I have a Ibanez 10G amp and it say it has an open back design for low response or something. What does it mean by open back or low response?
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Leaving an open gap in the back of a speaker enclosure is supposed to allow a fuller sound to be released. A lot of combos do this. Most cabinets are close backed to project better to the front.
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But, open back is where the cabinet or behind you combo is opened up, whereas closed back it is completely closed.

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You get alot of sound cancellation w open back cabs. And tend to not get the greatest bass response. But w combos its the norm, unless its a bass amp.

The open back does not lend itself well to distortion. Its why you may see blues or clean players use smaller miked combos. But never see a serious heavy metal player use an open back combo.