power distribution and sensitivity in a 3 way

[habanero]

Hi, I'm a newby designing an active 3 way system. It will be mostly used for domestic hi-fi, but I do like to play loud on weekends. I'm modeling the peerless HDS exclusive drivers, 8", 4" and tweeter. (830884, 830881, 810921) One reservation I have about them is the sensitivity of the midrange.

Woofer: 89.3 dB
Mid: 84 dB
Tweeter: 93 dB

As you can see the mid is much quieter than the others. Since this is an active system, I can always give the mid more juice, but I have some reservations about that. I'm not sure how much power it can handle. I'm certain that it can't handle as much as the woofer, so there must be a lot of woofer performance that will be sacrificed when the mid can't keep up. I'm also worried about power compression.

OK. So with that out of the way, here is my rookie power analysis:

I'm tentatively thinking of crossing at 500 Hz and 3 kHz. This gives me a rough power distribution goal of:

Woofer: 60%
Mid: 25%
Tweeter: 15 %

I've seen wide ranging opinions on power distribution. I got these figures from Rod Elliot but I really thought I saw them in Dickason. I can't find anything in that book tho. I'm guessing he can't find anything either and that's why he hasn't done an index after 6 editions.

So assuming drivers of equal sensitivity, and being generous: 200 W, 85 W, 50 W. Man I'd love to be putting that kind of power to work! However now I have to factor for sensitivity. Since the mid is the low point, I'll work out the power ratios required to adjust the woofer and the tweeter power requirements:

Woofer to Mid sensitivity difference: 5.3 dB = 10 log P
log P = 0.53
P = 3.4 (Eek! so it takes 85 W to produce the same SPL as 25 W into the woofer)

Mid to Tweeter sensitivity difference: 9 dB. No need to work that one out, it's 8 fold. This gives an adjusted power requirement:

Woofer: 60 W
Mid: 85 W
Tweeter: 6 W

Ouch. That is looking really anemic. I'm not sure if I can pump much more than 85 W into a 26 mm voice coil. I've done the relevant plots in WinISD Pro, but I don't know how single frequency SPL at 1 meter anechoic relates to music in my studio. The WinISD plots make it look like 25 W for the woofer gives me 102 dB without exceeding Xmax, and the corresponding 85 W into the mid will also achieve the same SPL. Will this be enough for me? I don't have the experience to put the numbers into perspective.

[rcw]

In the end it depends upon what you want from the system.
As it stands an eight inch driver will be able to produce adequate bass only down to 50Hz.

You can forget about room gain in the typical Australian case, open plan plasterboard and flow through natural ventilation etc. give the typical Australian room about as much gain as the middle of a paddock.

Measurements I have done indicate people are uncomfortable with an average of 85db. that includes peaks of 115db. over a wide range of music.

What I would put as a minimum requirement for hifi is that both mid and low of a stereo pair should each be able to produce short peaks of 110db. on the c weighting scale.

I personally would not use this mid driver and would use a more efficient nominally five inch, I would also use a subwoofer, as the bass speaker, although it might go lower, will strain to produce adequate bass bellow 60Hz.
rcw.

[PeteMcK]

"As it stands an eight inch driver will be able to produce adequate bass only down to 50Hz" - what's the logic behind this? I suppose it depends on your definition of 'adequate'?

"would use a more efficient nominally five inch" - inclined to agree

habanero, how much power you'll require depends on the size of your room for a given SPL. FWIW, I use 40 watt amps in a bi-amped system ~ 90dB sensitivity, it's more than adequate in my smallish listening room, even when I'm in party mode it fills up the back yard.
A few dB extra in the woofer is OK, means you're using ~ the same power once BSC is compenated for.

[rcw]

My comments about the bass driver are based upon Smalls power output expression.

P = kp Vd^2 f3^4

putting Vd = 1.22e-4, kp=3, f3= 50Hz.

this gives .279 acoustic Watts, giving

112=10 log(.278) = 1o6.5db. peak output.

Note that this is with a 112db. standard, these days ears are at least 3-4db. less sensitive than this.

In general for rooms up to 80m^3 the spl. at one meter is a reliable indication of how loud the system will sound.
rcw.

[sreten]

Hi,

The midrange level is fine for the woofer once you add in baffle step.

