Sealing ported cabs....

[xshawnxearthx]

is it safe and what should i use to do so?

i searched and didnt find anything. i have a 610hlf ampeg cab that i would like to seal up.

any suggestions?

 

[Emprov]

Why would you want to seal them?

 

[HeavyGroove]

I'm pretty sure that I've read in past threads that doing such a thing alters the tuning of the cab and therefore affects the performance of the drivers within the cab...most often making it sound like poo, or so I've heard.

 

[Thunder Pulse]

That cab is ported because it was designed to be that way. Whatever you hope to gain by making it a sealed cab ain't gonna happen.

 

[synaesthesia]

Yes but you can change the characteristics of the cab to suit your circumstances, particualrly in a physical situation when the port needs attenuation becuase it is a boomy room. Trace Elliot made port covers for some cabs, and in the audiophile world, many people stuff their ports with some fabric.

 

[A9X]

Stuff the ports with socks. See if you like it that way. Best thing is it's easily reversible.

 

[jsubbi]

A tuned port basically acts as another speaker at low frequencies, meaning it resonates (like blowing over the top of a coke bottle) and actually doubles your cabinet's output (+3dB). You can seal it, but you will have to use at least twice the power from your amp to achieve the same volume. If that exceeds your speaker's power rating, there is the potential for damage.

Sealed cabinets sound different because they can produce lower frequencies than the typical ported cabinet IF you use enough power and EQ. The speakers, though, also has to have enough throw (X max) and power handling, or you have to use twice the speakers (it's no accident an SVT cab has 8 x 10"s).

The ideal speaker design and cabinet for a sealed box is much different, though, than for a ported box. It's really hard to convert one to another. You're probably better off selling yours and buying a sealed design.

 

[greenboy]

Could just EQ out some content below 125 Hz or so. I realize using the EQ to CUT frequencies is a radical concept but -

 

[BillyB_from_LZ]

...I realize using the EQ to CUT frequencies is a radical concept but -

 

[neptoon]

tennis ball

 

[A9X]

A tuned port basically acts as another speaker at low frequencies, meaning it resonates (like blowing over the top of a coke bottle) and actually doubles your cabinet's output (+3dB). You can seal it, but you will have to use at least twice the power from your amp to achieve the same volume. If that exceeds your speaker's power rating, there is the potential for damage.

The port and enclosure form a Helmholtz resonator and it is only effective over a narrow band centred on the tuning freuency. The port can extend the LF response, but does not increase the efficiency of the cab. Reference efficiency is a function of the driver parameters, not the enclosure. So if you seal a ported box, all you will do is increase the frequency at which the LF starts, but reduce the slope of the rolloff from 4th to 2nd order. Above the rolloff you will be using exactly the same power as before to achieve the same SPL.

The ideal speaker design and cabinet for a sealed box is much different, though, than for a ported box.

That is correct in the main, but there are many ways to use drivers and many possible alignments depending upon the desired results.

 

[jsubbi]

Sorry if my previous post was incompete: the added output from your ports is narrowly centered around the tuning frequency which is usually around 40hz-60hz. Your speaker will be just as loud above that whether your cabinet is ported or sealed.

 

[Hoover]

The port can extend the LF response, but does not increase the efficiency of the cab. Reference efficiency is a function of the driver parameters, not the enclosure.

I think the reason ported cabinets are thought to be more efficient is because they generally produce an amplitude peak centered around the frequency that the port is tuned to. So, at least for a very narrow portion of the spectrum, the cabinet is louder.

So if you seal a ported box, all you will do is increase the frequency at which the LF starts, but reduce the slope of the rolloff from 4th to 2nd order. Above the rolloff you will be using exactly the same power as before to achieve the same SPL.

+1 Him speak truth.

 

[A9X]

I think the reason ported cabinets are thought to be more efficient is because they generally produce an amplitude peak centered around the frequency that the port is tuned to. So, at least for a very narrow portion of the spectrum, the cabinet is louder.

