GRS GPT 18 - New Budget Banger?

belgiansound said:

I don't have money to spare but I like your thinking. Can model them in winisd with a 100.000 L sealed cab!

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Assuming you are talking about infinite baffle without an enclosure, qts is generally the deciding factor of whether the woofer will work well or not. .707 means that the free air qtc will also be .707, which corresponds to a 12dB/octave rolloff at fs, which is generally considered the "best" because of how excursion relates to low frequencies.

With that said, it's very possible to use drivers with lower qts in an IB config, the rolloff will just be shallower, and excursion will not be as constant with voltage below fs. The 18PT-8 has a qts around .55, which is adequate, but another aspect that is recommended for IB designs is to have a lot of xmax because the efficiency is so high. This is not a characteristic the 18pt-8 has.

IB designs have a unique characteristic that their impedance rises exponentially until fs, with this in mind that means that the actually amp power also exponentially decreases until fs. Interestingly enough, this function directly translates to a linkwitz transform that fill in the rolloff. So essentially, any IB design can play flat to their fs without being overdriven thermally. The problem is that it also takes exponentially more excursion to achieve that response. Which is why high xmax is recommended. This type of design is quite unpopular though because after fs, the power very quickly rises, also, room interaction generally also follows a similar pattern to a linkwitz transform, so it is rarely needed. However, I shared it because it gives a good idea of the true efficiency of IB designs. Technically, if you had a woofer with infinite excursion and an fs of 1hz, you could have a system that plays flat to 1hz without thermal compression. The amp would still need to be able to supply crazy voltage, and the woofer would likely be very large or very inefficient, but it's just an example.

If you want to model a IB design, hornresp is probably the best way to go. Winisd can do it as well, but 100L is far too little, 10000L is probably more accurate for that woofer. You basically want to select sealed box, then increase the size until you achieve close to the drivers fs and qts for fsc and qtc values. Keep in mind that winisd does a weird thing where if you go too high, you will start to lose efficiency, so you can't just put an infinite number.

Overall, running 8 of them IB would probably work fairly well, but their are almost definitely more convenient options available. Think about it this way. Each 18pt-8 has 6mm of excursion, that means that all you need to match 4 of them is something with 27mm of excursion, or two somethings with 13.5mm of excursion. I don't know what is available is Europe, but here I could get a si ht18v3, or some cheap car subs that do the 13.5mm. Now probably not equally, inductance would definitely be higher, but whether I would choose mounting 4 subwoofers over 1, or 8 over 2 in your case would definitely be a pro I would weigh against that.

This speaker got me curious.

MKtheater said:

Keeping up? Can you compare on output and extension? You have SI, Skar, PA460 and these? Any comparisons?

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I don't have SIs, not yet anyway, LOL! I may or may not be getting my hands on those later this year or some time next year, we shall see.

As far as what I mean by "keeping up", what I'm saying is, because I can control the power going to each driver individually, I'm able to shut them off or power them on at will. With that, at least on the SPL meter, in REW, and (most importantly to me) to my ear/gut, I'm able to verify that a pair of GRS 18s do a decent job of delivering bass response to my seat. Now, they're not insanely dramatic, ZOMG I CAN'T BELIVE THIS MUCH BASS, but for under $70/ea (because I bought 4), shipped for free and installed in some MDF I had lying around (so the net-cost of them was literally $130~$140), they are impressive. Playing a pair of the PA460s in a similarly tuned enclosure side-by-side, there is definitely an improvement when adding the GRSes to the mix (when going from 2 drivers to 4, that is).

Now, I'm also not being ultra scientific with this because, I don't care that much. I can look at the sweeps, see good results, look at overall output, see good results, then sit at MLP, get good listening results, and I'm happy with that. I didn't do side-by-side compression sweeps nor did I stand next to them at-or-above reference to see how much more (or less) they're "working" when cranking out 10 - to - 15 Hertz sinewaves. That kind of apples-to-apples comparisons just don't interest me.