What's a good response curve

[amoeba86]

I've been playing with WinISD for a while now but i want to know what is an acceptable plot. Many people's subwoofer projects i have seen where they have been using a decent woofer are able to tune it too below 20Hz (these are normally for HT use). However when i get my plots if i'm to get a similar tuning frequency my plot dips down then raises before finally dipping again, most of the time the first dip is down near the -3db point so it would appear i'm losing a lot of response around 40-50hz. Ok for example when i plot my JBL-2245H 18" woofer the default plot that WinISD gives me automatically looks like this:

and when i tune it to 22hz:


here are the T/S specs:
fs:
20 Hz
Re:
5.8 ohms
Qts:
0.27
Qms:
2.2
Qes:
0.31
Vas:
820 L (29 ft3)
Sd:
0.130 m2 (200 in2)
Xmax:
9.5 mm (3/8 in)
Vd:
1,230 cm3 (75 in3)
Le:
1.4 mH
no (Half space):
2.1%
Pe (Max):
300 W Continuous Sine Wave

[Vikash]

I think the curve is only half the story. I mean would you really consider building a 900 litre enclosure (31.7 cuft)? You need to consider the other variables for the full story (excursion, group delay, resistance etc.). Some woofers simply won't go that low with a flat response, but the dips are a common phenomenon if you want to push it that far.

[amoeba86]

the enclosure doesn't seem too big for an 18" woofer really but i want to know if having those dips is alright because i've even input specs from woofers i've seen other people use and to get the tuning frequency they get i still get the dips. i just want to know if it's alright to tune it that low for a home theatre sub.

[roddyama]

For music, the f3 of 32Hz would have been quite respectable. You could have always move the fb down a few Hz to move the f3 into the 20’s at the expense of a little transient response. For HT the lower fb will work as well, but I wouldn’t recommend moving the fb below the fs of the woofer (which you haven’t done). You could run into excursion problems below fs.

The dip in you response curve won’t be a problem. My self, I would move the fb up to the 25Hz range. It will be a little “tighter” and still have significant output to the low 20’s.

In general, it is difficult to talk about the FR of a subwoofer to an accuracy much finer then +-3db when in a real listening environment. Room boundaries, speaker placements, and speaker to speaker proximity all play a significant roll in the total FR of the subwoofers. You’ll also have to factor in the low-pass x-over filer you will use at the upper range of the subwoofers. What frequency will this be? What slope are you planning? Will it be active or passive?

[amoeba86]

i'll probably end up getting a decent plate amp and use it's inbuilt crossover, thanks for the help.

[phreeky82]

i've whipped up a couple of test boxes to test the accuracy of winisd for a driver of mine. it was very good, but the curves seem to look worse than what they sound.

winisd shows my jbl in its current enclosure dropping off quite early, but i listen to it in a small room, so it makes all the difference.

u ought to go for a realistic box size (900 L is not realistic in my mind). for me, 300L is the absolute maximum, and if u can afford it, make a couple of boxes in different sizes, nice a simple cubes, just to test it. nothing better than having enclosures of different sizes lying around etc.

[OMNIFEX]

amoeba86

The goal in building a sub is getting the flattest curve
as possible.

Now, you will find there are times when using an
undersized enclosure will gve an oval hump on your
response curve. But, once your curve start moving
up and down like a rollercoaster, its unacceptable.

Your best bet would be to go for the 217.9 Liter
Enclosure, for, it will give you the flattest response
with no eq adjustments.

If want to use a 900 Liter Enclosure, throw an Eminence
Killomax 18 in the box. It will give a flat response from
100 - 21Hz, with a -3 dB @ 19.2 Hz Tuned to 22 Hertz.

[amoeba86]

i understand that the flatest response is good for sound quality however i'm using this for home theatre and i've seen other people tune their boxes to sub 20hz and have it sound good, as i've said i tried getting plots with woofers others have used such as the Adire Shiva and Tempest woofers and i get a similar shape when tuning low. I know 900l seems big but it's only just over 1metre in 2 of the dimensions.

[OMNIFEX]

Please Reread my thread. I've done some modifications
to it.

As for "I've seen other people do it" statement. Just
because they do it doesn't mean its right.

Best Regards,

[amoeba86]

I picked up the JBL cheapily and i can't really afford another woofer so i'll have to stick with it, since i can get mdf cheapily i'm gunna have a crack at the 900l enclosure. I don't mean to disregard all your comments but i really want some low bass for movies, i might make up the smaller 217l enclosure to see how that sounds.