Break-up in woofers

[Digital_Thor]

Hi happy DIY'ers
I've been building and experimenting for years - only with a fully active setup. I start to become very drawn to hard domes. First really good speaker that I had, was with a Ceramic Seas, Accuton midrange and SS 18W + 12" Alu subwoofers. I really enjoy the way that hard pistonic drivers can deliver fine detail an stay so very clean - when filtered and used well.
Now I build with the DXT - only waveguide I can find with hard dome and sensible price. Also there are many good design with this tweeter, that I can follow.
Now when it comes to woofers. I need something to fill in between multiple subs and my midrange.... from around 500hz down to 60-70hz - roling off slowly for better integration.
Now using two SBNRX23 a side - had 4 for cheap. They need a rather big box - maybe.... do not know how far down I can push them without loosing SQ.
I like that mid-woofer "smack" - which I think might be easier to get from a driver having a more pistonic behavior - mostly hard cones.
Looked at Dayton Audio RA225 - cause they seem to be able to play in smaller volumes and have an alu cone.
The SB plays well - but as far as I can see, the SB alu requires as big a box as the NRX.
I want a closed box and is purely thinking about whether a potential upgrade is worth it, since I'm not really getting close to the break-up region of the drivers.

 

[KSTR]

I want a closed box and is purely thinking about whether a potential upgrade is worth it, since I'm not really getting close to the break-up region of the drivers.

A good rule of thumb would be any sigificant break-up should start not lower than at 4x the crossover frequency. We don't want the motor distortion (2nd and 3d) being amplified by the effective gain of the woofer at those frequencies. Assume you have +15dB resonance at 1.5kHz and you play 500Hz, then the 3rd harmonic gets a free boost of 15dB, baam!

Personally, I would use a good 10" or 12" PA midbass papercone for that kind of duty. The frequency range of the driver is were the meat is in the music, so its contribution to the total is crucial. T/S-parameters are not that critical, don't be shy to apply EQ to tailor the thing to the target reponse. A series air-coil inductor can help in the stop-band and lowers amp noise (which also gets the boost from the woofer peaks).

 

[KSTR]

As for enclosure size and the EQ thing, select the enclosure size (and the woofer) to land at about a Qtc of 0.6, as long as the resonance frequency is in the vicinity of the XO and, preferably, lower. This make the recovery of the woofer from its own errors as fast and clean as possible (well-damped Bessel 2nd order lowpass function of the cone excursion).
Then apply any corrective EQ/filter to fit the response to whatever acoustic target, which would be most likely >= 2nd order. Single digit dBs of correction span are perfectly OK, assuming the amp has enough headroom for any significant boosts.

 

[Digital_Thor]

@KSTR Thank you
Yes - I'm very aware of the importance of mid-woofer duty. Midrange and tweeter is rather easy to measure and EQ with gated measurements. It's when we go below 5-600hz, that it starts to be tricky. Further - I see your point with one good 10" or 12". Maybe two 8" just makes it worse because of floor bounce and doubler effects, because of two sound-sources.
I meant the Dayton RS225.... that A should have been an S
It looks smooth and well behaved:
http://www.loudspeakerdatabase.com/Dayton/RS225
The SB23NRX seems to want around 90 liters to get a Qtc of 0,6. A bit large, when I don't need them to play deep.
My midrange is an 5" alu, with a brak-up around 9800hz. This should be fine, cause I cross-over at 2Khz ot my Seas DXT. The SB paper cone start breaking up around 1Khz - leaving 500hz in the "danger" zone. The SBNAC and CAC break up around 3Khz... which seems better for a Xover at 500hz.
200W in 8 ohms.... stable in 2-3ohms - dedicated for mid-bass duty with DSP.

