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|22nd January 2013, 18:40||#1|
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Impact of Dust Cap / Phase Plug on Bass?
I was having a look at the DLS 6a
when I noticed that instead of a regular dust cap, it had a weird contraption in the center which was unlike anything I'd seen before. Is it a phase plug ?
I first thought it would come in handy for manufacturing optimization - this would bring a large amount of commonality to the comps and coax manufactuing, and perhaps reduce manufacturing cost but then why add extra parts.
But more importantly. Now that there is such a thing in the middle, will it reduce the bass, because (following reasons are just gut feelings - no science behind them, really)
1. Less cone area and therefore less air moved
2. will there be good isolation between the front and back of the cone because of the gap in between?
so all other things being equal, would removing the dust cap of a speaker reduce bass?
I know phase plugs etc are used to improve the mids and highs, but I'm curious about the bass.
|23rd January 2013, 11:19||#2|
Join Date: Mar 2004
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re: Impact of Dust Cap / Phase Plug on Bass?
a. The dust cap in a regular woofer can either be made of cloth or other felxible material or a stiffer material (everything from aluminum to PVC has been used). Both have disadvantages and advantages that depend on the application. One of the disadvantages is the resonance of the cavity (betwen the dustcap and the magnet more so with ceramic/ferrite magnets).
b. A phase plug (derived from those used in compression drivers) is believed to help linearise high frequency the response of a woofer. Typically these only seem to help above 1500hz or so if at all so much of it is marketing. A typical (conventional) phase pulg is shown below
These phase plugs seem more useful when appliaed to wide range speakers like Lowther (see link).
The claim is that a typical dust cap created 2 point sources (in any plane) for the sound wave (one at each end along the edge of the dust cap). In reality there will be multiple sources (along the entire edge of the dust cap).
These multiple sources can create lobing above a certain frequency (depending on the size of the driver, dustcap etc..). If a woofer is expected to reproduce sound above this "lobing frequency" then a dust cap ca be detremental and is better replaced by a phase plug as it extends the usable frequency range of a woofer by maybe 1 octave (from 1500 to 3000Hz to typical 5-6" midwoofers).
c. The phase plug mentioned above however had some other side effects (a long story related to the edge of the voice coil around the phase plug) and eventualy morphed into a more odd looking phase plug as shown in the picture below.
The Cap used by DLS is derived from the above mentioned phase plug. The Scandanavian speaker industry is very small an nepotistic and everyone knows what everyone else is doing.
This Cap like phase plug has many claimants from SEAS (Scandinavian Electro Acoustic Systems) to DLS.
Oh and BTW the loss of the dust cap does not affect Sd to any large extent and hence will not affect the SPL capability of the woofer unless you are talking about the huge 4" dia dustcaps used by Dynaudio.
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