OTH40C 15" loaded Compact TH Flat to 40hz

[Hydrogen Alex]

Exactly!

If you're doing PA and you're in love with the "sealed" sound, just take a vented box with an Fb that's close to or lower than the lowest frequency that you're interested in producing, and EQ it to have the response of a sealed box in the passband above that. Then sit back and enjoy the same transients and much better performance

Of course no-one really does that, as they want to enjoy the extra low end output that vented boxes provide as well.

Hi Brian,
This is not the case at all, Chris is misreading the articles and confusing the seperate and additional electrical delay from passive crossovers (we all agree they are bad!) with the "core" problem of driver behaviour and driver loading / cabinet design.
As John Watkinson states in his article I linked to, NO ported / delayed resonance design can be time domain accurate.
It can attempt to minimise phase errors but all delayed resonance designs are designed to add out of phase sound energy to the main signal/impulse.

 

[Brian Steele]

Hi Brian,
This is not the case at all, Chris is misreading the articles and confusing the seperate and additional electrical delay from passive crossovers (we all agree they are bad!) with the "core" problem of driver behaviour and driver loading / cabinet design.

IMO Chris doing it the right way - he's approaching the issue at a system level. You can't just magic away the impact of the additional components required to reproduce the same SPL levels across the same passband when comparing two different enclosure types. Those need to be taken into consideration as well.

And BTW, all subwoofer designs involve some sort of resonance, including sealed systems. Can't magic that away either

 

[Hydrogen Alex]

" For kicks, I ran a simulation using Hornresp, and it went like this:
- High power 12", modelled in two different boxes. One was ported, and the other was sealed.
- EQ'd the sealed box to match the ported box in terms of frequency response (ie, flat to 40Hz, dropping quickly below that). They deviated by less than 1dB across the range, so we're comparing apples to apples here.

The results went as follows:
- The group delay curves matched perfectly above 55Hz. At 40Hz, the ported box had 25ms of group delay, while the EQ'd sealed box had 22.5ms.
- The in-band cone excursion of the ported box hit 8mm one-way at 60Hz. For the sealed box, it's 38mm at 40Hz.

Chris

This illustrates my point on driver suitability... Only a few drivers in todays Pro market are suitable (but still not optimised) for sealed box.
If you look at the Precison Devices driver in my sim you will see that the thermal limits and Xmax limits are perfectly matched. This is NOT the case with all your delayed resonace suited drivers.
Ie all the drivers you sim are totally unsuitable and will only reinforce your belief that ported / delayed resonace is the best.

 

[BP1Fanatic]

Would thermal limit be higher if you could vent the voicecoil to the outside of the sealed enclosure?

 

[Hydrogen Alex]

Hey BP1 Fanatic,

Yes it would... I dont know of any drivers that are mounted "inside out" in the commercial world but that could work in a crazy DIY set up?
It would be a great way to establish a reference and compare real world results and limitations with theory.
Personally I have never built really big sealed box / high power subs, just the 15 inch PD 1550's I use at home and they have never shown any signs of thermal overheating.
But how close they get to being too hot I will never know.

I dont want to go offtopic any more and apologies to OP for digressing, I just wanted to raise the issue of time domain driver / cabinet loading and encourage anyone seeking ultimate accuracy to explore the subject.
Cheers
A.

 

[Brian Steele]

I ran a Hornresp sim of a PD1550 in a sealed 44 liter enclosure. Basically the response is that of a midbass speaker with an Fb around 90 Hz and a rolloff that starts above 100 Hz, and is 18 dB down by 40 Hz. Sure you can use EQ / filtering to flatten that response curve and turn that midbass speaker into a subwoofer, but that EQ is going to add its own time domain issues. Here's what happens when you use a 60 Hz LP LR filter to convert it into a -3dB @ 40 Hz box, for example.

I don't get it.

 

[Hydrogen Alex]

Once upon a time LR was the best...

For sure Brain, 20 years ago your Linkwitz Riley filter would have introduced group delay...

But, thankfully, in todays world with the abundance of DSP (I use Audiolensehttps://juicehifi.com/ ) one can implement studio grade time domain correct crossovers and Eq which give you the best of both worlds with zero group delay... The Raison d etre of these programmes is to eliminate group delay!

