Okay, I did the homework for you. In net 190 liters, tuned to 38 Hz produces a 40 Hz -3 dB point. That's the smallest box I find that leads to 40 Hz F3, according to Unibox calculations, although this have 1 dB bump, centered at 95 Hz.
The SBB4 alignment in 227 liters tuned to 30 Hz leads to 41 Hz F3, this a flat alignment without a bump in the response.
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T/S max flat alignment:
Vented net volume (Vb) (L) = 20*Vas*Qts'^3.3
(Ft^3 = (Vb)/~28.31685)
Vented box tuning (Fb) (Hz) = 0.42*Fs*Qts'^-0.96
F3 (Hz) = Fs*0.28*Qts'^-1.4
(Qts'): (Qts) + any added series resistance (Rs)
Note that T/S theory peters out at the driver's effective upper [Fhm], lower [Flc] mass corners, so any bandwidth response plot beyond these points are strictly due to [inputted] inductance [mH] and/or mass roll off, with on-line freeware generally just flatlining it, so the box design may not be technically 'flat' to 200 Hz:
Fhm = 2*Fs/Qts' end of acceleration BW
Flc = Fs*Qts'/2 end of lower roll off BW [normally never used]
Qts' = 2*Fs/Fhm
Well, this means it sounds great without boom? The pioneers claimed there are too large box for reflex action which results in a disjointed low-bass. But when is too large? So Vb = Vas is not too large? Say for driver with Qts in the 0.2-0.4 range and Fb = 0.81 x Fs as you mentioned.IME.
IME, but like the pioneers I 'critically' damped them.
Hmm, not sure where you're 'coming from', so for now.............while the 'sweet spot' (mean) is a ~0.4412 Qts', a 'close enough' simple guideline is:
~0.403, Vb = Vas, Fb = Fs
~ <0.403, Vb = < Vas, Fb = > Fs
~ >0.403, Vb = > Vas, Fb = < Fs
Keeping in mind that one has to factor in the (huge) impact thermal power, inductive distortion can have on effective Qts, the dominant vented box alignment factor.
(Qts'): (Qts) + any added series resistance (Rs)
Hmm, not sure where you're 'coming from', so for now.............while the 'sweet spot' (mean) is a ~0.4412 Qts', a 'close enough' simple guideline is:
~0.403, Vb = Vas, Fb = Fs
~ <0.403, Vb = < Vas, Fb = > Fs
~ >0.403, Vb = > Vas, Fb = < Fs
Keeping in mind that one has to factor in the (huge) impact thermal power, inductive distortion can have on effective Qts, the dominant vented box alignment factor.
(Qts'): (Qts) + any added series resistance (Rs)
Attachments
Why do you need a SUBwoofer to play to 200hz?
If the lowest note on an 88 key piano is 27.5hz, why 40hz?
I just want to understand how to build a proper bass-reflex speaker. Maybe this isn't the best topic for it.
Anyway, JBL uses a Qts = 0.44 woofer (2216Nd) with an approx Vb = Vas and Fb = Fs x 0.81 alignment in its M2 reference monitor. What a coincidence!
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I have to cross it with a midbass that starts at 150hz
the nice thing about the fh500 is that it can be both a woofer and a subwoofer
flat from 40hz does not mean that 28hz will not be played, they will play at a lower level
in the PRO field bass below 40hz is not needed
I have used the 18FH500-8 as woofer in a three way, ~140l vented:
Worked pretty well, started with ~30 Hz tuning that was too low, sounded a bit stuck with low airflow at the ports. It came then well alive with a higher tuning 35-36 Hz, ports blowing now happily. The 140Hz is borderline, would I build it again I'll give it ~160-170l.
Here is the correspondent Basta simulation incl. Baffle Step:
Black curve is with 140l, red one with 165l (30% stuffing with damping material considered). Nice low group delay and slowly sloping response gives dry and precise bass. SPL limit was my ~40m² room, everything in the interior started rattling when the woofer was still doing only few millimeters of excursion. Had a DSP in my system, but there was no need for any low frequency equalization.
