Ported Enclosure Unloading?

[tinmanchris217 10+ year member]

Hey Guys,

I had a strange occurrence during gain setting of my old amp the other day. Normally, this topic doesn't belong in the Sub's section, but I think it's enclosure-related unloading.

Equipment:

2x Pioneer Premier ts-w2501d4 (2.5 cf per, tuned @ 50 Hz) - handles 1600wRMS for both

Lanzar Vibe 268 - puts out ~400w RMS @ 4 ohms, conservatively

Plenty of electrical for this amp

Gain Setting Time:

So according to basic math, this amp should have a V_max >= Sqrt(400*4) = 40V across the terminals. I get to about 30 V on a 50 - 60 Hz tone and the woofer quickly transitions from stable and strong, to loose and bottoming out.

I need to check the impedance of the woofers when this happens, but the phenomenon is repeatable, after you get above 30 V.

I can't find a reasonable explanation for the first 300w to be well controlled, and magically it gets loose as a moose. I'm not clipping the amp, and the woofers can handle much much more...

My question:

Since I have 10's in a big arse box, is it possible that the box dynamics to cause this?

Click to read more...

 

[av8ter 10+ year member]

Very interesting. I want to know what others think because I am able to do the same thing with my SE 12 in its spec box at 40-50hz...........

Kevin

 

[skmfkr 5,000+ posts]

your box is too big

 

[Mass Car Audio 10+ year member]

My question:

Since I have 10's in a big arse box, is it possible that the box dynamics to cause this?

yes a box that is way to big for your driver it will cause that.

 

[tinmanchris217 10+ year member]

Thanks for taking the time to relay your experience, Guru's!

Is there a physics-based, acoustical explanation for it?

My guess is too much in-box compliance. Transfer function of the back-wave has WAY too much overshoot, even though the sub is reasonably stiff, because it doesn't have in-box stiffness to support it. Basically free-air resonance...

So how would ya'll build the box differently? 1.2 - 1.5 cubes per woofer, instead of 2.5? Less than 64 sq in of port area?

 

[oxsign 5,000+ posts]

Wait wait wait, who do you think you are coming in here with all your big words and fancy talk?!?!? Do you know what forum your on?!?!?!

J/K, this is a good question though.

 

[terra 5,000+ posts]

In normal words, you're losing backpressure.

 

[tinmanchris217 10+ year member]

Sorry about that guys... I'm a mechanical engineering grad student... working on control systems...

Transfer functions are talked about once an hour, on the hour... or else I'm fired.

//content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

Yes, losing back-pressure. What size should I modify the box to?

 

[audioholic 5,000+ posts]

When a vented enclosure 'unloads', that is due to the signal material deviating too far below tuning. With such a high tuning, this is very possible with music, but when playing a single tone at or near (slightly) tuning, like you said you were, this isn't going to happen.

Too large of an enclosure, and/or too much port area can cause too little acoutsic dampening for a given power level. Basically speaking, the larger the enclosure, or port area, the quicker the sub can reach its xmax, much like a sealed enclosure system. Vented systems work similar, but signal frequency also affects back pressure on the cone due to port tuning. But in general, yes a larger vented enclosure allows for more cone travel, and decreases power handling of the system.

But, if its merely an example of too large an enclosure, the cone excursion should increase at a linear rate. A small jump in input power should not increase cone excursion a larger amount than did the same power increase at a lower excursion level. But, if the sub is 'unloading' due to the signal varying below tuning, excursion does increase drastically at a rapid rate, once the signal deviates below tuning. But as I said earlier, if you are playing a single tone, this should not happen. Even if the tone is below tuning, excursion will increase faster than normal, but it wont hit a 'plateau' where all the sudden excursion increases at a more rapid rate.

My guess, without really knowing much/enough about the situation, is you are clipping your amp. Im being too lazy to check specs or do any math tho, just saying that's what the symptom you describe sounds like to me.

Cheers, and good luck.

 

[audioholic 5,000+ posts]

Im a little bit medicated today, so I may not make alot of sense either. Just so you know, in case Im not.

edit: I had to edit this post 3 times for grammar

 

[tinmanchris217 10+ year member]

The amp is another possibility. I have 4.5 ohm load on it, according to DMM.

It's a 2 channel, class a/b with 60 amps of fusing.

