Monkey see, monkey do... two RL-P18s for me too!

[Big Worm]

Beautiful work Bradley! Can't imagine how good it sounds. Can't believe you are building another one either. But that should be amazing!

[Bradley]

SteveCallas wrote:

Bridging the amp should eliminate the clipping

2400 watts to one RL-P18 on WoTW?! A "mere" 750 watts was enough to freak me out and shake the whole house. While I'm all about "wretched excess", I'm not sure how much more power these subs are going to need. I'd like to use a single amp to power both subs, for the sake of simplicity, space, and fan noise. I've been considering a QSC PLX 1804 amp, which does 900 watts per channel into 4 ohms and uses a solid state power supply so it only weighs 13 pounds.

Rodny Alvarez wrote:

Nice work Bradley!!!

What finish are you going with?

I picked up some Duplicolor bed liner coating. Other guys have reported good results using it on their sub boxes, so I hopefully I will be pleased with it.

Bent wrote:

My appologies for being assumptive - as well as curt...
my eyes saw what they wanted to see, rather than what was really there.

I was rushed in my needless reply..., but also wanted to say the job looks very good.

Hey, no worries. I've had my share of "brain farts" and foot-in-mouth episodes.

[Bradley]

Question for you guys... as far as I can tell by watching cone excursion, the tuning on this first sub is d@mn close to 13 Hz. I was shooting for a 14.5 Hz tune, using 17 cubic feet net volume with a 35.25" long 8.375" internal diameter port. To the best of my knowledge, I got pretty close to those specifications in the actual "product", so... what happened?! Why is my tune so low?

I suppose it's possible that I put too much polyfil in, but I know I have less than 5 bags (20 ounces each) in there, so I'm guessing under 6 pounds of polyfill. Maybe I should take some out and see what happens.

[WillyD]

Checking the models in Unibox, it does look like a significant amount of stuffing could lower the actual tuning quite close to 13Hz...maybe 13.5Hz?

This is just using the diam and length of port data you supply, net volume, and lowering the Qa.

All that paint made the port diameter smaller, lowering the tune

...j/k

[Owen Bartley]

I think there are a lot of variables that you just can't account for. Maybe the displacement of the driver is a little more or less than stated, maybe your bracing displaces more or less, or the port. Maybe there's some cabinet flex, a bit too much or too little stuffing. Maybe the design software is off by a 3rd decimal place somewhere, or is subject to rounding inconsistencies. Maybe th atmospheric pressure where you are is different than where the software and drivers were developed, and that accounts for a small difference. I think if you're that close, you've done a pretty great job, and probably the only guys who are hitting their exact tune without a little luck are the manufacturers who can use some trial and error to dial things in before mass manufacturing.

In any case, you've got yourself an amazing sub, soon to have a twin(!), and I think you did a great job to come very close to your calculated tuning frequency.

[Bradley]

I know I shouldn't worry about it too much but here's the thing... to go from a 14.5 Hz tune with the same port to a 13.0 Hz tune would require around 23% increase in net cabinet volume, or a 28% increase in the length of the port (all other things staying the same). A 5% discrepency I wouldn't worry about. Even 10% I could understand, but over 20% seems like something must be wrong, so I just want to try to understand what is going on.

The cabinet fill definitely was/is playing a role, and I did not properly account for it in my design plans. So last night I yanked a bunch of the polyfil out and tested it again. The Fb came up to around 13.5 Hz. However, my crusty old piece-of- signal generator is producing something that looks halfway between a sine wave and a triangle wave, so that is probably skewing the results. I need to get a real signal generator or maybe use the soundcard in my PC as a signal source and try the test again.

Another reason I'm concerned about the tune: 13 Hz wouldn't be a bad tune for a larger box but at 17 cubic feet, it's far from ideal. Even my target of 14.5 Hz could be considered a little low relative to the size of the box.

It's not the end of the world or a crisis or anything. Just trying to understand what's going on and learn from it. With the second sub in place and a little EQ, I'm sure I won't be wanting for output and low end extension.

[WillyD]

Have you measured the FR? If you're getting relatively flat extension from the sub despite the lower tuning, I don't see how this is a problem at all. Its really more important that your 2nd sub matches the 1st.

[Bradley]

I haven't done much in the way of measuring FR yet. Definitely on the "to do" list. Unfortunately, the only FR I can measure is the "in room" response, so I won't know exactly how much response is due to the sub's natural FR and how much is due to room gain. This is fine in terms of end result and EQ-ing but makes it hard to relate the FR curve to the models in WinISD or Unibox.

