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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 10:40 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 11:00 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 11:26 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 1:15 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 1:16 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 1:51 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 3:26 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 6:06 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 6:13 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 5:15 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 9:49 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 6:11 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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Re: Ported speaker cabinet design and speaker frequency response curves

Postby IvanSC » Fri Sep 28, 2018 6:48 pm


I have two of the earlier green label 300s in a cab with a couple of smallish ports & I did almost zero work on calculating the effects of Q etc., but somehow I got lucky.

That said, years ago I DID get heavily involved in all this stuff and came to the conclusion that most of the science and deep thought that has been applied to this works great on traditional speaker enclosures, but all flounders a bit when applied to the dark and subjective art of guitar and bass amp speaker enclosures.

My considered advice is: Build it, suck it and see. Wood is still relatively cheap.
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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Hugh Robjohns » Fri Sep 28, 2018 6:53 pm

IvanSC wrote:....came to the conclusion that most of the science and deep thought that has been applied to this works great on traditional speaker enclosures, but all flounders a bit when applied to the dark and subjective art of guitar and bass amp speaker enclosures.

Or perhaps it's just that it's a less critical application that doesnt need to reproduce reference sources accurately and where some acoustic 'flavour' or 'character' is often considered desirable. ;-)

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Re: Ported speaker cabinet design and speaker frequency response curves

Postby ef37a » Sun Oct 21, 2018 12:05 am

Re SSG and amplifier damping? Yes, going to be a big factor and I don't think we know the amp? Valve guitar/bass amps have very little NFB (none at all for a lot of guitar amps) and so the OPZ is likely not less than an Ohm or three. Solid state should be better but not I would aver the vice-like grip of a DC300A?

I am also with Ivan, I am not sure the T/S parameters work that well at bass guitar levels? Bass rigs tend to be run close to limits. I have an email mate who services amps and he tells me THE most common component to fail in a bass rig is the speaker(s).

The "design" of guitar speaker cabs especially is really chunk of chalk, ball of string and the ideas counter to ALL that is high fidelity, e.g. they LIKE wobbly ply and DESPISE rigid MDF!

Always struck me as an amazing coincidence that SO many cabs share the side/front/back footprint with the head?

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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Wonks » Sun Oct 21, 2018 9:33 am

Dave, the amp is a TC BH250 (Mk 1). 250W into 4 ohms Class D.

The ports are now filled in (round wooden blocks with neoprene foam on both sides and stuck in with Gorilla glue) and the cab now sounds a lot better and far more balanced.

Amp and cab:
Image

Top of cab showing rounded cutaways in the handle corners that fit the amp's feet to keep it in place:
Image

Bass with cab to show its diminutive proportions:
Image
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Re: Ported speaker cabinet design and speaker frequency response curves

Postby ef37a » Sun Oct 21, 2018 11:30 am

Wonks wrote:Dave, the amp is a TC BH250 (Mk 1). 250W into 4 ohms Class D.

The ports are now filled in (round wooden blocks with neoprene foam on both sides and stuck in with Gorilla glue) and the cab now sounds a lot better and far more balanced.

Amp and cab:
Image

Top of cab showing rounded cutaways in the handle corners that fit the amp's feet to keep it in place:
Image

Bass with cab to show its diminutive proportions:
Image

Cannot see a figure for output resistance/damping factor. I am no expert but I think class D amps are not as low Z as conventional AB types not least because of the chunky RF filters in the output circuit?

I also found the images "failed to load". About a week ago BT.com told me I had to get another browser or switch to a different version of IE that worked but had fewer "facilities", could no images be a facility I have lost? Switching to Chrome or another browser they suggested would be a PITA but I would get used but which one? !!

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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Wonks » Sun Oct 21, 2018 11:46 am

The images might just be a matter of time, depending on your internet speed, as they are just .jpg files.
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Re: Ported speaker cabinet design and speaker frequency response curves

Postby ef37a » Sun Oct 21, 2018 12:05 pm

Wonks wrote:The images might just be a matter of time, depending on your internet speed, as they are just .jpg files.

Don't think so. Gave it a minute of twizzling then "Image failed to load".
I do only have 8.5M download speed and upload is a PITA 0.8 but I am hoping I shall get Fibre to Cab before very long? There is a cabinet only 50mtrs down the street.

I am NOT going for any hyper-speed fibre to home connection. 8 meg is a becoming a bit of a nuisance now but I shall get what I can and live with what I can't!

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Re: Ported speaker cabinet design and speaker frequency response curves

Postby Sam Spoons » Sun Oct 21, 2018 1:15 pm

first image is fine but the others failed. Opening the link in a new tab worked though.

edit :- they've opened in the post now......

edit 2 :- a mate had the 1x15 combo version of that TC amp when he played bass in a band of mine. He's a acoustic guitar player really and managed to make it sound woolly/boomy and uncontrolled. Most likely how he had set it up though rather than any inherent problem with the amp.
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