Understanding Vented Speaker Impedance

[Rick Sykora]

So circling back to the 224 Hz resonance, I realized I had addressed it in another thread, but needed to make a connection back to this thread.

Here it is: https://www.audiosciencereview.com/.../rebuild-of-purifi-spkx-speakers.35878/page-2

So have now shown resonances from drivers, vents and cabinet parts. Internal damping material can help with some of these resonances but not all. In some cases, damping is a mere bandage over the root cause of the resonance. Unless you know the root cause, you can be simply masking the real problem. If the root cause is more expensive to fix than a bandage solution, you may see some manufacturers choose the bandage instead. This is not always bad but may simply be an engineering tradeoff. These tradeoffs are made all the time in many products.

 

[Rick Sykora]

Lately, there has been some forum discussion around the impact of holes in a cabinet and their effect on the speaker. Had always planned to do this but had some more motivation to do and so here is how the impedance is affected...

The DUT is one of my Purifi 4.x speakers and they are a typical bass reflex design. The baseline measure is the green trace. The Purifi woofer is held in place by nutserts with 6 long bolts. So, removing a bolt results in a leak. The purple trace is the result of simply removing one bolt. You can see the effect is measurable but minimal. The blue trace is the result of removing 4 bolts. Now, a more significant change has occured. Notably, the box tuning has shifted from about 40 Hz to 45 Hz.

While the single leak is not likely audible, the multiple one has affected tuning. Will try to depict how audible the larger leak might be in a later post.

 

[Ze Frog]

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.

Wouldn't it just be easier to use a Dayton Audio DATS? That's been my method for years, am I missing something as to why people are using REW?

 

[IPunchCholla]

Wouldn't it just be easier to use a Dayton Audio DATS? That's been my method for years, am I missing something as to why people are using REW?

Well, I don’t have a windows box right now and I am somewhat loathe to get one with all the crap I have been reading about version 11. So I took the opportunity to learn a bit and make my own sensor and use REW.

 

[Rick Sykora]

As promised, here is the response sim for the cabinet leak depicted in post #22....

In this case, this seems likely to be an audible difference. Some might argue that the leaky case may be better. Position within the room has a major effect and so is within the range of typical vented speaker tuning tradeoffs.

 

[gnarly]

The DUT is one of my Purifi 4.x speakers and they are a typical bass reflex design. The baseline measure is the green trace. The Purifi woofer is held in place by nutserts with 6 long bolts. So, removing a bolt results in a leak. The purple trace is the result of simply removing one bolt. You can see the effect is measurable but minimal. The blue trace is the result of removing 4 bolts. Now, a more significant change has occured. Notably, the box tuning has shifted from about 40 Hz to 45 Hz.

While the single leak is not likely audible, the multiple one has affected tuning. Will try to depict how audible the larger leak might be in a later post.

Your experiment gives very similar results to what I've measured with any vented DIY I've built.
Port tuning, the impedance minima, has always shifted down in frequency a little bit as I've progressed from a leaky prototype, to finished tight construction.

I think a box tuning shift of 5 Hz is probably pretty audible. No so much because the speaker "goes 5 Hz lower", but probably more because of the gain that occurs at effected frequencies right at/below port tuning.

Given the vented/reflex has a 4th order roll-off, the gain be pretty significant...say 3-5db at any particular freq right below/at tuning.
I think your simulation above shows this nicely.....take say the 35-45Hz range for example...looks like about a 5dB gain to me..

 

[Rick Sykora]

Your experiment gives very similar results to what I've measured with any vented DIY I've built.
Port tuning, the impedance minima, has always shifted down in frequency a little bit as I've progressed from a leaky prototype, to finished tight construction.

I think a box tuning shift of 5 Hz is probably pretty audible. No so much because the speaker "goes 5 Hz lower", but probably more because of the gain that occurs at effected frequencies right at/below port tuning.

Given the vented/reflex has a 4th order roll-off, the gain be pretty significant...say 3-5db at any particular freq right below/at tuning.
I think your simulation above shows this nicely.....take say the 35-45Hz range for example...looks like about a 5dB gain to me..

