In at the deep end - SB 15 inch for sensitive bass

[almikel]

@@almikel

 

Hi Mike, thanks for your support. I had a lot of reading to do, to get through your 4 pages! It ended without finishing...

 

There's some really relevant exchange there, which was very helpful.

 

A couple of things stood out for me:

 

- nobody really questioned (or explained) why you'd put such huge drivers in such a tiny box

 

- that you found the cricket more interesting than building speakers 

 

I'll have a look at Matt's thread, and see if there are any answers...

 

Mine are starting from the bottom up (soon, anyway), expected to perform from 20 Hz up to wherever, so I expect a bit of brow furrowing to ensue very soon.

 

That 'Bitkote' looks interesting - is it very smelly? I once read somewhere that it's not good to use bitumen paint, as the solvents can damage drivers as they off-gas. But this is water-soluble and more modern. Just curious.

yes I noticed that I didn't have any finished photos when I looked at it yesterday!

Black drivers in a black (Duratex) finish, not much to look at (esp with iPhone pictures)

 

As Dave said, the TD18 models to a Qts (edit Qtc) of about 0.6 in a 60 litre box, and I use DSP to EQ from there.

Matt used a different driver (21"!).

 

I've not come across too many drivers that can get down to 20Hz without EQ - not sealed anyway

 

The Bitkote + gravel makes for a very inert box - but heavy - each box with driver is over 50kg.

Smelly to apply but not once dry.

 

cheers

Mike

Edited June 12, 2016 by almikel

[almikel]

Hi Brian,

your driver  SB Acoustics SB42FHC75 15" Woofer in a sealed box of 150 litres gives a Qtc of .707 and an F3 of 43Hz.

 

My approach would be run it sealed and bump it lower with a bit of EQ - but I'm lazy and greatly prefer the simplicity of sealed and active.

 

cheers

Mike

[BioBrian]

There's a lot for me to process, guys, thanks for all the wonderful input!

 

My head's still swimming from it all, and I'll get back to it ASAP.

 

The main stumbling block is trying not to get mad with WinISD for always telling me the driver info is incorrect. I would hope that it could tell you as soon as you enter the offending number, but I don't find out till I finish the project and open it up next time! I've tried various peoples' recommendations for the order to insert info, but it always goes bad. It just told me the best box size for Qtc of 0.707 was 1 litre - must be time to cool off.

 

There's a XO solution unmentioned until now - that of using a sub 'plate' amplifier, which has EQ built in. But is there a good one which would have a quiet Linear Power Supply that wouldn't undo what I'm trying to achieve with Computer Audio (ref: Tasso's thread)? I'm told they usually have a digital PS, and I'm not sure what the consequences of that are.

[almikel]

Hi Brian,

you need both sides of the Xover - the low pass for the woofer and the high pass for the mid.

Some plate amps may have both I think, or you could build the high pass as a passive.

 

As you've already discovered, crossover design isn't trivial, and getting acoustic summing correct needs to consider the natural driver rolloffs in their respective enclosures.

 

By Digital Power Supply I assume you mean "Switched Mode Power Supply" (SMPS)?

 

Some are good some aren't so good.

In the thread above you mention the river noise is quite loud in your environment.

 

Building a stereo with the lowest possible noise floor is a worthwhile goal, but if your background noise is say 40-45dB or higher, I wouldn't expect much of an issue with a plate amp with a SMPS.

 

cheers

Mike

[davewantsmoore]

The main stumbling block is trying not to get mad with WinISD for always telling me the driver info is incorrect. I would hope that it could tell you as soon as you enter the offending number, but I don't find out till I finish the project and open it up next time! I've tried various peoples' recommendations for the order to insert info, but it always goes bad. It just told me the best box size for Qtc of 0.707 was 1 litre - must be time to cool off.

 

It's a dark art  (requiring deep understanding of what the parameters represent to be able to 'get it' intuitively)

 

Paul comes to your rescue here:   http://redspade-audio.blogspot.com.au/2010/06/winisd-entering-new-driver-data.html

 

 

It just told me the best box size for Qtc of 0.707

 

 

There is a lot written about Qtc, which can be quite misleading.

