Box Volume & Port Size Question.

Hey guys, hope you're well. I just have a couple of questions regarding box volume and speaker port size.

I'm attempting to build a 2 way bass reflex bookshelf monitor for a university project. I know there is a lot of software out their but i'm trying to manually do as much as poss so I can explain my workings out so there's more to write about. I know its pretty backwards to validate it with software I just want to learn as much as poss.

I'm going be using the Dayton Audio RS150-8 for the low/mid woofer
Dayton Audio - RS150-8 6" Reference Woofer 8 Ohm

For the tweeter i'm going to be using the Dayton audio DC28FT-8 1-1/8"
Dayton Audio - DC28FT-8 1-1/8" Silk Dome Truncated Tweeter 8 Ohm

I've chosen to go for the QB3 box alignment.

I was just wondering, is it ok to use the QTS provided by the manufacturer, when using the box alignment table, or do you have to include the 'series resistances' you expect in the Qts value, however you go about doing that?

In regards to box volume (ignoring series resistance thing) I. Here's I have managed to get a VB of = VAS 16.6 L / A 1.9699 = 8.426823696634347 Litres

One thing that confuses me is, do I need to include factors such as the damping, crossover etc in this final box volume before calculating the port length? Or can I do all the calculation and then just add on the volume to adjust the overall box size without it affecting FB, port size etc?

I managed to get a port length of 7.94194 inches, but this seems quite big?
Imgur: The magic of the Internet

This is with a diameter of 2inches too. How can I go about shortening the length? I understand that I may have to increase box volume but i'm unsure how this will affect other parameters such as FB, and if I change FB how it will affect the rest.

I hope this makes sense. I have been trying to figure it out before heading to the forums but my brain is in a pickle.

Thankyou in advance.
 
Hmm, decided to get rid of the rest of it, eh?

Anyway, started a response last night, but found some errors in other's docs, so need to re-do some math first [a real challenge for me 🙁] to respond to calculating tuning Qs.

In theory, all added resistance should be included, so I usually 'ballpark' it by using Qes unless Qms is obviously low and/or if the driver specs an acoustically large cab tuned low [<] Fs, then the tuning error is typically inaudible in room.

On another forum a DIYer is 'circling the drain' in that he's been recalculating a new lower net Vb every time he calcs a new, longer vent to compensate for, so one way to work around it is to use a shelf vent for a 'one n' done' solution, though most folks just accept that tuning maybe off a Hz or two [or a lot more in some cases], which again is normally inaudible in room due to room modes and our poor bass hearing acuity.
 
Hey,

What d'ya you mean by rest of it haha? Sorry it's been a very long day

How would I add series resistances to the QTS provided by manufacturer?

Also, when calculating box volume to then figure out port size etc, do you do this with or without the damping, crossover volume etc? Or is that what you mean by most people just accept it may be off by a couple hz, although not noticeable anyway.

Ah ok, by shelf vent you mean a vent with a bit of an L shape?

Sorry for some newbie questions haha, hope ur well
 
You were asking Qs WRT cab, vent calcs.

[Qts']: [Qts] + any added series resistance [Rs]: Calculate new Qts with Series Resistor

[Rs] = 0.5 ohm minimum for wiring, so may be higher if a super small gauge is used as a series resistor plus any added resistance from an XO/whatever.

The math assumes an empty box [net volume = Vb], so any loss of Vb affects the vent design.

Not per se, but obviously can go up the back wall if required: https://boomspeaker.com/plg_upd/2018/08/Best-Subwoofer-Box-Design-for-Deep-Bass.jpg

Not based on the driver's specs:

net volume [Vb] [L] = 20*Vas*Qts'^3.3

box tuning [Fb] [Hz] = 0.42*Fs*Qts'^-0.96
 
I ran the numbers thru my software & got this...
The recommended calc is 9.4 liters, an fb of 56.94 hertz (really high)...a too tiny a port frankly, the trade-off of bass extension, an overdamped Qms...an F3 of 60.37 hertz, again too high, would need a subwoofer..
So I tweaked the tuning & got
An enclosure of 17 liters, an fb of 52 hertz, and -3.05 Db at 48.94 hertz...a 68mm diameter port, 183mm long.
Given 17 liters worth, a 183mm long port seems about right...
Much better than a sixty hertz bottom end...





--------------------------------------------------------------------Rick........
 
8.426823696634347 Litres OUCH!

I do wish people wouldn't imply false accuracy with so many decimal places. It's not a sign of being clever to use so many decimal places but actually a measure of laziness and ignorance about the real world. It's smart to go for the jugular - and that's the figure that works.

