Interesting article re: port length (1 Viewer)

[Greg Monfort]

Hmm, I thought I posted this the other day, but I don't see it. Oh well, here it is:
------------------------
Dan:
>The difference between a driver stalled and one
moving at half of Xmax can easily - and rapidly - add
100+ deg F to the core temperature (empirically
measured).
=====
Since heat rise goes up at the square of the
current*time, isn't this more like 100deg C, as pro
drivers routinely hit >200deg C?
=====
>In fact, I believe Harwood explicitly states that
his measurements show that as one exceeds a length
twice that of the diameter of the vent, air flow in
the vent is no longer symmetrical. Air flows into the
box easier than it flows out. You start to get box
pressurization, which displaces the rest position of
the driver, leading to yet more nonlinearities.
=====
This is my understanding. In the not too distant
past, acousticians believed that the larger the horn
mouth, the lower its distortion near cutoff. The
reality of technical enlightenment is that horns with
a much larger than 1WL/Fc mouth exhibit a similar
situation, in that the 'membrane' at the mouth exit
becomes so non-linear that it ripples back at a high
enough velocity to act upon the high throat
impedance, modulating the driver diaphragm more than
a too small mouth does.
=====
>Best bet: do a PR system. Eliminate most of the
issues in both cases. But I suspect that's what you'd
recommend anyway, Greg, what with your CBs doing bass
duty and all!
=====
That's a big 10-4! I'm to the point now where I
wished sonosubs weren't viable designs, so more PRs
were used, rendering threads like this moot.
I had experimented with PRs using Olson's work, and I
was underwhelmed to say the least. Decades pass where
I continue to optimize/minimize vent output/tradeoffs
with my limited education, and along comes Tom Danley
and explains what should have been obvious
considering all the prototypes I tested. I guess
that's what inventors do, highlight the trees in the
forest for the rest of us nearsighted types.
=====
TV:
>Oh, I have no doubt that SOME of the harmonics will
be driver induced, but having a straw port clamping
down on the fundamental is only going to ADD to the
overall thd levels...and at high output...thd will
increase a great deal.
=====
Straw?! You make it sound like I'm advocating small
vents. Everyone who's into quality performance agree
that vent velocity must be kept to Well, the tuning will affect the excursions just
above Fb...but IMO, it's the BW from about 1/6th
octave under Fb to about 1/3 octave above Fb(rough
approx...they change with many variables)that can be
highly compromised when under porting.
=====
Again, where have we advocated under porting to the
point of audibly increasing distortion? Sheer SPL
shouldn't be the sole goal in a quality music/HT
application.
=====
>Again, I disagree---I think the text is quite plain
about the need for high flow vents...in high output
subwoofer apps.
>Sure, any app taken to an extreme will seem somewhat
comical and less than practical. I'm not trying to
say every subwoofer should have a 96" long port...but
port compression is something that will degrade the
objective AND subjective performance of a subwoofer.
If you're using a basic 12" driver with a apex
jr---it's not going to be as big of a deal...then say
if you're using a mass12 driver with >500w.
=====
One more time, where have we advocated low flow/high
mach vents? I don't have a copy of the LDC, but
you've stated he advocates the vent area should be
equal to Sd in HO designs. Ok, let's sim the Mass
2012. It has a ~10" effective diameter, so according
to Dickason, we need a 10" dia vent. Just doing a
BoxPlot maximally flat alignment at rated power I get
7.32ft^3/17.6Hz. A 10"dia vent needs to be 85.6"
long.
Now which is it, a theoretically great performer
because Dickason says so, or comical/less than
practical, with strong organ pipe resonances because
that's what it is? You can't have it both ways.
Oh, but wait! Sims are for show, and 2m groundplane
is for go, right TV?
And right you are, as there's no way in this reality
that a 2012 has a strong enough motor to accurately
control that much box/vent mass anechoically flat to
Hey, you got a bunch of guys spending countless
hours insisting that *on paper*...going from a single
4" non flared port to dual 4" flared ports won't help
much and may actually HURT performance of a sub.(for
example).
=====
Well, I guess I've missed those posts, but I do agree
that there's a point of diminishing returns, and that
at some point they do hurt 'overall' performance if
too long, or have too much cross sectional area WRT
cab size.
=====
>I've found *paper* to be useful for one thing...and
I like the double-sided type for that.
>seriously,ALL of the actual data I've seen(from
about 100 hours of my own tests,many other serious
DIyers,various manufacturer findings,dickason's
text)all indicate high output woofing needs as much
port area as you can usually(pratically) fit in the
enclosure. How long would it have taken DP to test an
enclosure with two different porting styles...about
45 minutes?
=====
DP has been designing/testing as a professional for
at least 25yrs, or at least that's how long I've been
periodically reading his articles/posts, and has an
exceptionally keen understanding of the
forces/physics involved IMO.
Anyway, so you're saying that in your/their testing,
no matter how much vent area/length you use, THD
decreases with each increase, and that organ pipe
resonances never influence performance in any
negative way?
=====
>Dr. Hsu told me he lost 2-3dB going from the dual
vents of the Va, to the single port of the *TN*...but
he got tired of snaking dual >40" long vents into
enclosures all day.
=====
This sounds like a lot, but due to our hearing
inefficiencies at high SPL, it's barely noticeable,
if at all, in-room, especially at infrasonic
frequencies.
=====
>That's up to a 50%(!) loss in clean headroom,with a
*good* driver and a basic 4th-6th order enclosure.
=====
While 3dB is equivalent to half power electrically,
it represents very little in terms of percentage
acoustically. For example, using 120dB/400W has a
higher value, then -3dB would be 117dB/200W, or:
%change = 100-[(200-(400^-3/20)] = 99.59%, the
difference between the higher Vs lower SPL, so
100%-99.59% = 0.41% change acoustically, and why it
wouldn't be obvious in-room.
=====
>I bet he would agree that going from a flared 4"
port like the TN series has, to a single non-flared
4" port would drop clean lowend headroom another
1.5-3dB.
=====
And I imagine that before he did it, he'd satisfied
himself that it wouldn't be missed enough to
adversely affect sales either!