 

Q represents the shape of the frequency response.....   and so, if you have Qtc=X, all that is saying that the box is making the frequency response a certain shape.

• If you apply EQ to driver.   Qtc, no longer explains the system.
• If you put the speaker in a room, which modifies the response.   Qtc, no longer completely explains the system.

 

The rub?    If we're going to build a speaker and use no EQ on the woofer (a terrible idea), then the box size may play a big part in the frequency response.    If we are going to EQ the response to something  (eg.  if you were to place a highpass filter at 100Hz, on the previously shown response) .....   the Qtc, is meaningless and irrelevant.....  any concept of Qtc=0.5, or Qtc=0.7 sounding a certain way, goes completely out the window.

 

You see many people talking about the Q of their boxes  (which have high pass filters, or other EQ applied to them), like it means something.   They've midunderstood what Qtc is.

[davewantsmoore]

There's a XO solution unmentioned until now - that of using a sub 'plate' amplifier, which has EQ built in. But is there a good one which would have a quiet Linear Power Supply that wouldn't undo what I'm trying to achieve with Computer Audio (ref: Tasso's thread)? I'm told they usually have a digital PS, and I'm not sure what the consequences of that are.

 

Being able to set the correct crossover slopes and EQ,  has such a more drastic effect on the sound of your system than anything else like amplifier quality, or power supply quality, or what you use as a source.

 

If anyone questions this.   All you need to do is get a super-high-quality system and start adding EQ, or modifying the crossovers, and hear what happens.   The effect is MUCH larger than any component switch  (even switching a $100 component with a $10,000 component) .....  It's like the difference between having a mossie bite, and a broken leg.

 

That isn't to say that high-quality amps, power supplies, and digital audio don't make a difference.....    but you should never trade off the ability to implement a high-quality crossover design or EQ.

 

 

My advice from here.

• Keep your current speaker as a 3 way
• Implement a high pass filter for it, before the power amplifier
• Run a second output from your source/preamp to an electronic crossover (like a miniDSP or similar)
• Use a regular stereo amplifier for the subwoofers.   Select one with the grunt you need.   Eg.   400w/4ohm whatever.....    use a separate lower powered amp (ie. whatever you enjoy using - for your 3way)

Program your miniDSP so it does these things:

• Implements a low-pass filter on your 15", which matches the high-pass roll-off of your 3-way speaker (whatever that is)
• Implements a high-response for your 15" ...   You will find your 15" rolls off earlier than you want it to.    The miniDSP will be used to set the frequency (lower) and slope (probably steeper) where you want it.    This means you are not using the box size to set how low the bass goes  (because the bass won't go low enough anyways)
• Implements EQ for the 15" which corresponds to your room modes, etc.    Bass is typically +/- huge amounts in room.   You can fix this.
• Implements a couple of different "profiles" you can switch between.    Extra bass extension.   Early roll off bass.    A peak at 40hz.    Whatever you like.  This is your bass-tone control.

 

 

Advantages:

 

• You get acceptable bass performance with EQ
• You get a highpass response (ie.  bass extension) from your subwoofer that you want.   Rather than whatever you get just from the box (with your driver and no-eq... it won't be enough to be a "subwoofer")
• You keep your 3way speaker, "analogue"   (if you consider this desirable)
• You can pick an appropriate amplifier for your "subwoofers"  (eg.  I've been using a ME550-II and ME14 pre/power combo for my subs....  where as I need about 1% of that power for my mains)
• More that I've skipped over, but I have to run.

[almikel]

 

 

The main stumbling block is trying not to get mad with WinISD for always telling me the driver info is incorrect. I would hope that it could tell you as soon as you enter the offending number, but I don't find out till I finish the project and open it up next time! I've tried various peoples' recommendations for the order to insert info, but it always goes bad. It just told me the best box size for Qtc of 0.707 was 1 litre - must be time to cool off.

 

 

hi Brian,

Dave's correct regarding how Qtc becomes a bit irrelevant if you start using EQ to shape the response, but if you're only looking at sealed boxes, I have a simple spreadsheet taken from Rod Elliot's site (but written by True Audio) that does 2 things very well by inputting only a few numbers (many less than WinISD):

• plots the natural response of the woofer in the box without EQ
• shows the amount of EQ you need to push the response lower based on a Linkwitz Transform (LT)

This spreadsheet is my goto tool for woofer design in the first instance - enter driver Fs, Qts, Vas and box volume and you get a graph.

