sealed box, boost low freq?

[FroDaddy]

This is my first post here, so I should introduce myself as the latest resident newbie.

I've been planning a big ported box for quite some time now, but I've just stumbled onto this board from google when I was looking for more info on SS RL-p15 drivers. I spent all day reading about two DIY projects that were finished using two enclosures with these drivers. But why sealed?! When modeling them in WinISD, the ported enclosure seems to better the sealed in SPL and extension.

But the more I read on this board, the more I find the sealed + EQ being recommended? I modeled a SS RL-p15 in a 7cuft sealed enclosure with 1000w, and it was going over excursion under 25Hz. If you use the EQ to boost those low frequencies, wouldn't you destroy the driver?

Thanks, I know I should probably use the search feature but I wanted to post something. :B

[ThomasW]

Ported are slightly more efficient, and yes they have more output but at the tuning frequency. However if one tries to play frequenies lower than the port tuning frequency, the driver does what's called unload, and that destroys the it.

This means that one must protect the ported system with a subsonic filter to prevent unloading.

The reason this is now becoming a problem is that many of the newer movies have VERY low content. Action/Adventure movies for example Batman Begins, and War of the World have 3Hz content. This will destroy a ported system tuned to any reasonable tuning unless protected by a subsonic filter.

Sealed systems don't get the free boost from a port, but the physical nature of a sealed box acts as a acoustical filter and protects the driver from unloading at lower frequencies.

One chooses to boost the low frequencies of a sealed system based on the available power and the available cone excursion.

[Bent]

This means that one must protect the ported system with a subsonic filter to prevent unloading.

this is why I want to try a sealed sub next - I have the instructions and values from Adire to implement a subsonic filter in my ADA 1200 (it is very simple to do), I would rather have the security of a mass of air providing the resistance to over-excursion, rather than my pathetic soldering skills...

[cgr]

Sealed systems don't get the free boost from a port, but the physical nature of a sealed box acts as a acoustical filter and protects the driver from unloading at lower frequencies.

One chooses to boost the low frequencies of a sealed system based on the available power and the available cone excursion.

Does a passive radiator give you the free boost like a port and the driver protection of a port?

[ThomasW]

Does a passive radiator give you the free boost like a port and the driver protection of a port?

Ported design and PR based design are both vented alignments. As a result they have the same characteristics.

[cobbpa]

What determines how low a sealed sub can play? Fs? I know bigger box means less power & probably lower extension with less power (I think). I'm asking because I have a sub ported, tuned to ~30 hz, has an Fs of 26 hz. It's way louder than I need in my dorm room and wouldn't mind trying to get it to play down to 20 hz, but is this possible? I know if it is, a sealed box would do it. To note, I definitely have enough power to push it to its limits.

[ThomasW]

What determines how low a sealed sub can play? Fs?

The size of the box determines the tuning of a sealed box.

I'm asking because I have a sub ported, tuned to ~30 hz, has an Fs of 26 hz.

Drivers have a Fs, boxes have an Fb. You can't have an Fb of 26Hz if you have a box tuned to 30Hz.

wouldn't mind trying to get it to play down to 20 hz, but is this possible?

Yes, and if you put the design parameters into a program like WinISD or Unibox you can see what's needed to do that.

[---k---]

I built a sealed sub because it was much easier. I didn't have to worry about cutting the port length and the calculation of the volume as exact.

Also, the 7cuft enclosure that you built is huge! People look at my 3cuft enclosures and think those are large.

[Mark Seaton]

What determines how low a sealed sub can play? Fs? I know bigger box means less power & probably lower extension with less power (I think). I'm asking because I have a sub ported, tuned to ~30 hz, has an Fs of 26 hz. It's way louder than I need in my dorm room and wouldn't mind trying to get it to play down to 20 hz, but is this possible? I know if it is, a sealed box would do it. To note, I definitely have enough power to push it to its limits.

To add to what Thomas already said above, the un-EQ'd flat response of a sealed subwoofer is determined by the driver parameters and box size. Below some low frequency a sealed box will become increasingly less efficient. Without EQ, you are fairly limited with most drivers. If you add EQ, then how low you can push the response is primarily a function of how loud you want it to play. The driver size (Sd) and it's linear excursion limits (Xmax) dictate the maximum output at a given frequency. At some high power level you also have to watch out for burning up the voice coil.

In the case of your vented sub, the only way to get lower in frequency is to lower the tuning with preferrably a longer port. The response may start to sag at the low end, but in any dorm room you will be getting lots of gain at the low end anyway.

[Scott Simonian]

Also, the 7cuft enclosure that you built is huge! People look at my 3cuft enclosures and think those are large.

Heh. I consider my 7cuft vented Tempest a "medium sized" subwoofer.

Takes all kinds...