[Bradley]

OK... finally got my PC hooked up to the AVR via HDMI (for video) and optical SP/DIF for audio, downloaded drivers for the built in Realtek audio chip, and fired up REW. Here's my first in-room FR graph. I realize the SPL levels are a little high but I wasn't going for max SPL or anything. I'd have to figure out how to make REW start the sweep at no lower than 10 Hz for that!

[SteveCallas]

Bradley wrote:

2400 watts to one RL-P18 on WoTW?! A "mere" 750 watts was enough to freak me out and shake the whole house. While I'm all about "wretched excess", I'm not sure how much more power these subs are going to need.

You said you were noticing clipping on certain scenes - this is because you are only giving the sub 750 watts. It can take lots, lots more than that without any concern for damage - apparently a clipped signal poses a higher risk for damage, so you want to eliminate that. Bridge the amp until you start using both subs.

As for tuning, the port flare adds length to the port and 6lbs of stuffing sounds like a lot to me - assuming your volume calculations were perfect, those two things could account for the difference. However, your FR on the low end looks like an excellent starting point, I wouldn't mess with the tune at all. Before buying any type of EQ, see if you can flatten things out further by moving the sub a foot one way or another, and try opening/shutting various doors connected to that room. Also, play around with the distance settings in your processor to determine whether that dip ~60hz is crossover related or room related. If none of that does the trick, I'd say its almost a toss up whether you need EQ or not, as you are "relatively" flat naturally.

[Bradley]

Just to be clear... the clipping I referred to was simply the red clip indicator on the power amp starting to flicker. There was no audible clipping or bad sounds coming from the sub. I would never allow it to operate for any length of time with the amp clipping. Yes, I could bridge the amp and readjust the gain... but the main reason for the clipping, far as I can tell, is that the DTS track on WoTW is mixed riculously hot. I don't disbelieve what you are saying about the driver being able to take a lot more than 750W, but at the same time I don't want to find out "the hard way". I don't have any official subsonic filtering or HPF in place here (other than the power amp being rated -3 dB at 5 Hz), so I am a bit nervous about feeding it full power signals at or below 10 Hz.

A couple nights ago I removed a considerble amount of the polyfil. I'd guess there is about 3 pounds worth in there now. The tune now appears to in the ballpark of 14 Hz, using REW's signal generator to test it. I am pretty satisfied with the response curve so I have no plans to mess with the tuning any further.

I don't really understand what you mean about distance settings in the processor (AV receiver) and how that might cause a sharp dip in the FR. For the sake of testing the sub with REW, I set the crossover point in the AVR to 200 Hz and switched all the other speakers off, and recalibrated the SPL meter after every gain/volume or configuration change.

I do have a DCX 2496 sitting here, but I agree with you about hardly needing to bother with EQ.

[SteveCallas]

Ilkka tested a TC2k 15" outdoors using a 2400 watt amp and a 10-100hz sine sweep. It was tuned even higher, around 16-17hz, with no highpass filter, and it took all the power up to a 110db-115db sweep with no bottoming. The TC2K and RL-p15 use roughly the same motor, so the RL-p18 should be at least as capable in terms of handling power.

So that FR is with no mains in play? Hmm, the upper bass looks way too flat for that to be possible - TC motors have notoriously high Le, creating a slight rolloff above 50-60hz or so.

[WillyD]

So that FR is with no mains in play? Hmm, the upper bass looks way too flat for that to be possible - TC motors have notoriously high Le, creating a slight rolloff above 50-60hz or so.

Do you not remember the near-cone TC-2000 (in enclosure) measurement I did? Its upper end rolloff looked no worse than your Ava 18's.

And not all TC motors have notoriously high Le. The 3000, 5200's do. The TC-1000s and LMS do not. I'd say the Rl-p's are middle of the road. So yeah, that FR does look quite flat in the upper end, but hey, I don't believe many Rl-p 18s have been measured to determine their natural upper frequency response, so we don't really know.

[Bradley]

Interesting... so that test didn't involve any EQ and he ran the amp at full power into the TC2k? I've been trying to figure out how to make REW start the sweep at 10 Hz, but haven't found a way yet. Below 10 Hz that cone REALLY moves. As I'm sure you know, in the models the cone excursion goes nuts below tuning frequency and it takes a lot less than 1000 watts to exceed Xmech at 10 Hz. But I do understand that the driver is pretty tough and designed to take a certain amount of abuse. The suspension limit is about 41 mm, 5 mm short of smacking the back plate, so there's clearly a margin of safety built in there.

