Ported cabs sound different above vs. below resonance

[LetItGrowTone]

Ported cabs sound different above vs. below resonance.

A great way to hear this most clearly is to feed it a sine wave from an online signal generator like this one (being careful not to blow anything up). I took this signal from the headphone out of my USB interface, into a power amp.

For example, one of my ported cabs is tuned to about 47Hz. Start in the 50s and step down toward 41Hz and notice the change in voice. It starts out perfect (if you like the cab), but ends up different. It seems distorted to me. I'd describe it as sounding labored. You should try this if you like clean and beautiful.

Addenda:
After hearing the above, now switch from a sine wave to a note from the bass.
In the above example, at A1, the fundamental 55Hz and all harmonics are above the cab resonant freq.
At E1, the harmonics at 82Hz and above are above resonance, but the fundamental at 41Hz is below. I've always thought this sounds bad, but now I know how to fix it, and I have "fixed" two cabs by retuning them (while admittedly not knowing all the consequences).

I'll use the above to explain why the lows of for example the Heritage fridge, while rolled off a lot (which the cognoscenti say we need to do anyway, right?), always sounded great to me (and still sounds great EQ'd, within limits), while I was never perfectly happy with any ported cab unless modified, and never felt that EQing the lows fixed it.

Now I simply require that a ported cabinet be tuned below the fundamental of the lowest note I intend to play. That's settled.

 

[agedhorse]

That’s not necessarily true at all.

You are in part confusing porting with the resonance Q of a speaker. UNDER-damped cabinets will cause what you think you are hearing, critically damped likely will not.

Modifying the cabinet by lowering the tuning decreases the Q of the resonance but unless the other parameters are adjusted accordingly also create new problems (that may be as bad or worse than the original problem)

 

[MuseChaser]

My knowledge of speaker design is limited to hobbyist designing of home audio and theater speakers; I have no experience building sound reinforcement gear, so I'll state that out front. Always willing to be corrected by those more knowledgeable if anything I'm about to type is incorrect.

Generally speaking, ported speakers can allow a given driver to reach f3 (the point where the frequency response is down 3db) at a lower frequency, and potentially achieve a flatter response down to that point (some even have a slight rise as that point approaches, depending upon the Q) compared to a sealed enclosure. The tradeoff is that a ported design typically has a much steeper rolloff below that point; a sealed enclosure will usually have a gentle rolloff, still producing more significant energy at frequencies below f3 than a ported enclosure.

When I designed the speaker systems for the 7.1.2 home theater system I built (VERY closely modeled after a design that Joe D'Appolito did for Audax many years ago... I tweaked it here and there, but saying I "designed" the system is ...well... perhaps more than a bit self-congratulatory), I used ported speakers for the front L and R speakers and the center channel and sealed for the rear channels. I also designed a pair of sealed subs based on a single NHT 12" driver in each sub that weren't part of Joe's original system. Reasoning was...
1. I wanted as flat a response as possible out of the front L/R/C channels, and they'd all be crossed over at a high enough point so that they would not be fed any material at or below their respective resonance frequencies, so they wouldn't have benefited from a sealed enclosure's extra extension and smooth low frequency rolloff.
2. I wanted the subs to dig REALLY deep, and the gentle rolloff provided by the sealed enclosures produce significant energy down into the low 20s. The subs are responsible for anything below 80hz throughout the system.

I'm guessing what the OP heard and was describing was a speaker trying very hard to reproduce a frequency below it's designed resonance frequency, or its f3, and the steep rolloff below that frequency in a ported design made doing so problematic. Simply retuning the box or port may not help much if the box was originally designed to the specific speaker driver's parameters... if anything, it'll probably make things worse.

And again...I'm no expert... just some idiot who's built a bunch of speakers, read Dickason's book, subscribed to Speaker Builder for years, and still scratches my head sometimes. Corrections and other thoughts always welcomed.

 

[WayneP]

I'll use the above to explain why the lows of for example the Heritage fridge, while rolled off a lot (which the cognoscenti say we need to do anyway, right?), always sounded great to me (and still sounds great EQ'd, within limits), while I was never perfectly happy with any ported cab unless modified, and never felt that EQing the lows fixed it.

Boosting the lows via EQ can certainly help (within limits, as you say), but I’ve never found that to sound as good as a cabinet that “gets down there” naturally. Of course, it looks like you’ve already figured that out...

Regards,
Wayne A. Pflughaupt

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[Pings Bass]

Play a low E on your bass (clean, no EQ or FX) into an analyzer and note how the first harmonic is dominant. Bass guitar does not produce a lot of fundamental, so your cabinet doesn't need to either. This has been discussed at length.

 

[LetItGrowTone]

If the fundamental is distorted in a way that I don't like, and I can hear that, then it's a problem for me.
Well, I have solved it for myself.

 

[agedhorse]

If the fundamental is distorted in a way that I don't like, and I can hear that, then it's a problem for me.
Well, I have solved it for myself.

Well ok then, don’t let facts and science get in the way of your opinion.

 

[LetItGrowTone]

Anyone who wants to hear it can perform the simple test I described in the first section of the original post.

 

[agedhorse]

Anyone who wants to hear it can perform the simple test I described in the first section of the original post.

