Plugging speaker ports...any harm?

pearsall001

Just a question out of curiousity. Some speaker designs are sealed & others are ported. Now granted, the designer had specific goals in mind for their performance by choising one of the two designs. What is the real difference between the two? Does a ported speaker provide more bass than a sealed speaker? Does one design perform better than the other, & if you were to stuff the port holes how would it effect the speakers performance? Could that possible harm the speaker? I think it's all about moving air inside the cabinet. In a sealed design where does the air go or maybe it doesn't. I guess either design has it's +/-. Any thoughts?

TN_Polk_Lover

Here's what I understand about the differences:

SEALED ENCLOSURE (sometimes called "acoustic suspension") I think that this was first used by AR (Acoustic Research) and they may have a trademark on the term "acoustic suspension".

-Woofer is has a highly compliant suspension
-The air trapped in the enclosure acts as a "spring" and has the effect of damping or limiting the movement of the woofer

-Advantages:
*For a given low frequency cutoff point (the point where the bass response is -3db down) the enclosure can be smaller than a vented design.
*Easier to design vs. vented box.
*No "chuffing" noise produced by ports.
*The frequency rolloff in the bass is at a more gradual 6db/octave.

-Disadvantages
*Less efficient. Takes more amplifier power. The rear wave from the woofer is "trapped" inside the box and does not help contribute to the sound output.


VENTED ENCLOSURE (sometimes called "bass-reflex")
-The enclosure is vented. The vent can be one or more holes in the box, or sometimes a slot of a certain size. The vent(s) may also include tubes cut to a certain length that help "tune" the port.
-The concept of the vented box is that the output from the rear compression waves of the woofer come out of the box to re-inforce the sound from the front wave. The trick is to get the rear wave to come out "in phase" vs. "out-of-phase" with the front phase. If you take a woofer and just hang it in mid air, it will have very poor bass, because the rear waves cancel out much of the output of the front waves. The early bass-reflex designs were often very "one-note" boomy. Before the design power of the computer, some bass reflex speakers only reinforced the bass at one particular frequency.

-Advantages:
*More efficient. Less amplifier power is required vs. sealed box.

-Disadvantages:
*Harder to design, although with today's computer power, much of the design can now be computer modeled.
*Can sometimes have "chuffing" noises from the ports.
*Usually requires a larger enclosure for a given bass output vs. sealed box.
*-3db point might be lower than sealed box, but the rolloff from there is steeper at 12db/octave. This can be good or bad. If you are trying to integrate with a sub-woofer, the fact that the main speaker has a steeper roll-off might be an advantage. On the other, hand, if you don't have a subwoofer, a sealed box design will seem to go lower in the bass because the roll-off is not as steep.

A variation on the vented box is to use a passive radiator. Polk used these on some of the older SDA's. The passive radiator is excited by the rear waves from the woofer (or woofers) and then transfers that energy to the air outside of the box. There is no "chuffing" noises with a passive radiator.

I hope that this helps. I have been fascinated by speakers all of my life and have read about them. There is a book about speaker design that they used to sell at Radio Shack. I believe the author is David Weems, but I could be wrong. I used to have it years ago and I actually built a pair of small sealed box speakers using RS drivers -- 4 inch woofers and 1 inch soft dome tweeter. I even wound my own coils out of magnet wire for the crossover networks. I used them for years, connected to a Yamaha R9 receiver. I still have the receiver.

Refefer

+1 That's a pretty good reason about it.

Another huge advantage of sealed boxes is that the group delay is very small, making for much tighter, more precise bass.

Vented enclosures can also be much more precisely tuned, and to much deeper volumes. They are not particularly hard to design with software out there.

pearsall001

TN Polk Lover - nice write up, very nice! You know a hell of a lot more about this than I do. I was dying out of curiousity to try plugging the ports on my speakers, just to see what would happen. I have a pair of AAD 2001 monitors and they are extremely good at pounding out deep tight bass. They spec out at 30HZ-30KHZ (8ohm, 86 sen., 50-300watt recommended) and believe me they sure as hell perform as stated. Well, after stuffing the ports the bass just died, I couldn't believe it. It was still very tight but not nearly as deep as running them without the ports stuffed. So as far as my experiment goes, they will not be receiving the stuffed port treatment. It was neat though hearing the immediate difference in sound.

venomclan

Hi Pear,
I agree with all said so far. Do not be afraid to seal the ports with different types of materials and in different volumes. "port tuning" can be rewarding but very tedious. As you now know, finding the magic happy medium is very difficult. It also depends on the speakers design, bracing and the amps you use. Driver size and speed also play a huge factor.

My B&W's came with port plugs about 3 inches long that are to be used at one's discretion depending on placement. If the speaker is too boomy in one application, the plugs can help. I find that stuffing the ports with a softer material can help the best. Most speakers come with some type of internal damping like polyfill or closed cell foam. I had the best results semi-plugged with poly fill, but your mileage may vary. Spiking your speaks will also yield different results.
Venom

Deadof_knight

by plugging a ported box you are essentially detuning your system the B&W are an exception Im sure. Typically when you plug a port you have just made a sealed enclosure too big or more to the point a slight loss in bass efficiency. The enclosure now plays lower in most cases but will have a muddy sound in the upper bass region.

Excellent write up TN

VR3

Rule of thumb - unless designed for it, if you feel you need to alter the design of a product - its time to get something new.

I learned that the hardway.