Enclosure Types and Comparisons

There are quite a lot of different enclosure types. The three most common are Sealed, Bass Reflex and Bandpass. In this paper, I will write a little about each of them and I will demonstrate some main differences between them. Group delay (which can be looked upon as how good transient response the speaker has or how "fast", "quick" etc it is) and frequency response is what we will mainly look at when comparing the different enclosure types. 

 Short description of each type

Sealed Box

The sealed box is the simplest box of the three mentioned. It is simply a box with a driver in it. The air inside the box works as a suspension for the speaker. Depending on the size of the box different frequency responses can be achieved. With the differences in volume comes differences in cone excursion, and group-delay, as well as other things.

Benefits with sealed boxes are:

• Easy to build (they are not so sensitive if the volume isn't precise)
• Best (low) group-delay/transient response which means they are very tight if designed properly
• They can take a lot of power throughout the entire frequency range

Drawbacks with sealed enclosures are:

• Efficiency is relatively low
• Frequency response can suffer in some cases in the lowest octaves/frequencies

Bass Reflex Box

The bass reflex box is a little more complicated than the sealed box. In addition to using a determined amount of air to reproduce frequencies, it uses a port that helps in reproducing the lowest frequencies. The port can be "tuned" to reproduce different frequency responses. Variables are port length and port area (i.e. diameter for a cylindrical port).

Depending on what frequency you want the port to be tuned to, the length will be given for a given area by using certain formulas. Since there are a few different theories to calculate port length for a given tuning frequency I will not give a formula here. Instead you can contact teamROCS Technical and we will help you out. A lot can be said about the bass reflex box, but let's go into pros and cons.

Benefits with bass reflex boxes are:

• Extended frequency response
• Higher efficiency
• Higher power handling above the port tuning frequency

Drawbacks:

• Harder to build and to get right
• Lower power handling below the port tuning frequency
• Worse group-delay/transient response than a sealed box (but better than bandpass) but still very acceptable if designed properly

Single Reflex Bandpass Box

The single reflex bandpass box is one kind of bandpass box. It has merely a port to reproduce all the bass frequencies. There are other types of bandpass boxes, but since they in most cases will affect sound quality negatively I will not mention them here. The single reflex bandpass is the bandpass that will affect the sound quality least negatively of all bandpass types.

Since all the sound will come from the port, it is obvious that the port needs to have free flow and that the sound is not distracted by any metal, fabric or other material. Furthermore, as you can see on the picture above, it has two chambers which makes it quite hard to build and get right. Depending on the volume of the chambers and the length and area of the port (tuning), different frequency responses can be achieved. The bandpass can be built to give a very high efficiency at a very narrow frequency range, or it can be designed to have low efficiency at a very wide frequency range. Something in between is common.

Bandpass boxes are in most cases only for the experienced people. If not constructed properly with correct chamber volumes and port variables, it will in 99% of the cases sound A LOT worse and A LOT lower (SPL-wise) than a simple sealed box. It is always a good idea to check with the manufacturer of the speaker for volumes and port lengths/areas if you decide to build a bandpass.

Benefits with bandpass are:

• Can be built to have very high efficiency which makes it good for high SPL
• Can be built to have great frequency extension

Drawbacks are:

• Hard to build
• Worse group-delay/transient response compared to the other boxes

Comparisons between the mentioned three speaker types

Below I will try to demonstrate the difference in frequency response between the different enclosure types. In the comparison, the same driver has been used for all three types. The simulation has been done by using LSPCad for Windows v3.00. I have made a comparison of "typical" frequency responses and I have set the volume of the enclosures to be the same. Power was set to 300 watts. The graphs show free-air simulated responses. In-car responses would be a whole different story, but since the car would affect the sound in the same way for all three enclosures it is neglected here.

As you can see the bandpass has the highest efficiency, but lacks some in extension. The ported enclosure has somewhat lower efficiency but improved bass extension. Compared to the sealed box it has about the same extension but higher efficiency. For this driver in a 1.5 cu ft enclosure, the ported enclosure might be the best alternative. However, it should be noted that the sizes are not necessarily the optimum and it should be noted that other factors such as group delay should be considered. For the above boxes, the following group-delay figures were calculated:

Sealed: 6 milli-seconds @ 38Hz
Bass reflex:10 milli-seconds @ 25Hz
Bandpass: 14 milli-seconds @ 50Hz

Some people feel that group-delay figures lower than 20 ms is acceptable and the least that can be noticed. If that is true, you would not be able to hear any difference in transient response between the above simulated speakers. However, other people say that 10ms is the limit. The best thing is to compare different boxes for yourself and determine what you like. Personally I have yet to hear a bandpass enclosure that sounds as good as a sealed enclosure.