Both are moot points though as we all agree that low
vent mach is a necessary goal.
=====
>This will probably be one of those *grey* areas that
few *woof* types will ever completely agree on. At
the end of the day, you have to use the
data/experience you've gathered as your guide, and I
know you understand I have to do the same. If
someone(like TN) measured a SVS subwoofer and found
it performed badly from Fb to 1/3 octave above
fB...what would i say? GM told me to design it like
that?
=====
If I designed it, it wouldn't perform badly, and your
seeming inference that it would based on my
suggestions/advice presented on this thread/forum is
totally without merit.
=====
>I have to be able to say...hey, that's MY design,
and if it measured bad, it's MY fault 100%. When I
spend ALL summer outside Groundplaning...it's not
because I'd rather NOT be crusing in the SS396,or out
on one of my ATVs.
=====
Been there, done that, but if I had a better
understanding of the physics back then of my
accumulated data as I do now courtesy of DP, TD, DW,
DH, EG, KK, et al, and affordable sims/testing
programs/gear, I could have saved myself a s**t load
of tedious work.
Oh well, it's alway been just a (sometimes) paying
hobby for me, so I don't regret it as it's always
been as much about the trip as the destination.

=====
JG:
>(I didn't paraphrase wrong,did I, Greg?)
=====
Not IMO.
GM
------------------
Loud is beautiful, if it's clean

 

[Greg Monfort]

TV:
>>>Twice the internal area, over 2.3 times the input/output area, and the flaring, and he gained roughly 78% more flow. In other words, doubling the vent area (and more than doubling termination area) did NOT double linear flow.So even if you only gain 25-33%, it will subjectively sound like 2x the bass output!
====
You need to study them a little more closely and use the more accurate Robinson-Dadson curves, as the opposite is true. Our hearing efficiency drops in the low/high BW, particularly at higher SPLs, requiring much more SPL to perceive a doubling of intensity (volume). A 2dB gain WRT 110dB/20Hz is ~inaudible to most folks.
====
>But (and I think everyone agrees on this)...dynamic source material does need about 20dB of *extra headroom* to sound "live". So if you listen to music at a nominal 85dB level...you should have the capability to produce sudden dynamic peaks at 105dB without the system *compressing* at all.
====
30dB for music, though at sub tuning frequencies, anywhere from 18dB-32dB depending on whether or not organs are used. And of course, 20dB for HT.
GM
------------------
Loud is beautiful, if it's clean

 

[Tom Vodhanel]