 

[Digital_Thor]

Something like this maybe....
https://sica.it/prodotto/8-s-2-5-cp/

 

[MrPeabody]

Since you use multiple subs and only need the woofers to reach down to around 50 Hz, you have a lot of drivers to choose from, that you wouldn't have if you were wanting to build an acoustic suspension speaker capable of playing flat to, say, 35 Hz without EQ. And since you say "active", I assume that you don't mind using EQ to make up for any early rolloff. This opens things up to where you have a ton of choices and can choose based on other criteria. What are the priorities? How much does cost matter? Size of enclosure? Ability to play loud without a ton of distortion? If my preference were for aluminum, I would look for one that exhibits a lot less breakup than most aluminum woofers do. If you don't want to rely heavily on EQ (and on the amplifier power needed for it to work, to overcome the inefficiency that accompanies the rolloff), you still want an F3 at maybe 50 Hz, which still puts some constraints on driver choice. It depends on much you're willing to rely on EQ. A useful formula for estimating F3 is .73 x Fs/Qts. If you want F3 to be no higher than 50 Hz, then Fs/Qts needs to be kept below approximately 70. But this is not a difficult criterion to meet - there are plenty of woofers that do. Paper formulations vary greatly. Paper can be just as stiff as aluminum, and when it is, it has similar properties including breakup. The Scanspeak 18W/8535-01 seems a very desirable driver in most every respect. It definitely does have the rising response characteristic, but the response above 1 kHz isn't nearly as ragged as many aluminum woofers are. The thing that would likely be most difficult to suppress with a typical crossover is the +2 dB bump at 700 - 800 Hz. If you want aluminum, less costly, and somewhat bigger, the SB23NBACS45-8 may deserver a look. If you don't mind going even larger, to about 266 mm, the Dayton RSS265HF-8 is probably worth a look. The biggest drawback to it is probably poor sensitivity. Which might not matter to you.

 

[HammerSandwich]

The Monacor SPH-165CP just tested at Dibirama has incredibly clean H2 & H3, even compared with larger drivers. (Wish Dibirama showed H4 & H5 as Hificompass does.) Sensitivity's only mid-80s, and frequency response goes crazy above 1.5kHz - it's very weird for a 6". Impedance & THD graphs remain clean, though, so I'm really unsure what's happening. @KSTR?

Parallel a pair in 15-20L for 4ohms & 90+dB sensitivity. With 24dB/octave crossovers & minor EQ, they'd cover 60-500Hz very well.

 

[KSTR]

The Monacor SPH-165CP just tested at Dibirama has incredibly clean H2 & H3, even compared with larger drivers. (Wish Dibirama showed H4 & H5 as Hificompass does.) Sensitivity's only mid-80s, and frequency response goes crazy above 1.5kHz - it's very weird for a 6". Impedance & THD graphs remain clean, though, so I'm really unsure what's happening. @KSTR?

Parallel a pair in 15-20L for 4ohms & 90+dB sensitivity. With 24dB/octave crossovers & minor EQ, they'd cover 60-500Hz very well.

The impedance graph is heavily smoothed that's why we don't see much of the ragged response mirrored into the impedance.
Low single-sine H2/H3 at higher freqs helps but at higher excursions IMD will be raising its ugly head.
That is not to say this driver won't work here, it might be perfectly fine in midrange quality with a 4rd order acoustic XO at 500Hz, it will be just a bit harder to get good phase tracking in the octave above the XO... some correction of the developing peaky edge at 1.5kHz is certainly required.

 

[Digital_Thor]

Thank you all - much appreciated
I have a box extra - with 3 18W8545 in parallel. They play nicely in 60 liters combined - but has the surface area equal to the two SB23NRX. I'm mostly trying to learn how to read all the details and maybe also buying/building something else.... if I can find good proof that it actually matters. No need to fool myself or my wallet - if the solution lies somewhere else

 

[HammerSandwich]

The impedance graph is heavily smoothed that's why we don't see much of the ragged response mirrored into the impedance.

