The following article was written by Anthony Lucas for Bass Gear Magazine.
Are sealed or ported enclosures better for bass guitar? What are the differences between them? In this article, I will compare the two most common types of bass guitar enclosures and try to highlight the benefits and shortcomings of each. It took me a while to decide what details to cover, and I soon realized it might require a book to cover the concepts of cabinet design. It would take a couple of articles this size just to introduce the terminology. For the DIY guys and players interested in obtaining more knowledge, there is a wealth of information available online to learn more about cabinet design. There is also plenty of software available online to help you with calculations. Whether you find information about car audio, home hi-fi, pro audio or bass guitar, the principles are basically the same. This article will be used for the details I feel will help bass players the most.
First, let’s think about the role of the speaker. A speaker produces minimal output in free-air (outside of a cabinet). Sound is produced from the front and the rear of a speaker’s cone. These sound waves must be separated to achieve usable output. If not separated, the output from the front and rear of the cone would cancel each other out. This is further accentuated with lower frequencies, and the main reason for mounting a speaker on a baffle. The baffle also absorbs vibration created by the speaker. If not mounted to a baffle and simply placed on a solid surface, the vibration created from the speaker’s movement may be louder than the sound it emits. A baffle that does not resonate with the speaker must be made from a solid and thick material.
Shaping the low-end (bass frequencies below 300Hz) is the reason for choosing a particular enclosure type. A speaker’s cone response dictates the signature shape or sound of the enclosure beyond 200-300Hz (figure 1). Any speaker will produce sound in any cabinet, but optimizing the relationship between the speaker and the enclosure is the key to good bass. In the early ‘70s, two engineers, named A.N. Thiele and Richard H. Small, devoted considerable effort to show how specific speaker parameters define the relationship between a speaker and a particular enclosure. These are known today as Thiele-Small (or T/S) parameters. They are a means of comparing speaker performance and finding optimal cabinet conditions.
Speaker designers manipulate T/S parameters when customizing a speaker for a particular cabinet or when meeting certain design goals. I’ll leave the specifics about T/S parameters and how they are interconnected for another day. But, for now know that trade-offs are involved in every aspect. If you want deeper bass, punchier bass, tighter bass, more snap, pop, or whatever the desirable adjective, something else will be sacrificed to obtain it. Every amp and cabinet manufacturer has methods to achieve their signature tone. Speaker performance and cabinet design are equally crucial parts. Designers must prioritize what performance or sonic characteristics they desire from a product and determine what aspects of the speaker and cabinet will make it a reality. Output level (or SPL), power handling, frequency range, and size and weight are all considerations. When one is improved, other factors may suffer. The most difficult part is finding a middle ground. We want to have it all, but unfortunately, it’s not always so simple.
Here’s how a sealed enclosure works. The back of the speaker is completely sealed off from the front. The air inside the enclosure acts as a spring, which helps control the movement of the cone. When the speaker moves out, the pressure inside is decreased. When the speaker moves in, the pressure inside is increased.
A sealed cabinet is considered a punchier, more accurate sound. Sealed cabinets are much easier to design and build than ported enclosures and are typically smaller in size. There is also much more room for error in design and construction because a small change to the internal volume doesn’t affect the lowest audible frequency significantly (Figure 2Figure 3). Sealed enclosures have better transient response, which means the system will respond more quickly to a sudden change. This is one reason why they sound punchier and more accurate. When you’re playing a run, it sounds more articulate with better note separation. Sealed enclosures also have good power handling capability and gradual frequency roll-off.
The disadvantages of sealed enclosures are lower efficiency and poor deep or extended bass output. The sealed design will never play as low as the resonant frequency of the speaker. It seems like the easy answer would be to design speakers for sealed enclosures with low resonant frequencies, right? That’s certainly one part of the puzzle, but a lower resonant frequency can negatively impact other design and performance goals. Speakers with lower resonant frequencies are typically low in output and narrow in frequency range. Another disadvantage is higher distortion. Maximum cone movement occurs at the resonant frequency of the enclosure (Fc). Basically, the speaker is working harder where the cabinet is most demanding. Dampening improves below Fc, so control of the cone and mechanical power handling of the speaker are good.
Ported enclosures are also referred to as vented, or bass-reflex enclosures. This design requires a more scientific approach, and there is less room for error in design and construction. Ported cabinets allow for an extended bass response. The result sonically is more “rumble” and deeper bass tone. A port (or vent) is used to tune the enclosure to a specific frequency (Fb). The surface area and length of the port are crucial to the tuning. The Fb of the enclosure does not change with speaker selection, but F3 does. The port uses the speaker’s rear output to enhance the speaker’s front output, which increases bass output (or SPL) above F3 (see Figure 3 again). This minimizes the movement of the speaker cone, so mechanical power handling at and above the tuning frequency is very good. The port is actually producing most of the output at the tuning frequency and the speaker’s excursion is minimal. Distortion is lower at this point due to less cone movement.
There are some disadvantages to ported enclosures. Transient response is poor compared to a sealed enclosure. The result is decreased accuracy. Also, there’s less control below the box tuning, which allows the cone to move more freely. This can result in damage to the speaker mechanically, a phenomenon known as over-excursion.
A poorly designed ported enclosure can cause all sorts of problems. Tuning the enclosure too high can be a problem. This can create a ringing at Fb and result in a one note wonder with inadequate frequency range (Figure 5). While mechanical power handling is typically a good advantage of a higher tuning, remember that the enclosure is not helping the speaker below Fb. If there is a sudden peak at a lower frequency there could be a potential for over-excursion problems. The speaker’s cone will literally jump out of the box. Low tuning can also generate problems. A large enclosure is required for a lower tuning. This can severely lower the speaker’s mechanical power handling. Loose, rumbling bass with no definition or “punch” may be the result sonically. You sacrifice the effectiveness of the cabinet and the speaker (refer to Figure 4, Figure 4-1 and Figure 4-2
A port without adequate surface area can create unwanted noise at higher volume… even when the tuning and size of the enclosure is good for the speaker. If the velocity of the air travelling through the port is too high, a noise often referred to as “chuffing” may occur.
We live in a day where power is relatively cheap and high power handling is a big selling point. Whether it’s the enclosure or the speaker, be aware that there are other specs to consider before you make a purchase. High power handling and lower output may not be the best bang for your buck. In contrary, a high SPL enclosure or speaker may not require as much amp to reach the desired volume levels. Factors for high a high output cabinet are the speaker’s SPL, power handling, and size, and a cabinet that is well built and free from air leaks.
A parameter called EBP (calculated by dividing FS by Qes) is often used to determine if a speaker is best suited for a sealed or ported enclosure. An EBP close to 100 usually indicates a speaker is best suited for a ported enclosure. An EBP closer to 50 usually indicates a speaker is best suited for a sealed enclosure. A speaker with an EBP 50-100 might work well in both types of enclosures. This is purely a general rule of thumb. Many great designs violate this.
You will often hear me talk about modeling a cabinet. I use software to calculate or predict a speaker’s performance in a given cabinet. It is an invaluable tool for cabinet designers and for recommending speaker components. The graphs used in this article are the result of one of those tools. Of course, anytime you’re dealing with TONE, there’s some voodoo or black magic involved. I’ve seen many successful designs that didn’t look so good on paper. There is definitely some art and science involved in the voicing and EQ of the amplifier. I also believe that sometimes you just get lucky and find that perfect combination of speaker voicing, cabinet, amp, and instrument and it just doesn’t matter if it works out technically. See you next time.