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BioBrian

Gestating large sensitive passive 15" augmenting subs

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On 13/10/2019 at 10:31 PM, davewantsmoore said:

A lot of people seem to misunderstand this.

I understand it completely - I just prefer to start with a sealed box that doesn't require EQ cut to get a flat anechoic response.

Given I'm happy to use EQ, I realise box size is mostly irrelevant

 

On 13/10/2019 at 10:31 PM, davewantsmoore said:

but there is not necessarily any reason to choose a box which delivers a Q=0.7, like it is some magic number

Agreed

If you can get away with a larger box (lower Qtc) you will need less EQ boost down low, and smaller boxes just need more power/EQ...although room effects will swamp everything.

You end up just EQing the room response (within driver Xmax limits and amp capability)

 

cheers

Mike

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7 minutes ago, almikel said:

You end up just EQing the room response (within driver Xmax limits and amp capability)

achieving this in an "all passive" setup is the challenge

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On 16/10/2019 at 12:18 AM, almikel said:

I understand it completely - I just prefer to start with a sealed box that doesn't require EQ cut to get a flat anechoic response.

Given I'm happy to use EQ, I realise box size is mostly irrelevant

It isn't irrelevant ..... the point is that one should go for the box which loads the driver so it has it's highest performance (most linear in/out movement under all conditions).

 

This isn't necessarily the one which produces a flat frequency response.

 

If you have "EQ" (which could be many things like a port/PR or a horn, or mass, or stuffing, or passive or active components bending the input signal) then you can choose the box for best performance, independently of the frequency response.

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2 hours ago, davewantsmoore said:

It isn't irrelevant ..... the point is that one should go for the box which loads the driver so it has it's highest performance (most linear in/out movement under all conditions).

As you know, in a sealed box design the excursion of the driver won't change regardless of box size - just the power and EQ required to hit a target response.

I could have halved/doubled/quadrupled the size of the sealed boxes for my TD18s for no change in their linear in/out movement - assuming I have the power and EQ to hit the target frequency response.

3 hours ago, davewantsmoore said:

If you have "EQ" (which could be many things like a port/PR or a horn, or mass, or stuffing, or passive or active components bending the input signal) then you can choose the box for best performance, independently of the frequency response.

For ported and horn speakers (I've never looked at PR, but I imagine similar) you have to design/choose the box volume that interacts appropriately with the port/horn/PR to achieve a target frequency response - IMO you can't choose a box for best performance independently of the frequency response.

A smooth "in room" frequency response is the end game after all.

 

cheers

Mike

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On 17/10/2019 at 10:36 PM, almikel said:

As you know, in a sealed box design the excursion of the driver won't change regardless of box size - just the power and EQ required to hit a target response.

Yes, but neither of these things are particularly "important".    The power is importantas you can't use "too much", and the amount of EQ needed is irrelevant.

 

What is important is the acoustic load on the driver and how the driver "moves in and out".   This starts to become a key factor as you approach the system resonance, and starts to become practically "the only" factor as you move below it.

 

This is dictated by the mechanical system, box size being one very big factor.   (Note)

 

People choosing an arbitrary (eg. large) box size because "I'm going to use EQ".... and people choosing a very specific box size because "I'm not going to use EQ" .... are overlooking the optimisation of the mechanical system.

 

(Note) another big factor is the electrical damping applied in the amplifier.   Which is where "current drive" comes into it (but that's another story).

On 17/10/2019 at 10:36 PM, almikel said:

I could have halved/doubled/quadrupled the size of the sealed boxes for my TD18s for no change in their linear in/out movement

In the mass controlled region yes.... at resonance, or in the compliance controlled region of the motion, no.

 

For you driver in 100L above resonance is ~80Hz, and below is ~40Hz (with the peak in between) ......  so you might want to rethink your assumption, as in the region you're using the driver, the mass of air doesn't have a big bearing on it's performance as the amplifier isn't "in control".

 

Granted, the mass of air you have chosen might be the "right one" (or close enough, it's not super sensitive) .... but double/quadruple/half the size of the box, no.

 

On 17/10/2019 at 10:36 PM, almikel said:

IMO you can't choose a box for best performance independently of the frequency response.

You can (or you at least have a freer choice) if you have EQ.

 

Let's for example say that the "best" box size for a vented cabinet performance (not frequency response), produced a response which had too much peak in the passband.    You couldn't use this if you had no EQ.  As the peak in the frequency response dominates the sound.   You'd have to adjust the box to get a flat response.

