Meniscus 1284 Excursion limit confusion

[scottandmo]

Hi I am new to this whole DIY subwoofer thing but I have read a lot and am have the components to build my first box. I know my driver is optimized for a sealed box but I am building two boxes for a carpeted concrete basement theater area that is 15'x15'x8.5' opening in to a 20x15 rec room. Basically these subs will be used mainly for HT with a little music, so I am considering building a sealed and sending the front L/R signal through it, and then building a ported sub for the LFE sub preout on my amp so i can get some low 20Hz thump.

2x Daytion SA240, 2x Meniscus 1284 (supposudly same driver as ACI SV12)

can't find meniscus T/S params, so basing these on SV12:
Meniscus 1284
Fs 17.50 Hz
Re 2.98 Ohm
Qms 8.35
Qes 0.41
Sd 466.0 cm2
Vas 250.0 l
Xmax peak 12.70 mm
(Le) 1.45 mH
(Le2) 0.00 mH
(Re2) 0.00 Ohm
Nominal Power 150.0 W
Pe 500 W

I constructed these excursion graphs in unibox and it appears to exceed the xmax limit at about 30ish Hz sealed or ported at only 150W. I am confused about this because its supposed to have a 500W max and an Fs of 17.5 and I am using the optimal box sizes along with the unibox's option to optimize Vb for wanted Qtc. I don't know if i am using Unibox Correctly or even have the correct driver params so I guess that could be the problem. I attached the graphs. Anyone know what I am doing wrong? Or am I asking too much of these drivers.

[tinitus]

Consider SPL
I suppose 150watt would be pretty loud

[scottandmo]

The spl is under 110db at 150 watt which I don't think is an overboard.

[PB2]

What box volumes are you using?

For vented, try pushing up the tuning freq to 20 or 25 Hz - it is a trade-off. Check the amplitude response and reduce the box volume if there is peaking at the higher tuning freq. Then check displacement limited output.

For sealed consider that any driver with the same cone area and Xmax will have the same mechanical volume displacement limitation (VD), the power it can handle is a function of how efficient it is.

Threre are two limits, thermal, and max displacement, you are running up against the displacement limit.

Also, consider that most drivers will do at least 20% (depends on the driver) more Xmax than rated if you consider that the non-linearity is soft, and that we tolerate a good amount of distortion at LF.

Pete B.

[tinitus]

Originally posted by scottandmo
The spl is under 110db at 150 watt which I don't think is an overboard.

Im no sub expert and maybe shouldnt respond here

But I would expect that at 110db SPL on subs the SPL on main speakers should be a bit higher, so that your general SPL is expected to be a bit higher

Dont know how loud 110db really is, but I guess its not very pleasent fore very long

[Ron E]

True, you can blow the woofer up mechanically or even thermally with a 250-500W amp. Most woofers have a mechanical limit that is much less than their thermal limit. Mechanically, you could probably toast this driver in 100 milliseconds with the power you have available, but thermally if the 500W rating is a true IEC rating, it will likely only happen if you tolerate huge amounts of clipping distortion for 10-20 seconds. All the above assumes you are listening to test signals...

With most music, You can't deliver a clean 500W RMS into a driver without perhaps 2kW or more of amp power, so take your amp power, divide by 5-10 and that is the true amount of clean RMS power you can put into a driver. If you don't have that much headroom, there are going to be some warning signs (distortion) for most sensible people.

The problem is, music and movies are not a test signal, you could be cruising along for 20 minutes at a reasonable level and the wham comes an explosion or a louder passage that fries your woofer mechanically. With vented woofers, excursion goes up below tuning, so one way to protect them mechanically is to cut them off below tuning with a subsonic filter.

Oh, and 110dB is quite loud - if your frame of reference is a car system, you will be disappointed with a home sub unless you have radiating area and power in proportion to the difference in size between your living room and car cabin. A pair of 8's in a car can sound quite nice, but to get the same bass in a typical living room, you'd need probably 6x 15" drivers or more.

[scottandmo]

thanks, you guys have helped me understand the mechanical (or displacement) limit versus thermal. I was playing with the values and noticed that the graph was the same no matter what wattage I set because it is always graphing the MAX excursion. So really isn't that helpful since it's not telling me how much power I can apply before it actually hits the mechanical maximum. I'll look for some other software that can model what the excursion limit will look like at a particular wattage.

[GM]

In Unibox? You must not be updating the plots each time you change the wattage.

GM

[PB2]

Originally posted by scottandmo
thanks, you guys have helped me understand the mechanical (or displacement) limit versus thermal. I was playing with the values and noticed that the graph was the same no matter what wattage I set because it is always graphing the MAX excursion. So really isn't that helpful since it's not telling me how much power I can apply before it actually hits the mechanical maximum. I'll look for some other software that can model what the excursion limit will look like at a particular wattage.

I think you've got that backwards, if the thermal wattage rating of the driver is set very high, then you will be looking at the mechanical limit say below 100 Hz where we are interested.

I set the power rating to 2 or 10KW when I just want to see the mechanical limit.

Thermal I just estimate from the physical design, wind length, diameter, metal versus thermally insulating former.

Pete B.

[scottandmo]

Ok, i figured out that the graphs weren't updated because I was clicking the optimize Vb button but that wasn't updating the graphs. Now its says that 12mm (my max) excursion will be reached at 20 Hz if I supply 40 watts. My amp says "Stock" Values - Flat to 30 Hz, -1.5 dB @ 20 Hz, -3 dB @ 13 Hz. -3db basically means half power, right? So that means at 13 Hz my amp could supply up to 120 watts which would definitely cause over excursion, the graph shows 23mm at that wattage. This would generally apply to most 12" drivers, right? I would have to find one with 23mm excursion to safely use it with my amp it seems?