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Post subject: Size of back panel
Posted: Sun Sep 02, 2012 5:08 pm
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I was looking at the back of my Rivera this afternoon and I noticed the back panel is larger than is typically found on a Fender combo amp. I was just wondering if anyone has ever experimented with changing the size of the lower back cover and what affect, if any, it might have.

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Post subject: Re: Size of back panel
Posted: Sun Sep 02, 2012 5:25 pm
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I assume you mean the amp has an open back like this (Yes, I know this is not your amp):

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Since an open back allows some phase cancellation of the audio output due to interaction between the front and rear speaker wave, ideally the size of the opening would be used to tune the low frequency response of the amp (ignoring close wall or room interaction). The larger the opening, the greater the phase cancellation at low frequencies and the less bass you have. Make the opening smaller and phase cancellation decreases giving you more bass. Close the speaker off entirely and no phase cancellation takes place. The size of the opening also controls the "spaciousness" of the amp sound. Hence open back speakers tend to sound larger and closed back speakers tend to be beamy. There are formulas to determine how much phase cancellation occurs and at what frequency it begins. I'm not sure if the amp manufacturers are that precise, as it is a fairly complex interaction.

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Post subject: Re: Size of back panel
Posted: Sun Sep 02, 2012 6:05 pm
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+1

That's the theory, in a nutshell.

Great job, Bill (as usual).

8)

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Post subject: Re: Size of back panel
Posted: Sun Sep 02, 2012 6:46 pm
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Thanks, fellas. That's pretty much what I expected to hear. I have already observed that a closed back cab is bassier. My Rivera is a little heavier on the bottom than most Fenders I've owned. The lower panel is roughly twice the size of a Fender lower panel.

I ask because I've been toying with the idea of building a DR some time during the winter. I'd buy a kit for the chassis but since I'm not too shabby with wood tools I'm considering building the cabinet myself. As a result of my ponderings I've been noticing cabinets and thinking about designs.

What's the scoop with insulation? I haven't seen it in years but I used to see some cabs with and inch or two of fiberglass on one or more interior panels.

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Post subject: Re: Size of back panel
Posted: Sun Sep 02, 2012 7:52 pm
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The fiberglass insulation is used to absorb mid and high frequency standing waves that occur within the cabinet and can affect speaker movement. The standing waves really only occur in sealed/acoustic suspension cabinets or ported/bass reflex cabinets and other similar designs. Not many standing waves in an open back cabinet so the insulation is not necessary.

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Post subject: Re: Size of back panel
Posted: Sun Sep 02, 2012 7:57 pm
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Cool. Thanks, Bill.

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Post subject: Re: Size of back panel
Posted: Sun Sep 02, 2012 8:17 pm
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There are a lot of books out there on hi-fi (showing my age :lol: ) speaker cabinet design. Never really seen much as far as guitar amp speaker cabinet design but there is probably something, somewhere on the Internet.

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