The speaker cabinets on guitar amps come in two main forms: open and closed. Most combo amps, those that have the amplifier and speaker built into a single cabinet, feature open-backed cabinets, whereas most separate amp cabinets are fully enclosed. It's useful to know a little bit about the design properties of each type of cabinet so that you can decide for yourself if you need to add some dampening.
Fully Enclosed Cabinets
Fully enclosed, or sealed, speaker cabinets, like the classic Marshall "4x12," are designed to maximize the forward power from the speakers. They are designed and constructed to deliver a specific frequency and power response, and so everything from the speaker size and power rating to the internal volume of the cabinet is carefully planned. The sealed construction dampens the sound by limiting the mechanical movement of the speaker cone, while they also normally feature internal dampening material to eliminate any undesirable frequency responses. Additional dampening is not required in sealed cabinets.
Open-Backed Combo Amps
Combo amps are popular because of their portability and versatility. Everything is self-contained in a single unit, meaning you wheel it out, plug everything in and you're ready to rock. The open-back design allows the speaker cone to move freely and so it produces a lively open tone that reverberates in every direction thanks to the ability to project sound out of the back as well as the front. Sometimes this property is not desirable, though, and dampening can be appropriate either to help project more sound forward, alter the tone or to quieten the volume while still driving the amp's electronics.
Dampening With a Blanket
An easy way to add dampening to your open-backed cabinet is to use an old blanket as a removable cover for the rear of the speaker enclosure. Cut a piece of the material to slightly larger than the overall area on the back of your amp and attach it to the top with some gaffer tape. The blanket hanging down over the back will subtly limit the speaker cone movement and muffle the sound coming from the back of the speaker enclosure. The result will be a slightly fuller tone through the front and a sense of greater control over the sound.
Dampening With Upholstery Foam
Upholstery foam is a more substantial means of adding dampening to an open-backed combo. Measure the dimensions of the cavity at the back of your amp and cut a piece of foam just slightly larger; 1/2 inch all around is probably fine. This piece of foam can then be squeezed into place whenever you feel the need for extra dampening.
Whichever option you choose, always think of safety first. The electronics on a combo amp can get very hot, especially if you have a valve-driven model. Any extra dampening that encloses the electronics can lead to overheating that may damage your amplifier or even cause a fire. Wherever possible, you should only add the dampening over the speaker enclosure and avoid covering the electronic components. If this is not possible then try to minimize the time you drive your amp with the dampening attached.
- Hemera Technologies/PhotoObjects.net/Getty Images