Understanding Vented Speaker Impedance

[RickS]

As some of you are aware, have been playing around with my Purifi SPK5 parts and working to determine a final configuration for them. As I have been trying various vented alignments, reminded me that I planned to share some useful observations about speaker impedance measures. I am multitasking projects but Covid just put a damper on going out in public here and thought I would start the thread.

A couple of quick caveats before I start:

1. While most of what I will present will be generally applicable, will the using the SPK parts to demonstrate some things and so may not be always be applicable to other speakers. The SPK woofer is a 4 ohm impedance, lower Q woofer and the tweeter is a Mundorf AMT with a very flat 4 ohm impedance. The crossover is pretty typical but nothing that would prevent me from using it for this thread.
2. Some of what I will show may be hard to apply without a baseline for comparison and frankly experience in speaker design. Am hoping to help members better understand some aspects of the impedances shown in Amir's reviews. That said, conditions matter. This includes the measurement voltage. software, environment and condition of the speaker being tested. In my case, my measurements are being done are at room temperature, using REW with a 100 ohm sense resistor using a Sound Blaster Audigy card to drive the speaker at 100 mV.

I have an advantage that Amir usually does not have as Purifi provides a baseline measure...

Here is my measurement in comparison...

So why do they not match exactly? As I said earlier, conditions matter. Note that I used the Purifi SPK4 as a baseline. My SPK does not have the same damping and has the crossover mounted internally rather than externally. More importantly, I can say the build is essentially valid as the key peaks and valley frequencies match. The key ones being the leftmost peak (vent related frequency), the next dip (system tuning frequency) and the next peak (driver related frequency). The other key inflections also match at 800 Hz, 2000 Hz and 8000 Hz. These indicate a good match of the tweeter and its crossover to the baseline.

Note the impedance magnitude difference is most likely due to my cabinet being a bit leakier than the reference Purifi one. I anticipate I will be able to reduce the box issues in upcoming steps. Also, as will examine later, there is a slight blip in the trace around 240 Hz. As Amir will often point out in his reviews, this indicates a (undesirable) resonance.

 

[RickS]

Due to logistical issues, am not copying @witwald dispute over the the descriptions of the peaks in the previous post. We are reviewing the descriptions offline and have enlisted some top talent to help check the accuracy. While this progresses will continue with additional measurements that are not impacted by these definitions. Stay tuned for more!

 

[IPunchCholla]

when you are working on a project like this, do you also use REW to calculate the Small-Thiele values? I found that to be kinda sketchy, (I used the two weight method), but since I had no other way of getting them (Pioneer doesn’t archive that info) I used what I got.

I guess it proved out ok in the end, as the custom crossover I built worked as it should, but I’d be grateful for any pointers you had about that process.

Following with interest as my first DIY is a ported cabinet build to rehouse my old drivers.

 

[RickS]

when you are working on a project like this, do you also use REW to calculate the Small-Thiele values? I found that to be kinda sketchy, (I used the two weight method), but since I had no other way of getting them (Pioneer doesn’t archive that info) I used what I got.

I guess it proved out ok in the end, as the custom crossover I built worked as it should, but I’d be grateful for any pointers you had about that process.

Following with interest as my first DIY is a ported cabinet build to rehouse my old drivers.

As you have found, conditions matter. While it would be advisable to verify the driver parameters, am usually modeling using manufacturer data and seeing how close the real world measurement are to the model. Typically, they are close and then can focus more on assuring that the final results for each speaker match closely.

Most of my orginal work has been with active crossovers and have more options for readily tweaking to get good matches. Passive design is not as flexible and so component precision is more key. With quality passive kits, have generally had good outcomes. As cabinets are part of the getting good consistency, careful cuts and assembly are crucial. Measuring to verify your build is essential to knowing your end result performs well. Even if you get great components, you need to validate that the build is correct and solid. Simple miswiring, bad connections or marginal cabinet seals happen even when you are meticulous.

 

[IPunchCholla]

definitely agree about remeasurement! I was super excited when the predicted model based on my measurements closely matched what I got. I honestly can’t tell that music sounds any different with my new crossovers, but my DSP gained 10dB of headroom upon re-measure!

 

[RickS]

For the simplest case, thought I would show the how changing the port length affects impedance. So here is the SPK box measured with the various port lengths so you can see how that affects the chart below:

Except for the red trace (the infamous snake), you can see the main difference between something as 8 inches down to 4 inches is mainly a change to the impedance of the first peak. The longer the port, the smaller the impedance. The snake is a special case and will deal with it later. As you may guess, the snake no longer fits inside the cabinet, so all these measures were done with the port outside the cabinet to keep a consistent internal volume. While this may be interesting from the academic perspective, note that the port normally does take up some internal cabinet volume and so does have an effect on the tuning. So, here is a look at the same set of ports when mounted internally:

Now the relationships are a bit more complex. With the change in cabinet volume affecting the impedance in more and less intuitive ways. No snake here obviously. The green is still the longest port, the blue is the next and the orange is the shortest (and slightly smaller diameter).

After doing all this I realized this could be much more meaningful if you could see the change in the bass response as the port length changes. While I could measure the response in nearfield, a sim in VituixCAD makes this much easier to illustrate. So, will show this next...

 

[Dumdum]

Why do people use abbreviations that mean something to them? SPK = speaker? Brand? Write speaker if it’s that… putting it in capitals makes it no more or less meaningful

When people use abbreviations they provide a glossary… your scribbling Irritated me sorry, I just stopped rdng

 

[fpitas]

Nice to see some DIY stuff here, especially for woofer design. A lot of people on a tight budget might be inspired to make their own subs.