 

If you had EQ you could.

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For the bench tops I used some oil-based paint from a can that might be 20 years old. Bad choice - the last 2 weeks has been literally "watching the paint dry". Today was the first time I could think of sanding - just knocked the tops of the brush-marks off, and all the insects that had such a long opportunity to live out their dreams in such luxury.

 

4 sheets of 24mm ply came out of hibernation for the baffle pieces. With no helpers within miles, I had to get a bit clever with ropes and pulleys - sure there's more of that to come.

 

The benches turned out fairly flat - enough for the job - but with 2 benches "the same" height, I still had to do a fair bit of shimming under the legs to get them on the same level for cutting 8' x 4' sheets.

 

So we are on the way at last.

 

708956668_CuttingforGestiesstarts.jpg.767cb4341634a84dd5e8aaaaadd0f4d7.jpg

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Happy to help lift things Brian. Dont be shy

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2 hours ago, Upfront said:

Happy to help lift things Brian.

Thanks again Linc. You're on call every day for the next 4 years - OK?? Nah, let's maybe save it for the big sub crawl thing if we get that far... Can't be long now...

 

I did get the baffles laminated, now 48 mm thick, and had a few happy moments today,  routing the driver holes. Except, despite careful measurements, the drivers don't fit! Hmm, measurements, gotta love 'em...

 

Will have to do that nasty business of screwing the cutouts to the table inside the holes again, centring baffle around the pivot hole, and trying to enlarge the driver holes equally in all directions - with only 1/2 a mm all around to play with. And not damaging the base that the driver screws to.

 

At least the baffles are a bit lighter with big holes in them.

 

1538659784_Screwsbetterthanweightsforlam.jpg.d5497bd3bfc9cc340ab3d2c7501d586d.jpg

 

619340795_Driverrouting.jpg.32db1a3e3f1768edc9d0d01a6eb373b7.jpg

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On 13/10/2019 at 11:31 PM, davewantsmoore said:

given our more acute hearing at bass frequencies, it's likely.

Huh? cough*Fletcher Munson*cough.

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1 hour ago, A9X said:

Huh? cough*Fletcher Munson*cough.

Yes, exactly.  Fletcher Munson....

 

 

Hint:   As you move lower in Hz....  a phon becomes a lower and lower value when expressed in dB.   This means that at low frequencies, you can hear differences in (dB) level more acutely than at higher frequencies.

 

Ergo, dB levels at bass frequencies are important.   1 or 2dB difference in level at low frequencies is more important than the same error at high frequencies

 

 ..... but, bass frequencies is where we typically have the worst accuracy of levels (for many reasons).   This is why when people hear a system where the bass is highly accurate (in dB levels) that it sounds so much better.

Edited by davewantsmoore

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3 hours ago, davewantsmoore said:

Ergo, dB levels at bass frequencies are important.   1 or 2dB difference in level at low frequencies is more important than the same error at high frequencies

? are you sure ??

  I can easily live with 1 or 2 db difference at low frequencies, but a 2 db peak around 3-4K can make a speaker sound crap

 

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6 minutes ago, afa said:

? are you sure ??

  I can easily live with 1 or 2 db difference at low frequencies, but a 2 db peak around 3-4K can make a speaker sound crap

I agree wholeheartedly with that. But I've never heard this fabled perfect dialled-in bass, so I'm nothing.

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Brian....you are not nothing... you are the "Messiah"  (Monty python ref):)

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9 minutes ago, afa said:

? are you sure ??

Yes.

 

9 minutes ago, afa said:

  I can easily live with 1 or 2 db difference at low frequencies, but a 2 db peak around 3-4K can make a speaker sound crap

I do know what you mean..... and I don't disagree that peaks in the low Khz, range can stick out quite a bit, and make the music sound quite unnatural, especially on a speaker with bad polar response in that range.

 

That may be our preference.... but the fletcher munson chart shows you how your ear actually works.

 

Your sense of loudness (in phon) spans a lower range of decibels at LF, than it does at higher frequencies.

 

Playing with an equaliser, and various noise/tones, it is quite obvious.

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11 minutes ago, BioBrian said:

I agree wholeheartedly with that. But I've never heard this fabled perfect dialled-in bass, so I'm nothing.

See my response above.

 

Our experiences and preferences might not make it seem so ...... but that is actually how our ear works.

