Trouble summing port/woofer

[Bryan S]

Hi all, I'm new to the forum but have been using and learning REW for a few months. Lately I've been using REW to measure a prototype speaker (Radian 5210) to move the files over to Vituix for crossover work. The measurements have been going great, but when I try to sum the port with the woofer using trace arithmetic, it looks like hell, with a sharp spike null at 70hz. Looks catastrophic! The woofer is measuring just great and as expected, but the port has a double peak, which I wonder about. I'm going to post a screenshot showing the woofer, the port, and the summed pair. Can anyone help me understand why this null is showing up? FYI I dropped the port level 6.4 db to align more closely with the tail of the woofer FR. My only thought is I might need to try a different port?

 

[sam_adams]

You must measure the driver and the port separately, nearfield after Keele. The port should be measured with the mic centered on the port and flush with the baffle. The port output scaling equation can be found here. There is a scaling calculator here.

Driver nearfield:

Port nearfield:

Combined response:

Driver nearfield impulse:

Port nearfield impulse:

Combined impulse:

 

[Bryan S]

Thank you Sam, those are some very friendly looking measurements. I've measured as you suggest, and this is where I've ended up. Headed to a holiday picnic right now but I'll apply the scaling tool you've suggested and see where it leads compared to my arbitrary 6.4db reduction of the port. Not entirely arbitrary, it was based on a suggested method of aligning the tails at 20hz, so certainly not scientific.
Here's another shot including phase info, which shows an inversion right at the null, but I don't know what to do to remedy this, the port and the woofer seem to be cancelling each other out at that frequency. FYI this was a shot taken at an even greater reduction of the port just to see what happened but the result is pretty much the same just not quite as dramatic:

 

[Bryan S]

As it turns out, the calculator shows -6.4db for the port, so below are the results of the sum, and then the sum when I invert the polarity of the woofer. But I don't know how I can invert the woofer polarity in relation to the port, is there some way to do that? Makes everything smooth out like magic!

 

[sam_adams]

You are trying to align the woofer response with the port response using the alignment tool. You cannot do that. There is no way to separate the woofer response from the port response so that it may be altered in the time domain. To visualize the combined response of the woofer and the port you need only to measure each individually, compensate for the difference in SPL because of their individual size—which determines the contribution by each to the combined response—and then sum them together using the Trace Arithmetic function. If you have an .mdat of correctly executed nearfield measurements, please post it along with the diameters of the driver and the port.

 

[Bryan S]

Thanks Sam, I'll post those .mdat files shortly (pretty clumsy on this windows rig which I only bought for this purpose). I understand your point about not being able to alter the time response of the port, so that makes me wonder if the port is wrong? I was trying to push the bass response to 50hz and was initially given the port dimensions by USSpeaker, who used his in house modeler. It's a 4" port, no tube, just a hole in the cab. What I'm curious and concerned about is the second peak from the port at 150hz +/-, and if I might be able to get rid of that with a different port configuration or with more filling in the base of the cabinet? I've built only from plans before and this is a giant wide open new world, learning three new softwares and not having much experience to know what the cues are when I look at the measurements.

 

[sam_adams]

You will need a tube to put into the hole in the cabinet (Some example parts.). How long it has to be is determined by the frequency that you want to tune the cabinet to.

 

[Bryan S]

I've used a couple of calculators, here's the result of one of the calculations. It shows a pretty short port length, not far off from what I have, which is resonating at 51hz with a 1.8cm port length (the plywood baffle). For a 50hz tuning the calculator shows a pipe length of 4cm. It seems like my tuning is pretty close already, but the problem is the second peak at 150hz, I'm not sure where that's coming from, but it seems troublesome. I'm wondering if I can somehow deal with that second peak resonance (more cab filler?) how things might be more friendly?

 

[DanDan]

Best ask John PM if Trace Arithmetic is simple numerical or does it do Vector Summing..... I remember Fuzzmeasure could combine Impulse Responses....

 

[sam_adams]

Ideally, you would want the port tune to be much lower than 50 Hz. A tuning that high will lose a large amount of output below that point.

Tuning to thirty Hz gives a port length of about nine-and-one-half inches with a simple straight port with no end flares or baffles. Twenty-eight Hz gives a port length of about eleven-and-one-half inches—probably too long for a straight pipe in the box so it would need a bend in it. Twenty-five Hz would require a tube with a bend in it that would be about fifteen inches long. Tuning any lower probably isn't really feasible—given the size of the box—since the port volume is starting to take away a large percentage of the box volume

Once you have the port tube in place, you can then add the fill of your choice to the box to adjust the tuning frequency a little bit lower. Don't expect changes in the tune of more than one or two Hz lower.

 

[Bryan S]

Do you think tuning the port lower might show a smoother response, with less (or no) 150hz peak? I know it's got to change it just not sure. Also still very curious about the 70hz null happening where the port and woofer are 180 degrees apart, is there any way to deal with that? I thought it might be the resonant frequency of the woofer, but when I took measurements with the DATS, I get 79hz, though Radian states 67 (in a 25 litre vented box as opposed to my 90l box).
I'll head to the lumberyard tomorrow and pick up some 4" PVC and cut a few test lengths and measure.
Thank you Sam.

EDIT: Just wanted to add some info - the Vas for this driver is very small, it's designed for stage monitors and smaller pro audio enclosures, like 1 cu ft, so I'm really pushing things by putting it in a 3cu ft box, that's why I kept the port tuning so high, this driver doesn't go that low. I don't know if that makes a difference.

 

[Bryan S]

OK, today I went and got a piece of 4" ABS and made a few port tubes, corresponding to 34hz, 38hz, 40hz, and 42 hz. I did sweeps of all of them, with and without fill. The fill made a big difference in smoothing things out, so I used these sweeps for my modeling, since without the fill I'm not even in the game. Below there's the NF woofer with damped cabinet, the port calculated at 34hz and reduced 6.4db, and the summed pair using trace arithmetic. It looks really good to me except there's a 10db slow rolloff, which I figure I'm getting because it's all port supporting the bass below 100hz, so it's propping things up. I don't know where the phase null went, but I worked this up twice to make sure and this is the result:

 

[Bryan S]

I wonder if anyone would mind looking at the graph above (summed port+woofer) and give me some impressions? It seems most of the time when people are discussing baffle correction the sum ends up making a hump much lower down near F3 or the port tuning, but I've got a real drop, so that means that I'd have to attenuate the whole curve and try to pivot it down from 50 hz, which will kill sensitivity, am I right about this? Just basically wondering if this looks like a pass, a fail, or a maybe..............the whole reason for building these is to match my 45 SET amp, and I need to keep every decibel I can!

 

[sam_adams]

Without knowing more about the driver(s) and enclosure, it is hard to make recommendations as to how you should proceed. In any event, build recommendations are a bit off-topic for the REW support forum but you might want to start a new discussion in the DIY Speaker forum and include all the necessary info regarding the speaker design in that discussion.

 

[Bryan S]

Without knowing more about the driver(s) and enclosure, it is hard to make recommendations as to how you should proceed. In any event, build recommendations are a bit off-topic for the REW support forum but you might want to start a new discussion in the DIY Speaker forum and include all the necessary info regarding the speaker design in that discussion.

OK Sam you're right about it being off topic here, I started the thread with REW questions and it turned into a general speaker design thread. I'll take it elsewhere.