8" woofer selection for high performance 3-way with bass boost

pawelp

Member
2006-05-21 9:47 am
I am designing a 3-way with DSP correction performed on Sharc. The biggest decision that I am facing now is the choice of woofer. From the preferred physical dimensions for the enclosure there are some limitations:
- 30 litres volume (maybe up to 40 ltrs if stuffed properly),
- 8” woofer.
For the sake of quality of measurements I have decided to go for the closed box design.

One of my key targets is to use DSP to boost the bass to get possibly linear FR down to 25 – 30 Hz. I have been analyzing the topic for some weeks and for now I have come to a conclusion that one important factor is the maximum linear excursion, as this defines the max SPL for bass.
But an even more important thing for me seems to be using DSP bass boost as little as possible since this e.g. limits the overall dynamics of the speaker. So I thought the best selection criterion for the woofer can be the level of reference SPL in the 30-ltr-enclosure at some low bass frequency, say 27 Hz. I made some preselection and landed with the following woofers based mainly on their ( low) f3 in a sealed box, suitable Vbox, max linear excursion (in some cases) and reasonable pricing.
So the list is for now is:
- Visaton TIW200 XS, 12,5 mm linear exc. max
- Wavecor 223BD01, 10,5 mm
- Peerless HDS-P835026, 6mm
- ScanSpeak 8555, 6,5mm
- Seas L22RNX, 6mm
I calculated the reference SPL for each of the drivers using the formula that takes into account the differences in Re: 112dB +10*log10(n0*8ohm/Re)
where 112 dB – 1W radiated into semisphere at 1m
n0 = 9.64*power(10,-10)*Fs3*Vas/Qes
Because I factored in Re I think I am effectively making the assumption that I have power in abundance which improves a little efficiency for drivers with low Re, right ?
However this calculation did not help me a lot as a bit surprisingly most of these drivers land at 30 Hz at similar efficiencies (within 1,5dB, Scan 21W8555 being the best). This could actually speak in favour of more efficient drivers with higher f3 like Seas LR22 since it has at least noticeably higher efficiency at 40-50 Hz where significant bass energy is located (more than in 25-35 Hz) ?
I am also a bit unsure if the efficiency calculated this way (factoring in Re in particular) is really reliable enough since in reality amp power is pumped into the driver impedance not Re ?
What do you think of my line of analysis and conclusions ? What would you take into account on top of/instead of what I did ? What would be your drivers of choice for this kind of application ?

Regards, Pawel
 
What you really need to do is use something like WinISD for simulating the responses. When concerning drivers used down low, in and around resonance, the impedance and phase angles can swing wildly and this makes the situation down there less easy to come to terms with.

From a simple point of view though, all you need to do is observe the maximum linear excursion of the driver vs the drive voltage. This will tell you if your amplifier has enough voltage swing to reach the drivers linear limits without clipping. The only other thing you really need to pay attention to is whether or not the amplifier is then capable of delivering the required current, within that range of voltage swing, to drive the loudspeaker without self destructing, or being prematurely limited due to it's protection kicking in. This is where you might need to look at the spots where the phase angles are high and see if they are combined with a reasonably low impedance.

From a maximum SPL point of view in a sealed box, the volume displacement of the driver in question is the only thing that matters. Put simply the larger this is the louder it can go. So for the same cone area, the drivers with the larger xmax will be able to hit higher SPLs. What changes is the efficiency of the system, ie how much power you need to get to that maximum SPL. For a given driver, if you increase the box size, the max SPL remains the same, but the power required to get there goes down.

This is all a balancing act. If you've got lots of power then efficiency isn't so much of a concern and you can go with a small box. As you noted however, this will increase power compression as the dissipation in the motor will go up as the power requirements for a given SPL at a target frequency increase. For normal music listening I wouldn't be too concerned with this, unless you like to listen at very high levels or to music with tons of low content.

There is always the SW223BD01 from Wavecor and the RS225 from Dayton Audio. With its balanced drive
motor the SW should have very good performance and with its large surface area and linear excursion will have good max SPL figures, but it is expensive, the same is true for the Scanspeak revelator bass drivers. The RS225 would be my primary go to 8" driver for any 3 way, unless big bucks, ie Scan revelator or something like the SW, was on the table.
 
