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Damaging Your Expensive Ported Cab?


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#1 Phil Starr

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Posted 15 December 2016 - 02:10 PM

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....a_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.

#2 Phil Starr

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Posted 15 December 2016 - 02:12 PM

OK I've reserved this for answers/responses to anything you post up, I wanted to keep the original post shortish

#3 Phil-osopher10

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Posted 15 December 2016 - 02:35 PM

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?

#4 chris_b

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Posted 15 December 2016 - 02:56 PM

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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#5 Bill Fitzmaurice

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Posted 15 December 2016 - 04:33 PM

View PostPhil-osopher10, on 15 December 2016 - 02:35 PM, said:

Is the only way to prevent damage a low pass filter of some description?
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.
http://www.gollihurm...AND_RUMBLE.html

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'
+1. If this was an issue reports of blown drivers would be rampant.

Edited by Bill Fitzmaurice, 15 December 2016 - 04:34 PM.

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#6 Ghost_Bass

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Posted 15 December 2016 - 04:51 PM

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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#7 Passinwind

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Posted 15 December 2016 - 05:18 PM

View PostPhil-osopher10, on 15 December 2016 - 02:35 PM, said:

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

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

#8 Phil Starr

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Posted 15 December 2016 - 05:22 PM

View PostBill Fitzmaurice, on 15 December 2016 - 04:33 PM, said:

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.
http://www.gollihurm...AND_RUMBLE.html

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.

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.

#9 DangerDan

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Posted 15 December 2016 - 05:22 PM

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.

#10 pete.young

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Posted 15 December 2016 - 05:36 PM

View PostDangerDan, on 15 December 2016 - 05:22 PM, said:

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.
Micro thumpinator for me.
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#11 DangerDan

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Posted 15 December 2016 - 05:40 PM

View Postpete.young, on 15 December 2016 - 05:36 PM, said:


Micro thumpinator for me.

Cheers for the shout Pete, will check it out!

#12 sratas

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Posted 15 December 2016 - 05:44 PM

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

#13 Marty Forrer

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Posted 16 December 2016 - 02:51 AM

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.

#14 Phil Starr

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Posted 16 December 2016 - 08:51 AM

 sratas, on 15 December 2016 - 05:44 PM, said:

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

 Marty Forrer, on 16 December 2016 - 02:51 AM, said:

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.

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.

#15 sratas

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Posted 16 December 2016 - 09:55 AM

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

#16 alexclaber

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Posted 16 December 2016 - 10:02 AM

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!

#17 Downdown

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Posted 16 December 2016 - 01:37 PM

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?

#18 Kev

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Posted 16 December 2016 - 02:14 PM

 alexclaber, on 16 December 2016 - 10:02 AM, said:

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!

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

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Posted 16 December 2016 - 02:33 PM

No movement no sound

#20 alexclaber

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Posted 16 December 2016 - 03:08 PM

 Downdown, on 16 December 2016 - 01:37 PM, said:

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?

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!

#21 Passinwind

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Posted 16 December 2016 - 03:31 PM

 alexclaber, on 16 December 2016 - 03:08 PM, said:

It's much more complicated than you think!

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

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Posted 16 December 2016 - 03:34 PM

 alexclaber, on 16 December 2016 - 10:02 AM, said:

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!

Good news, I'll stop reading this thread now!
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#23 Bill Fitzmaurice

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Posted 16 December 2016 - 07:58 PM

 alexclaber, on 16 December 2016 - 03:08 PM, said:

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.
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, 16 December 2016 - 07:58 PM.

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#24 Downdown

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Posted 16 December 2016 - 08:05 PM

 alexclaber, on 16 December 2016 - 03:08 PM, said:

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!

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?

#25 Bill Fitzmaurice

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Posted 16 December 2016 - 09:58 PM

 Downdown, on 16 December 2016 - 08:05 PM, said:

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?
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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#26 Chienmortbb

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Posted 16 December 2016 - 10:05 PM

 Passinwind, on 16 December 2016 - 03:31 PM, said:

Yep. And as usual, there's no free lunch.
Designing speaker cabinets is what I call squeezing a balloon. You squeeze out something you don't want and something else squeezes out somewhere else. Charlie (passinwind) has designed an HPF and we have had many happy hours emailing back and forth about it. The real issue with speakers like anything else in life is that whatever you do, the laws of physics apply. Any designer or manufacturer that says otherwise is either kidding himself, you or having his balloons squeezed by the Marketing department.

As for the HPFs built into most amps, they are not right for bass. Most PA amps have the HPF set too high and the HPF needs to have a very sharp slope to avoid affecting your bottom end. Let hope everything we open this Christmas is not like this thread (a can of worms).

#27 Chienmortbb

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Posted 16 December 2016 - 10:06 PM

 Bill Fitzmaurice, on 16 December 2016 - 09:58 PM, said:

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.
Bill at some gig volumes the death clank of the voice coil is lost amongst the general cacophony.

#28 Chienmortbb

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Posted 16 December 2016 - 10:14 PM

 Downdown, on 16 December 2016 - 08:05 PM, said:

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?
The problem is there are many problems associated with the super low frequencies. Phil has outlined the most extreme. That is where the speaker/cabinet combination is no longer under the control of the amplifier. It may be only in very extreme conditions that this is a real problem but it is a problem nonetheless. The other issues are, subsonics that steal you amplifier headroom, and a room boominess that you cannot dial out with EQ (although neither of these (except maybe excessive use of EQ) are damaging to the speaker).

#29 molan

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Posted 16 December 2016 - 10:45 PM

As a retailer I've been involved in the sale of a huge number of cabs from people including:

Aguilar
Bergantino
Hartke
Carvin
tks Engineering
MarkBass
Bag End
ATS
Gallien Krueger
Phil Jones
Warwick
Mesa
Genzler
Plus lots of other pre-owned cabs

The designs have been hugely varied with porting front & rear and fully enclosed cabs.

Prices have ranged from as low as about 50 up to 1,300.

In all this time, across all these products and all these price points and cabinet designs I can only ever remember one returned with a blown driver.

Maybe our customers don't push their cabs hard or maybe, with modern designs and construction, this isn't really much of an issue any more. . .

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#30 bazztard

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Posted 17 December 2016 - 04:30 AM

+1 for a HPF set at 30Hz or 40Hz
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