Add a desired "Transformed" F3 and Q, and it calculates the Linkwitz Transform and shows the graphs of before/after and LT gain required

 

It only works for sealed enclosures, and doesn't show driver excursion or required amplifier power, but for simply seeing what a particular driver does in a particular sized box it's brilliant.

 

PM me with your email address if you want the spreadsheet.

 

Back to Dave's comments on Qtc, I agree completely - but I've headed over to the darkside where I happily apply DSP EQ and 96dB/octave filters to get the response I want.

You appear to be someone who would choose to apply EQ as little as possible - so if building sealed enclosures IMHO target a volume to give a Qtc of .707 - that will give you the flattest passband and the lowest F3.

A smaller box will give you a peak before rolloff (which may need to be EQ'd out), and a larger box will have slower rolloff, but droop in the pass band (which may need to be boosted via EQ to get a flat response).

 

As Dave said, if you choose to use the EQ path (which I would recommend), then box size becomes a bit irrelevant (within the constraints of amp power and driver excursion when you go for a very small box) - but I always target a sealed box size to give a Qtc of 0.707 and go from there.

 

I can't provide any input on ported boxes - they're too hard to design and integrate well into a room IMHO (I'm a lazy DIYer) - just remember that the port is just another way to add EQ - I'd much prefer to add that EQ electronically.

 

cheers

Mike

[BioBrian]

Success at last! Tried Red Spade method yet again, this time Rms, not Qms, and it worked. Never been so happy to see a green light!

 

So the basic choice still remains, if I ignore all the great advice above:

 

 

The figures only show on one option; the sealed one is with Qtc of 0.707, giving 124.16 litres. I assume .707 will make the driver most comfortable, so agree, with present understanding, with Mike there. The vented one is QB3, and I confess not knowing what the difference is - just that I didn't like the sound of 'ultra boom-box', or whatever they called the one I thought was supposed to be right.

 

Adding EQ to a sealed box is an option - I have a gutsy Parasound A21 power amp (250W/8Ohm). But sheesh, the vented one is what I've been dreaming of - it's hard to make sense of the other, from this angle!

 

Must remember to breathe... The box could be smaller, meaning it might be able to match the shape of the pyramid above, tempting, tempting...

 

Might have a play with how a HP filter wrecks or otherwise the 3-Way before giving up on that. It actually played some orright bass yesterday, with Janine Jansen's (borrowed) Strad sounding absolutely fantastically as they do. Modern Decca recording of Prokofiev's 2nd violin concerto; if anyone wants to know what all the fuss is about with Strads, I'd recommend this for a listen. (I even noticed that above how pleasing the speaker sounded).

 

Just a general thing about my noise floor - the river was particularly loud the other day when we were all in flood. It can get quite frightening, with huge boulders crashing down, shaking the earth and house - they sound like cannon fire. My adventures with Computer Audio have been a real insight into possibilities of improving source: the '192' files amazing, noticebly better, within any constraints of the rest of my system. We can differentiate, shut out and forget. They call it psycho-acoustics, don't they - the acoustics of a psycho??

Edited August 20, 2016 by BioBrian

[davewantsmoore]

Back to Dave's comments on Qtc, I agree completely - but I've headed over to the darkside where I happily apply DSP EQ and 96dB/octave filters to get the response I want.

You appear to be someone who would choose to apply EQ as little as possible - so if building sealed enclosures IMHO target a volume to give a Qtc of .707 - that will give you the flattest passband and the lowest F3.

 

... but it is only going to give an F3 of ~40hz.     Not a subwoofer  

 

If we use EQ to push the response lower...   then the Q of the system will be that of the resulting response .... we could have used any box size.

 

The only real considerations are:

• If we use a too small box .... we reduce the low-frequency efficiency
• If we use a too small box ....  there may be a peak in the response, and this can make the EQ more difficult to design.