The days of using Matlab to try and force LR analogue crossovers into time alignment across all frequencies are over!

Today over 90% of studios / home producers use FabFilter - Quality Audio Plug-Ins for Mixing, Mastering and Recording - VST VST3 AU AAX AudioSuite this which is a classic example of modern time accurate filtering.
This didnt exist 20 years ago.

 

[Brian Steele]

But, thankfully, in todays world with the abundance of DSP (I use Audiolensehttps://juicehifi.com/ ) one can implement studio grade time domain correct crossovers and Eq which give you the best of both worlds with zero group delay... The Raison d etre of these programmes is to eliminate group delay!

And those can be used with other enclosure types as well. And those other higher-order types have better performance wrt excursion requirements and power dissipation.

Like I said, I don't get it. But if it makes you happy, fine .

The room is going to mess up the response at bass frequencies anyway

 

[Hydrogen Alex]

"And those can be used with other enclosure types as well.."

But why use a square wheel...?
Why choose to start out with flawed drivers and cabinet loading when the physics and maths clearly show they are flawed?

Use the software to enhance a fundamentally accurate design, rather than try to repair a broken design.

Also, a correctly designed low Qts driver in the appropriately sized sealed box will easily outperform any time delayed design in the most important area of all... It will sound accurate and enhance the listener's experience.
Should that not be the goal?

 

[BP1Fanatic]

What sounds good to YOU might not sound good to someone else. I don't like the distorted sound of sealed and ported enclosures. I prefer BP4's (sealed BP or FLH) and BP6's (series or parallel BP6 & TH's [positive, straight, or negative flare]). Having all the output coming out of vents, ports, or horn sounds magical to me.

 

[chris661]

For sure Brain, 20 years ago your Linkwitz Riley filter would have introduced group delay...

But, thankfully, in todays world with the abundance of DSP (I use Audiolensehttps://juicehifi.com/ ) one can implement studio grade time domain correct crossovers and Eq which give you the best of both worlds with zero group delay... The Raison d etre of these programmes is to eliminate group delay!

The days of using Matlab to try and force LR analogue crossovers into time alignment across all frequencies are over!

Today over 90% of studios / home producers use FabFilter - Quality Audio Plug-Ins for Mixing, Mastering and Recording - VST VST3 AU AAX AudioSuite this which is a classic example of modern time accurate filtering.
This didnt exist 20 years ago.

Not so fast.

FIR (linear-phase) filtering still incurs a time penalty.

It's fine for playback applications, but a 10s of ms system-wide delay would be unacceptable for DJs or live sound.

That's assuming, of course, that you can find something that'll implement FIR filters well below 100Hz - it needs a lot of taps!

With a really high sampling rate (384kHz+), you can cut down the delay, but we're once again getting towards the super-expensive end of an already high-end arena. To give you an idea, Powersoft's X-series run at 48kHz sampling rate. I don't know of anyone making PA gear bothers to run at those super-high sample rates, but since sample rate is inversely correlated with filter delay, it's your best bet.

If you would like to introduce very large FIR filters, is it not then possible to compensate for the phase shift of a ported box?

I'd strongly recommend that you fire up Hornresp and run a few simulations. Ported boxes aren't as evil as you think.

All designs are a compromise, so your insistence that sealed boxes are the epitome of sound reproduction vexes me.

Chris

 

[Hydrogen Alex]

Very fast actually... Well under 10ms delay!

My 4 year old lap top (i7 core) gave me over 100,000 taps and my new ASUS ZenBook Pro UX580GD (i9 core) and runs both Smaart Live Smaart v8 and Audiolense and I can implement up to 220,000, yes two hundred and twenty thousand taps!

This link
The Complete FIR Filter Guide for Loudspeakers & Audio | Eclipse Audio
explains in more detail gives you the formulas and you can extrapolate to calculate your own system preferences. Ie An 85Hz FIR / Minimum phase 96KHz sample rate and approx 18,000 taps per filter with sub 10ms delay solves all the issues you list... They are only issues when you dont know how to solve them!

 

[Hydrogen Alex]

IIR for min delay not FIR

PS
Ment to say IIR / minimum phase not FIR for low bass under 10Ms delay.

