[Arctos]

I am trying to understand why unibox tells me that, no matter what driver I use, I am always going to overdrive my woofer at lower frequencies.  The only way I can get this to correct is if I reduce the nominal power to a very small figure (like 1 watt).  Am I missing something?  I have been very careful in setting the parameters and using a recommended Vb.

[_scotty_]

Arctos, it sounds like you are running into displacement limited acoustic power rating and displacement limited electrical power rating. Your woofer has a certain amount linear excursion and power handling capability.
When you feed the numbers in the program tells you that for this woofer in this enclosure you run out of excursion at a certain frequency and the max loudness at this frequency is x. The program is probably right.
What woofer do you have and what is its' Xmax and Vd. You may need a larger woofer or one with a larger Xmax figure or both to not run into problems.
You may also require a woofer with a higher thermal power rating in addition to the above parameters to stay out of trouble.
Try increasing the Xmax figure and power handling figure and see what the program does with different numbers. For example Xmax= 18.9mm,
RMS power 350watts,Fs22Hz,Vd.949L.Sd.050sq.m.,Qts..413,Qms9.563,Qes.433,Vas4.46cu.ft.
These numbers should give a very different result. An SBB4 3.94cu.ft.enclosure,Fb20.63,F3-23.1Hz,Anechoic response output@20Hz of 109dB,
in room 118dB@20Hz with room gain assumptions.Power handling is 400watts with linear response to about 18Hz.
Hope this helps sorry for length. Scotty

[Arctos]

Scotty,
Thanks for the detailed info!!  Well, here are the the specs of my Audax HM210Z0 drivers.  These things are pretty high quality and I am following a set of tried and true plans for the KLS-9 speaker design:

Z = 8 ohms
Fs = 30 Hz
P = 70 watts (this is what I input into the Unibox "nominal power" cell)
E = 91 dB
Re = 6.3 ohms
Xmax = 4.25 mm
Qms = 6.45
Qes = 0.32
Qts = 0.31
S = 230 cm2
Vas = 108 liters
Lbm = 0.47 mH

I'm not sure what you mean by "Vd".  The cabinet volume I am using is 60 liters, which is a little smaller than what the original design calls for, but not by much.  As I said above, this DIY plan has been successfully implemented by many DIYer's and was designed by Noel Keywood (known for designing some nice speakers).

Thanks,

RDS

[JoshK]

If you speakers give you Vd you got some problems!  

[JohnR]

Yep, woofers in most all domestic speakers are excursion limited at low frequencies. This is not necessarily a bad thing, as the power will be spread over a broader band than just 30-80 Hz... but yes, most domestic speakers compress and distort with any reasonable power input in the bass region.

With those specs, looks like 10W keeps it within Xmax down to 30 Hz.

[Arctos]

Josh, you're killing me!!

John, what do you mean by "domestic" speakers?  Also, the description given by a company that used to sell kits for this speaker (World Audio Design, UK) states "amplifiers of 30-80watts are suitable partners".  Are they mistaken, or am I missing something?

Thanks,

RDS

[JohnR]

John, what do you mean by "domestic" speakers?

Speakers that people use in their homes. 6-8" woofers, direct radiators (not horns).

No, they aren't mistaken, but if you put 80 watts into that driver at 50 Hz it will compress and distort. That's "normal." That 80 watts will come in useful at higher frequencies.

[_scotty_]

Arctos,many people add a subwoofer to their "full range" speaker systems
to overcome the excursion limit on their woofers. This is always an option
for the future if you want louder and lower bass reproduction capability.
Vd is the total linear displacement of the woofer in liters. The woofer in the earlier example will displace almost a liter of air without distortion.
It is not enough, but it is a start in the right direction.
         Good luck with your project. Scotty

[Arctos]

Thanks guys,
I suppose I shouldn't complain since I should be able to get over 100 dB out of these and still be in the "safe zone" from the standpoint of excursion.  If they are 88 dB sensitive I can go to 103 db at 32 watts and not push them past Xmax.

I get the concept of using a sub to get the low frequency stuff (I have a Hsu VTF-2 in my H/T system), but does that mean that I have to add some kind of low frequency filter to my crossover network to keep from blowing the driver or getting massive distortion if I push it too far?

Regards,

RDS

[_scotty_]

Arctos,it depends on the low frequency content of the music you play and
how loud you play it back. If you add a sub to increase the headroom available in the low frequency region you will have to filter  low frequencies out of the mains to do any good. When I ran separate
subs I used a small value foil and film cap in series with the main amp  for a 6dB rolloff at 115Hz to keep the lows out of my main speakers. This is a very inexpensive and effective xover. The sub was actively crossed over at 50Hz at 18dB/oct. the hole was filled in by the room and the response was acceptably flat at the listening position.
The speakers that I now use have two of the of the drivers that I gave you specs on in each speaker xovered at 150Hz at 12db/oct. I can reach at least 115dB at 20hz without distortion being a problem. I probably will never use this capability as I value my hearing too much. This system sees double duty with movies so the headroom
is good to have. You are in a good position to be able to judge whether
or not a sub adds any value to the presentation by substituting the sub you have into the system and judging the results.  Let us know how your project turns out. Scotty