[davewantsmoore]

I assume .707 will make the driver most comfortable

 

In short no.     Q= X will make the frequency response have a certain shape.      Q refers to the amount of peak in the response where it begins to roll off.

 

It is wrong to say Q=X will make it "more comfortable".... it will only make it have a different frequency response.    What frequency response is "best"?   There's lots of factors in that question...  but there is no correct answer really.    You have to decide what response you want.... and then chose a way to get there.

 

 

 

The vented one is QB3, and I confess not knowing what the difference is - just that I didn't like the sound of 'ultra boom-box', or whatever they called the one I thought was supposed to be right.

 

The different types of vented boxes, and just again, different Qs, and slightly different frequency responses.

 

WinISD are just using the "traditional" names for some of the box types.....    but it does a real disservice.    By "naming" them.   People read things into those names.... and they assume that the different box types have something to do with performance ...  and that one must be "better" than the other, etc.

 

the vented one is what I've been dreaming of

 

Why do you want a vented box?   The vent in the box, is just another form of EQ.     It's EQ which is difficult to design .... difficult to modify after built....  and difficult to make high-performance.

 

Everything which modifys the response of the raw driver is "EQ".

• The box.
• A vent in the box.
• Changing the input signal.

 

It is the final response which matters.  

Edited June 12, 2016 by davewantsmoore

[almikel]

 

 I assume .707 will make the driver most comfortable, 

Hi Brian,

the driver doesn't care, unless you're hitting its limits...

...that said, excursion = distortion, so keeping excursion low helps to minimise distortion..

 

On and above the tuning frequency of a ported enclosure, the excursion of a ported driver will be less than sealed (up till the port is not doing anything)...

...go below that frequency with a ported box and excursion increases rapidly - to driver destroying levels  - any ported enclosure driven to high volume IMO should have a high pass filter as protection.

Sealed enclosures don't need this, and are much easier to design and integrate with other speakers/room etc.

 

Mike

[BioBrian]

I have no doubts about the wisdom and experience being offered, it's just that I'm a careful shopper. I sincerely appreciate the advice!

 

Having got this software to work, I can now illustrate why I am still pursuing this thought.

 

At the background is efficiency. Vented means cheap. The design is done already - by WinISD. Box size, vent diameter and length (possibly multiplied by 0.6, following Troels' studies), excursion worries put to rest, etc etc.

 

The sealed option leaves no choice but to force the driver into fighting the air volume of a smaller box using electrical power, digital processing, another amplifier, and a lot more studying and gadgetry, which suits some but which I personally have an aversion to. Also is the issue that the present room is not it's final resting-place, so eons of battling with the likes of REW is unlikely to make me happy along the way.

 

Going vented, it seems there's no problem with driver excursion:

 

 

Just for general interest:

 

 

This one worries me, but I don't understand much about the implications:

 

 

And this one needs explanation too:

 

Edited June 13, 2016 by BioBrian

[BioBrian]

Dave, what does a line-level HP filter look like? Just a cap/coil as in a XO? I like the idea, but eg Siegfried's Orion circuitry is a bit scary. 

[davewantsmoore]

it's just that I'm a careful shopper.

 

No worries.    I should probably add, that much of the posting is from the perspective of "explaining things"  , and ensuring that (which might not necessarily need to be understood) .... and so "no misleading information is left lying around on the internet".    ie.   People google things, and they find threads like this ....  and I wouldn't want them to get the wrong idea about something   (oh, the time I've wasted following misleading advice from the 'net).

 

 

Anyways.    What I'm trying to say Brian, is don't get the impression from my posts that you're too far off track.     

Edited June 12, 2016 by davewantsmoore

[davewantsmoore]

At the background is efficiency. Vented means cheap. The design is done already - by WinISD. Box size, vent diameter and length (possibly multiplied by 0.6, following Troels' studies), excursion worries put to rest, etc etc.

 

Vented boxes are a lot more complex than they look.    WinISD can be a drastic oversimplification.     I would honestly say that if you're not going to build and measure multiple boxes, or you're not going to build a recipe from someone who has  (ie.  use the exact same box, driver, amplifier, etc) ....  then be skeptical.

