Plugging Speaker Ports

[T-Bone]

Hey all in audioland... T-Bone here.

I was curious to know what the effect of plugging speaker ports would be. I heard that it increases bass response, but that seems rather counter-intuitive to me.

I have a set of rear-ported bookshelf speakers, and was interested in giving this a try if indeed it does increase bass response.

Does anyone know what a good plug material would be?

Thanks for any help... T-Bone

[markw]

Got some socks laying around? Try it. Seriously. It won't damage anything.

It will change the bass response of the system. Maybe you will like it, maybe not.

[happy ears]

My understanding this is usually down when using a sub. When you plug the port you change the box from a bass reflex to acoustic suspension design. It will usually raise the point that the lows will start rolling off to better merge with a sub. A sock will work to try it out. Try and use what sounds better to you. For myself I would rather more accurate bass than mudding lower sounding bass. Small speakers just don't get that low without some trade offs in sound quality but there always is the exception.

Enjoy the music life is to short.

[Jon Lane]

A port in the cabinet means its been designed as a Helmholtz resonator (http://www.jsdsound.com/enclosures.htm) and the air in the port moves in phase with the driver at the driver/box's resonance. Short version: Its a critically tuned system designed to reduce driver motion almost to nil at the system resonance, aiding low bass output and improving mechanical power handling above roughly cutoff. Stuffing it, depending on the driver's closed box "relationship" with the (now) sealed air volume may or may not convert the system to a usable sealed alignment.

If you use a powered subwoofer AND you cross over this newly converted sealed speaker at a standard HT crossover point, like 80Hz, you MAY find you have a better integration between the two. If so, ripple thru the crossover region will be reduced and the the system may mesh better than had you left the satellites to go as low as they were designed to as a ported system.

Swan speakers come with port blocking plugs, the intent of which is to do exactly what I've described. But we also specifically design our systems to be convertable too.

Your mileage may vary. You may wish to ask the manufacturer how they've designed their systems before undertaking this modification.

Jon Lane
The Audio Insider.com

[skeptic]

I do not think that this is a good idea. For a system designed with a port, blocking it will usually increase the bass resonance frequency and the amplitude of the resonance. The enclosure is designed as an integral part of the system. You also change the electrical characteristics (impedence) of the speaker. The idea of supplying plugs to partially or totally block off the bass port is hardly new and some speakers as far back as the 1970s offered them. IMO, most manufacturers whether they use one type of design concept or another, carefully optimize driver and enclosure to get the best performance possible. This might be the most extended low end response possible, the least distortion, the greatest efficiency. Blocking the port does not change the design of a speaker from ported to acoustic suspension. An acoustic suspension design has a driver with very carefully chosen parameters and a sealed box of a specific internal volume. It also may use the right amount of internal damping material to smooth the response as well. If you want to experiment with the bass response of a loudspeaker beyond merely trying different placements, an equalizer would be a far better choice. There are some specifically designed to help fine tune just the bass response.

[Jon Lane]

Blocking the port does not change the design of a speaker from ported to acoustic suspension.

Not necessarily -- we'd have to define what constitutes an acceptable range of A.S. Q's before we can make this statement. Some driver parameters permit acceptably switching from ported to sealed by removing the port and leaving the volume unchanged.


If you want to experiment with the bass response of a loudspeaker beyond merely trying different placements, an equalizer would be a far better choice.

An EQ can overdrive either a ported or sealed system in much the same way mistuning the enclosure can. While the topic is too complex to be described simply here, remember that the acoustic properties of an enclosure/driver system are analagous to the electrical properties of an equalization circuit...

[happy ears]

Jon Lane
"Not necessarily -- we'd have to define what constitutes an acceptable range of A.S. Q's before we can make this statement. Some driver parameters permit acceptably switching from ported to sealed by removing the port and leaving the volume unchanged."True Jon calculations must be preformed to find if the volume is acceptable. Also true that there are many drivers, which may used for an A.S. or in a ported enclosure. Usually a ported enclosure will have a larger volume. As we were talking about small bookshelf speakers the change in volume between the two is not as dramatic as larger speakers such as floorstanders. Yes driver parameters are still important but a 10 – 20% oversized sealed box is fairly forgiven if the driver will allow both sealed and ported design. Still not perfect as Skeptic stated, as we have not increased fill volume (damping material) to properly damp driver. However it is unwise to port a sealed enclosure. Yes this is very basic and for those that wise to acquire more information there are many books available.