How high does the woofer need to go?

If its effectively a subwoofer in the same cabinet as a 2-way (ie, ~100Hz crossover), pick the one with the most excursion, eq it to death. This does assume that you're giving the subwoofer a seperate amplifier, and it has enough power (a few hundred watts a side will be plenty).

If you pick a more efficient woofer, it'll need more eq in the bass to get the same f3 as a less efficient woofer (Hoffman's Iron Law), but won't need so much power higher up in its range.
The extra eq vs efficiency probably work out similar, so I'd discount efficiency from your calculations.


IMO, a lot is going to depend on your crossover point. I'm not sure I'd trust some of the dedicated subwoofer drivers with going up ~500Hz cleanly, but I wouldn't want to hammer a midbass to death with subwoofer duties, if the crossover was ~100Hz.

Chris

PS - I'd advise aiming for more like 30Hz - going much lower is the realm of the big drivers, and you'll have to trust me when I say that 8"s don't like trying to go that low.
 

pawelp

Member
2006-05-21 9:47 am
How high does the woofer need to go?
My initial plan is 250Hz. And the xover will be >50 dB\octave so it will be heard at all north of 600-700Hz.

pick the one with the most excursion, eq it to death.
Well, that conclusion has been bouncing in my head for some time but I was hoping there is more to woofer design than this (too) simple rule :(

If you pick a more efficient woofer, it'll need more eq in the bass to get the same f3 as a less efficient woofer (Hoffman's Iron Law), but won't need so much power higher up in its range.
That's the thing that surprises me a lot about Hoffman's Iron Law. So if I want more efficiency in lower bass the only way to get it is to live with lower overall efficiency ? The high-end expensive drivers achieve nothing more than el cheapo in this respect - sophisticated magnets, etc ?
The extra eq vs efficiency probably work out similar, so I'd discount efficiency from your calculations.
Looks like my calculations say the same. So it would even back my surprising thought that higher f3 with more efficiency is better for my equalization than the reverse combination. Then the only pity that e.g. nice in this respect and inexpensive L22RNX has little Xmax.
For Dayton RS225 suggestion - it does not have high Xmax either so what mailnly speaks for it - good distortion specs ?

PP
 
Any driver will be stressed if you push it :D

IMHO, 8"ers are too small to offer deep bass (<30Hz) reproduction at satisfying levels. As these terms are so subjective, its up to the builder to decide what's enough.

For 250Hz, its a little borderline, though I think I'd go for more excursion. You're trying to get deep bass from relatively small cones, with no LF support from the cabinet. Excursion really will make the difference here. For ported, any transmission line variant, etc, etc, other parameters will come into play more.
For the sealed box, you need lots of excursion, and enough power handling to reach Xmax without cooking the driver.
Low distortion would be nice, but I'd expect this to be a given for the drivers you're thinking of using - none of them are particularly cheap.

With regards to Hoffman's Iron Law, in my head it works something like this...

(sensitivity)/(f3 in a given cabinet) = some constant.

There may be some logarithms and powers in there, but that's the basic idea.
The constant is determined as
k=z(1-e^(price*x))
Where k is the constant,
z is the best (highest) possible constant
x is another constant, related to how well the driver is manufactured, magnet strengths, suspension/cone design etc etc.

So, you get a graph like this:
[IMGDEAD]http://www.cis.rit.edu/class/schp730/bmri/chap-2/images/1-e-xt.gif[/IMGDEAD]

Price is on the x-axis, effective performance (given as k here) on the y-axis.
The graph tends toward a value (z) on the y-axis, but never quite gets there - if you spend infinite money, you'll get absolutely everything available from that driver size.

So yes, spending an extra £100 on a driver might give you a couple of Hz more extension and the same overall sensitivity as the less expensive driver, but you're on diminishing returns - the next £100 might only get you 1Hz lower.