>>>TV:
>>>Twice the internal area, over 2.3 times the input/output area, and the flaring, and he gained roughly 78% more flow. In other words, doubling the vent area (and more than doubling termination area) did NOT double linear flow.>You need to study them a little more closely and use the more accurate Robinson-Dadson curves, as the opposite is true. Our hearing efficiency drops in the low/high BW, particularly at higher SPLs, requiring much more SPL to perceive a doubling of intensity (volume). A 2dB gain WRT 110dB/20Hz is ~inaudible to most folks.One more time, where have we advocated low flow/high
mach vents? I don't have a copy of the LDC, but
you've stated he advocates the vent area should be
equal to Sd in HO designs. Ok, let's sim the Mass
2012. It has a ~10" effective diameter, so according
to Dickason, we need a 10" dia vent. Just doing a
BoxPlot maximally flat alignment at rated power I get
7.32ft^3/17.6Hz. A 10"dia vent needs to be 85.6"
long.>Now which is it, a theoretically great performer
because Dickason says so, or comical/less than
practical, with strong organ pipe resonances because
that's what it is? You can't have it both ways.
Oh, but wait! Sims are for show, and 2m groundplane
is for go, right TV?

 

[John E Janowitz]

TV writes:
"Hey, you got a bunch of guys spending countless hours insisting that *on paper*...going from a single 4" non flared port to dual 4" flared ports won't help much and may actually HURT performance of a sub"
Why would you not flare a single 4" port when comparing to a pair of 4" flared ports? In some cases, a single 4" flared port could perform just as well, or better than a pair of flared 4" ports. Take an extreme example. Single 4" diameter flared port 40" long compared to dual 4" daimeter 80" long flared ports. In this case you would obviously see the dual ports perform much worse than the single port. They would be much more lossy. You would likely lose 3dB on the low end with equivalent input power. Going from a single 20" long port to dual 40" long ports would likely give the same effects, just to a smaller scale.
TV writes:
"I'm not really a huge fan of PRs. If you cite Nousaine's extensive work for comparison...the few PRed subs in his database generally performed quite poorly compared to like priced ported subs."
Well with all due respect to most of the major manufactures, most of them have not implemented PR systems properly. Those systems that are implemented right have performed very well. I am not sure if the knew Klipsch subs have actually been measured yet by TN, but these would be a good example of a properly designed PR system. Others would be Deon's Beast subwoofer, the Contrabass, and the Power15 sub that TN measured for me. IMO the reason most commercial PR based subwoofers don't perform well is because of the additional cost of the PR's. If you have to add in the cost of PR's instead of ports, then other corners have to be cut to bring the price of the system back down to comparable ported subwoofers.
John

 

[Tom Vodhanel]

>>>Why would you not flare a single 4" port when comparing to a pair of 4" flared ports?

 

[Hank Frankenberg]

Sheesh! You boys need a non-sub related hobby The threads with DW, TV, GM, JJ et al discussing the technical aspects have been very informative/educational for me and I appreciate their time spent on this forum. Reading this thread and stepping back from the trees to see the beauty of the forest, I realize that I sufficiently understand the applicability of Sealed, Ported and PR'd, given the end user's performance and physical limitation parameters. Therefore, I sincerely THANK YOU gentlemen for the past many months of your postings.
TV, If I weren't a D-I-Yer, I would buy one of your subs for its price/performance value and for your rare dedication to customer satisfaction. I will certainly recommend your subs to people who are after best low extension/SPL and don't have a WAF problem with them.
DW, I have bought a Tempest and an AVA250 and will buy more drivers/amps as family/friends and maybe even a couple of customers (heh-heh) might desire. As with TV, I'll buy your products because of price/performance value and also your great customer service.
Enough. I'm going to take half a day off this pm and watch a movie or two and enjoy my Tempest sonosub dual-flared single 4" ported sub and

every time the house rumbles.
Regards,
Bond...James Bond

 

[Jack Gilvey]

That's a big 10-4! I'm to the point now where I
wished sonosubs weren't viable designs, so more PRs
were used, rendering threads like this moot

What fun would that be?
Isn't there a difference between "adding more subs" to gain 3db , and adding more ports to an existing sub? (Due to cited losses?)
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[David Judah]

For a DIY project, I think I would rather risk the pipe resonance/vent loss issues of a larger port than the compression issue of a smaller port. After all, it's easy enough to change if there are problems.
In a consumer, mass-market application, I guess I would have to be a little more conservative.
DJ

 

[Tom Vodhanel]

>>>As well you should. It seems that GM is referring to perception of LF changes occurring at very high levels, not 75-85 db. A 2-3 db change is much higher,percentage/perception-wise, at 85db than at the 110-120db level Greg specified.

