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Phil Starr

Damaging Your Expensive Ported Cab?

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Nowadays it is rare to see any bass cab around that isn't ported, but they all have a basic flaw which isn't widely advertised. If you don't know about it then you could end up with a large repair bill.

Ported cabs for bass are generally tuned to 40, 50 or 60Hz , or somewhere in between. the way they work is simple. They are tuned to the frequency where the speaker starts to cut out as it's impedance rises. As the speaker cuts out it's output is replaced by the output from the port giving you very roughly a 3dB boost over the lowest octave. BUT you don't get something for nothing and the cost is what happens below 40Hz (or 50,60 or whatever, depending upon the make and model you use) .

Below the resonance of the port the port just becomes a big hole in the cab. Down to that frequency the air acts like a weight and damper on the cone, suddenly that is all removed and the cone is free to move with little resistance. as a result any signal below 40Hz is likely to make the cone move way beyond the limits the speaker is designed for, even with just a few watts going through the speaker. With the coil outside of the magnet it rapidly heats up and it may even start drumming on the back of the magnet, either way complete failure won't be far away.

Don't believe me? Try going to the Eminence website http://www.eminence.com/pdf/Beta_12A-2_cab.pdf and have a look at the designs they have for the Beta 12" speaker which is a 250W speaker. Have a look at the design for the large bass cab, they recommend only 75W into their 250W speaker and even so the graph shows the cone moving beyond its 4mm limit at 40Hz. this is a speaker widely used in Eminence equipped bass cabs.

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Is the only way to prevent damage a low pass filter of some description?

How do you work out the frequency the cab is tuned to, with very little in the way of tools, ie a bass, a cab and an amp?

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Is this really a big problem?

I don't hear much about speaker failures these days, not at the rate they failed in the 60's and 70's.

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[quote name='Phil-osopher10' timestamp='1481812543' post='3195366']
Is the only way to prevent damage a low pass filter of some description?
[/quote]Most amps have HP filtering built in. If yours doesn't it will be obvious by excessive thump noise, which is cured with a rumble filter.
[url="http://www.gollihurmusic.com/faq/38-HIGH_PASS_FILTERS_GETTING_RID_OF_THE_MUD_AND_RUMBLE.html"]http://www.gollihurm...AND_RUMBLE.html[/url]

Ported cabs are no more prone to damage than sealed. If anything you're more likely to over-power a sealed cab, as they have less sensitivity in the lows than ported. Ported cabs also have minimum driver excursion at Fb, where the port is doing all the work. Finally, xmax is not where voice coil damage occurs. That would be xlim.
[quote]I don't hear much about speaker failures these days, not at the rate they failed in the 60's and 70'[/quote]+1. If this was an issue reports of blown drivers would be rampant. Edited by Bill Fitzmaurice

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Ahh! The joy of having class-D amps with built-in HPF to ramp up the "wattage"... all the lows, none of the farts :D

Never blown a driver but i have had to replace a tweeter twice, i should ease up on the B3K... ;)

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[quote name='Phil-osopher10' timestamp='1481812543' post='3195366']
Is the only way to prevent damage a [b]low pass[/b] filter of some description?
[/quote]

That would cut highs, and therefore be more relevant to the tweeter.

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[quote name='Bill Fitzmaurice' timestamp='1481819614' post='3195449']
Most amps have HP filtering built in. If yours doesn't it will be obvious by excessive thump noise, which is cured with a rumble filter.
[url="http://www.gollihurmusic.com/faq/38-HIGH_PASS_FILTERS_GETTING_RID_OF_THE_MUD_AND_RUMBLE.html"]http://www.gollihurm...AND_RUMBLE.html[/url]

Ported cabs are no more prone to damage than sealed. If anything you're more likely to over-power a sealed cab, as they have less sensitivity in the lows than ported. Ported cabs also have minimum driver excursion at Fb, where the port is doing all the work. Finally, xmax is not where voice coil damage occurs. That would be xlim.
+1. If this was an issue reports of blown drivers would be rampant.
[/quote]

I'm writing this in response to a handful of people who don't know all you do who have blown drivers. No it isn't an epidemic but it happens regularly to folk on these pages and it is preventable. I'm thinking that one preventable failure is one too many and provoking some debate will help in understanding.

