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Ported speaker cabinet design and speaker frequency response curves

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Ported speaker cabinet design and speaker frequency response curves

Postby Wonks » Wed Aug 15, 2018 9:40 am

I'm struggling to find any info on how a speaker's frequency response (as shown on a manufacturer's website) is modified by the cabinet design. There are software programs which will give a cabinet frequency response if you put in the Thiel-Small parameters and the proposed cab dimensions and port size but these generally show a nice flat response line for mid and upper frequencies with all the interest being how the bass rolls off or is boosted around the cut-off frequency depending on the cab design. And a speaker's frequency response is far from flat in the mid and treble frequencies.

So do you then just 'map' the cab response on to the basic speaker frequency response, so if the cab response is -6dB down at 40Hz, then you adjust the speaker curve so that it's also -6dB down from its value at 40Hz (and so on for all frequencies)? I know there are other items like port desig and just how loud a signal the speaker is reproducing, that will affect the basic cabinet response curves further, but as a first approximation to how the installed speaker will respond (or what the cab will sound like if a different speaker is installed), I'm assuming they are OK.

I have bought Vance Dickason's Loudspeaker Design Cookbook (7th edition) in an effort to find the answer myself, but whilst it has lots of interesting information, you quickly get bogged down in minutiae of using various software programs and references to unreadable small diagrams without seeming to answer my question.
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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Hugh Robjohns » Wed Aug 15, 2018 10:00 am

This is one for Phil, obviously, but basically yes, the cabinet response adds to the driver's response.

So the LF roll-off from a conventional bass driver will be 6dB/Octave. And the LF-roll-off from a sealed cabinet is also 6dB/Octave. Consequently, the overall LF roll-off from a driver in a sealed cabinet is 12dB/Oct final slope.

A ported cabinet's roll-off is usually 12dB/Oct, giving an 18dB/Oct final LF slope of the combined system.

Obviously, the alignment of the driver's and cabinet response's turnovers will determine how quickly that final slope is reached -- if their roll-offs points are separated in frequency for some reason you could get a slope that starts at 6dB/Oct (or 12) and then steepens...

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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Wonks » Wed Aug 15, 2018 10:26 am

Thanks Hugh. I've got a small 10" bass cab that was made and designed by a friend, but it's rather boomy, so sealing the ports will be a first attempt to reduce that. But the speaker itself has quite a strong bass end according to the Celestion data sheet, down by about 8dB from the mids between 100Hz and 30Hz and only really rolling off below 30Hz according to the graph (though the graph's X axis labelling I find a bit dodgy).

https://celestion.com/product/163/bn10300x4/
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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Martin Walker » Wed Aug 15, 2018 12:15 pm

Wonks wrote:Thanks Hugh. I've got a small 10" bass cab that was made and designed by a friend, but it's rather boomy, so sealing the ports will be a first attempt to reduce that.

You could try stuffing the ports with acoustic foam first, to give you an idea of the bass end flattening improvement, before tackling any woodwork.


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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Wonks » Wed Aug 15, 2018 12:16 pm

As I don't have any spare acoustic foam lying around, it's going to have to be some rags!
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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Martin Walker » Wed Aug 15, 2018 12:51 pm

Wonks wrote:As I don't have any spare acoustic foam lying around, it's going to have to be some rags!

That would work fine - as long as you bung them in, to make a bung ;)


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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Wonks » Wed Aug 15, 2018 2:26 pm

So, when a triangular MDF grille corner support splits in half after taking the grille off, and you then glue it back on, and the weight you used rolls off and crushes the paper dust cap/dome, how to you pull the dust cap back out if it won't respond to a powerful vacuum cleaner? :oops: :shh:

In other news, stuffing the ports did help control the bass end. I preferred the sound of the mids with the ports open, but the bass was so much more controlled with them closed that that's the next step.

After pulling that dust cap back into shape.

Grrrrr.
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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Wonks » Wed Aug 15, 2018 5:06 pm

Dust cap pulled back. I noticed that the suction tube had a hole in the side, so I taped over that and the increased suction pulled the dome out again. Very slightly creased but it's the right shape now.
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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Wonks » Wed Aug 15, 2018 5:13 pm

Yes, I'm a sucker.
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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Martin Walker » Thu Aug 16, 2018 4:15 pm

But a very talented one ;)
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Re: Ported speaker cabinet design and speaker frequency response curves

Postby The Korff » Fri Aug 17, 2018 8:49 am

Wonks wrote:So, when a triangular MDF grille corner support splits in half after taking the grille off, and you then glue it back on, and the weight you used rolls off and crushes the paper dust cap/dome, how to you pull the dust cap back out if it won't respond to a powerful vacuum cleaner? :oops: :shh:

I know you've fixed it, but I've had success using gaffer tape — lightly press it onto the edge of the dented bit and then pull, gently teasing the dent out (it takes a few goes). There's probably a residue-free alternative to gaffer but I didn't have it to hand at the time.
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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Studio Support Gnome » Sat Aug 18, 2018 5:11 pm

Boomy cab is usually either insufficient internal damping, or poor cabinet panel damping or both, but add the driver to the equation as it can also be a combination of those elements with an inappropriate driver choice..... how the magnetic motor assembly is built has a significant impact on the damping factor of the driver assembly as a whole.... if there is significant excursion (as there often is for a bass driver) and there is travel outside the core area of the flux field , then the amplifier has less control on the cone and the bottom end is looser....

so... how the magnet is shaped and mounted has an effect on the result....

you need to be looking not just at instantaneous response but waterfall plots of the response over time.....
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