Eight inch woofer testing, report

[JonMarsh]

I am new to the forum so... Hi All!
I came here while looking for some tests of SB20PFCR30.
svp2000 says above " 3rd harmonics are just dominating" and "Not much bass in tested conditions". What are these statements based on? Show me the measurements?
DaveFred I read you eliminated SB20PFCR30-8 from your tests as "out of the league". Could you explain what you do not like about this driver? Do you agree with the statements above?
I am planning my next design to be a TL box. The box itself is planned to be based on Ekta-TL design by Troels. I am just planning to replace the drivers with some more economic ones. Naturally I will make the necessary changes in the box and XO design to accommodate for the new drivers. I just like the concept of the original box, the way the mid driver compartment is blended into TL and box design.
And frankly SB20PFCR30-8 was the first choice. This driver is very attractively priced in Europe and gets very good reviews. Most people say it has very good linearity, so where does the statement about high 3rd order harmonics come from? It is recommended as a driver with very good, low bass, and here I read "not much bass" .... hmmm.
This driver is very very very popular but it is very difficult to find any reliable information about it, especially well made measurements. DaveFred I wish you would put it into tests, show the distortions, verify T-S parameters. Maybe it is out of your league but I think it deserves a few minutes of your time because of its popularity.
Show me any other 8" driver in the $50 range that can compete with this one. I did not find any other driver suitable for a descend quality diy design (I consider only drivers with at least 90dB sensitivity). Sorry, but $200 drivers are out of my league.

Well, I really advise you to go back and read the first page of this thread more carefully... this was one of the first drivers test, posted in #15 and #16, all the data you are claiming is missing is there, including the test condition that the drivers under test are being evaluated for their suitability of operation in a 1.5 cu ft enclosure, I.E., 42.5L. This is Dave's requirement. You may have different requirements. T/S parameters cannot be ignored or wished away, though of course it is your prerogative to use whatever driver you prefer that fits your budget and preferences. This one didn't fit Dave's requirements.

Free air impedance plot



Note rapid rise of inductance- this is a good clue for inductivity modulation and higher distortion in the midrange. Note the blip at 800-900- cone or suspension resonance.

Impedance in enclosure






Distortion SPL Plot






Distortion Per Centage Plot





RAW response in enclosure

[Ally Fantastic]

Thank you for a quick reply.
I overlooked that post. The distortions are indeed large compared to other 8" drivers measured here.
Would you have a recommendation of an 8" driver suitable for a TL design with low budget? I am targeting drivers with at least 90dB sensitivity.

[JonMarsh]

That's likely to be a tough combination, because in my experience TL designs favor higher Qts and Qes than drivers that deliver 90 dB sensitivity. It's sometimes a reference that drivers with light cones and extended response only in a rather large enclosure are called Monkey Coffin drivers.

You can see design differences even within a limited range for specialty supplies like PuriFi- there 6.5" drivers will tend to work fairly well in smaller enclosures and I think several would work well in a TL, BUT, the 8" series is another animal altogether - much lighter cone than some other 8" class drivers, with similar Xmax, like the RSS210HF or RSS210H0 series. This raises the stakes considerably for enclosure size. If you're doing a three way, then distortion performance up to 300-400Hz will sometimes be adequate without shorting or faraday rings. but for a two way, like Dave's application, thats not true, so you need to pay attention to the impedance behavior over the planned operating range.

[Reet]

Don't expect much from an 8" driver in $50 range these days. SB20PFCR may suit your budget, but don't expect the world from it or any other driver in that price range. I built a TL with Scan Speak Discovery 8" and think it's a great 8" woofer, but a bit above your price range.

[Ally Fantastic]

I appreciate that SB20PFCR is hard to beat price-wise and I will not find anything that can even come close. I see from the measurements that 3rd harmonics are at 9% level, that's huge when compared to other drivers tested here. I did not expect that considering the speaker gets very positive reviews. What would be the next descent quality 8" driver, well priced, suitable for TL? As I say, I look at sensitivity of min 90dB, say 89dB as min. There is Scan Speak Discovery 18W/8424G00 which is still in my price range, though it is smaller and it has a bit high Qts and Fs. I wonder what its distortions are.
Although, I have to say that I have have doubts whether the methodology of the measurement allows direct comparison of the results.The measurements are performed at 1W/2.83V, while in my opinion they should be calibrated for the same SPL level at the listening point. 1W pumped into a speaker with 85dB sensitivity does not produce the same SPL as 1W pumped into a speaker with 91dB. What are going to be the distortions when you pump 6dB more power into the 85dB speaker? Because that what the listener hears, SPL not watts. I know, this is the way you normally measure that parameter, but can you then compare two different drivers?
The thing is that those more expensive drivers are usually low sensitivity drivers. It have personal aversion against using low sensitivity drivers. In my opinion low sensitivity is just a way of hiding the fact that you do not know how to build a good driver. Just use a resistor, pump hundreds of watts into it, it will make a sound at some point, and it will be flat over the whole frequency range....
About the Qts. I have to say that I personally did not experiment with that, I was just selecting drivers recommended in literature. And the recommendation was always low Qts, Qts=0.3 ... 0.35 as a sweet spot ("A Derivation and Analysis of the Transmission Line Speaker Enclosure" by Perry Marshall, page 4). Qts above 0.5 as being completely not suitable for any TL or BR enclosure.