HTH

Chris
 

pawelp

Member
2006-05-21 9:47 am
Well, I think when choosing a woofer 3 things matter most:
A. FR - i.e. how flat it is and does it have troublesome resonances
B. Distortion figures
C. How well it performs in bass which actually is two things - C1. how efficient in low bass it is (low bass extension) and C2. how much air it pumps - i.e. cone area x linear excursion (low bass max SPL).

In my opinion A is not important if I do DSP correction, B is difficult to compare/controversial in impact on listening experince and more expensive drivers should have more less it right.
C2 has merit but is less important in my case as I want to test at moderate volumes. C1 is the most important thing for me that is why I started the topic with efficiency analysis.

Chris, after a deeper look I can see that there is little to the magic constant "k" - from the equation it is simply reversely proportional to Qes.
So it looks that there can be more efficient drivers that will manage relatively low Fs and a advantegous combination Vas and Qts to have better efficiency.
But it also seems that the Hoffman Law hints the differences will be rather small and probably they might not be worth paying for.
So for now I am leaning slightly towards Wavecor 223db01 mainly due its 10,5mm excursion although with quite miserable efficiency that is only boosted by low impedance/Re. So tough choice actually. The best is actually Visaton, but this one I will have for a comparison anyway, I need a second driver to run the contest.
Second choice is ScanSpeak 21W8555, that has the best efficiency in low bass the main disadvantage being low Xmax.
If the bufdget was the main driver I'd go for Seas L22RNX - not a big difference in parameters and over 30% lower price in Poland at least. And that's probably the value leader for me together with Peerless HDS, though very little popular one.

There was a doubt whether steep xovers are good. Well, the only disadvantage is time-domain distortion - i.e. ringing, except that all strong benefits. The problem is not many people actually heard them as they are not the mainstream technology.
 
The rs225 has very low distortion, especially when used below 300Hz, it is really one of the better 8" non sub type bass drivers out there and considering its cost, make it a very attractive choice.

If you want better but with a similar type of driver then you really have to go to the 22W revelators from scan and these cost a small fortune compared to the 225.

I have used the 225 in a 12 litre sealed cab and eq'd with a Linkwitz transform to give 30hz extension, it works well and sounds great, just don't expect it to get to party levels. 30l will give you more efficiency, but 30hz, in a small sealed cabinet, at high spls is really too much for a standard 8".

I think the bottom line here is that the 22W, the SW223, the TIW200 and the RS225 will all do the job well, with what? About 2-3dB difference maximum SPL between them? The only downside to the RS225 is that it doesn't handle power as gracefully as the others, so if you really want to hammer them, then pick something else.

Visaton list the excursion limit as 12.5mm but this isn't the xmax figure. The TIW200 has an 8mm top plate and a 30mm long coil. 30-8=22, giving the driver 11mm of one way xmax when calculated in the same way that the other manufactures do it.

The 225 is an obvious choice if you need low cost.

The TIW lists 84dB @ one 8 ohm watt for its sensitivity. This is equal to the SW223s 87dB at 2.83Vrms, as an 8 ohm version would have 3dB less sensitivity.
 
Why not a Seas L26ROY? Will give a qtc of 0,62 in a 30 liter box and will easily reach 250hz. A little pricey but it will do a lot better than a std 8".
The problem is more complex than you (or OP) think, but you are right. Taking just these two drivers for a comparison, the Visaton TIW200XS (8") and Seas L26ROY (10") (besides the Seas having 3dB more sensitivity in the full range), the Visaton likes a box double the size of the Seas in BR mode what is in favor of the Seas L26ROY. This shows us that size of the enclosure plays a significant role in choosing a woofer what was not mentioned previously behind F3, sensitivity, distortion, et all. But if we use a sealed system in a similar sized box, in around 40L for the 2 drivers, the behavior is not the same or as expected. The first driver Seas shows F3=59.0 Hz and sensitivity at 30Hz=80.3dB (~90.3 dB-10). Max. SPL/1.00m is; 107.3 dBx1 driver, 110.3 dBx2 drivers. QTC = 0.565.
For the second Visaton driver, in the same enclosure (40L), F3=48.4 Hz and sensitivity at 30Hz=77.2dB (~87.2 dB-10). Max. SPL/1.00m is; 101.9 dBx1 driver, 104.9 dBx2 drivers. QTC = 0.714.
If we go with a smaller box (30L) then the Visaton starts going out of the race (it likes a bigger enclosure) with it's QTC = 0.778 (vs. VB = 30.0 L, QTC = 0.626 for the Seas L26ROY).

