2 hz tuning - pros and cons - Page 7

I must say I can understand diy's frustration.

I think this thread has gone 'round and 'round because of the dozens of comments on port compression, when the simulations have shown, and not been refuted that I can see, that there won't be any, except maybe with a track or two, like F'in Irene which I believe has some strong 5 Hz rotor blade noise.

It's also been pointed though that such a small port can't add significant useful output, and this is borne out by the simulations as well, which show an increase of a couple of dB.

Going off on a tangent, sort of, many times I've puzzled over why is that a couple of dB is not a lot?

That represents about a 60% increase in efficiency, which anywhere else in engineering is huge.

I think the answer is that it may very well be significant if that's the difference between loud enough and not quite loud enough, or loud enough with a hotter VC and more compression than if you didn't have the extra 2 dB.

To those who say 2 dB isn't a big difference in loudness, I guess that's personal, as many times the 1 dB steps on my volume control seem too coarse.

OTOOH, the above applies to midrange freq but perhaps not the low freq we're talking about here.
Edited by noah katz - 12/11/12 at 10:27am

Not to go too off topic, but 'they' told Columbus that the world was round but the shore of India was too far away and he'd never make it, because even back then there was a consensus on the circumference of the Earth and the journey was seen as being too long. Columbus disagreed, but he was in fact the one who was wrong... but he was lucky. Possibly as lucky as anyone in all of history. He'd be dead if the America's weren't there. http://www-spof.gsfc.nasa.gov/stargaze/Scolumb.htm

I don't think there's an undiscovered country hiding somewhere inside the physics that define the workings of a conventional subwoofer. If you want 2 Hz, you need a fan-based design whereby displacement naturally (and dramatically) increases as the frequency decreases... or you'll have to build out your entire wall with subs (and still likely not match the fan).

Edited by imagic - 12/11/12 at 10:24am

Why do you think the air in a port ever behaves as compressible?

Oh, and diy, about you're analogy between port air and fluids in pipelines:

I don't really see the comparison, as the point of a port is to get some extra energy with resonance, and the point of a pipeline is to transfer material without much importance to the energy required.

"I've asked at least a dozen times where you guys think the model breaks down. I've received no answer at all."

chuffing is where turbulence begins.

turbulence is where you get complete compression. the port fails and for all practical purposes, you have a sealed enclosure.

"Not true sir. Both the red line and the blue line in Flare It show the ONSET of turbulence, not the complete failure point."

blue line is the onset of turbulence aka chuffing.

red line is failure of the port.

"Yes, most of the people here have shown they are knowledgable but the problem here is they are not smarter than a simulation when it comes to predicting port compression, although they seem to think they are."

your model isn't prediciting compression. it is a t/s model.

"Like I said many posts ago, I don't expect audible chuffing until the core get up to 20 m/s or higher."

well, there is your major failure point.

"Turbulence begins way way before chuffing. Chuffing is simply turbulence so bad it sounds like a fart."

there is your problem right there.

"Increasing velocity even further, the air in the "core" of the port becomes turbulent. Flares are unable to help with this problem, which represents the limiting velocity for the port. By this stage the port is beginning to present a different load to the system, resulting in de-tuning and subsequent loss of output, known as compression."

http://www.subwoofer-builder.com/flare-testing.htm

this is what is meant by core limit. chuffing is just where you begin to hear some noise but the port is still functioning.

This effect can be observed with a kitchen funnel and rice grains, but also in freeway traffic. Go to 2:20 in this video:

"Wouldn't that be analogous to including Flare It results directly within the program? I admit that would be cool for ALL the simulators to have but since we can use Flare It separately I don't see any need."

sure, but you asked why the software was a failure. my answer was that it is based on t/s parameters only. t/s parameters don't include collo's work on port chuffing. as a result, relying on the simulation software only can fake one out.

"Yes, ok, that's what I said in the first place, many many posts ago."

i meant that your thinking that 20m/s was your failure point, not that 20m/s is your failure point. at 10m/s, you are at the core limit for a 2" port and will be compressing and de-tuning the sub. as such, the t/s models that have been posted along the way are no longer valid. in the latest one, that appears around 7hz. in my winisd model it is around 9hz. so somewhere in that region, the models are no longer valid.

I have not changed my stance on this. And I have attempted to explain multiple times why the simulation at high power is not representative of the real world result and also why this type of system doesn't really offer much of anything to justify its increased bulk and space occupied. Simulations scale in a perfectly linear manner. They do not for the most part attempt to address large signal deviations that occur in the system due to a large variety of factors. They do not simulate driver thermal compression, impedance rise, motor and suspension non linearities, vent compression, etc. You go from 1 volt input to a 10v input and you get a perfect increase of 20dB and then increase it from 10v to 100v and get another 20dB increase in output. This does not happen in the real world. Any speaker measured at high power will start to show shifts in the response and compression or even expansion of output as the parameters shift and with vented systems vent compression occurs. Simulations are very basic representations of much more complex and varied real systems. The more output you ask from the system the further the result deviates from the model due to these unaccounted for factors.

The model is fine and works as predicted at low power.

This one breaks down anywhere that the port is going to be pushed past 10m/s airspeed where significant vent compression is going to occur. How much we cannot know exactly with out having the actual system built and measured but all evidence points to there being compression. There are plenty of compression tests of vented subs available from Illka, Avtalk, myself and others. It is not being overblown. What is being overblown is the extra 1 maybe 2dB if we are generous of output all of it below 10Hz mind you, from a system many times the size of a comparable performing sealed system while ignoring things like vent noise and pipe resonances. It just doesn't make sense to build anything like this unless you have an IB sized space available.
Edited by Ricci - 12/11/12 at 12:31pm

Solution is simple. Build it, and present your findings

"I'm thinking 10 m/s is the onset of minor turbulence, minor compression, minor detuning."

for practical purposes, the point of onset of minor turbulence is where the port begins to audibly chuff. once the port is audibly chuffing, you are already getting minor compression and minor detuning, but it is usually acceptable. you are talking about exceeding the core limit, so you are talking about significant compression and detuning.

this is what all the other guys have been trying to say from the beginning.

"How fast is your vacuum sucking air?"

you do realize that electric motors make little to no sound and that most all the noise from a vacuum cleaner is related to all the various air turbulences?