According to my math, 60 amps, means 770 watts of input power at 12.8V. 60% efficiency gives me a maximum of ~460w RMS available. This may be too optimistic though, since it is a VERY crappy amp. The fusing may be excessive, and the efficiency might not be that high.

The voltage this causes across the bridged 2 channels should be 45 V. The 30 V that the "clipping" would happen at, means the amp is only producing 200w @ my 12.8 V battery power... which is thoroughly depressing.

Thank you for the science lesson though, medicated thinking can be good for creative though. //content.invisioncic.com/y282845/emoticons/crazy.gif.c13912c32de98515d3142759a824dae7.gif

 

[tinmanchris217 10+ year member]

Did some search work, and BigBangTheory's box for the 2 Pioneer SPL 10's is:

2.0 cubic feet and 38 sq. in, tuned to 33

If the amplifier is not clipping out, I think I'll try going down to 1.5 cf per woofer, and allowing for future volume removal down to 0.8.

Does anyone have any experience with Pioneers (AQ SD's or similar mid-level 10's) to help guide me?

Thanks,

Chris

 

[Kyle_Keating 10+ year member]

Hey Guys,
I had a strange occurrence during gain setting of my old amp the other day. Normally, this topic doesn't belong in the Sub's section, but I think it's enclosure-related unloading.

Equipment:

2x Pioneer Premier ts-w2501d4 (2.5 cf per, tuned @ 50 Hz) - handles 1600wRMS for both

Lanzar Vibe 268 - puts out ~400w RMS @ 4 ohms, conservatively

Plenty of electrical for this amp

Gain Setting Time:

So according to basic math, this amp should have a V_max >= Sqrt(400*4) = 40V across the terminals. I get to about 30 V on a 50 - 60 Hz tone and the woofer quickly transitions from stable and strong, to loose and bottoming out.

I need to check the impedance of the woofers when this happens, but the phenomenon is repeatable, after you get above 30 V.

I can't find a reasonable explanation for the first 300w to be well controlled, and magically it gets loose as a moose. I'm not clipping the amp, and the woofers can handle much much more...

My question:

Since I have 10's in a big arse box, is it possible that the box dynamics to cause this?

i would really encourage you to get an imdpeance meaurment of that box, you might not have a 50Hz tune,

a couple theories.

60Hz is likely the point at which the driver is going to experience the most displacement in that system (slightly above tuning). if the coil leaves the gap in that case, you get a large drop in back EMF and a surge in current, this is equivlent to loading a port (electrically speaking), expect only in this case, there is nothing to load against so the excursion skyrockets due to the current draw and you'll see cone's displacement nearly double with just a few addition volts and the distortion will be very large.

second theory, 60Hz could produce a flux modulation in the DC magnetic circuit with a high enough voltage across the coil, flux modulation usually is affected by a very acute frequency (60Hz is common), and if that's the case, you might be modulating the motor with that voltage level such that the electromagnetic field of the coil offsets itself enough to shoot below the gap and bottom out - clank! A simple shorting ring would solve the problem - it boggles my mind why manufactures do not use shorting rings under gaps - they improve sound quality and can even improve SPL - no reason to omit them really, they cost only a few dollars.

 

[tinmanchris217 10+ year member]

So I tested a few things this morning.

Originally, I did gain setting with the subs connected, assuming that a Lanzar Vibe has an unregulated power supply.

Today, I tried it with subs disconnected. I was able to get the 40 V across the bridged speaker terminals with the gain up at 50%.

My Pioneer Premier headunit (bragging 5 V pre-outs) was doing around 0.5 V with the volume at 45/62. 2 points for accuracy....

My new questions:

1) Does the Lanzar Vibrant series really have a regulated power supply (like JL, and others)?

2) Is it possible to begin clipping the amp at ~50% gain, with fairly low pre-out voltage?

 

[n2audio 5,000+ posts]

I couldn't find t/s data on the 2501's, but the 2502's have a very low vas, at ~2.0cf that sub is probably starting to exhibit free-air behavior.

all amp power supplies have some degree of regulation. I would expect the cheap lanzar's are on the loose side.

I wouldn't think the amp would clip at 1/2 gain with a 1/2volt signal. The manual says 250mV to 5v on input sensitivity. I guess it could be a very non-linear gain adjustment.