 

[Hydrogen Alex]

One guy on the Smaart Live forum uses Audiolense for a big front of house rig and he runs everything through a buffer memory in his lap top so he can use FIR / IIR and or hybrid Minimum phase filters and he eliminates the delay totally.

He says the audience never notice the slight lip sinc / delay sound effect of the vocalist up to 50ms or 60 ms, but he does not have video screens.
With all home cinema / music video the buffer cures all filter delay and you can use all 200,000 plus taps and get "full fat" FIR / 96Khz @ 65 Hz plus all other crossover points.
The best solution is to run the audiolense or whatever software you use for the whole system.

 

[chris661]

For sure Brain, 20 years ago your Linkwitz Riley filter would have introduced group delay...

But, thankfully, in todays world with the abundance of DSP (I use Audiolensehttps://juicehifi.com/ ) one can implement studio grade time domain correct crossovers and Eq which give you the best of both worlds with zero group delay... The Raison d etre of these programmes is to eliminate group delay!

In a previous post (quote above, for clarity), you've suggested that we use FIR filtering down into the low bass (where your sealed subs would be EQ'd), and now you're saying that IIR filters will be needed down there, to avoid latency.
Which is it?


I see you've also avoided my question: why not use very long FIR filters to fix the time-domain problems* of a ported box?

* Not that we've discussed the audibility of the group delay at frequencies that low, nor the venue's RT60.


In the end, you're suggesting the use of huge piles of speakers, amplifiers and processing in order to work around something that's not considered an issue.

I didn't want to drag this discussion out, but I really have tried it both ways, and only one of the ways is feasible for sound reinforcement purposes.

Chris

 

[Hydrogen Alex]

"its hard to fill a glass that is already full"

In a previous post (quote above, for clarity), you've suggested that we use FIR filtering down into the low bass (where your sealed subs would be EQ'd), and now you're saying that IIR filters will be needed down there, to avoid latency.

Which is it?

I see you've also avoided my question: why not use very long FIR filters to fix the time-domain problems* of a ported box?

Chris

Maybe you didn't take the time to read my answers... In the link?
" : Minimum-phase FIR Filter
Previously we showed how both IIR and FIR filters use sample delays (as well as coefficients) to achieve their intended changes in the frequency response. A minimum-phase filter effects EQ whilst adding the least amount of delay to the audio signal. (This is one of the reasons long FIR filter based EQ in PA systems is typically minimum-phase.) A characteristic of a minimum-phase filter is that its impulse response has larger coefficients at or near the start of the impulse response. The following two plots shows a minimum phase FIR filter that effects a HP near 100 Hz and some EQ. Whilst the FIR filter length is 42.7 ms, the effective delay is negligible. "

I have provided references and expert witness articles from internationally respected sources which explain all the theory, give practical examples and list the techniques... What more do you need?!
With all due respect Chris, and as you state you are in the "working mans club" PA hire business... You are not an internationally recognised Professor or audio designer so I think any independent ( not in the ported box business!) reader will decide whos "experience" carries the most weight!

I am not expecting to win any converts from the established PA industry, more likely just encourage a few readers to research the facts.

Sorry I dont have the time to walk you through the various reports I linked to but "its hard to fill a glass that is already full"

 

[chris661]

Maybe you didn't take the time to read my answers... In the link?
" : Minimum-phase FIR Filter
Previously we showed how both IIR and FIR filters use sample delays (as well as coefficients) to achieve their intended changes in the frequency response. A minimum-phase filter effects EQ whilst adding the least amount of delay to the audio signal. (This is one of the reasons long FIR filter based EQ in PA systems is typically minimum-phase.) A characteristic of a minimum-phase filter is that its impulse response has larger coefficients at or near the start of the impulse response. The following two plots shows a minimum phase FIR filter that effects a HP near 100 Hz and some EQ. Whilst the FIR filter length is 42.7 ms, the effective delay is negligible. "

I have provided references and expert witness articles from internationally respected sources which explain all the theory, give practical examples and list the techniques... What more do you need?!
With all due respect Chris, and as you state you are in the "working mans club" PA hire business... You are not an internationally recognised Professor or audio designer so I think any independent ( not in the ported box business!) reader will decide whos "experience" carries the most weight!

I am not expecting to win any converts from the established PA industry, more likely just encourage a few readers to research the facts.