Agreed, and the 150-ohm scale minimizes even the 7kHz breakup. Looks like I was posting well past bed time!

Good note about IMD, too. A pair of 165s would have ~2x the excursion of a 12", so it's definitely a factor.

 

[Digital_Thor]

I always liked light and hard drivers. But is there any proof that we can use all the parameters as a guideline for active systems. Because I know that most people focus on passive constructions. But if I have an active solution and direct control of the driver, is their then anything I can use as an factor for most dynamic sound in the very important midwoofer region?

 

[Digital_Thor]

Read some more and experimented some more too with my system. Two things strike me as important for the perception of a dynamic bass sound. Firstly it is very important for the harmonics in the midrange to match the woofer. Secondly it seems to sound better when a woofer is mounted in a closed box, big enough to reach minimum Q=0,7 - maybe even 0,6 or lower.
My question is then - how small a box can I use to both get a good low Q and also get an optimum dynamic response - while keeping it elegant?
Still need a good flat response from around 70-500hz. The two SB23NRX that I have now - might be fine - but require around 140 liters to reach a Q=0,6. Two Dayton RS225 should be able to reach Q=0,65 in just 60 liters.
Is there a physical compromise to this - and/or is a non-issue in my case?

 

[Digital_Thor]

Also looked at an older set of Seas L18 or L22 - which jumps from tiny 20 liters to 80 liters(both with a Q=0,65) - for 2 drivers pr. side.

 

[Wolf]

I would go Dayton RSS over either of those Seas woofers. The motors are much cleaner.

Another option would be Peerless XXLS aluminum woofers.

 

[Digital_Thor]

Thank you. But how do I know that the motors are cleaner?
I already have 4 x 12" XXLS subwoofers, but those would not be good to 5-600hz.
My whole concern started with looking on the new SB23CAC, thinking that this might be a good option by the highish Qms. But the more I read, the more it seems that this is not the whole story to achieve good bass

 

[Wolf]

Okay, so your bass is covered well. The RSS motors have 3 shorting rings. The RSS210 can easily play to 900Hz.

Looking at 3rd party testing is one way, or take advice.

 

[Wolf]

More detail, look at the Harmonic Distortion measurements. The Seas will have higher 3rd order, and that's not so good.

 

[Digital_Thor]

Right now I play with Seas DXT, Dayton RS125 and then the two SB23NRX. + 4 subs. I find the RS125 rather easy to tame. So that is why my attention was drawn to the RS225. Wouldn't the RS210 with a 3 time heavier cone, be less detailed than the RS225? Also, it cost twice as much

 

[Digital_Thor]

Thinking that when I have the RS125 as a midrange, my overall SPL is still not going to be huge. So I might be fine with "just" one SB WO24P as a woofer, since I'm going to cross around 70hz to multiple subwoofers. Having only one woofer might ease up on the filter design - even when active - compared to two woofers.

 

[Tom Danley]

"We don't want the motor distortion (2nd and 3d) being amplified by the effective gain of the woofer at those frequencies. Assume you have +15dB resonance at 1.5kHz and you play 500Hz, then the 3rd harmonic gets a free boost of 15dB, baam! "

This is an excellent point and it is exactly as you describe, a signal level or passive crossover / low pass filter cannot undo that magnification of the distortion..
On the other hand, a proper acoustic low pass filter after the radiator but before radiation can reduce the total distortion..

A further comment would be that breakup behavior is also chaotic and non-linear and so tends to change significantly with increasing levels (gets worse).
A 1W response measure gives one thing while often a 10W measure gives a worse indication. With the aluminum cone Dayton woofers, the 8" and 10" inch versions I have tested have an aluminum cone and a LARGE resonant peak which when used as a woofer alone, is still audible with music.

A driver engineer once told me the reason we mostly use paper based cones is, it is the lightest, strongest thing that has enough internal damping and is cheap enough.
Best Regards
Tom Danley
Danley Sound Labs