Not really. Where the port is producing it's max output, the driver is giving it's least, so they sum relatively flat.

Here's a K140 box sim I did a while back.
http://www.talkbass.com/forum/attachment.php?attachmentid=70700&d=1191883417

You can of course make a whole variety of alignments that may produce a peak at the bottom, but there are usually other issues with them.

 

[jsubbi]

That's it A9X, you made me break out my Loud Speaker Design Cookbook by Vance Dickason.

Ch. 2 Vented-box Low-frequency Systems

2.10 Definition

(excerpt) Compared to closed-box systems, vented enclosures possess several unique characteristics:

A. Lower cone excursion near the box resonance frequency, resulting in relatively higher power-handling and lower modulation distortion. This attribute makes vented enclosures rather attractive for use in two-way loudspeakers. It is accompanied, however, by high excursion potential at frequencies substantially below resonance, making the design extremely sensitive to subsonic noise. Fortunately, this problem can be easily overcome with a low frequency filter.

B. Lower cutoff using the same driver.

C. In theory, a + 3dB higher efficiency for the same volume closed-box system. While this is not particularly significant in practice, driver design requirements of lower required cone mass and less required voice coil overhang do contribute significantly to increased efficiency for a given magnet assembly.

Hmmm...a "+3dB higher efficiency." I knew I've heard that before.

 

[billfitzmaurice]

Sorry if my previous post was incompete: the added output from your ports is narrowly centered around the tuning frequency which is usually around 40hz-60hz. Your speaker will be just as loud above that whether your cabinet is ported or sealed.

While the output of the vent is centered around the tuning frequency there's much more to it than just that. A properly matched cab and driver will allow the driver to operate more efficiently over a range at least an octave above the box frequency. This chart shows the ubiquitous Eminence Beta 10 in 1.5 cubic feet, sealed, and vented with a 50 Hz Fb.

This is not an SPL chart. It's a chart of what we bass players are usually most concerned with: Maximum Output.
The difference at 50 Hz, which is where the highest power demand lies for electric bass, is 8dB. That's the same difference as ten watts versus seventy. But the overall increased output capability of the vented box extends all the way from roughly 35 to 175 Hz. That's over two full octaves.

Could just EQ out some content below 125 Hz or so. I realize using the EQ to CUT frequencies is a radical concept but -

True, as far as frequency response is concerned. OTOH, an overdriven driver in a sealed cab can give a desireable tone that EQ can't emulate. But to get it at a useable level you need a lot of drivers and a lot of watts pushing them.

 

[greenboy]

Could just model a bunch of popular bass cab drivers in appropriately-sized/tuned ported cabs in WinISD Pro and sim the same drivers in sealed cabs of the same size, and compare what's going on in output, phase, group delay, excursion, etc. It'll reveal a lot more about the subject and its variability than any number of textual simplifications.

But in general, the output of a typical ported bass cab is higher at frequencies most bass players want to hear a significant amount of, since that is usually the region most affected by porting.

EDIT: I see Bill put up a typical WinISD example while I was typing.

 

[Hoover]

Where the port is producing it's max output, the driver is giving it's least, so they sum relatively flat.

In theory perhaps, but I've never seen a vented bass cabinet that measured flat at the port frequency!

 

[billfitzmaurice]

In theory perhaps, but I've never seen a vented bass cabinet that measured flat at the port frequency!

That's the fault of using a cabinet volume too small for the driver. Don't blame the theory, blame the improper execution of it. A Beta 10 will go absolutely flat to 44Hz, if you give it the 4 1/2 cu ft box required.

 

[Hoover]

That's the fault of using a cabinet volume too small for the driver. Don't blame the theory, blame the improper execution of it.

Right...and I'm sure you'd agree that improper execution occurs all the time in the MI business. Hence my original contention: People *think* ported cabinets are more efficient because so many of them have an amplitude peak in the low end. It may just be pure coincidence, but it only takes a small handful of coincidences for disinformation to become "conventional wisdom".