port resonances of mains in a 2.1 setup

[pjug]

In general, high-passing a speaker will do little to remedy port issues (whether they are caused by pipe resonances, standing waves, or something else) at frequencies above the HPF frequency, because the HPF will do nothing to reduce the amplitude of excitation frequencies.

In general, the only effective solution to such problems is acoustical, i.e. stuffing, plugging, rebuilding, etc. etc.

Yes. I am using a minidsp to HP filter the WOW1 and the resonances seen above 400Hz are still coming through same as shown in Amir's plots.

I wanted to play around and see if I could reduce these resonances. Assuming that these were port resonances, I was thinking of 3D printing some inserts to shape the port for LP acousic filtering. However, given what I am reading here it seems likely that this would be a waste of time. So now I am thinking of trying to add more stuffing inside. These speakers are heavily padded on the walls so I don't think there is much to do there. There is some room for some polyfill. Would adding some fill be expected to make any difference of do you think this is a waste of time as well?

This fiddling with the WOW1 is really just an academic exercise as far as I'm concerned. When I measure the speakers at the listening position, the cabinet resonances are pretty much lost within the room effects. I have to EQ anyway if I want a more even response so I'm not sure there is any real benefit in trying to get rid of these with more stuffing.

 

[andreasmaaan]

Would adding some fill be expected to make any difference of do you think this is a waste of time as well?

Adding (enough) polyfill would likely tame some of the peaks you're measuring there. However, it may not be possible to significantly reduce the peaks without introducing other unwanted effects. If you're going to experiment, I would suggest suspending the damping material between the walls of the enclosure rather than layering it directly over the material that's already covering the walls.

What's the length of the port?

 

[pjug]

Adding (enough) polyfill would likely tame some of the peaks you're measuring there. However, it may not be possible to significantly reduce the peaks without introducing other unwanted effects. If you're going to experiment, I would suggest suspending the damping material between the walls of the enclosure rather than layering it directly over the material that's already covering the walls.

What's the length of the port?

Thanks. It's slot ported. The speaker depth dimension is 9 inches, so the slot is roughly 8 inched depth. I am not sure I have a good idea on how to suspend the damping material, or how easy this would be compared to just pressing in some stuffing. The only access is through the cutout for the little 4 inch woofer. I do have a lot of different types of both open and closed cell foam about 1" thick. Maybe I could cut this to divide the cabinet. I guess I'd have to cutout in the center of the foam and slip it over the woofer.

 

[Koeitje]

I've plugged the ports on my M106's with the foam plug that came with the speakers and high-pass at 80hz or something. If I got time I'll see if I can measure the difference. My thought process was that I don't need the extra extension that the port provides when I high pass it anyway.

 

[pjug]

I've plugged the ports on my M106's with the foam plug that came with the speakers and high-pass at 80hz or something. If I got time I'll see if I can measure the difference. My thought process was that I don't need the extra extension that the port provides when I high pass it anyway.

Previously I tried putting some foam in the slot port of the WOW1. The result was that it muted the main port peak below 100Hz but had very little effect on the higher frequency resonances. Or (hard to remember I should save my measurements) maybe it was that it just didn't take care of the problem as well as I was hoping. I can do it again if you want to compare what happens to what you get with the M106.

 

[dasdoing]

I suspect clothing/socks is better then foam for the port

 

[Putter]

I suspect clothing/socks is better then foam for the port

What is the problem with using a rubber stopper to plug the port? I assume there is one because I've never seen it done. Offhand it would convert a woofer designed for porting (high QTS if I remember correctly which means the woofer has limited excursion among other things) to an acoustic suspension (low QTS and floppy) which it would not be designed for.

I mention this because I have a pair of Eckman 505 speakers (curbside find) that were apparently basically a store brand that were boomy disco speakers that became listenable with the rubber port plug. I've probably answered my own question, but would appreciate any additional insight.

Thanks.

 

[dasdoing]

What is the problem with using a rubber stopper to plug the port? I assume there is one because I've never seen it done. Offhand it would convert a woofer designed for porting (high QTS if I remember correctly which means the woofer has limited excursion among other things) to an acoustic suspension (low QTS and floppy) which it would not be designed for.

I mention this because I have a pair of Eckman 505 speakers (curbside find) that were apparently basically a store brand that were boomy disco speakers that became listenable with the rubber port plug. I've probably answered my own question, but would appreciate any additional insight.

Thanks.

that is probably the perfect material to block sound without risk of creating vibrations.

 

[MrPeabody]

There are various reasons in favor of plugging the port when placing a high-pass filter on a small ported speaker. There isn't much downside. As you point out, the woofer isn't optimized for use in a sealed enclosure. This would matter with a subwoofer, but not so much with a small speaker. A subwoofer driver intended for use in a ported enclosure isn't going to deliver deep bass when it isn't supplemented by the port (or passive radiator). With a small speaker it likely won't matter; the thing that is more likely to matter is that the enclosure may be larger than is to be desired unless something is done to reduce the interior volume. If nothing is done to reduce the interior volume, the interior volume will be greater than the volume at which the system Q would be .7 or thereabouts. If nothing is done to reduce the interior volume, you end up with a system Q that can be much lower than is suitable, possibly lower than .5. Not even this will necessarily be a problem, however the potential effect is that the bass rolloff, which will be unusually shallow, will start at higher frequency. You could potentially notice a slight loss of SPL in upper bass. If so, this is correctable by doing something to reduce the interior volume, but this would likely involve some experimentation. The practical solution is to block the port at its inner end, so that the volume of the port itself does not end up being added to the enclosure volume. For someone not opposed to removing the woofer so as to be able to plug the port at its inner end, this would be the way to do it.

