8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

[kaidomac]

I just inherited some 3/4" MDF and wanted another project to mess around with, so I got 4 of the 8" GRS BOFU-replacement drivers on my last PE order:

http://www.parts-express.com/pe/showdetl.cfm?partnumber=292-430

Last year, rogoll posted his WT3 results on those drivers:

http://techtalk.parts-express.com/showthread.php?223685-WT3-Results-vs-Published-Values

I would like to make 2 pairs of 48" floorstanding speakers - one set ported, one set sealed (I have some 1-1/4" PVC handy for ports, plus some PVC elbows). Just a standard tall rectangle cabinet design is what I'm looking to do. I'm looking for some help on the dimensions - I'm guessing I should probably go with the WT3 results? Although I'll be keeping the whizzer cones on the front for these projects.

[diy speaker guy]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

I can model it for you easy enough but personally I wouldn't do anything without measuring the drivers. There's a HUGE discrepancy between the manufacturer spec and the measured example you posted. I've never seen such a big difference between published and measured, it will make a huge difference in the model.

[chrisn]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

I can model it for you easy enough but personally I wouldn't do anything without measuring the drivers. There's a HUGE discrepancy between the manufacturer spec and the measured example you posted. I've never seen such a big difference between published and measured, it will make a huge difference in the model.

Mine measured similarily out of spec also. I would build them sealed, in a large box, with the driver close to listening height if you aren't usng a tweeter.

[kaidomac]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

Mine measured similarily out of spec also. I would build them sealed, in a large box, with the driver close to listening height if you aren't usng a tweeter.

What did you end up doing with yours?

[kaidomac]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

I can model it for you easy enough but personally I wouldn't do anything without measuring the drivers. There's a HUGE discrepancy between the manufacturer spec and the measured example you posted. I've never seen such a big difference between published and measured, it will make a huge difference in the model.

Is there a big difference in the final box size, or would they be more or less in the same range? (currently don't have anything handy to measure with)

[kaidomac]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

...with the driver close to listening height if you aren't usng a tweeter.

Also any tweeters you recommend? If I like these, I may do some more. I'm finding that I really like the full-range sound, although as with my Aura NS3 projects, a tweeter would definitely help!

[diy speaker guy]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

Is there a big difference in the final box size, or would they be more or less in the same range? (currently don't have anything handy to measure with)

A difference of .4 or .85 qts is a pretty big deal. If it's .85 for real it probably shouldn't be used in anything other than sealed and maybe possibly OB - assuming you are shooting for relatively flat response. If it's closer to .4 you have a lot more options.

[ameuba10]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

I dont mean to semi-thread jack but, are these good for a dipole design?

[diy speaker guy]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

I dont mean to semi-thread jack but, are these good for a dipole design?

Maybe, you would have to model it, but it's best to have accurate specs first. It would almost certainly work for a mid driver. It would be a bit of a stretch to make it play full range, although it could probably do it on a big baffle with a bit of filtering. It wouldn't be able to go very loud in that situation, but that's OB.

If you want OB bass on a slim baffle, I'd look to the other grs drivers with higher qts and use them in multiples. Unless you have active crossover and dsp, in which case you can do whatever you want with your choice of driver.

[kaidomac]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

A difference of .4 or .85 qts is a pretty big deal. If it's .85 for real it probably shouldn't be used in anything other than sealed and maybe possibly OB - assuming you are shooting for relatively flat response. If it's closer to .4 you have a lot more options.

Alright then - noob 101: if I remember right, I need a mic & some software, right? How is testing typically done?

[diy speaker guy]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

Alright then - noob 101: if I remember right, I need a mic & some software, right? How is testing typically done?

No, we want to measure impedance. You can make your own testing jig if you have a resistor and a couple of 3.5 mm jack ends. Or you can buy a woofer tester.

On the other hand I can just model for an average. Surprisingly even the high measured qts spec models well in a ported box.



That's 8 x 11 x 46.5 inches internal, I assume you dont' want to go too much bigger than that. It's got a 2.5 inch port, .75 inches long. Shown at xmax, 12 watts. Moving up to 12 x 12 flattens out response nicely but that's a big box.

The first graph shows the enclosure with the measured specs. Making the box larger will fill out the low end.

The second graph shows excursion, and gives an idea of tuning.

The third graph shows port velocity is under control.

