Enclosure Dilemma: Ported vs. Sealed
Copyright © 1997 by Cameron Noakes
So you just purchased that new set of drivers, and now your ready to build the enclosure. Whether for your car or for a new set of home speakers makes no difference - the bass response is the main focus of nearly every speaker system available today, and is the foundation for the rest of the system response. Manufacturers compete desperately with each other, enticing the buyer with advanced systems using sealed, ported, push/pull, various bandpass, transmission line, and even more exotic configurations. Speakers that have poor, weak, smeared, boomy, or colored bass response degrade the entire system and often make for an all around unsatisfying musical experience. Ofcourse, the quality of midrange and high frequency response is very important as well, but the DIY amateur speaker designer must decide on the type of enclosure to build to house the woofer. You will notice that this website spends most of it's resources explaining the different types of enclosures for reproduction of bass. This article will briefly analyze the two most popular types of enclosures, the ported and sealed box.
Why does everyone focus so much on the bass? It most likely is due to the fact that bass you can actually feel is a very visceral experience, much like watching a movie at a high quality theater. Ever been in one of those Sony SDDS Theaters where they really crank it? You become involved in the experience more with the help of breath taking dynamics, incredible sound depth, and the shocking realism that the soundtrack conveys through the high power sound system. The soundtrack and special effects would be virtually non-existent without the dynamics of the high quality sound system. Go rent the same movie you saw at that spectacular theater and watch it on your TV, with the sound emanating from the tiny built in speakers of the set. Not quite the same experience. The same can be said for your home or car audio system.
Rather than rely on a store bought system, the DIY loudspeaker builder understands that with just a little attention to detail and a basic knowledge of acoustics, a speaker system can be realized and built to custom specifications that meet personal goals and expectations. Why would the average manufacturer produce a really good speaker system when the average person just doesn't care? It is a sad fact that most people are satisfied with their home "rack system" stereos or the stock factory "Whizzer cone" speakers that came in their low watt factory car stereo. Many manufacturers do indeed build superb speaker systems - and you will pay dearly to acquire them. If you are like me (a cheapskate), you learn to do it yourself.
Most dynamic drivers come with a specification sheet with certain parameters that define the acoustical and electrical behavior of the speaker. These are often called "Thiele/Small" parameters, named after the two engineers who did much research trying to standardize and define driver/enclosure relationships. If you are like most amateur DIY builders, you probably don't actually test your drivers yourself to determine T/S parameters, but rely on the spec sheet that is included with the purchase. Although testing the drivers is the best way to determine parameters, many people do not have the knowledge or equipment to accomplish this simple task. The manufacturer gives you all sorts of numbers on the spec sheet, but what the heck does it all mean? And what type of enclosure is actually best? Unfortunately, there is no such thing as a "best" enclosure. Every enclosure type has it's advantages and disadvantages, and the final decision will have to be based on factors involving driver parameters, application, and listener preference. The best way to start is to take the manufacturer's recommendation; if they advise that the driver is best used in a ported box, there is probably a reason why.
Lets look at a popular, affordable 8" driver that has a paper cone and foam surround, and lets call this Driver A. The manufacturer recommends that these drivers be used in ported enclosures, and for good reason!
The T/S parameters we are interested in are as follows for Driver A:
Fs (free air resonance) = 43 Hz, Qes (electrical Q/damping of the driver) = 0.352, Qms (mechanical Q of the driver) = 12.11, Qts (combination of Qes and Qts, total Q of the driver) = 0.342, Vas (equivalent air volume of speaker compliance) = 1.02, Xmax (peak linear excursion) = 0.15".
