Types of enclosures – speaker boxes – easy explanation
Among the many existing types of enclosures, which one is the best for you ?
There are many types of enclosures out there. But first, let’s start off by clearing up some things. When we talk about enclosures / boxes, it is common sense that we are going to use it for bass drivers. Tweeters usually come capsulated and do not need enclosures. Midrange drivers need their own chamber, mainly to separate it from the bass driver (so the pressure from the bass doesn’t interfere with the midrange’s cone), but volume is not to be neglected, as it does affect the sound signature. The role of a speaker box is the separate the waves produced by the front of the speaker, from the waves produced by the back of the speaker. If they meet, they will cancel each other, since they are out of phase, and will lead to poor bass response. This happens especially to the lower frequencies (bass).
Now, since we got that cleared out of the way, let’s ask a question : How much does the enclosure affect the sound quality ? The answer is : a lot ! I’m not exaggerating by any means, if I say that an exotically priced speaker, with poorly designed enclosure, will sound worse than an average speaker in a great box.
Which enclosure is best suited for you ? That’s for you to decide !
Types of enclosures / boxes, starting from simple to complex:
Sealed / Closed enclosure
A sealed enclosure is exactly what it sounds like. A box, of whatever shape you wish, that is air tight. When the speaker moves, the air does not escape the box, it only alters the pressure inside it. This is the easiest enclosure to design and build. You only need to calculate the internal volume of the box, which is done with little effort. Stuffing the box with sound dampening material will help absorb stationary waves produced by the back of the speaker and yield better results.
- If space is an issue, sealed enclosures are the smallest.
- Easy to design.
- Easy to build.
- Design errors don’t have big impact on overall sound.
- High power handling.
- Great transient response (plays with little effort short duration sudden sound waves, like drums).
- Smooth roll-off of 12 db / octave.
- Low efficiency.
More about sealed boxes here.
The theory of infinite baffle is that you place a woofer onto a board or baffle, and the baffle extends in all directions so much, that even the longest sound waves don’t reach the edge of the baffle. This way, the waves created by the back of the speaker never meet the waves created by the front of the speaker, no resonances, no diffraction. With a good speaker, this sounds like the recipe for a great audio system. The concept is hugely impractical and cannot be perfectly implemented in real life.
Real world infinite baffle enclosures are merely a particular case of sealed box. If you make a sealed box large enough, the air inside the box will not alter the compliance of the driver and there you have it : infinite baffle. However this does not eliminate the diffraction and resonance issues, like a true infinite baffle. You can go crazy with your imagination and picture a home infinite baffle enclosure, where you have a wall of woofers and the “sealed enclosure” is the room next to the wall. This is a highly engineered setup, with lots of practical drawbacks, but with the potential to sound very good.
- Doesn’t need much power.
- Usually less distortion compared to other enclosures.
- In an ideal infinite baffle setup (which is actually a finite baffle, but sufficiently large), there are no resonances and no diffraction issues.
- Hard to separate back waves from front waves in a basic, real life setup (like a car).
- The woofer can reach maximum excursion easily, so you need to be aware not to damage it.
Bass reflex / Ported / Vented
This is the cookie cutter enclosure, the meat and potatoes when it comes to speaker boxes. Why ? Because it offers a great balance between sound quality, output/efficiency, design and build difficulty. There is a lot to talk about bass reflex enclosures, but I am going to keep it as simple as possible. This type of enclosure has a port, which is usually cylinder shaped (or rectangle).
The port has a predefined length and diameter (these dimensions are important as they set a certain resonant frequency). Now to sum it up really quick : some frequencies that come out the back of the speaker, reverse their phase and come out the port. This way, they add with the waves from the front of the speaker (because they have the same phase) and increase the output sound.
This is a very rudimentary explanation and it’s not all fine and dandy. For example, waves below the tuned frequency of the port, don’t reverse their phase and when they come out the port, they cancel the waves in front of the speaker. So below the tuned frequency of the port, the vent acts like a hole in a sealed enclosure. Considering all types of enclosures out there, this is by far the most popular.
- Higher efficiency than sealed. On paper it’s 3 db.
- Speaker can reach lower frequencies, outside its frequency response.
- Reduced distortion (speaker doesn’t need to move as far near resonance frequency).
- Not as good transient response as sealed.
- More difficult to design and build compared to sealed.
- Bass reflex port can become noisy at high volumes.
- Larger than sealed.