You are attributing a cause rather than just the effect. iME, the cause you are stating is not the actual cause. I don’t disagree with your premise for very under-damped cabinets, but because the fundamental is very much masked by the harmonics (which are in fact a distortion of the fundamental itself), it’s much more complicated than you suggest.

 

[LetItGrowTone]

Thank you.
One of the cabs I'm talking about is reportedly - according to the cab designer - "close to critically damped". I don't know about the other one, but it's qualitatively the same with regard to this change of voice from above to below the cab resonant freq.

You are attributing a cause rather than just the effect. iME, the cause you are stating is not the actual cause. I don’t disagree with your premise for very under-damped cabinets...

True, I have no expertise.

...but because the fundamental is very much masked by the harmonics...

This is why I suggested listening with just a sine wave.

My claim was "ported cabs sound different above vs. below (cabinet) resonance". This seems to be a fact (step the test freq above and below the resonant freq, as though doing a "rice test").

That it sounds *worse* below resonance is, I admit, just an opinion; but I don't think anyone would say it sounds *better*.

 

[Wasnex]

For example, one of my ported cabs is tuned to about 47Hz. Start in the 50s and step down toward 41Hz and notice the change in voice.

Above the range where the port is active, the driver essentially makes all of the sound.

The port is tuned to start becoming active in the range where the driver starts to cut off. As the frequency drops towards Fb, the port makes more and more of the sound and the driver makes less and less. Also, the increasing resonance of the port suppresses driver excursion.

As the frequency drops below Fb, two important things happen: 1. The output of the port drops off quickly. AFAIK this is typically at a rate of 24dB per octave for ported systems. 2. The loading that suppress driver excursion drops off quickly; meaning the driver unloads and cone excursion increases dramatically. To summarize, below Fb response quickly decreases and driver excursion quickly increases. So there is not much point in operating the design very far below Fb...also no point in trying to use EQ to extend the response below Fb.

To protect the driver a steep HPF should be applied. AFAIK it's fairly common to put the HPF at F6 of the design (the -6dB point), but this varies from design to design.

 

[LetItGrowTone]

I don’t disagree with your premise for very under-damped cabinets, but...

Thank you.
I understand why you said that in reply.
But I didn't try to say that the cause of this voice change is that the cone(s) become uncontrolled. I just said it happens and is audible.

The cones are in a different dynamic environment above vs below, right? (as described by Wasnex, above, thank you). I'm not trying to study this at this moment (because I'm not able). I just said that a voice change happens and is audible and suggested a simple test.

 

[LetItGrowTone]

@Wasnex, isn't it not surprising that the cab's voice would be different above vs below that transition?

 

[agedhorse]

No, you said:

I'll use the above to explain why the lows of for example the Heritage fridge, while rolled off a lot (which the cognoscenti say we need to do anyway, right?), always sounded great to me (and still sounds great EQ'd, within limits), while I was never perfectly happy with any ported cab unless modified, and never felt that EQing the lows fixed it.[\quote]

For both ported and sealed cabinets, a HPF would be (very) beneficial for your needs.

Just play what you like and don’t worry about the arguments or being right.

 

[LetItGrowTone]

...also no point in trying to use EQ to extend the response below Fb.

Yes.
I said that but I want point out (to another poster above) that I didn't mean the *quantity* of 41Hz; I meant that EQing could not repair this change of voice that I'm talking about.

 

[LetItGrowTone]

The fridge has no such transition from above to below (port) resonance.
That was the explanation I intended, sorry.

 

[Wasnex]

I'll use the above to explain why the lows of for example the Heritage fridge, while rolled off a lot (which the cognoscenti say we need to do anyway, right?), always sounded great to me (and still sounds great EQ'd, within limits), while I was never perfectly happy with any ported cab unless modified, and never felt that EQing the lows fixed it.

How many ported cabs have you tried, and where in relation to Fb did you apply EQ.

You can definitely extend the low freq cutoff of an Ampeg 810 using EQ. A sealed cab typically cuts off at a rate of 12dB per octave. This approach eats up power quickly, but I thought it sounded good with my 810 pushed by my SVT.

I also have some experience with Bag End ELF (now called INFRA), which uses a high-tech version of this approach. I built a system with four sealed 18" ELF subs. ELF claimed flat response to 8hz. The caveat is the subs don't play very loud to 8hz. I thought the system I built sounded best with the subs cut off no lower than 40hz. I am pretty sure a single quality ported 118 Sub would bury the four ELF subs.

It doesn't really make sense to try and extend the low freq cutoff of a ported design too far below Fb. Efficiency and power handling below Fb drop off quickly.

 

[LetItGrowTone]

@Wasnex, sorry, I only meant to say that EQing doesn't fix this change in voice.

 

[Wasnex]

@Wasnex, isn't it not surprising that the cab's voice would be different above vs below that transition?

I don't personally find it surprising. Whether I should be surprised is a different matter...I don't have a deep enough understanding to really say.

I was actually under the impression a tonal change would occur in the transition where the driver is cutting off and the port is taking over. Your talking about the lower transition where the system is cutting off.

 

[LetItGrowTone]

I was actually under the impression a tonal change would occur in the transition where the driver is cutting off and the port is taking over. Your talking about the lower transition where the system is cutting off.

I'm talking about the point at or very near where the "rice" stops hopping.