Sorry T-bone you will get less bass response not more. However it may be a better match in a HT system with a sub, only trying will really tell you. That brings up the question why do speaker manufactures design small ported speakers that where designed to be used with subs. Personally they would be a better fit to cut off lower bass response in the 60 to 80 hertz range. Even though most if not all HT receivers have built in crossovers it is still better if the satellites natural roll off and end were the sub starts. I still find less more accurate bass is preferable to lower muddy bass response with large peaks in the lower end. But there are always exceptions.

[skeptic]

"Not necessarily -- we'd have to define what constitutes an acceptable range of A.S. Q's before we can make this statement. Some driver parameters permit acceptably switching from ported to sealed by removing the port and leaving the volume unchanged."

While it is true that plugging the openings in a ported speaker can created a half assed acoustic suspension speaker of a sort, this is a very poor way to go about it. Not only is the cabinet size not optimized nor the damping material but the concept flies in the face of the difference between the kind of drivers used. Ported designs rely on the mechanical restoring force of the driver itself to return the cone to its neutral position when voltage is zero. Acoustic suspensions rely to a far greater degree on the difference between the inside and outside air pressure by definition. True acoustic suspension woofers therefore have much looser suspensions than those optimal for other designs.


"An EQ can overdrive either a ported or sealed system in much the same way mistuning the enclosure can. While the topic is too complex to be described simply here, remember that the acoustic properties of an enclosure/driver system are analagous to the electrical properties of an equalization circuit"

This is correct. An EQ is such a powerful device that it must be used with great caution. However, it has some great advantages over retuning an enclosure and that is that it is easly adjustable, it is usable over the entire audio range (except for models dedicated specifically for tuning woofers.) IMO, this is a far better way to optimize a sound system then to be readjusting the enclosure once it is optimized for a particular set of drivers. The best of both worlds takes advantage of both devices in synergy. Unfortunately, there are very few systems designed this way. Equalization in the crossover network where power levels are high is a far less satisfactory method from an engineering point of view but given the idiotic preconceptions of "audiophiles" far preferable from the marketing point of view.

[WmAx]

Ported designs rely on the mechanical restoring force of the driver itself to return the cone to its neutral position when voltage is zero. Acoustic suspensions rely to a far greater degree on the difference between the inside and outside air pressure by definition. True acoustic suspension woofers therefore have much looser suspensions than those optimal for other designs.The different 'generalized' characteristics dictate the feasible frequency response. The penalty for using a woofer that has an EBP(EBP = Fs / Qes) greater then a value of 50 in a sealed alignment, is a higher cutoff frequency. The the time it takes to return to 0 amplitude settling point is deteremined by the frequency response of the system(which is determined by the pysical interactions of the speakers properties and the enclosure, plus the added interaction of the high pass crossover filtering bass with an active crossover, if such crossover is activated). For speakers that have front mounted ports, their can be added benefits(when a subwoofer is used), since the midrange residual output that leaks from this port, usually signficantly, will be eliminated by plugging th port. Of course, ideally the frequency response plot should be known when the port is blocked, so that one will know the lowest feasible frequency to integrate with the subwoofer(s).

-Chris

[Jon Lane]

While it is true that plugging the openings in a ported speaker can created a half assed acoustic suspension speaker of a sort, this is a very poor way to go about it.
True for highly specialized vented or sealed systems -- they can't be converted to the opposite design easily, if at all.

But converting a vented alignment to a sealed Q of say, 0.6 (or even Q=0.5, which is critically damped and a superb choice to cross over to a subwoofer) is eminently usable. IOW, there are drivers whose sealed and ported alignments are within a close enough margin to allow them to be used in either type of enclosure.



