How do you reduce/prevent ringing in sealed subwoofer?

[KEW]

I recognize that sealed subs generally have less issues with impulse response than ported, but looking at the data bass chart waterfalls, it seems like sealed subs can exhibit rather substantial ringing.

Is there a way to model or predict ringing?

Does increasing the size of the cabinet have any effect?

 

[Swerd]

I recognize that sealed subs generally have less issues with impulse response than ported, but looking at the data bass chart waterfalls, it seems like sealed subs can exhibit rather substantial ringing.

Is there a way to model or predict ringing?

Does increasing the size of the cabinet have any effect?

It depends on which ringing you're talking about. The type you might see in a waterfall graph can come from a variety of sources, including but not limited to an inadequately braced cabinet, or a resonance in the structure of the driver frame or motor assembly.

But there is another source of ringing, that you can model and predict, and it is related to the cabinet size. You can see it directly with an impulse response curve.

Read post #5 (from Dec 7, 2014):
http://forums.audioholics.com/forums/threads/frequency-response-graphs.92881/#post-1062303

"The total system Q of the woofer combined with cabinet is a ratio of values (with no units) that defines both the shape of the frequency response curve roll-off. It also determines the amount of damping to oscillation or ringing after the signal stops. This is also spoken of as “transient response”. Qtc is the term for sealed cabinets, and the similar Qts term is for ported reflex cabinets."

As Qtc gets to 0.7 or lower, the woofer's motion is damped enough to prevent ringing.

If you have a specific sub woofer in mind, look up its T/S parameters, and find its Fs, Qts, and Vas values. Plug them into an online calculator and choose the Qtc value you want. It will calculate a number of values, and predict a frequency response curve for the low end of the driver. It won't directly show an impulse graph, but you can play with various Qtc values and see what happens.

Calculate Closed Enclosure (choose Qtc)
http://www.mh-audio.nl/CBC.asp

There are other calculators here http://www.mh-audio.nl/spk_calc.asp that let you choose the volume of the cabinet instead of the Qtc.

 

[highfigh]

I recognize that sealed subs generally have less issues with impulse response than ported, but looking at the data bass chart waterfalls, it seems like sealed subs can exhibit rather substantial ringing.

Is there a way to model or predict ringing?

Does increasing the size of the cabinet have any effect?

Vented subs tend to ring far more than sealed because the cabinet doesn't dampen the motion, especially below the vent tuning frequency and because the Q of the cabinet/vent is higher than what's typical in a sealed box. As Swerd posted, a sealed box that is designed to be well-damped (Qb at .707) will not ring but if the Qb is lower, its sound is considered 'dry' because it doesn't ring at all. High Q subs are considered 'boomy' and if you have heard car systems that could be called a 'one note wonder' because they only do one frequency well, this is usually done by designing the box for high Qb. Music with a very limited use of the bass range benefits from this because the user usually wants everyone to hear his big potent system from as far away as possible. It's also used in competition systems because it's the low frequencies that make the SPL meter see it as "louder" than another system that actually sounds good. These one not wonders don't score well in the RTA or sound quality section when the judges know what they're doing.

 

[Steve81]

Time for some practical examples:
The Creative SDX-15 tested in a 1.25 cu foot enclosure, which results in a high Q alignment. You can see what this does in the time domain as well as the basic frequency response. You can compare this to the B&C 21SW152-4 tested in a 4.5 cu foot enclosure, which results in a relatively low Q alignment.

Moving on, I'll point out one other factor I haven't seen mentioned yet: EQing to extend the response. A fine example of this is the SVS SB12-NSD. Looking at the base frequency response, you see something you'd expect from a vented alignment, i.e. extension down to the low 20Hz range, with a steep rolloff. It takes a great deal of boost to achieve this FR as well as a high pass filter to protect the driver from damage. You can see what this does to the time domain by glancing at the group delay and impulse response.

 

[lsiberian]

In my experience as long as you are in between .5 and .7 Qtc you will be fine in a sealed design. I'm not aware of bracing having any real impact on Qtc. The frequencies produced by a sub are typically below the threshold of panel activation. The primary concern for me is box strength which is definitely important with a powerful driver. Vented subs definitely ring more, but a properly designed one uses that ringing to enhance a lot of source material. However even the very best vented system I've heard butchered the music from my teen years(loudness wars rock, etc). It made the symphonies epic though. I kind a like the idea of having one of each for flexibility.

 

[TLS Guy]

Ported subs DO NOT necessarily resonate more than sealed subs. Both designed badly will be bad news.

The real problem is far too many high Qts sub drivers around.

Now the end design will have a Q higher than the driver.

If you choose a a driver with the T/S parameters in the sweet spot, and use a sensible alignment, then a ported enclosure can be very tight and as good or better than most sealed units and a lot more efficient, and far better at coupling to the room.

So how does this happen with so many sealed subs.

Well it all comes down to money. If you have a sub driver with T/S parameters reasonable and want a sealed sub, you will find a very high F3.

So then you need a driver that can handle gobs of power, (none take what they say really) and have to use lots of Eq to extend the bass response.

So if you take one of those awful sloppy drivers, you have a lower F3, need less power and less Eq. Do they sound good, no.

In the final analysis, as I have stated so many times before, there is no free lunch.

If you build a bass system smaller than the laws of physics demand, then you will have a compromised low frequency system in at least one or more ways.

If you don't care how big the enclosure is, it is a fairly easy matter to design an almost perfect bass system

Bottom line, is that between money and size, the route to problems is paved.

One last thought, if it were not for the above issues, no one in their right mind would build a low bass system with a driver in a sealed box. That is absolutely the worst way to use a moving coil driver in an a system for the last octave.