Sealed 30L as requested:
Seas L26ROY, F3=55.8 Hz
Visaton TIW200XS, F3=48.0 Hz
Conclusion, better low-end extension with Seas L26ROY, just because of QTC. :)
 
If the goal is to get the lowest possible f3 excluding bass boost, I'd approach that this way.

1. Calculate f3 of each of the candidate drivers in a closed box with Qtc= 0.71. Qtc = 0.71 results in the lowest possible f3, relative to Qtc higher or lower (in an unassisted system). Record box volume corresponding to Qtc = 0.71 in each case.

2. The maximum SPL that the system can produce is a function of f3 and the volume displacement Vd. Vd equals the effective surface area of the cone of the driver times Xmax. The equations are

SPL 1W/1M = 112 + 10LogPar(p)

Par(p) = 0.85*f3^4*Vd^2

Simple, huh! Thank Richard Small.

Regards,
Pete
 
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tinitus

diyAudio Moderator R.I.P.
2005-11-24 1:47 am
with sealed design, the lowest possible -6db or-12db is more interesting than f3
and ok if it pushes f3 higher up
not so much with vented though

actually, with the modern tools I believe its much easier to design good vented than sealed
sealed is still a tricky game
also not easy to find good woofers for sealed box
most are designed for vented
 
If I were doing a 30-40 liter sealed box with a 30 hz f3 I would use a Dayton RSS265HF-4 woofer. Yes, a 10" driver. Driver surround noise will be lower and it will play louder at low frequencies with less effort. Because a larger diameter driver moves less for any volume level, there will be less Doppler distortion too. Finally, you will need only modest boost to hit 30 hz flat.
 

andy19191

Member
2005-04-17 11:42 am
-
I am designing a 3-way with DSP correction performed on Sharc. The biggest decision that I am facing now is the choice of woofer. From the preferred physical dimensions for the enclosure there are some limitations:
- 30 litres volume (maybe up to 40 ltrs if stuffed properly),
- 8” woofer.
For the sake of quality of measurements I have decided to go for the closed box design.

What do you think of my line of analysis and conclusions ? What would you take into account on top of/instead of what I did ? What would be your drivers of choice for this kind of application ?
If you are looking for flat down to 30Hz in a 30-40 litre sealed enclosure an 8" is too small. To flesh that out I would suggest having a look and perhaps even a listen to commercial speakers that take a similar approach. For example the K&H (now Neumann) O 300 and its recent replacement the KH310 use an 8" in a sealed box. The measurements are on the right hand side and show the clean SPL you can get with an arrangement like the one you propose. With off the shelf drivers I suspect you will be pushed to reach the O 300 performance. Clearly to listen at standard levels (peaks of 105-110dB) a subwoofer is required.

To get clean down to 30 Hz at standard levels you need to use a bigger driver and with 30-40 litres size that should not be an issue. Under 20 litres is a typical volume for an 8" and so two sealed would be an option if width is a limiting parameter although it will still be lacking in clean SPL at the lowest frequencies. Which is where ports help a lot.
 
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Hi Pawelp,

Highest output I can think of that fits the box size, sophistication, and driver size requirements is the JL 8W7. It has a full 19mm of 1-way coil overhang and can do [email protected]@30hz into 2pi space (requires ~400W) with full linearity in a sealed box around the size you want to build. Aside from perhaps other "super" competition drivers you aren't going to find an 8" that can do this. The way the big surround occupies the space of the mounting rim (covering the mounting screws) is an interesting innovation and increases the usable Sd of the 8" size platform while giving it a nice clean look on the install. I'll even go so far as to say that this mounting method cleans up the look so much that it counteracts the fact that they still have a logo on the cone (albeit small enough to be considered tasteful).

As a direct radiator, reaching 250hz with low distortion should not be a problem for a W7. They have very linear motors and light weight cones (all things considered).

Only problem: ~$350 US