Call it 8.4 litres. And indeed were you to be calling it 8 1/2 litres you'd be there to practical accuracy. I really challenge you to make a speaker which demonstrates an audible difference between a box volume of 8.2 litres and 8.6 litres.

If people stick to the sort of accuracy they might have done with a slide rule - sorry that shows my age - then actually one's fairly well aligned to the sort of scale of accuracy that one meets in the real world.

Likewise with -3.05 Db at 48.94 hertz hearing the difference between -2.8dB and 3.2dB or the difference between 48.8Hz and 49.1Hz. Likewise whether anyone will hear a difference between a port length of 7.8 inches and 8.1 inches, let alone 7.94194.

Lots of figures just to look clever isn't clever at all and merely belies a complete lack of understanding of what you're actually analysing. Human hearing isn't that precise.

Best wishes

David P
 
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Based on the manufacturer's data for this speaker model and not on an actual measurement of the RS150-8 speakers that you have ( which can be different), I would suggest that you build an enclosure; which allowing for the volume occupied by all the solid parts, i.e. magnets, frames, braces and the vent pipe, will amount to a working internal volume of 15.66 Litre. The vent length required for an Fb of approx. 45Hz, using a 2'' I.D. pipe is 5.7'' ( 145mm).
 
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If people stick to the sort of accuracy they might have done with a slide rule - sorry that shows my age - then actually one's fairly well aligned to the sort of scale of accuracy that one meets in the real world.

Yeah, I used one till I got an HP-97 under duress from the boss to join the 20th Century and to run the '81 Margolis-Small T/S program.

As for accuracy, most DIYers rely on [wrong] published specs to two decimal places and are usually happy with the results once damped in room even though the box alignment is too often quite audibly far off in reality, but with nothing to compare to other than what sounds good to them........Still, thanks to the marketeers, WWW, if we don't use T/S it can't be good since the pioneers were clueless, which of course is ludicrous in the extreme.
 
hello ,

I am trying to calculate the good dimension of my rectangular port (for after buy a good woofer) . I already have a big cabinet with 2 rectangular vents of:
26cm x 3.4cm x 28.5cm (each)
as I have 2, one on each side
is it correct to say that this corresponds to 1 single event of:
26×3.4×57 (28.5×2)?
 

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The tuning won't be 100 percent dead on anyways if you factor in other (somewhat subtle) variables which affect it ie. altitude, barometric pressure, air temperature and humidity. Driver TSP parameter drift caused by varying VC temps can also further offset it, but thats not common for home audio drivers. These first mentioned things can have a noticeable impact on tuning if the stack up of variables shift against your favor. Most poeple just aim for a specific port length correction factor and let things fall where they may. Usually in the practical world I end up with a shorter port length than originally calculated.
 
thank you for your advice, I confess that I don't know too much about port settings!

in troel gravessen's "loudspeakers III" project, the rectangular port is this one;
40x342x350

I have a cabinet of the same volume, around 110-120 liters
if i change my 2 ports with the following size
26×3.4×54 (27×2)
is it fair to say that it comes to the same thing?

Troel :
in troel gravessen's "loudspeakers III" project, the rectangular port is this one;
4x34,2x35

I have a cabinet of the same volume, around 110-120 liters
if i change my 2 ports with the following size
26×3.4×57 (27×2)

is it fair to say that it comes to the same thing?

Volume loudspeaker port
40 x 342 x 350 = 4 788 

Volume of my speakers port
26 x 3.4 x 54 (2x 27) = 4774
 

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in troel gravessen's "loudspeakers III" project, the rectangular port is this one;
40x342x350

I have a cabinet of the same volume, around 110-120 liters.
The Loudspeaker III is is 45 (W) x 50 (D) x 100 (H) cm, 225 liter gross volume, double the volume you say yours are.

The volume of the port does not determine the Fb (Box tuning Frequency), the relationship between port length and depth and the cabinet's net volume do.

You can determine the Fb by running a sine wave through the speaker, at Fb, the speakers excursion (and impedance) will be at minimum, increasing above and below Fb.

If you want a higher Fb, reduce the port length, lower Fb, increase the port length.
 
The volume of bass cabinet is 125 liters for the Loudspeakers III (there is a closed box for Mid too )
The port is 40 x 342 x 350

I have 115 liters (i think) i have 2 port
260 x 34 x 270

Thank you for your explanation
 
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