 

[Jack Gilvey]

This "110-120dB/20hz" figure is somewhat mythical for commercially available subwoofers.

But that level (110db,anyway) seems a reasonable goal for someone with a Tempest or a Mass (this is the DIY forum,after all

)) and,as such, I think it interesting to discuss the perceived benefits of increased porting to gain a few db at that SPL, vs. the drawbacks as mentioned in the article and by John.
Dickason says a few intersting things about porting. He does recommend vent sizes nearly equal to the driver size for high-power apps ("stage performance"...that's us,right?), and goes on to point out that the drawbacks of such necessarily long ports (resonances) are not nearly as severe as the non-linearity caused by undersizing. For home use, (at moderate levels) he says a 4/1 vent to driver diameter ratio yields reasonable linearity.
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[DanWiggins]

GM wrote:
==========
Since heat rise goes up at the square of the current*time, isn't this more like 100deg C, as pro drivers routinely hit >200deg C?
==========
Yeah, well, it should... My bad, wrong symbol! Of course, on your way to a 100 deg C rise, you WILL pass through a 100 deg F rise...
GM continues:
==========
=====
>In fact, I believe Harwood explicitly states that his measurements show that as one exceeds a length twice that of the diameter of the vent, air flow in the vent is no longer symmetrical. Air flows into the box easier than it flows out. You start to get box pressurization, which displaces the rest position of the driver, leading to yet more nonlinearities.
=====
This is my understanding. In the not too distant past, acousticians believed that the larger the horn mouth, the lower its distortion near cutoff. The reality of technical enlightenment is that horns with a much larger than 1WL/Fc mouth exhibit a similar situation, in that the 'membrane' at the mouth exit becomes so non-linear that it ripples back at a high enough velocity to act upon the high throat
impedance, modulating the driver diaphragm more than a too small mouth does.
==========
Correct. In fact, this one reason why well designed PA drivers have extremely stiff cones - to avoid excessive throat impedance from distorting the cone and causing excessive distortion. In fact, I know of one compression driver (with a soft dome, no less) that basically "collapses" at frequencies above 10 kHz. The ring of the voice coil still moves, but the rest of the soft dome essentially sits still, over-pressured by the throat and horn reflections.
Acoustics is a curious thing; air is extremely compressible, and a lot of the basic "accepted" guidelines of standard physics don't quite hold up. In fact, air itself can go nonlinear at pressures greater than 1.05 atm. This does NOT bode well for small boxes with extreme-excursion drivers, unless they are carefully designed to operate in that type of an environment (the THD from such operation can be several percent; note this is from the air itself compressing, and NOT the driver!).
GM goes on:
===========
That's a big 10-4! I'm to the point now where I wished sonosubs weren't viable designs, so more PRs were used, rendering threads like this moot.
I had experimented with PRs using Olson's work, and I was underwhelmed to say the least. Decades pass where I continue to optimize/minimize vent output/tradeoffs with my limited education, and along comes Tom Danley and explains what should have been obvious considering all the prototypes I tested. I guess that's what inventors do, highlight the trees in the forest for the rest of us nearsighted types.
===========
PRs are a great option. They can provide the benefits of huge vents, with none of the drawbacks (of course, they do have their own set of problems). Given a choice between vents and PRs, I personally select PRs, too. However, the cost factor is one that really needs to be considered. That and the fact that they don't mix with traditional sonotube designs, like you point out...
GM finishes with:
==========
Straw?! You make it sound like I'm advocating small
vents. Everyone who's into quality performance agree
that vent velocity must be kept to

 

[Tom Vodhanel]

>>>Agreed. And I'll accept that a 3db difference would be discernable at that level. But as far as our Mass12 or Tempest project is concerned , those wouldn't be levels at which we'd benefit from increased porting, are they? I don't know when port compression would begin to set in for the two projects Terry wrote up (or even how you'd find out...10% THD?), but would we be port limited at the 85-90db level, so that another port would give us our 3db?>>I agree completely. Additionally, it's worth pointing out that, even the subs that can do 90 dB SPL @ 20 Hz anechoic according to TN measurements should be good for approximately 105-107 dB SPL in-room (using a total 17 dB gain at 20 Hz from the room).