The 18FH500 is a unicorn, very high bandwith, very low mass and resonance frequency. Maybe the only 18" you can use for 2-way duty with a big horn.
I observed some special behavor with the drivers: I have never heard and believed in 10s or 100s of hours "burn in time" of drivers or speakers. I give new drivers a 20Hz shake near xmech for 5-10 minutes, checking for rattling or noise and then measuring TSP. Never heard changes or evolving in sound afterwards of assembled speakers then.
The 18FH500 was different, it really needs some burn in. Give it a shake for a whole day initially, and/or even give the spider a tenderly massage with your fingers.
But, and this is remarkable, the driver needs 5-10 mins. to come alive every time again when you start using it! When the stereo was off for a day or so, sound was a bit thin every time the speakers started playing, and needed a bit time to come up and get full sound. My wife observed it without any hint from me, and to avoid brain adaption effects I made some tests by starting playing, then leaving the room and coming back after 10 mins. It was no total change in sound or so, but clearly perceptable.
I've used a lot of typical rubber surround HiFi drivers, but the these are the only PA drivers > 12" I used, so I don't now if this is typical behavor of lare PA drivers with stiff cloth surrounds.
But in sum: great divers that give you more SPL your ears can stand, and perhaps the best 18" you can use for HiFi application!
Best regards
Peter
Worked pretty well, started with ~30 Hz tuning that was too low, sounded a bit stuck with low airflow at the ports. It came then well alive with a higher tuning 35-36 Hz, ports blowing now happily. The 140Hz is borderline, would I build it again I'll give it ~160-170l.
Here is the correspondent Basta simulation incl. Baffle Step:
Black curve is with 140l, red one with 165l (30% stuffing with damping material considered). Nice low group delay and slowly sloping response gives dry and precise bass. SPL limit was my ~40m² room, everything in the interior started rattling when the woofer was still doing only few millimeters of excursion. Had a DSP in my system, but there was no need for any low frequency equalization.
The 18FH500 is a unicorn, very high bandwith, very low mass and resonance frequency. Maybe the only 18" you can use for 2-way duty with a big horn.
I observed some special behavor with the drivers: I have never heard and believed in 10s or 100s of hours "burn in time" of drivers or speakers. I give new drivers a 20Hz shake near xmech for 5-10 minutes, checking for rattling or noise and then measuring TSP. Never heard changes or evolving in sound afterwards of assembled speakers then.
The 18FH500 was different, it really needs some burn in. Give it a shake for a whole day initially, and/or even give the spider a tenderly massage with your fingers.
But, and this is remarkable, the driver needs 5-10 mins. to come alive every time again when you start using it! When the stereo was off for a day or so, sound was a bit thin every time the speakers started playing, and needed a bit time to come up and get full sound. My wife observed it without any hint from me, and to avoid brain adaption effects I made some tests by starting playing, then leaving the room and coming back after 10 mins. It was no total change in sound or so, but clearly perceptable.
I've used a lot of typical rubber surround HiFi drivers, but the these are the only PA drivers > 12" I used, so I don't now if this is typical behavor of lare PA drivers with stiff cloth surrounds.
But in sum: great divers that give you more SPL your ears can stand, and perhaps the best 18" you can use for HiFi application!
Best regards
Peter
Nice build!
How come your model doesn't match your build?
Your model shows two 5" ports at the BASE of the enclosure.
Your build shows the 2 ports above the driver.
Why do you need higher air speed for the enclosure to sound good?
The question is, why did produced low air speed with the lower Fb. Maybe because the box is undersized/borderline? Just thinking.
To add something, for example B&C usually recommends Vb around Vas and Fb around Fs for drivers with Qts roughly above 0.25 and below 0.4.
Say: https://www.bcspeakers.com/en/products/lf-driver/18-0/8/18tbx100
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