- Steep roll-off of 24 db/ octave.
While port usage is very popular, a passive radiator (a speaker without the magnet and coil) is the alternative. When the active speaker moves, the passive radiator moves in a push-pull fashion. It yields the same results, pretty much, with some differences of course. The passive radiator doesn’t make extra noise, but has limited extension.
More about bass reflex boxes here.
There are a few types of enclosures when it comes to bandpass (4th / 6th / 8th order bandpass). The speaker is out of sight, inside the cabinet and the sound comes out only through the port(s). For the 4th order bandpass, one side of the speaker is placed in a sealed enclosure, while the other side is placed in a ported enclosure. In this type of enclosure the woofer plays louder than bass-reflex, but has a narrow frequency response. You can make the woofer play a broader spectrum, but that is at the expense of efficiency. 6th order bandpass has both chambers ported, while 8th order bandpass has an additional ported chamber.
- High efficiency. Theoretical +5 db compared to sealed. Even higher efficiency for 6th and 8th order.
- Low woofer excursion.
- Good choice for high SPL applications.
- Enclosure can get impractically big.
- Extremely difficult to design, with no room for error (especially for 6th and 8th order).
- If tuned for efficiency, sound quality is very poor.
- If pushed to the limit, you don’t hear the woofer “struggling” (as it is inside the cabinet) and might kill it unknowingly.
More about 4th order bandpass boxes here.
Unlike sealed or bass reflex boxes where the inside of the box is left as is (except for damping material and / or bracing), the transmission line design creates a labyrinth on the back side of the speaker. The idea is, that the back waves generated by the speaker, travel through this labyrinth, which has a fixed length, which is directly correlated to the wavelength of the resonance frequency of the speaker in free air. This way, when these waves come out, they are in phase with the waves generated by the front of the speaker. The hard part is to fill this labyrinth with damping material of different thicknesses and densities. You have to stuff it in various patterns, until all of the upper frequencies are absorbed. Among all types of enclosures out there, this is one of the most difficult to design, because it is very unpredictable.
- Great low frequency response.
- Can reach subsonic frequencies.
- Not so sensitive to positioning.
- Because of the extra elements inside the cabinet, it is more complex to produce.
- Hard to calculate the design.
- The boxes can reach impressive dimensions.
- The woofer moves, more or less, freely and can reach maximum excursion easily (careful not to blow it up).
More about transmission line boxes here.
Horns are excellent devices to improve the efficiency of the driver. Although we are discussing types of enclosures, the horn is actually a coupler for the speaker and not an actual enclosure in itself. By following the shape of the horn (from narrow to broad), the sound waves make a smoother transition from the driver to the air. Horns are used for tweeters and midrange drivers because when you go down in frequency, the horn has to be bigger. Few centimeters might be enough for a tweeter, but for woofers, we are talking about meters.
Folded horn enclosures are a good choice for woofers, since they make use of space in an efficient manner. You don’t need to stick a massive “trumpet” in front of the woofer, you just have to make the path of the sound waves narrow near the driver (the throat) and broader near the exit (the mouth). In the case of folded horn, the speaker sits inside the cabinet and the waves are following a snail shell shape (more or less) path. Same technique as the “trumpet” horn, but more space efficient.
- Up to +10 db more efficient than sealed .
- Excellent for outdoor, very large rooms.
- Big enclosure.
- Poor off-axis response (the speaker needs to fire directly at you for best frequency response).
- Not so good for the very deep notes.
More about folded horns here.
Of course, there are a lot of enclosure types out there, but my interest was only for the most popular ones, and just the general facts. By focusing on the main points, you can make up your mind easily and choose one that suit your needs. For the inexperienced enthusiast, the decision lies between sealed and bass reflex. If you want a small box or maybe you want to start with something very easy to build, then sealed is for you. If you want more output and like more bass, you should go for bass reflex. The other types of enclosures should be studied in detail, as they are more difficult to design and build.
- Loudspeaker Design Cookbook 7th Edition by Vance Dickason (Audio Amateur Pubns, 2005).
- How to Build Speaker Enclosures by Alexis Badmaieff and Don Davis (Howard W. Sams & Co, 1966).
- Newnes Audio and Hi-Fi Engineer’s Pocket Book by Vivian Capelm (Elsevier, 2016).
- The Audio Expert: Everything You Need to Know About Audio by Ethan Winer (Focal Press, 2012).
- Image source : link.