 

[DanWiggins]

TV writes:
==========
Well, you were promoting the tempest 38% Sd gain as something ABSOLUTELY worth talking about a couple days ago?
That would = what...2dB or so of POTENTIAL increase? Now a 3dB increase is *nearly nothing* when it has to do with my porting concerns?
Wow, what a difference a couple of days makes?
==========
I guess the distortion reduction from extra displacement isn't of concern? Sure, you may not gain more perceived loudness, but the extra linear displacement will result in lower distortion at any given SPL level.
Additionally, I would think that you would be in favor of higher displacement and SPL output, since that seems to be your point - higher SPL is good?
TV continues:
==========
>>>It's worth remembering that the ear/brain combination is logarithmic. It typically takes an increase of 7-10 dB (8 dB is often cited) for your ear-brain to "hear" a doubling in volume. In other words, you have to bump the pressure by 5 to 6 times, to have an audible increase in volume by a factor of 2. That's one contributing reason why a 2-3 dB gain is really difficult to hear.
Dan Wiggins

 

[Tom Vodhanel]

>>>I guess the distortion reduction from extra displacement isn't of concern? Sure, you may not gain more perceived loudness, but the extra linear displacement will result in lower distortion at any given SPL level.Again, I don't think you're quite following what I (and Greg) are talking about - I'm sorry if I haven't been clear enough. At low levels, a 3 dB change is noticable>Given this, the concern between a subwoofer that starts vent compression at 110 dB SPL in a room (about 93 dB anechoic, if we go with your 17 dB of gain at 20 Hz from anechoic to in-room), and one that experiences vent compression at 113 dB SPL in room should be very difficult to hear.

 

[DanWiggins]

Tom,
Here's a link to some measured results showing 110 dB SPL in room at 20 Hz: http://www.hometheaterforum.com/uub/...ML/002838.html
I trust Terry's measurements are acceptable? This is for a single Tempest, too...
Dan Wiggins
Adire Audio

 

[Tom Vodhanel]

Dan's original text
>>>and Terry's measurements show that all three sub systems he's tested recently (the two Tempest based subs, and the Mass vented unit) can reach these levels, too. >With no regards to actual THD measurements? Sure.>Tom,
Here's a link to some measured results showing 110 dB SPL in room at 20 Hz: http://www.hometheaterforum.com/uub/...ML/002838.html
I trust Terry's measurements are acceptable? This is for a single Tempest, too...
Dan Wiggins

 

[Greg Monfort]

>>>TV:
>78% more port flow should get you a 2-3dB gain,(at least)near Fb..wouldn't you think?
====
It would depend on what the design is, but even if it did it wouldn't be a 78% increase as you stated.
====
>>>You need to study them a little more closely and use the more accurate Robinson-Dadson curves, as the opposite is true. Our hearing efficiency drops in the low/high BW, particularly at higher SPLs, requiring much more SPL to perceive a doubling of intensity (volume). A 2dB gain WRT 110dB/20Hz is ~inaudible to most folks.I'm not really a huge fan of PRs. If you cite Nousaine's extensive work for comparison...the few PRed subs in his database generally performed quite poorly compared to like priced ported subs.
====
I assume you mean consumer subs, which don't interest me at all, and is irrelevant to this forum/thread. Has he measured a Contrabass, or a well designed/built DIY PR sub that performed poorly? Or Deon's Beast?
Anyway, why are you knocking something you haven't tried? At least I assume you haven't since you're resorting to using TN's work as a reference instead of your own.
====
>sims are like bench racing...lots of stuff looks good on *paper*. maybe I'm "old school"? I'm not comfortable advocating something unless I've built it and measured it...repeatedly. Lots of folks like to point to a sim to back up a performance claim. that's like saying my *hotrod* will do 10 second 1/4s because it should on paper.
====
Ok, you repeatedly quote Dickason to make your point about large vents, and are "not comfortable advocating something unless I've built it and measured it...repeatedly.", but AFAIK you've yet to build/measure a Vd = Sd design, much less repeatedly, so how can you be "comfortable" with it?
====
>Oh? So going from 2 of a subwoofer like the PW2200 to 3 of them(50% increase in headroom...3dB)...would be 'barely noticeable'?
>I'd disagree with that. You're not going to need the extra headroom that often, but when you did...I think it would EXTREMELY easy to pick out the 3 pw2200s from the 2 in a blind test.
====
Going from two to three isn't +3dB. You need four for that, and at high SPL transient peaks, which is what this is all about, no, 3dB more isn't all that obvious. Not the fundamentals down around Fb, which is what we're discussing here. The extra shaking/rattling of various things around the house might be obvious though, or port noise if it has any, for whatever reason.
====
>>>>That's up to a 50%(!) loss in clean headroom,with a
*good* driver and a basic 4th-6th order enclosure.
====
Wrong. If you're going to spout percentages, please learn how to calculate them.
====
>While 3dB is equivalent to half power electrically,
it represents very little in terms of percentage
acoustically. For example, using 120dB/400W has a
higher value, then -3dB would be 117dB/200W, or:
%change = 100-[(200-(400^-3/20)] = 99.59%, the
difference between the higher Vs lower SPL, so
100%-99.59% = 0.41% change acoustically, and why it
wouldn't be obvious in-room.

