begging, mindya, just offering square meters in Germany...
Pit
Pit
Unfortunately I do not own a Morgan 3 wheeler.
Why are they so highly rated ... because they have the desirable characteristic of power over-steer, so you go around a race track sideways......
Also they break all the engineering rules but still work.
Here's a very good modern variation on the theme:
http://news.windingroad.com/countriesmarkets/euro/more-trike-trickery-the-blackjack-zero/#more-3683
The nearest similarity (i.e. classic design now very expensive) in hi-fi terms is the Tannoy dual concentric although those do not break the rules.
Unfortunately I do not own a Morgan 3 wheeler.
Why are they so highly rated ... because they have the desirable characteristic of power over-steer, so you go around a race track sideways......
Also they break all the engineering rules but still work.
Here's a very good modern variation on the theme:
http://news.windingroad.com/countriesmarkets/euro/more-trike-trickery-the-blackjack-zero/#more-3683
The nearest similarity (i.e. classic design now very expensive) in hi-fi terms is the Tannoy dual concentric although those do not break the rules.
You can also reduce your port to a 2 in dia. You have a relatively small box for a 3" port. Like other people said, Ports can behave like TLs when they are large. Unibox is a good program that models the issues you have quite well.
You should also retry stuffing, The sock and shirt can be quick and dirty, but if they are clumped up too tight they can block all sound from your port. Follow what other people use for TL, they add wool to a density they find experimentally.
if you are worried about wind noise, make sure that your ports are flared and that you have a baffle on the internal opening of the port.
You should also retry stuffing, The sock and shirt can be quick and dirty, but if they are clumped up too tight they can block all sound from your port. Follow what other people use for TL, they add wool to a density they find experimentally.
if you are worried about wind noise, make sure that your ports are flared and that you have a baffle on the internal opening of the port.
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