I've kept the technical side down to a minimum here but speaker breakdown is complex, well a little anyway. The problem is not always overheating or always Xlim (the mechanical limits to excursion for those who don't do the maths) some of the cases I've looked into look like the result of thermal runaway where the coil is leaving the magnetic gap for significant periods meaning the temperature rises are greater than they would be using AES testing procedures.

I've also suspected thermal compression is an issue when a speaker fails. the temperature of the coil rises and the sensitivity of the speaker falls ( I know you've spoken about this in past posts Bill, this is aimed at the interested onlooker) the response in the middle of a gig is to increase the power to the speaker and sometimes to boost the bass.

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I've been looking for a decent HPF, what are people using? At the moment im using the depth setting on the fishman emulation on the b3, but id love a dedicated 'always on' solution.

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[quote name='DangerDan' timestamp='1481822554' post='3195482']
I've been looking for a decent HPF, what are people using? At the moment im using the depth setting on the fishman emulation on the b3, but id love a dedicated 'always on' solution.
[/quote]
Micro thumpinator for me.

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[quote name='pete.young' timestamp='1481823419' post='3195491']

Micro thumpinator for me.
[/quote]

Cheers for the shout Pete, will check it out!

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Very interesting, but I know there are not many reports of certain cabs failure. My barefaced bb2 can sustain prolonged sub bass abuse, I checked...trust me. and Alex Claber did not recommend a HPF setted too high, lt's say above 30 hz because the driver lowers its excursion and this can impair cooling. At least this is what I understood. Maybe it all depends on the whole design and quality. Waiting for opinions from manufacturers

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I have two Barefaced One10 cabs and have cranked them up to earsplitting volume with and without an FDeck adjustable HPF. My double bass has much more low bass in it than my P bass, hence the A/B with the filter. To tell the truth I noticed negligible difference with or without the filter, regardless of what hz was selected. This tells me that a well thought out and designed cab and driver combination should handle any load.

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[quote name='sratas' timestamp='1481823889' post='3195498']
Very interesting, but I know there are not many reports of certain cabs failure. My barefaced bb2 can sustain prolonged sub bass abuse, I checked...trust me. and Alex Claber did not recommend a HPF setted too high, lt's say above 30 hz because the driver lowers its excursion and this can impair cooling. At least this is what I understood. Maybe it all depends on the whole design and quality. Waiting for opinions from manufacturers
[/quote]
[quote name='Marty Forrer' timestamp='1481856681' post='3195785']
I have two Barefaced One10 cabs and have cranked them up to earsplitting volume with and without an FDeck adjustable HPF. My double bass has much more low bass in it than my P bass, hence the A/B with the filter. To tell the truth I noticed negligible difference with or without the filter, regardless of what hz was selected. This tells me that a well thought out and designed cab and driver combination should handle any load.
[/quote]

A 30Hz filter is sensible, it is too low to make much if any noticeable difference to the sound, as you've observed. A lot of bass amps attenuate the lowest frequencies anyway. If you are running a relatively clean signal path with a double bass you wouldn't expect too many problems. If Alex is recommending that as a frequency I'd imagine he has done all the sums for you. There are other advantages than speaker protection about removing subsonics anyway so if it doesn't affect your sound and you've already splashed out why not use the HPF?

If your gear is doing what you want reliably year after year then you've clearly made a whole host of sensible decisions, matching your speakers to the right amp and your particular needs. Most people get there but there are a few casualties on the way.

As ever the weakest link in the chain is usually the fleshy organic bit pulling on the strings. I've probably a slightly distorted view because I inhabit the technical bits of the forum mainly offering help in the repairs section to people who's speakers have blown. It usually comes down to 'oh no you didn't really do that, did you'. I'm hoping this thread will reduce the casualties.