 

[Scgorg]

Why do people use abbreviations that mean something to them? SPK = speaker? Brand? Write speaker if it’s that… putting it in capitals makes it no more or less meaningful

When people use abbreviations they provide a glossary… your scribbling Irritated me sorry, I just stopped rdng

SPK is simply the name of the speaker model (SPK5 to be specific). It's a reference design by purifi to show an application of their driver.

 

[Dumdum]

SPK is simply the name of the speaker model (SPK5 to be specific). It's a reference design by purifi to show an application of their driver.

For people that don’t know that it might be worth prefixing a post with it, thanks for explaining

 

[RickS]

For people that don’t know that it might be worth prefixing a post with it, thanks for explaining

Fair enough, so altered the first post to reflect the original model designation for better context. Was using SPKx as it really is no longer a 5 and as I experiment, it is even more generic. So shortened to SPK. Will see if it helps as find many simply do not read or miss content anyway.

 

[RickS]

Nice to see some DIY stuff here, especially for woofer design. A lot of people on a tight budget might be inspired to make their own subs.

While agree this content is important to DIY, my target is the larger audience that may look at Amir’s review measurements and not know what an impedance chart provides versus a single number.

 

[fpitas]

While agree this content is important to DIY, my target is the larger audience that may look at Amir’s review measurements and not know why an impedance chart helps versus a single number.

No doubt. Keep up the good work.

 

[RickS]

So as promised, here is a look at the response changes corresponding to various port lengths. As earlier when I measured the ports outside the cabinet, here am keeping the volume constant and only varying the port length. Let's start with impedance comparison...

So, as I change the length from 4 inches all the way to 16, you see the progression. The solid line is 16 inches and the one with the highest peak is 4 inches. If you compared to my earlier measurement post, you could see VituixCAD models the impedance well for the different port lengths. Let's see how each different port affects the frequency response...

The gold trace us the longest port and the light blue is the shortest. This is 4pi space response so am showing the impact of an example baffle step effect as it plays a part in picking the tuning and crossover design to flatten. Should note that the shallowest response (gold) roughly corresponds to the Purifi SPK5 and the next shallowest is the SPK4. At this point, there is no crossover included in this model and no room response compensation either. My build SPKx build thread includes the crossover and also looks more closely at my choices.

P.S.

Given the visual above, thought it might be useful to mention how the bass response might be flattened. With an active crossover, could add a shelving filter or a low Q notch to add gain to the response between 100 Hz to 1000 Hz. With a passive crossover, would have to look at where the baffle step hump is and apply filtering to reduce it. Since the passive baffle step compensation can only reduce the output, is often why you will see lower sensitivity for some speakers than you might expect given the rated driver sensitivity.

 

[RickS]

So, looking again at the different frequency responses as different port lengths are utilized. One might ask, so how does this affect what I hear?

Well, the answer is, it depends. In this case, it depends quite a bit on how close to walls they are being used. Both Amir and I tend to listen with the speaker away from walls. So, it should be little surprise that Amir felt the SPK5 bass (gold line) was lacking. On the other hand, I thought the other extreme was a bit boomy, but did not result in one-note bass based on my listening. For my SPK, am leaning towards the 6- or 8-inch port but am still working on other tradeoffs. As some may recall, most of these have some degree of port resonance to consider.

 

[fpitas]

Placement and room modes are the "elephant in the room" for subs, or any bass response.

 

[IPunchCholla]

Placement and room modes are the "elephant in the room" for subs, or any bass response.

Especially since many of us don’t live in places where the rooms could fit an elephant.

 

[fpitas]

Especially since many of us don’t live in places where the rooms could fit an elephant.

Just as well. Things would get messy very quickly.

 

[RickS]

Now that you have seen how the port length can affect the response, let's take a look at some of the popular port tweaks and how they change the impedance (and overall box tuning). First will establish a new baseline using the 6 inch port and then will put a very light amount of poly-fill inside it and see what happens:

Green is the baseline and the blue is with the poly-fil added. If you recall, the effect here is much like a vented design with a leak. Despite what seems a small tweak, the result is a significant impact to the box tuning. Let's try some other tweaks....

Here we start with the baseline in green and lined the diameter with 4mm acoustic felt for about half the length. This has a more subtle effect as shown by the red trace. It is subtle but still significant as it acts like the port has been lengthened. Looking at my earlier posts, this comparable to making the port another 2 inches longer. Not sure how it affects the port resonance but may have some advantages (as my box no longer could fit an 8 inch straight port), so will look at this later.

Next, I kept the felt in place and added a port plug that shipped with my Mirage Omni 150s. Now we transition to the purple trace. This now looks much like a (leakier) sealed cabinet. This makes sense as the intent would be to move the speaker rolloff to a more gradual sealed box response.

 

[RickS]

So, looking a bit more at lining the vent with felt, here is what I observed...

The baseline trace is red and is the 6-inch vent without lining. As expected (since is not the vent), the 240 Hz resonance is still present.

The next trace is the orange one and it corresponds to an earlier one where I lined the closer half of the vent with felt. Is not bad above 50 Hz but is ugly under 50 Hz. Perhaps the ugliness is due to the opening becoming more constricted as air moves towards the out of the vent...

Finally, the aqua trace tests my hypothesis about the constriction by moving the same felt to the inner half of the vent. Here we get a smoother curve with the predicted benefit of a bit lower box tuning (without having to lengthen the port). Perhaps yet even more benefit could be obtained by lining the whole vent? Will look at that next...

For this test, decided to try another baseline (blue trace) and used the original larger diameter pipe supplied with the SPK5. As you can see, this shifted the tuning to about 50 Hz. The green trace is a fully lined vent and looks very comparable to the better half-line one with the smaller diameter vent. Tuning is now lower (about 42 Hz) but does not produce a much more significant change otherwise.