Guitar Cabinets - Open Back vs Closed back
- A General Discussion -
Open BackAn open back guitar cabinet has either no rear panel or a partial rear panel. The rear sound wave emanating from the speaker driver will be heard. This has advantages and disadvantages as will be discussed.
Closed Back (sealed)Closed back guitar cabinets are sealed enclosures, where the rear wave of the speaker will not be heard - as it is trapped inside the box. Again, there are advantages and disadvantages to this design.
Frequency ResponseAssuming the same driver is used, in general terms an open back cabinet will have less bass and a more pronounced midrange, the higher frequencies are less effected, but they can appear brighter due to the lack of bass.
A sealed back cabinet will typically have more bass, and overall will have a warmer tone with the higher frequencies relatively dim.
However, different drivers will exhibit wildly different behaviors. Some will sit well in a sealed enclosure, some better in an open back. Typically speakers with a more powerful motor will work better in a sealed enclosure, while weaker motors will have a smoother response in an open back.
DispersionDispersion is the sound radiating pattern. Open back cabinets will disperse sound nearly in all directions - whereas a sealed box will exhibit a much more directional dispersion pattern.
Stage monitoring is a benefit to open back cabinets, where the sound will radiate out to the crowd and back onto the stage as well. A sealed enclosure will better direct sound out to the crowd while the sound will not be heard as well by the other musicians on stage.
As open back cabinets radiate sound from the rear, it creates a situation where placement relative to rear walls becomes a factor. Even tilting the box up will alter the frequency response The sound becomes more "open" and less direct, apparantly filling a larger area. Open backs outdoors, however, lose many of their benefits as there may be little to no rear wave reflections reaching the listeners.
ResponsivenessResponsiveness can be understood in terms of slapping the strings... a more responsive rig will have a POW sound that is short and percussive, where a very unresponsive rig would have long boom.
While this is incredibly driver dependent, a sealed back enclosure will generally have a tighter sound and will be much more responsive. The lower frequencies are more affected, but even the upper midrange can gain responsiveness in a sealed box.
An open back can totally lose the definition of individual notes in the lower frequencies, this may or may not be a concern depending on what is being played. Sometimes a muddy sound is desirable, and some drivers designed for open back cabinets remain quite responsive in their ideal enclosure.
Power HandlingWith the speaker supported by a "spring" of air, the sealed box will naturally handle more power. It will be much more resilient to frequencies that are below the normal operating range of the guitar, as well. While there may be less heat dissipation - which can be a concern when there is heavy guitar for long periods - this is generally not an issue.
A sealed box will typically have a more linear response through the operating range. As more power is added, the speaker continues to get louder until thermals and mechanical limits are approached. With an open back these limits (particularly in the lower frequencies) are easier to reach and exceed.
The increased power handling allows a closed back box to be cranked with less concern, and more power to be sent to the speaker, which will of course result in more output.
PracticalityThe ability to easily transport and easily setup is imperative. Ideally one trip is made and setup happens in a matter of seconds.
The storage gained by having an open back can be a wonderful thing. The inside of the cabinet is a great place to store cables, stompboxes, picks, tools, etc. Care must be taken, however, to not puncture the speaker cone by poking something through the rear.
Open backs are inherently lighter as well, as the strength of the enclosure is less of a concern than a sealed box.
Open back cabinets will cover a wide range of playing situations, where a wide dispersion pattern may be needed, a quick setup/teardown is needed, and weight is a concern.
ConstructionFor the DIY'ers, an open back cabinet is much simpler to design and build - with a large margin of error. A sealed enclosure, if too small, may create too much midbass and not enough low end... and if it is too large power handling will suffer as well as weight will increase.
Materials for an open back are relatively not of great concern, construction can be a bit lackluster and great results can still be achieved
In a closed back, however, materials are of great concern. An air leak can create a whistling sound and alter frequency response. A weak enclosure can either be blown apart or resonate in very undesirable ways. A closed back cabinet also needs a method of getting the signal from the amp into the box - which can also complicate things.
ConvertibleIt is certainly possible to design an enclosure with a removable rear panel. This way either can be tested. This may seem ideal but having a removable rear panel will generally make the enclosure a bit less strong than if the rear panel were glued in. It, as such, is more likely to resonate and is is not likely this resonance will create desirable results. Air leaks are also more of a possibility. A bit of engineering can resolve this, of course.
The enclosure size will have to be designed to favor the closed back, so desirable results can be obtained when the rear is on. This will likely create a larger enclosure, but it depends on the driver utilized.
This also opens the possibility to create partial blockage, to "tune" the cabinet. This is certainly the ideal way to prototype a box, and when the greatest results are achieved the final enclosure can be built with the newly discovered information in mind.
SummaryBoth designs have their ideal situations... with open back being very commonplace in the guitar world as it will cover a wide range of situations and is in general the more practical choice. Closed back enclosures offer a different sort of tone - the tighter response and greater power handling/output may be worth the extra weight and loss of practicality.
While this post can help you decide in terms of what to look for, you really should listen to both cabinet designs in a few different situations. Be aware of what your driver is built for and, if possible, build your own convertible box and try all of the above.
www.linearteam.dk has a great enclosure design tool called WinISD - this can help you to find the ideal size for your sealed enclosure. It can also help design ported and bandpass enclosures, although the properties they exhibit are generally undesirable for emanating guitar tones.