The first thing to look at is the Efficiency Bandwidth Product for this driver. The EBP can aid you in selecting the proper enclosure for any speaker. To get this, we use the following equation:
EBP = Fs / Qes = 43 / 0.352 = 122
An EBP of 122 is indicative of a speaker best used in a ported enclosure. If the EBP had been lower, say between 50 and 90, you would get decent performance in a variety of enclosures - a "Jack-of-all-Trades" driver. EBP below 50 are generally best for sealed boxes. Don't misunderstand, you can put any driver into any enclosure, but you might be disappointed with the results. EBP is simply a guide, however, and good performance has been attained with drivers seemingly unsuited to a particular application.
Using a scientific calculator and the formula's (see Useful Conversions and Formulas) to configure an optimum ported enclosure, the Vb for Driver A comes out to be 0.703 cu. ft., with a cutoff frequency (F3) of 50.21 Hz and tuning frequency (Fb) of 47.43 Hz. Driver A in an optimum sealed enclosure with a Qtc of 0.70 would require a Vb of 0.32 cu. ft., and an Fc and F3 of 88 Hz! Obviously, this driver is optimized for ported enclosures, as evident by the EBP and indicated by the manufacturer. But what else is there that makes this driver a poor choice for a sealed box?
Look at Driver A's Xmax of 0.15". With such a short linear throw, it is not well suited for sealed box duty. Speakers specially designed for sealed boxes will generally have a greater Xmax than others. For comparison, another popular 8" driver which is optimized for very small sealed boxes, Driver B, has an Xmax of 0.343". This is not a law, however, and many very good ported drivers have a fairly high Xmax. Also, Driver A has a free air resonance of 43 Hz, which is rather high for a sealed box, and most drivers that perform best in sealed enclosures will have a low Fs. Remember that a sealed enclosure with a Qtc of 0.70 will generally have a resonance and cutoff of approximately the same value, and these will always be higher than the actual free air resonance frequency. Thus, it makes sense to design a sealed enclosure around a driver with a low Fs to begin with. Drivers designed for sealed box applications will generally have slightly smaller magnet structures, and therefore lower damping than equivalent ported drivers.
Conversely, If you try and use a driver designed for very small sealed enclosures (higher Qts and Xmax, low Fs) in a ported enclosure, you will often find that the port length must be very long to tune the box to Fb with a port diameter large enough to avoid vent noise. A good example of this is the above Driver B, which if put into an optimum ported enclosure of 0.50 cu. ft. would need a 3" diameter port 35.9" long to tune the system to an Fb of 31.47 Hz!
Ported systems are all around good performers, and most commercial home speakers use some type of ported enclosure. Automotive subwoofer manufacturers also like the ported enclosure, and most design drivers for this type of installation. The tuned port in these systems increases efficiency by nearly 3 dB in an optimum enclosure, and the roll-off frequency can be much lower, often by as much as 1/3 - 1 octave below a sealed enclosure. Think of the extra 3 dB as equal to the output you would get using twice the amplifier power on the driver. Add several ported drivers together and you can achieve impressive SPL's indeed! Because of the ports damping characteristics on the driver above Fb, distortion levels are also lowered because driver excursion is less. So, with nothing more than a properly designed optimum vented enclosure, you have very efficient bass reproduction with several advantages over an optimum sealed enclosure.
So, why in the world would anyone waste the time designing and building a sealed enclosure? Like anything, there are compromises in all designs, and ported systems are no exception. A very clear advantage for a sealed enclosure is simplicity - you can get good performance with nearly any driver with an EBP of less than 90 in a simple sealed box. Enclosure volume is not critical with these designs, and a volume change of +/- 10 - 20% will not adversely affect the sound. Ported boxes must be fairly precise in volume and tuning. Consider also the fact that sealed boxes have very gentle roll-off characteristics after F3 at -12 dB/octave. Because ported system cut-off is a steep -24 dB/octave, often lower bass can be realized with a sealed design even if the ported F3 is lower. (Note: attenuation slopes after cutoffs are almost never exactly these values, and may be steeper or more gentle depending on alignments).