By the way, the rule that is conventionally used to decide the best use for a given driver is based on the ratio of Fs to the electromotive portion of the damping (Qes). It is actually better to base it on Fs/Qts. This ratio practically determines the theoretical F3 for a sealed enclosure. For a driver to work well in a sealed enclosure, and produce adequately deep bass, this ratio ideally needs to be less than roughly 50. For a driver to take best advantage of the port in a ported enclosure, this ratio needs to be greater than roughly 100. Often it is said that drivers with this ratio falling in the middle, between 50 and 100, can be used for either type of enclosure. Perhaps, but this won't be an optimized design. For a sealed enclosure, you can estimate F3 by multiplying this ratio by .73 (this is a good multiplier for Qtc values in the typical range from about .65 to .9). For a small speaker that won't be expected to produce deep bass because a subwoofer is used and a high-pass filter is added to the small speaker, the F3 value isn't important, and because the F3 value isn't important, neither is it important for the Fs/Qts ratio to be optimized for sealed enclosure use. All that is going to matter in this case is for the enclosure volume to be small enough to insure that Qtc will be not less than .5. (The conventional school of thought is that Qtc should be about .7, but there are varying opinions.)

 

[Juhazi]

Why calculate/estimate, when we have cheap calibrated mics and freeware available? (Minidsp UMIK-1 and REW)

One point I want to remind of, is that a speaker's natural response and HP/LP filter deteremine the acoustic slope of crossover. We must measure both mains and subwoofer(s) (in semi-nearfield groundplane method) and find suitable delay (usually for main speakers!) to get right match! Most unsuccessful attempts are based on failing in these fundamental issues... Multisub is even more tricky to do right!

 

[dasdoing]

made a little test today, and couldn't find any obvious advantages. had some surprising variation in the midrange, but not significatly enough for me to dig deeper

 

[witwald]

Why calculate/estimate, when we have cheap calibrated mics and freeware available? (Minidsp UMIK-1 and REW)

Calculations are useful as they can often provide insights into what is going on. They are also handy for designing new systems, rather than needing many experiments to get to a workable design. Of course, measurements are also important, as they can help to verify the calculations, as the calculations often involve a number of approximations and assumptions. The real system might stray from those, so measurements can be used to fine tune the theories being used. Using both approaches, calculations and measurements, will help to achieve a higher-performance design.

 

[ernestcarl]

made a little test today, and couldn't find any obvious advantages. had some surprising variation in the midrange, but not significatly enough for me to dig deeper

Was this for rear ports? And was it zero improvements whastoever, or just not enough to justify?

 

[restorer-john]

I think the arrows labeled "port resonances" are actually the 3 fundamental internal resonances of the cabinet being "heard" through the port, not actual resonances in the port

I agree. Cabinet base-top, side to side and baffle to rear panel. It seems very little effort is made these days to damp/control those resonances and the smaller the cabinet, the further into the midrange these aberrations go.

 

[MrPeabody]

It has never been apparent to me whether the midrange response peaks coming from the port were internal cabinet resonances or "organ pipe" effects of the port itself, or something else. I don't know, but perhaps these thoughts are useful:

1. If the peaks have a strong harmonic relationship, then unless the internal dimensions of the enclosure are harmonically related (stupid design!), the peaks are more likely indicative of the organ pipe effect of the port.

2. Since the organ pipe effect is associated with the change of acoustic impedance at the ends of the pipe, which causes reflections, it follows that if the peaks in the midrange coming from the port are due to the organ pipe effect, they should be more conspicuous in pipes that end abruptly at both ends, compared to pipes that are rounded at both ends.

3. The woofer will not conduct the internal standing waves from inside to outside nearly as effectively as the port will, for the simple reason that the impedance associated with movement of the woofer diaphragm is much greater than the impedance associated with air flow through the port. For the woofer, there is in addition of the impedance of moving the air in front of the speaker, the electrical impedance and the mechanical impedance, both of which are very significant, especially the electrical impedance. Were it not for this, evidence of internal standing waves would be every bit as apparent in the woofer response as in the port response, and this would be a way to determine whether the peaks seen in the port response were internal cabinet resonances (i.e., if they are, they will be seen in the woofer response). But this test doesn't work, because the two impedances are very different. On this basis it is likely true that internal cabinet resonances are a more significant problem with ported speakers than with sealed speakers, and thus it is more important for ported speakers to have lots of internal damping vs. sealed speakers. (Personally I've always believed that the ports themselves should be lightly damped, however my experience is that if you put even a little bit of loose poly in the port it will kill the bass. I've wanted to experiment with stretching a cloth membrane over one or both ends of the port, but I've never gotten around to that.)

4. One way to investigate the question would be to construct a ported speaker with all three dimensions the same, i.e., a cube. If the internal resonance is escaping through the port, it should be easy to see this in the measurements.

 

[dasdoing]

Was this for rear ports? And was it zero improvements whastoever, or just not enough to justify?

front slot port. stuffed a towel in it. it didn't realy improve anything. no change in group delay above my crossover, and de FR had some 1dB variation around 600-800Hz. note that I meassured at LP though, since this is the only meassurement that matters to me.

EDIT: and the mirange variation could just be because I had the towel hanging out a little in front; the slot is at the bottom. also the FR abviously was atenutated below 200Hz

 

[beagleman]

https://us.kef.com/pub/media/wysiwyg/documents/ls50/ls50_white_paper.pdf

KEF talks on this subject. Not sure it helps or not