What happens in a Ported sub when unloading?

[warnerwh]

I read but am not sure I understand that when a sub reproduces a sound below it's tuning frequency that it "unloads". Does this mean there's not enough pressure in the cabinet to support the driver so it can exceed it's Xmax?

Is it easy to damage a driver when playing frequencies below the tuned frequency and does the unloading become worse the further below the tuned frequency?

Thank You

[cjd]

A good box modeling program will give you an excursion graph. A really good one will let you compare everything, including excursion, between different box types. Unibox does this, not sure what others do.

It really is simplest to understand if you model it up.

Excursion is controlled at (and directly around) tuning frequency. Let's take the 15" RS sub as an example, tuned to 19Hz. Up to about 26Hz excursion is about the same as sealed. It then swoops down to 19Hz and rebounds. By 10Hz it is about 2.5 times the excursion of the sealed.

C

[spmachina77]

i'm sure somebody can be more informative than me....but i'll give it a shot

a sub in a sealed box will generally move more than the same sub in a ported box....,
say we have a box tuned to 32hz, vs a sealed box

the vented subs exurusion will drop around the tunning frequency, but then go up very quickly as the frequency drops, subs tuned low are usually a lot better, but if you tune it high for spl, say 40-45 hz, then the unloading happens a lot higher (where more bass is)

anyways here is a pic with the same sub in a (1.25cf sealed box, vs a 2cf vented box tuned to 32hz) with 1000rms

[warnerwh]

Thank you guys. The graph really helped. I certainly like the behavior of the sealed sub on paper better. The ported sub goes bonkers below the tuning frequency to the point I wonder how the driver will deal with it.

The reason I asked is because the sub I'm building will be tuned to 16hz. It appears to be a good compromise the best I can tell. My concern is that apparently there's movies with frequencies down to 10hz or even below. When you watch a movie driving the subs fairly hard and a 10hz frequency comes in on the sub I'm designing I hope the drivers can take it ok. There will be two RL-P 15's driven by 600wrms.

[---k---]

Well, the obvious answer is to add a HP filter to protect the driver. Unfortunately this is easier said than done.

Hopefully, you will have enough headroom with this sub, that at the volume level you listen to, there won't be enough source material below tuning to bottom the sub. I've done the War of the World lightening scene that supposedly has 5hz material at well above normal listen levels with my sub, and it didn't bottom. So, I don't worry much about it. But, it is a very real possibility that the driver will bottom and be damaged. So be careful the first time running some of these types of scenes. If you run into problems, add another sub.

[warnerwh]

Ryan: When my friends wife sees this beast I'm building I'll be sure to mention that adding another sub is an option. :rofl: I can hardly wait for her to "thank" me. I'm sure she has no idea what's coming into her living room.

The good thing though is that my friend will love it. :T He already likes demoing his system for people. Once this beast is set up I suspect it should be able to scare the bajingos out of folks. I've already warned him to remove anything of value on tables and walls as he likes cranking his system.

[spmachina77]

2 subs is always better than 1 (just cost more, and takes more space)

keep in mind though, since you are tuning to 16hz the sub still unloads but at a much lower frequency, it doesn't meet witht he sealed box until it hits 13-14hz, and very little content is found below that. A high pass filter is generally more important on higher tuned boxes. Car subs are usually fine at 30hz, cause music doesn't really have really really low bass like movies.

[Jack Gilvey]

http://www.hometheaterhifi.com/volume_5_2/cmilleressayporting.html

[chasw98]

Well, the obvious answer is to add a HP filter to protect the driver. Unfortunately this is easier said than done.

It will become much easier very shortly! :T

[warnerwh]

My friend has a Cinenova Grande like mine which has a high pass second order filter that can be set to 20hz. Would this be a good idea or no? Thanks

[chasw98]

My friend has a Cinenova Grande like mine which has a high pass second order filter that can be set to 20hz. Would this be a good idea or no? Thanks

I have 2 of the Grande's and have looked at the filters they provide. #1 - They are not real accurate as far as setting them goes (might be 18 Hz or might be 25 Hz at the lowest setting). #2 - The rolloff is not really steep enough for driver protection. Also, with most of the subs being built around here you would be losing a lot of deep information by setting the filter that high.

Chuck

[warnerwh]

I doubt he'll get himself into trouble but the way he likes to crank it a low frequency could come and it could be bye bye drivers. In his room which is about 18x25 with an opening to the kitchen and hall I suspect he'll have more than adequate headroom. The sub will be sitting against a short wall several feet behind the couch so two meters away he should be able to hit 110db fairly easily without stressing the drivers, I hope. Fortunately if he blows one they're easily replaced except for the 300 each price tag.

It's hard to determine what someone else is going to do. Now I know how speaker makers must feel sometimes.

[---k---]

It will become much easier very shortly! :T

Ahhh! I thought about mentioning your project, but hadn't heard much about it recently.

[chasw98]

Ahhh! I thought about mentioning your project, but hadn't heard much about it recently.

SHHHH.... don't tell anyone. :W