Yes, should add, that the original alignment is not a typical one. This was an SPK5 that I put the crossover inside and then had to rework the port to fit. This is what made it closer to the SPK4.

More importantly, although my tests showed a shift in the box tuning due to the leaks, simply changing Ql in the sim did not cause a comparable shift. I had to add that manually.

 

[Ze Frog]

Well, I don’t have a windows box right now and I am somewhat loathe to get one with all the crap I have been reading about version 11. So I took the opportunity to learn a bit and make my own sensor and use REW.

Ah, I see, I don't blame you, not a fan of Windows myself and it seems to get worse each time.

Sounds pretty cool making your own device /sensor for non windows, could be a market for that. Should imagine that was an interesting journey.

 

[valerianf]

Thank you @Rick Sykora for providing all these useful measurements.
About the BR port tuning, what is the end configuration that you kept?
Are the 2 impedance peaks of equal high the best compromise?

 

[Rick Sykora]

Thank you @Rick Sykora for providing all these useful measurements.
About the BR port tuning, what is the end configuration that you kept?
Is the 2 impedance peaks of equal high the best compromise?

Equal is the rule of thumb case. You see the current state in post #22. The original SPK5 had a long twisty port and would not allow internal mounting of the crossover, The SPK4 calls for 9 inch port and is still too long, The posted impedance measures use a 7 inch port...

Bear in mind the internal crossover has reduced the cabinet volume somewhat and have changed out the original (very dense) damping material. I have measured it in-room and my mods address most of the complaints Amir had about the SPK5. It sounds pretty good to me but do have on stands and away from walls. Overall, I like my Directiva r1s better though.

 

[restorer-john]

Wouldn't it just be easier to use a Dayton Audio DATS? That's been my method for years, am I missing something as to why people are using REW?

Probably because it's free and people are cheap-asses.

So, you have a leak and the tuning/impedance changes. Nothing groundbreaking. Put your speaker in a cold room vs hot and then test it, you'll see a much greater change than a few screw holes. Run it hard, heat it up and then retest- see the same thing.

 

[Rick Sykora]

A simple way to see the difference air temperature affect can affect sound is an online wavelength calculator. You can set a frequency and pick speed of sound at 20 C and then redo at 40.

 

[restorer-john]

A simple way to see the difference air temperature affect can affect sound is an online wavelength calculator. You can set a frequency and pick speed of sound at 20 C and then redo at 40.

I was not really considering speed of sound, but the variations caused by the driver suspension being hot or cold and how it affect the impedance/tuning.

 

[restorer-john]

Random 6.5" wooger (free air).

23.2C degrees cold:

Same wooger after 10 seconds with a hairdryer (surround and spider):

 

[Rick Sykora]

I was not really considering speed of sound, but the variations caused by the driver suspension being hot or cold and how it affects the impedance/tuning.

Yes, the material properties in varying temps can affect performance. Reminds me to call out how impedance measures can vary based on voltage level too.

For that matter, even though I have seen some decent correlation between speaker design software and reality, am almost always tweaking bass reflex tuning after I build and test.

 

[restorer-john]

...am almost always tweaking bass reflex tuning after I build and test...

Danny from GR should be offering "port upgrades" for all those "cheesy" thin-walled plastic ports. Custom tuned, screw length adjustable for different climates/elevations with low turbulence inner and outer edges.

 

[Rick Sykora]

Danny from GR should be offering "port upgrades" for all those "cheesy" thin-walled plastic ports. Custom tuned, screw length adjustable for different climates/elevations with low turbulence inner and outer edges.

He does not do below 200 Hz you know.

I find thick cardboard tubes easiest to adjust and finish. I usually flare the exit and have not found a need to do an internal flare Although my Purifi SPKs came with some, they are an unusual size. If you follow port spacing and location rules, it helps. Not sure you can still buy, but did acquire a Precision port once. It sits on the my parts shelf collecting dust!