The fourth graph shows the same box with the manufacturer's published specs. That lump on the low end is easy to flatten by making the port longer, but you rapidly run out of excursion (and max spl) as you lower the tuning. As it is, it's already quite low spl once you take away the baffle step loss.

These graphs do not show diffraction, so there's no baffle step. If you want to run this full range you might need some bsc, if you don't want to do that you might try putting them in room corners to boost the bass up.

[chrisn]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

What did you end up doing with yours?

I was going to try them as a full range in a multi use box I never built. The specs were too far off (Qts .96, FS 63HZ, VAS .95ft ) too use as intended. I listened to them in a small test box, with a filter too flatten the response. It has no bass and little treble. I would have paired them with a Vifa BC25SG15 if I went further with them.

[diy speaker guy]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

I was going to try them as a full range in a multi use box I never built. The specs were too far off (Qts .96, FS 63HZ, VAS .95ft ) too use as intended. I listened to them in a small test box, with a filter too flatten the response. It has no bass and little treble. I would have paired them with a Vifa BC25SG15 if I went further with them.

You can "fix" the specs a bit by completely blocking all the driver basket holes with fibreglass. It needs to be quite tight, with no "air holes" - air has to go through the fibreglass. The more you impede the airflow, the more qts will drop. Some of the other specs might change a bit too but it's mainly qts that's going to change. Polyfill won't work nearly as well as fibreglass - you can try it if you can't stand fibreglass but it isn't as effective. Fibreglass is messy and this method makes it hard to get the driver through the baffle hole but it's effective. It won't get your measured qts down to the manufacturer published spec but it might make your drivers usable.

There are other variations on this theme that you can try as well, in addition to or in replacement of the fibreglass. Some people use layers of semi breatheable cloth on the backside of the baffle to similarly impede the airflow.

[chrisn]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

You can "fix" the specs a bit by completely blocking all the driver basket holes with fibreglass. It needs to be quite tight, with no "air holes" - air has to go through the fibreglass. The more you impede the airflow, the more qts will drop. Some of the other specs might change a bit too but it's mainly qts that's going to change. Polyfill won't work nearly as well as fibreglass - you can try it if you can't stand fibreglass but it isn't as effective. Fibreglass is messy and this method makes it hard to get the driver through the baffle hole but it's effective. It won't get your measured qts down to the manufacturer published spec but it might make your drivers usable.

There are other variations on this theme that you can try as well, in addition to or in replacement of the fibreglass. Some people use layers of semi breatheable cloth on the backside of the baffle to similarly impede the airflow.

Interesting idea. I had some mid drivers that had the basket closed off with a felt like material. I may give this a try just too see what happens.

[kaidomac]

Re: 8" GRS 8FR-8 Full-Range Drivers - need help with 48" cabinet design!

No, we want to measure impedance. You can make your own testing jig if you have a resistor and a couple of 3.5 mm jack ends. Or you can buy a woofer tester.

On the other hand I can just model for an average. Surprisingly even the high measured qts spec models well in a ported box.

That's 8 x 11 x 46.5 inches internal, I assume you dont' want to go too much bigger than that. It's got a 2.5 inch port, .75 inches long. Shown at xmax, 12 watts. Moving up to 12 x 12 flattens out response nicely but that's a big box.

The first graph shows the enclosure with the measured specs. Making the box larger will fill out the low end.

The second graph shows excursion, and gives an idea of tuning.

The third graph shows port velocity is under control.

The fourth graph shows the same box with the manufacturer's published specs. That lump on the low end is easy to flatten by making the port longer, but you rapidly run out of excursion (and max spl) as you lower the tuning. As it is, it's already quite low spl once you take away the baffle step loss.

These graphs do not show diffraction, so there's no baffle step. If you want to run this full range you might need some bsc, if you don't want to do that you might try putting them in room corners to boost the bass up.

Actually the bigger box, the better :D Thanks for the detailed info! Maybe I'll try both designs - an 8 x 11 and a 12 x 12. So for the 2.5" port...if I use .75" MDF, all I need to do is a cut a hole?

It sounds like these would be pretty easy to make...a tall box with a cutout for the speaker, a hole for the port, and I'd assume some foam inside? For $14 a speaker, I'm not too concerned with audiophile-grade sound quality, mostly I want to get my hands dirty in the shop with some more easy starter projects ;) I've been having a lot of fun tinkering with my Aura NS3's and wanted to try something a little bigger, but not much more complex since my woodworking skills are still growing