 

 

Dave, what does a line-level HP filter look like? Just a cap/coil as in a XO? I like the idea, but eg Siegfried's Orion circuitry is a bit scary. 

 

His is an "active" line level high pass filter.

 

Yes.  A passive line level high pass filter is simply a cap/coil.

[davewantsmoore]

The sealed option leaves no choice but to force the driver into fighting the air volume of a smaller box using electrical power, digital processing, another amplifier

 

I do understand the aversion.... but

• The "fighting the air volume" issue.  Doesn't need to be an issue - use a larger box.
• There is more distortion from the "air volume" in a vented box  (the air volume moves through a hole - its a whole extra order of complexity for the mechanical system - and is not ever as linear as a sealed box)
• Some sort of EQ for the bass is really going to help achieving high performance.   It's unlikely you can get a good frequency response without it in-room.
• A seperate amplifier for the bass, is usually a good idea, simply due to the differing requirements you typically have.

 

 

The charts you have posted, all don't mean a lot.   There's really two big things to consider.

 

Sealed v Vented.     You're trading off lower cone movement, for the air inside the box being forced out a hole.

 

Generalising about which is better is difficult.    You need to work out how much excursion you are going to get with a sealed box, at your maximum output....  and the decide if this is too much.  Modern drivers are very very linear.... and it would be wrong to assume that (for example) if the vented box is operating at 30% of "xmax"  (maximum linear cone movement), and the sealed was operating at %70 of xmax ....  that the vented is tangibly better.

 

The reason vented boxes are popular (and useful) .... is when you cannot get enough linear output from a sealed box.

 

Frequency response.    The charts you've posted, like group delay, phase, cone excursion .... are all directly related to the frequency response.     Excursion also varies with box size (and vented v sealed) but it also depends strongly on the low-end roll-off of the system (ie. the frequency response).      So to "fix" all of these, you need to get the right frequency response.    How are you going to get the frequency response you want?    You could design a few different vented boxes, to see which one hinges the bass rolloff, just exactly where you need it in your room (and potentially related to your exact speaker placement) .....   but even if you do this, and especially otherwise - you'll need EQ to get a really good result.

Edited June 13, 2016 by davewantsmoore

[davewantsmoore]

Also is the issue that the present room is not it's final resting-place

 

Another good reason to have EQ IMO.

 

IMO

• Put line-level passive high pass on amplifier for 3-way.    This filter doesn't have to be "exact".    I would do that in 'digital' ... but I appreciate the aversion.

• Assuming you can get enough SPL .....  Put 15" in sealed qtc=0.5-0.7 box (ie. a bigish box)
• Use a separate amplifier and EQ to make the response what you want in-room
• Use that EQ to add a low pass filter to the 15" that mirrors the highpass response of the 3-way  (this is why you didn't have to get the passive highpass "exact", because you can tweak the low pass of the subwoofer to match it)
• BBQ

[Guest]

 

Another good reason to have EQ IMO.

 

IMO

• Put line-level passive high pass on amplifier for 3-way.    This filter doesn't have to be "exact".    I would do that in 'digital' ... but I appreciate the aversion.

• Assuming you can get enough SPL .....  Put 15" in sealed qtc=0.5-0.7 box (ie. a bigish box)
• Use a separate amplifier and EQ to make the response what you want in-room
• Use that EQ to add a low pass filter to the 15" that mirrors the highpass response of the 3-way  (this is why you didn't have to get the passive highpass "exact", because you can tweak the low pass of the subwoofer to match it)
• BBQ

 

BBQ 

[BioBrian]

 

Umm...

[BioBrian]

Is anybody willing to have a go at explaining the bottom graph ('maximum power') in Post 112?

 

I entered the 300W figure from the spec sheet as Pe, but don't know what line means, other than presumably it requires less power where the dip is.

 

Also, the maximum excursion line shows less than 3mm for both vented and sealed. This would seem to negate any concerns, but I'm not sure whether it is in reference to full 300W, or 93dB as per data sheet, or what.

 

The data sheet that I've been drooling over since before I bought the woofers shows only 5dB drop from full output, from 25 to 30 Hz. This is with a (presumably very large) IEC baffle, with no air compression/stretching behind. Putting the driver in a wall would be as good or better, preserving the full dynamic expression, presumably without self-destruction, down to 22.5 Hz (Fs) at least??