 

You don't need "perfect bass".... and you won't get it anyways ........   but if you just use an EQ to lift/reduce the bass some, and then do the same at higher frequencies  (with test tones/noise ... not music) .... then you will see.

Edited by davewantsmoore

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8 minutes ago, davewantsmoore said:

especially on a speaker with bad polar response in that range

Depends on your listening environment, if its crap on axis its going to be crap on your ears regardless

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"The human auditory system is sensitive to frequencies from about 20 Hz to a maximum of around 20,000 Hz, although the upper hearing limit decreases with age. Within this range, the human ear is most sensitive between 2 and 5 kHz, largely due to the resonance of the ear canal and the transfer function of the ossicles of the middle ear"

Edited by afa
syntax

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22 hours ago, BioBrian said:

I did get the baffles laminated, now 48 mm thick, and had a few happy moments today,  routing the driver holes. Except, despite careful measurements, the drivers don't fit! Hmm, measurements, gotta love 'em...

 

Will have to do that nasty business of screwing the cutouts to the table inside the holes again, centring baffle around the pivot hole, and trying to enlarge the driver holes equally in all directions - with only 1/2 a mm all around to play with. And not damaging the base that the driver screws to.

Turns out that the outer diameter was possibly OK - I expanded it a little, but the driver still wouldn't sink down into the hole. The problem was, because the outer diameter was the same, I'd used my notes from the previous build, to do the inner diameter the same. Wrong...

 

I mentioned somewhere above that these "sub" versions have a wider roll-surround. It looks like they kept the same cone, but re-built the chassis, so now the lip that the driver screws onto is only 20mm, instead of the previous 23mm. So the chassis legs wouldn't fit.

 

Another re-alignment later (2 very large lumps of very delicately-surfaced plywood around the pivot circle), the 12 x 50mm bit swept into action and cut a 20mm lip, which let the drivers down to full depth.

 

A couple of hours I'd rather have spent otherwise, but at least it puts off the horrible choices of vent hole routing. ("Why rout a vent hole, if you can put it off till tomorrow?").

 

The largest rounding-over bit I've found that will fit into my router has a rounding radius of 7/8", or 22mm, for the new-age among us. This is a whole lot bigger than 8mm, about the biggest possible if I use the vent-screw-on-cover idea. I'm resolved to go with the larger flare, and later improvise some fly-wire magic and/or vent blocker. It'd certainly look better.

 

As discussed before, the 150mm (6" in real money) diameter vent means at least 6.8 times better than necessary, according to the mighty online calculator, so if nobody steps in and rescues me, this is what might happen tomorrow. Single 6" round vent, with a 22mm radius flare on both ends.

 

I did do some comparison with the SVS whizz-bang vented 16" sub, featuring 3 x 3.5" vents (quite long - much more friction). Mine have 93% of their vent area, drivers probably have a lot less cone area (Sd), and I don't have to pay a million customers refunds if they all play Bassotronics at full dial.

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6 minutes ago, afa said:

Thanks Arthur, that could save a lot of work. (Screws? Can't see how it is attached).

 

Price? - They must've seen me coming 😞. (Actually, I have no idea what this emoji is doing).

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Its glued in, yes its pretty expensive but i got mine a while back when it was on clearance sale (cheaper) from a now defunct online business and i thought one day it might be useful and it was.

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22 hours ago, soundbyte said:

Might a bit late in the process but is this software of any use to you?

 

https://www.subwoofer-builder.com/flare-it.htm

https://www.subwoofer-builder.com/flare-it.exe

Thanks Russell, this adds another level of stress-reduction for me.

 

Not sure if I'm doing this right, but I got a 20 Hz vent speed of 6.8 m/sec from the WinISD sim, and entered it into this Flare-it calculator. Looks like we're well covered. We don't even need flares (as my mother insisted in the '60s). I like the affirmation in green down the bottom.

 

482952780_Flare-itsnip.JPG.88ceb898af08b006a1b5a54346a4d3c5.JPG

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On 31/10/2019 at 8:41 PM, afa said:

Depends on your listening environment, if its crap on axis its going to be crap on your ears regardless

A single axis measurement can be quite misleading.   That is not to say that the direct sound is unimportant .... only that especially if the polar response is bad (which too many speakers are) then you will notice the issue you mentioned.

 

ie. the issue is not so much due your level sensitivity at khz range..... but due to other cues in this range from a small room (reflections, delayed, etc.)

 

As I said, the level sensitivity of your ear is clearly explained by FM chart .... and you are (dramatically) more sensitive to level differences in the bass.

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