Edit 13 Jan 2025:
JonMarsh I did some research on what you said about low frequency extension and selecting a driver with a higher Qts. You are correct that I should be looking for a driver with a higher Qts, something closer to 0.45-0.5. How high can I go here? The higher the better? What is the trade off?
Based on the report I mentioned above "A Derivation and Analysis of the Transmission Line Speaker Enclosure" by Perry Marshall the Qts itself does not seem to be enough. Large cone mass Mmd and large driving force BL have to go along. Which is also what you have said above.
I am normally using Hornresp to simulate TL. Hornresp does not seem to simulate this low frequency extension effect that depends on Qts, Mmd and BL. Which is something I did not know before. What software do you use?
Anyway there must be some trade-offs here, there always are. It is usually the case that deep bass, deep low frequency response does not go on a par with good impulse, a tight "punch". You trade one for the other. From the design of BR enclosures we know that one has to shrink the volume (below the optimum from F3 point of view) in order to get good impulse control, small group delay. What does it look like in the case of TL and light/heavy cone, small/large Qts? In the case of a TL enclosure, should I go for low Qts (0.35) and small cone mass if I want a tight bass, tight and warm "punch" or should I go for a bit higher Qts (0.5) and large mass?
In the case of TL enclosure, does going for large Qts (0.5) and large mass mean poor impulse? And going for low Qts (0.35) and small cone mass means tight "punch" but worse low frequency extension?

[technodanvan]

I can't comment on how Dave did his measurements, but isn't distortion typically measured at a reasonably high SPL? Like, just under clipping the microphone level? Calling JonMarsh to confirm.

Low sensitivity v High sensitivity - it's a give and take. An extreme example could be subwoofers - typically fairly low sensitivity due to mechanical compliance and moving mass necessary to control the diaphragm and achieve an appropriate resonant frequency. Extreme Xmax drivers (both woofers and subwoofers) probably require a higher moving mass - i.e. a stronger cone - to ensure it minimizes deformation under stress. Though of course exotic materials may reduce this someone, at a literal cost.

[Ally Fantastic]

Well, the measurement of SB20PFCR30-8 is a bit confusing. The graph shows 100dB SPL, but the title just above it "Distortion 2.83 volts, 315mm,"
DaveFred
Can you measure also HiVi M8N-1B 8" Damped Aluminum/Magnesium Cone Woofer, please?
And STX W.20.140.8.MC and STX W.22.200.8.MCX_v3 ?

[Reet]

Ally Fantastic 100dB at 315mm is about 90dB at 1m.

[Ally Fantastic]

JonMarsh Following the information from your post that larger Qts drivers perform better I run some simulations using Hornresp.
I selected drivers with small and large Qts, with small and large Mms. The results are in the table below.
All speakers were simulated in the same tapered TL enclosure 190cm long, cross-section 320/100 (in/out).
The simulation results are all over the place not allowing me to draw any conclusions.
The Wavecor with large Qts, large Mms and large BL according to your post should perform the best. But the results are disappointing.
I just can figure it out which T-S parameter makes Scanspeak's frequency response to go that low.
And what makes SB20PFCR30-8 to look so bad? It's T-S parameters are similar to Scanspeak's. How can speakers with similar T-S parameters perform so drastically different?

speaker | F6dB | F3dB | Sensitivity | TL length| T-S parameters
SB20PFCR30-8, 38Hz, 46Hz, 91.4dB, 190cm , small Qts, small Mms, small BL
WF223BD02, 32Hz, 39Hz, 89.6dB, 190cm , large Qts, large Mms and large BL
18WU/4741T00, 30Hz, 35Hz, 91dB, 190cm , small Qts, small Mms, small BL