Sorry I dont have the time to walk you through the various reports I linked to but "its hard to fill a glass that is already full"

... Except here we're talking about EQing subwoofers. From what I've seen, if you use minimum-phase filters to EQ a sealed box, you'll get the same group delay curve as a ported box with the same frequency response.
If you use linear-phase filtering and a lot of taps, you incur a delay penalty anyway.

If you combine the two, as the paragraph suggests, you don't magically get away with it. The group delay will vary according to the rules of minimum-phase systems, where the minimum-phase EQ is being applied.


Having written several articles for one of the largest publications in the industry, and had interesting talks with people across the world who have taken the time to get in touch with me to discuss the issues further, I do have a little international recognition.
I'd also encourage you not to worry too much about the scale of my business. It's been a while since I was messing about with sealed boxes in small venues.


Besides the time-domain comparison (which I'll come back to below), I don't think there's any argument that ported boxes represent a better option in terms of distortion and efficiency. They need fewer drivers and less power compared to a system running with only sealed boxes.

If you like, we can settle this with some measurements.

What would a reasonable test be?

My hypothesis is this: a ported box and a sealed box with minimum-phase EQ will have the same group delay if they have the same frequency response.

I'd suggest that testing that would be pretty straightforward, and I have the equipment to do so:
- Start with a ported box, measure frequency & phase response
- Block the ports
- Measure again
- Create EQ curve that perfectly maps the sealed response on to the ported response
- Measure again

If you're happy with that as a test, I'll get started tomorrow.

The cabinets I'm thinking of using contain a Faital Pro 10HX230. I'd like to use those because I have rubber bungs which match the circular ports in the cabinets.
Alternatively, I can fit a 10FH520 - I think they have the same baffle cutout etc.


Chris

 

[Hydrogen Alex]

Except here we're talking about EQing subwoofers...

Chris, have you read any of my posts or the links and references...? Here is an extract from the article I linked to and quoted from in previous posts... It shows you how to use DSP crossovers and Eq at 100Hz to crossover subs with "negligible delay"

For the third time:
" ... to achieve their intended changes in the frequency response. A minimum-phase filter effects EQ whilst adding the least amount of delay to the audio signal. (This is one of the reasons long FIR filter based EQ in PA systems is typically minimum-phase.) A characteristic
of a minimum-phase filter is that its impulse response has larger coefficients at or near the start of the impulse response.
The following two plots shows a minimum phase FIR filter
that effects a HP near 100 Hz and some EQ.
Whilst the FIR filter length is 42.7 ms, theeffective delay is negligible.

Please do read the full pdf (attached) in case you cant follow the link.

 

[Hydrogen Alex]

APE: Air Pressure Events... Compression or Rarefaction

...

Having written several articles for one of the largest publications in the industry, and had interesting talks with people across the world who have taken the time to get in touch with me to discuss the issues further, I do have a little international recognition.

Chris

Having looked on your website (nice!) I could not find any links to your articles, please post the links.

Re your offer to test drivers designed for ported cabinets in sealed boxes... Why would you want to do that?
I am assuming you know that your suggested drivers are designed for ported boxes?
Also, as it appears you have not actually read any of my references, the advantages of sealed box over all ported / delayed resonance designs is the superior quality and realism... "Quality over quantity" as I stated in earlier posts.
So your offer to "prove" that a ported box makes a louder noise than a sealed box is stating the obvious...
All of the peer review science and real world examples by expert designers with genuine international credibility that I have listed prove that sealed box is more accurate ... Are you understanding this?
Cheers
Alex.

 

[Brian Steele]

I am assuming you know that your suggested drivers are designed for ported boxes?

The PD 1850 you referred to previously is also designed for ported boxes. In fact, the data on the driver specifically states "Appropriate for applications as diverse as scoop bins, conventional reflex cabinets and horn loaded systems."

Funny, it doesn't mention anything about being designed for sealed systems though. So why are you harassing Chris about choosing a driver that was "designed for ported boxes" when for your sealed example you clearly chose a driver that wasn't designed for sealed boxes ?

Anyway, fascinating argument. Like those characters that insist that class A amps are the most accurate, while the rest of the world is moving to the use of class D amps instead. Maybe being the last word in accuracy really isn't that important for PA use.