On the downside, a sealed driver reaches maximum excursion at resonance, which adds considerably to the distortion produced at high output levels. A ported system receives maximum damping at resonance, with minimal driver excursion. The port "takes over" at Fb, and the driver hardly moves at all. This minimizes low frequency distortion, but ported systems progressively lose damping from the enclosure the lower you go below Fb, which amounts to a loss of control for the driver at very low frequencies. This is definitely a factor to consider if you are planning on building a ported system much smaller than optimum, as the tuning frequency may approach 50+ Hz, a frequency generally considered quite usable by most listeners. Subsonic filters can help with this unloading experienced with some ported systems.
While a ported enclosure is damped by the port at resonance, the sealed enclosure is wholly dependent upon the air trapped behind it in the enclosure. Once placed in the enclosure, the Fs turns into Fc, or the box/driver resonant frequency. The air in the box acts as a restoring force upon the driver cone, with a larger Vb usually meaning the driver can respond to lower frequencies (to a point). The following table illustrates the different damping alignments and driver characteristics with a chosen Qtc for sealed:
Note: Qtc can never be lower than the driver's Qts, so choose wisely.
Some advantages and disadvantages of each type of enclosure are listed in the following table:
There are several possible alignments attainable by different tuning configurations and driver parameters when building a ported system. These are beyond the scope of this article, and I highly recommend Vance Dickason's "The Loudspeaker Design Cookbook" for very detailed descriptions of these. The formula's for configuring an "optimum" ported enclosure listed on this website provide an alignment that gives a flat response to cutoff, a fair transient response, and good power handling. For home speakers, it is just a matter of what your design goals encompass, and the sound you require from the final product. Ported systems and their variations often perform very well in most home systems, and if tuned properly can generally outperform similar drivers in sealed enclosures. If you like vinyl, you might want to try and acquire a subsonic filter, which will remove any low frequency noise generated by the turntable that may upset your ported loudspeakers.
Sealed designs have several advantages for the car. One is enclosure size, as optimum ported systems generally must be larger. Building an optimum ported enclosure for the home is easy, as you usually have more than enough space for that big box. If you try and design an "optimum" ported box for your car, chances are you will find out quickly that the size of the enclosure will be unacceptable. Most people value cargo space, and unless you are willing to sacrifice all or most of it, large drivers in ported enclosures are not an option. To achieve ported enclosures in vehicles, most designs call for a much smaller box than optimum. This has the end result of much worse transient response, a higher F3, and the potential to unload at low frequencies normally seen with demanding music. Ported systems behave much like a sealed enclosure if you make the box smaller, and you will find that a large peak will appear and grow in the response curve around Fb the smaller and smaller the enclosure becomes. This makes the system boomy, with that one note unmusical quality seen in so many systems today. A compromise ported enclosure may be more efficient than sealed, but you lose a lot in the way of quality.
With all the new "small box" drivers available today for sealed enclosures, very good performance can be achieved with minimal space used in the vehicle. A true optimum enclosure is a reality with a Qtc of 0.707, with superior transient response and a low F3. Cabin gain is also something to consider, and with the gentle roll-off characteristics of the sealed enclosure, cabin gain multiplication at the lowest frequencies becomes greater with these designs. This can be good or bad, depending on your viewpoint. Small sealed enclosures need more juice to achieve comparable SPL's to ported, so make sure you have enough amplifier for the task if you choose this route. I have yet to see true "small box" ported drivers comparable to those available for sealed enclosures, but they might be out there. A driver specially designed for high performance small ported enclosures for the automotive environment seems like a good idea. But if you have the room for a larger box, ported enclosures can certainly provide some very real performance advantages over sealed enclosures.
Designing and building speaker systems is fun and rewarding. Few hobbies offer the chance to combine both art and science into a project with the end result something that can be enjoyed daily for years. The age old question of which enclosure is best will continue to be asked by builders and designers, and like anything else the answer is "It Depends."
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Revised: 11 Jan 2003 07:33:44 -0600.