Take a look at the 3 ways here : you'll need something similar ...

http://www.rjbaudio.com/projects.html

(Even though your going active, EQuing for BSC etc. is important)

/sreten.

edit : one way of doing it is bi-amping actively at 300 to 400 Hz
bass to midrange and a passive mid/treble croosover, in this
case identical power amplifiiers would work well. Just like a
passive speaker the bass unit ideally should be around 5dB
more sensitive than the midrange. Padding a tweeter back
to the midrange improves power handling (unsurprisingly...)

[habanero]

Thanks for the responses, I think I'll go with a larger mid. I'd love to go with a larger woofer too, but in my budget, I haven't found many drivers that model well in a 60L reflex cabinet. My cardboard cutout testing has shown that a 1m tall 60L tower is the largest cabinet with sufficient SAF!

The HiVi D8.8+ looks great in WinISD. In fact, I was looking at putting two of them in 80L. I had a hard time finding any kind words about them though.

[rcw]

If you are doing a three way you can use two cheaper eight inch drivers to give you a genuine usable 30Hz. cut off, details here..

http://sound.westhost.com/articles/cscaling.htm

This type of alignment effectively doubles the amount of bass at the lower cut off, and the extra driver with mutual coupling means that two drivers can produce 9db. more output than a single one can.
rcw.

[habanero]

@sreten, thanks for the link, there are some pretty impressive looking frequency response specs for speakers using 7 and 8 inch woofers.

@rcw, Aha, I was just about to ask you what you thought the author meant by... then I checked. You are the author! Or at least share the same initials.

I get the idea of a filter assisted alignment, and I follow the part about raising electrical Q by increasing output impedance, but the part about compliance scaling just seems like hand waving to me! I need to go through the equations and figure out what they mean. I suppose as long as you scale the parameters you have control over, such as the ones describing the enclosure (or electrical Q), then it makes sense. So far I have been totally relying on WinISD to do the math for me.

I see that you got two 8 inch drivers to play nice in 60 L, and that is my goal. The CSX drivers don't seem to be available anymore, and I can't get any 8 inch drivers to model nicely unless I use a larger box. Since compliance scaling increases the box size, I can't use it unless I find better drivers, or an explanation of why my Vb is always around double what the manufacturers recommend.

Incidentally, I also see that your WinISD plots show a hight frequency rolloff beginning at 500 Hz. Some drivers do this and others don't. I have seen mid bass drivers that are often crossed at 3 kHz show a steep rolloff above 500 Hz. I know that can't be true. Do you know if this is a bug that can be safely ignored?

Well thanks, I had written compliance scaling off before as interesting, and probably something I would try for some satellite speakers, but now I have renewed enthusiasm. OK off I go to spend countless more hours modeling speakers...

[rabbitz]

Good music 8"-10" woofers are getting harder to find.

One of the best was the Vifa M22WR-09-08 which gives a usable 30Hz in a box a tad bigger than 30 litres but is no longer available.

There are still some Vifa P21WO-39-08 available but your best bet would be the Vifa M26WR-09-08. WES still has some of these in stock. A 10" is going to be better than 2 - 8" and these have a very strong following from builders.

I never had a great deal of success getting great extension with the Peerless 8" HDS series but found the 5" - 6.5" drivers very good. One I haven't tried is the SLS series 830667 but have no idea how good it sounds as I stopped selling drivers before it came out.

Jaycar have some good options and Andy G has used these to great effect and not shabby on the bass at all.... very good in fact but I have no idea what is now available. Some uses can be seen here.
http://members.optusnet.com.au/~gradds55/index.html

I'll agree with the others and go with a 5" mid rather than a 4" mid. This will give you a chance to cross over lower, say between 250Hz-400Hz, and the trick is to cross your woofer at around the baffle step frequency so the woofer provides all the BSC and not any electronics killing the mid or tweeter.

Don't believe all you see in WinISD as there is a lot of results that doesn't relate to the real world or is inaccurate. There are other modelling programs out there and a search on this forum will find quite a few. Usually the optimum box size supplied by the software is not necessarily the best as you have to fiddle to suit the intended application, priorities etc.

Enjoy your project.

[PeteMcK]

Re:"Jaycar have some good options" - they don't have much of interest now, try Altronics or SoundLabs which has Visaton & Wavecor drivers
http://www.altronics.com.au/
http://www.soundlabsgroup.com.au/c/V...s/Visaton.html