 

Putting it in a large vented box would preserve most of this dynamic range, if fact increase SPL at low frequencies, the negative being chuffing air through ports (in my case, very large ones, using 2 x 102 mm sewer pipes), and of course the problem of the vented air being out of phase with the driver, whether the vents are at the front or rear. (But how much does that matter, if you're actually getting it?}

 

Putting it in a sealed box restricts the magic straight away - what I've been trying to avoid - surely cutting the tops off the amplitude is a form of distortion. Like crossing the Wellington plateau with good shock absorbers - missing the real thing . To cut off this magic and rebuild it with processors and extra amplification would seem like an unnecessary waste of energy, and could end up the aural equivalent of having a shower in a raincoat. As I'm still 'intuiting' it!

 

I'm getting that the response at my targetted early 20s is going to be more 'linear' using sealed, but with the vented there's the option of chucking all the science out the window, and using a (large) pair of socks to try different aperiodic tunings, which apparently work. (Blocking the vents in a 235 litre box would be pretty much on the way to 'infinite baffle'?)

 

Just another question about EQ Dave - in what you've been suggesting, is it just raising the level incrementally towards 20 Hz, or do you mean the whole shebang with DSP treating modes, etc?

 

And I appreciate your efforts to keep this part of the internet correct and informative. I (try to) support that idea.

Edited September 1, 2016 by BioBrian
spelling

[acg]

@@BioBrian, I would hesitate to use the cap in series like that and use series resistance instead.  If you have to use a cap I think it is usually better to shunt it.  Line level HP can be series resistance and shunt inductor...more likely to work well unless I have missed something else about the application.  The maths is simple to calculate the required values when used in an application such as this [First Order HP Hz = R_ohm/(2.pi.L_H)]...what cutoff are you after?

Edited June 13, 2016 by acg

[BioBrian]

@@acg - thanks for your input.

 

My maths is ancient and unreliable, but it gives me, for a XO point of 100 Hz, 1 Ohm series resistor, and 1.6mH shunt inductor. Or 2 Ohm and 3.18 mH.

 

I'm not sure what the series resistor does - wouldn't it impede the whole frequency spectrum?

[acg]

No worries.

 

The whole point of a filter is to act as a voltage divider and a voltage divider is usually comprised of two resistances one each in series and parallel (shunt).  To form a voltage filter we need to swap a resistor (that treats pretty much all voltages the same) for something that does not treat all voltages the same.  In this case the inductor is reactive and has higher impedance for higher frequencies, so it is more likely to take low frequencies across it (shunted to ground) than high frequencies that keep flowing on down the line thus forming a high pass filter.

 

Keeping component values small is a good rule of thumb so pick a small inductor (I am told they are not easy to get right at small values) and calculate the resistor that you need to match it for a 100Hz high pass filter and it should work.  Your previous calculations were fine so just do the same thing again.  

 

EDIT:  Note that this is a first order filter...not sure if that is suitable or if you want a higher order.

Edited June 13, 2016 by acg

[BioBrian]

I'm still not sure - do you mean the first or the second scenario of these sketches?

 

 

To my understanding, the resistor in the first one would impede the entire spectrum. The resistor in the 2nd one (commonly seen in passive XOs) would control the influence of the inductor - how much of the calculated LF would be dumped to ground. Maybe?

 

Yes, I think 2nd order would suit the task best. This is for the bottom end of the 3-Way, which has an 8" woofer in a closed 40 l (Qtc .707) box. Other than stuffing a lot of bricks in it, I think the only options are active, or passive XOs.

 

I'd like to experiment with passive, but the expense, and the warnings of failure, are quite strong influences.

 

This line-level filter would seem like a nice idea, but lots of people get pretty lively about putting caps in the signal path!

 

But my question about the role of a series resistor (as per the 1st part in the drawing) is 'of the moment'.

 

There's also the choice of components - their size, voltage rating, type, etc. Completely out of my experience.

 

 

[davewantsmoore]

 

Umm...

 

Yes.

 

... but the values of C and L will be much much smaller than the ones you've got in the pic