Yes. Absolutely 100% guaranteed my mains are switched off, and there is no EQ in effect that I am aware of.

[Bradley]

Adjusted the mic position by moving it one chair over and pointing it up, maybe 60 degrees from horizontal. Previously the mic was horizontal and pointing straight ahead toward the projection screen and sub.

This graphs looks a lot better in the 60 Hz region.

[Ilkka]

SteveCallas wrote:

Ilkka tested a TC2k 15" outdoors using a 2400 watt amp and a 10-100hz sine sweep. It was tuned even higher, around 16-17hz, with no highpass filter, and it took all the power up to a 110db-115db sweep with no bottoming. The TC2K and RL-p15 use roughly the same motor, so the RL-p18 should be at least as capable in terms of handling power.

Steve,

~16.5 Hz tune that I measured causes smaller excursion in 20-30 Hz range than a lower tune. Lower tune has more protection only *below* the tune.

So that FR is with no mains in play? Hmm, the upper bass looks way too flat for that to be possible - TC motors have notoriously high Le, creating a slight rolloff above 50-60hz or so.

The roll-off isn't that obvious in larger enclosures. If you look at the 270L I measured, it's only around 4-5 dB from 50 Hz to 200 Hz. Also look at Bradley's scale: 10 dB/div.

[Ilkka]

Bradley wrote:

Interesting... so that test didn't involve any EQ and he ran the amp at full power into the TC2k?

No, I wasn't able to run full ~2200 watts into it. With the sweeps, 110 dB at 50 Hz (nominal) was the highest that I was able to run all the way down to 10 Hz. That equals to around 600 watts per simulations. The driver didn't yet bottom out below the tune but it wasn't very far. Simulation shows around 55mm of excursion at 10 Hz but obviously that is just a small signal simulation.

When I raised the drive level by 5 dB (to ~1900 watts), the driver at least soft bottomed out at around 30 Hz. If it wouldn't have bottomed there, I'm 100% certain it would have bottomed out below ~15 Hz.

But on a program material, I wouldn't be too worried to use, say, 2000W amp with these drivers, especially the ones with this newer tall profile surround.

I've been trying to figure out how to make REW start the sweep at 10 Hz, but haven't found a way yet.

Unfortunately one can not adjust that.

[Blaser]

Bradley, Just had my daughter's birthday's party yesterday and was cranking a lot out of these subs using 2000W amp/sub for about 2 hours non stop...and no problem at all

[brucek]

I've been trying to figure out how to make REW start the sweep at 10 Hz, but haven't found a way yet.

Because there is no adjustment to the start of sweep, only end of sweep...

Do an REW electronic sweep of your AVR in a loop from the soundcard to check its low frequency extension. It may be providing free HPF protection.


brucek

[SteveCallas]

Willy wrote:

I'd say the Rl-p's are middle of the road. So yeah, that FR does look quite flat in the upper end, but hey, I don't believe many Rl-p 18s have been measured to determine their natural upper frequency response, so we don't really know.

Right, for all we know it could be very linear up top. Now that I think about it though, he's taking that measurement from his seat - which is absolutely the only place that matters - but to get an apples to apples comparison, we'd need a close mic sweep.

Bradely, any way you could do a close mic sweep (position the tip of the mic 3-4" away from the center of the cone) with something like a folded up blanket behind and or around the mic?

[Bradley]

brucek wrote:

Because there is no adjustment to the start of sweep, only end of sweep...

Do an REW electronic sweep of your AVR in a loop from the soundcard to check its low frequency extension. It may be providing free HPF protection.

I did that and the sound card is pretty d@mn flat. It was down not even half a dB at 10Hz.

Maybe it's time to buy some op amps and bread board up a 4th order activer HPF.

[Bradley]

SteveCallas wrote:

Bradely, any way you could do a close mic sweep (position the tip of the mic 3-4" away from the center of the cone) with something like a folded up blanket behind and or around the mic?

Here you go. I plugged the sound card directly into the power amp for this one to make sure no crossover was being done through the AVR. Judging from the big dip at 13 Hz, that would appear to be the tune, correct? Strange that it is still so low.

[Ilkka]

Bradley wrote:

Here you go. I plugged the sound card directly into the power amp for this one to make sure no crossover was being done through the AVR. Judging from the big dip at 13 Hz, that would appear to be the tune, correct? Strange that it is still so low.

Yes, the tune seems to be around 13 Hz. And the roll-off above 50 Hz looks as expected.

[brucek]

Interesting how the amps roll off below 10hz appears to be protecting you.......... doesn't appear that you require an HPF........