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Oh...I use a HPF. ..the micro thumpinator. It dies wonders to the tone and no,it produces a clearly audible clean up of the tone. The mesa walkabout pushed hard is capable of huge subsonic content,i guess no built in hpf is used in the amp circuit

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I'm struggling to understand all the details, but surely if a manufacturer designs a ported cab in which the driver could potentially be damaged by frequencies below x (or above y) then it would be sensible for the manufacturer to include the appropriate filters within the cab to ensure that these frequencies cannot be present at levels that could cause damage?

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[quote name='alexclaber' timestamp='1481882546' post='3195884']
This is a very complex issue but one thing I can say for sure is that if you own a Barefaced cab you do not need to worry about this!
[/quote]

Good :) I always get a bit antsy when I see the speaker cones dancing around!

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[quote name='Downdown' timestamp='1481895423' post='3196063']I'm struggling to understand all the details, but surely if a manufacturer designs a ported cab in which the driver could potentially be damaged by frequencies below x (or above y) then it would be sensible for the manufacturer to include the appropriate filters within the cab to ensure that these frequencies cannot be present at levels that could cause damage?[/quote]

You can't filter out these frequencies with passive speaker level components, it has to be done at line level with active components. Most bass amps have some kind of high pass filtering and most power amps have switchable filters.

The strange thing about this issue, is that the worst problems I've had with cone over-excursion were with cabs which were tuned to 31Hz, so you were never driving them with frequencies below the tuning frequencies. You'd think from reading Phil's original post (which is broadly correct) that these low tuned cabs could never suffer such problems.

That experience set me down the road of finding out what really matters with the inputs bass guitar cabs can handle and also the sounds they're expected to generate. It's much more complicated than you think!

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[quote name='alexclaber' timestamp='1481900888' post='3196140']
It's much more complicated than you think!
[/quote]

Yep. And as usual, there's no free lunch.

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[quote name='alexclaber' timestamp='1481882546' post='3195884']
This is a very complex issue but one thing I can say for sure is that if you own a Barefaced cab you do not need to worry about this!
[/quote]

Good news, I'll stop reading this thread now!

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[quote name='alexclaber' timestamp='1481900888' post='3196140']
the worst problems I've had with cone over-excursion were with cabs which were tuned to 31Hz, so you were never driving them with frequencies below the tuning frequencies.
[/quote]That's not surprising. With 31Hz tuning excursion in the maximum power band width, from 50 to 70Hz (even with a low B ), will be considerably higher than with the usual 45-50Hz tuning. 31Hz is appropriate tuning for a PA sub, but not for an electric bass cab, not even a 6 string with a low F#. That's perfectly obvious to anyone who's ever seen a spectral analysis of the output of the electric bass, but I'd say that's rare even within the community of acoustical engineers, let alone bass players. Edited by Bill Fitzmaurice

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[quote name='alexclaber' timestamp='1481900888' post='3196140']
You can't filter out these frequencies with passive speaker level components, it has to be done at line level with active components. Most bass amps have some kind of high pass filtering and most power amps have switchable filters.

The strange thing about this issue, is that the worst problems I've had with cone over-excursion were with cabs which were tuned to 31Hz, so you were never driving them with frequencies below the tuning frequencies. You'd think from reading Phil's original post (which is broadly correct) that these low tuned cabs could never suffer such problems.

That experience set me down the road of finding out what really matters with the inputs bass guitar cabs can handle and also the sounds they're expected to generate. It's much more complicated than you think!
[/quote]

Fair enough, but without going into the techie stuff, is it a real problem with ported cabs and if so why are there no warnings about it in the manuals - or have I missed them over the years?

Perhaps only some cabs are susceptible, but even so, I'd have thought that the ones that are would have a pretty clear warranty disclaimer in their manuals, as they do regarding power handling limits. I doubt I could return a cab under warranty if I'd blown a driver with too much power so why should blowing one by driving with too-low frequencies be any different?

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[quote name='Downdown' timestamp='1481918748' post='3196349'] I doubt I could return a cab under warranty if I'd blown a driver with too much power so why should blowing one by driving with too-low frequencies be any different?
[/quote]For you to approach xlim with most drivers they'd sound really bad. It's ignoring the warnings of impending doom that often result in it.

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