Not only is the cabinet size not optimized nor the damping material but the concept flies in the face of the difference between the kind of drivers used. Ported designs rely on the mechanical restoring force of the driver itself to return the cone to its neutral position when voltage is zero. Acoustic suspensions rely to a far greater degree on the difference between the inside and outside air pressure by definition. True acoustic suspension woofers therefore have much looser suspensions than those optimal for other designs.
Actually the reverse is true. Given two otherwise identical drivers, the one with the lower Q (more compliant) is the one used in the vented box, while the stiffer driver (higher Q) is used in the sealed box. Example: A driver with Q=0.35 fits a classic vented alignment, while a driver with Q=0.5 is far more suited for acoustic suspension. Very high grade drivers designed for vented boxes can have high enough compliances to render their total Q's under 0.2. Were you to put such a driver in an acoustic suspension box, you'd get virtually no bass -- they require venting.



IMO, [equalization] is a far better way to optimize a sound system then to be readjusting the enclosure once it is optimized for a particular set of drivers.
Unless, of course, the speaker system is designed to be convertable...



Equalization in the crossover network where power levels are high is a far less satisfactory method from an engineering point of view...
Yes, and EQ'ing the bass region of a passive xover IS nearly impossible due to the power levels involved.

Xover EQ typically is reserved for tweeters and midranges where power levels are more managable. There it's also essential in the vast majority of passive loudspeakers because, well, they're passsive and there's no wisdom adding the cost and inflexibility of an outboard line-level equalizer. If I can transparently and reliably EQ the tweeter level for the cost of a couple $0.25 resistors, I won't burden my customer with an active component costing him a couple hundred and adding it's own colorations

On the other hand, there are superb bass EQ's on the market, such as Phil Marchand's BASIS, which includes an entire suite of bass contouring adjustments. If you know how to use it, it's a terrific way to remodel your system's bass (within the limitations of the driver, of course.)

Jon Lane
The Audio Insider.com

[skeptic]

"But converting a vented alignment to a sealed Q of say, 0.6 (or even Q=0.5, which is critically damped and a superb choice to cross over to a subwoofer) is eminently usable."

If there's one thing that drives me nuts, it's a guy who would sell you a pair of speakers for say $2000 and tells you if you want to hear deep bass, buy a subwoofer and you're on your own figuring out how to integrate it with the rest of the system. IMO, that's HIS job. Actually, if I was confronted with this problem, I would feel compelled to buy two of them and place the main speakers directly on top of them because of the impossibility of avoiding insurmountable placement problems in the crossover region with just one. I don't consider any loudspeaker system which does not cover the entire audible range to be high fidelity.

After much thought, I came to the conclusion a long time ago that the best way to understand woofer/enclosure interaction is to condider Newton's second law of motion as applied to forced resonance. It is not only 100% accurate and has been completely solved for you but you see exactly what is happening. The problem with every ported design is that no matter what size or length the opening, it gives rise to a series of resonances at multiples of the principle resonance frequency and anti resonances at every multiple of the frequencies halfway between. IMO, because K is a function of f, the design is inherently unsalvagable. In an acoustic suspension design, K is not a function of f, but of the result of Boyle' and Charles' gas laws P1V1=P2V2. The restoring force is linear and constant with frequency. The only problem left for the designer is to choose fs of the system, control damping adjusting K, B, and M. The results are predictable and straightforward.

I will stick with active equalization as the best and most effective method for the trained ear to get the best possible tonal accuracy from a sound system.

[Jon Lane]

If there's one thing that drives me nuts, it's a guy who would sell you a pair of speakers for say $2000 and tells you if you want to hear deep bass, buy a subwoofer and you're on your own figuring out how to integrate it with the rest of the system.

Actually, if I was confronted with this problem, I would feel compelled to buy two of them and place the main speakers directly on top of them because of the impossibility of avoiding insurmountable placement problems in the crossover region with just one.

I think it's time I bow out of this discussion. The issue actually seems to be with a combination of 0.1 channel convention, audio salesman, and manufacturers of multiway systems without perfect waveform alignment (yet the wavelengths of concern are over a dozen feet long...) That, along with a partial analysis of resonators and acoustic suspension systems (the latter do not technically offer a perfectly linear restoring force, by the way, being inherently single-ended ;) ) says that this may be too much of a variable topic to solve here and now. :cool:

Jon Lane
The Audio Insider.com