 

[TerryC]

Here are my thoughts the THD and SPL issue and from testing the subs I've built.
My Mass 2012 is severely underported. I ran some quick tests last week and the results are embarrassing to say the least. The 4" port does indeed raise THD by insane levels near tuning. High above tuning with say with a 60hz signal its almost impossible to get it to read 10% distortion and even then the whole room is buzzing and rattling I'd guess its adding a few % itself.
The Dual mass I built easily has less THD from the posts although I've never tested thd with is, port noise is VERY low at it's 17hz tuning with its 4 X 4" ports.
That said, I really wonder how much it matters with real world listening situations. What I mean is whenever you listen to a movie with 20hz info in it, its for only a quick few seconds and accompanied by lots of other frequencies that will mask/hide the high levels of THD near tuning frequencies. I would guess this is the reason all commercial subs get away with super small ports without anyone complaining about the chuffing or maybe the masses never push their subs hard enough to bring on compression or chuffing? I semi tested a REL Storm III before and it gets all sorts of praise form many golden eared folks, compared to all the DIY subs I've heard its port noise must have been 10X higher than anything I've heard DIY, I've used the term "whistle" to describe it in the past.
About SPL levels and 3db differences. I agree with Tom that an overall calibration of -3db less will make a noticeable difference at almost any listening level. I also agree with Dan and Greg that if both subs are calibrated to reference and both subs are producing 115db at 30hz and their only off by -3db from one another at 20hz then it will be impossible to nearly impossible for anyone tell them apart with movies or music at any level. I'd even guess it can be even higher than -3db of a difference as long as they are equally matched throughout the rest of the 25hz and up frequencies.
I have Lewis coming over tomorrow to hear the sealed Tempest, if you guys would like I'd be happy to run any tests you guys might think of that might prove interesting including 10% thd.
I'd really like to stress that even though my mass sub has really high measurable thd with fixed frequencies near its tuning it is very and I mean very hard to say anything negative about it. This is because when using it with movies or music as the source material I have yet to actually hear the THD in those situations.
But this is DIY and if your box has the room I'd easily suggest porting on the bigger side IMO. From my measly testing results I've yet to hear Pipe resonances in any of the other subs I've built that use longish 30" ports either.
Terry

 

[Tom Vodhanel]

>>>A single pw2200 maxed out around 91dB/20hz in TN's testing.
====
Your point being? What does a corner full of consumer subs have to do with a discussion on vent size limitations and their tradeoffs?>I assume you mean consumer subs, which don't interest me at all, and is irrelevant to this forum/thread.> Has he measured a Contrabass, or a well designed/built DIY PR sub that performed poorly? Or Deon's Beast?>Anyway, why are you knocking something you haven't tried? At least I assume you haven't since you're resorting to using TN's work as a reference instead of your own.>Ok, you repeatedly quote Dickason to make your point about large vents, and are "not comfortable advocating something unless I've built it and measured it...repeatedly.", but AFAIK you've yet to build/measure a Vd = Sd design, much less repeatedly, so how can you be "comfortable" with it?>Going from two to three isn't +3dB. You need four for that,>No, that's not what I said, or implied, and you know it. This thread is about vents, remember? Vents only work around Fb, right? and Fb tends to be

 

[Jack Gilvey]

I think that referring to "calibrating" subs differently is missing the point. When referring to headroom, we need to consider two subs calibrated to the same reference (say,75db), but one of the subs can peak at 110db, while the other can peak at 112-113 db on transients. One sub is not calibrated lower. It seems obvious, and has been agreed, that a difference of 3db at 75-85db SPL would be discernable.
Will that peak difference be worth the "cost"?