I began to get some feedback about my typical BR preferences, saying that I'm tuning the box too low. This will damage the woofer, said my friends. I have never had a problem with any driver blowing out, because I only build speakers for living-room use, no discotheque, and it's possible I've simply been lucky.
So, I decided to show you various alignments and seek your help. What would you do? Please note that I'm looking for an approach which works for my context -- domestic hi-fi, not auditoria or discos. All these are done in Unibox, for the SB Acoustics SB17NBAC aluminium cone 6.5" midbass. My doubt would apply to any bass reflex box for any other driver too, of course.
This is the first alignment -- 25 litres, Fb 35 Hz. If you've not used Unibox, then I'll draw attention to the red line -- that's the line which is the basis of the objections my friends are raising. The red line shows the max SPL for each frequency based on excursion limit. The thick blue line is the actual modelled SPL which the driver will generate (diaphragm + port) if the driver had infinite excursion. Therefore, this graph tells me that if I operate at rated power (60W), then I'll hit excursion limit at about 100Hz. Below 100Hz, the output of the speaker will be excursion limited. At about 40Hz, you can see that the gap between the excursion-defined limit and the modelled SPL is maximum -- it's about 16dB.
The next one is 15 litres, with Fb of 40Hz. Here, the max gap between the modelled SPL and excursion-defined SPL is about 10-11dB, at 50Hz or so.
The third one is 10 litres. In this one, the gap between the two has reduced to about 7dB.
In this one, we have 8 litres, Fb of 38 Hz, and the F3 has now gone up to about 60Hz.
What would you use, and why? I would use either the 25-litre alignment or the 15 litre one. How seriously does one need to keep the modelled SPL close to the excursion limit? And if I am expected to keep it very close, then what's the point of having such a high F3 -- it becomes a midrange driver, not a midbass, almost.
Looking for your insights.
So, I decided to show you various alignments and seek your help. What would you do? Please note that I'm looking for an approach which works for my context -- domestic hi-fi, not auditoria or discos. All these are done in Unibox, for the SB Acoustics SB17NBAC aluminium cone 6.5" midbass. My doubt would apply to any bass reflex box for any other driver too, of course.
This is the first alignment -- 25 litres, Fb 35 Hz. If you've not used Unibox, then I'll draw attention to the red line -- that's the line which is the basis of the objections my friends are raising. The red line shows the max SPL for each frequency based on excursion limit. The thick blue line is the actual modelled SPL which the driver will generate (diaphragm + port) if the driver had infinite excursion. Therefore, this graph tells me that if I operate at rated power (60W), then I'll hit excursion limit at about 100Hz. Below 100Hz, the output of the speaker will be excursion limited. At about 40Hz, you can see that the gap between the excursion-defined limit and the modelled SPL is maximum -- it's about 16dB.
The next one is 15 litres, with Fb of 40Hz. Here, the max gap between the modelled SPL and excursion-defined SPL is about 10-11dB, at 50Hz or so.
The third one is 10 litres. In this one, the gap between the two has reduced to about 7dB.
In this one, we have 8 litres, Fb of 38 Hz, and the F3 has now gone up to about 60Hz.
What would you use, and why? I would use either the 25-litre alignment or the 15 litre one. How seriously does one need to keep the modelled SPL close to the excursion limit? And if I am expected to keep it very close, then what's the point of having such a high F3 -- it becomes a midrange driver, not a midbass, almost.
Looking for your insights.
There is the temptation to tune so low that the response ends up falling away and then rising to peak again before rolling off, like your first option.
You might also like to look at their group delay.
You might also like to look at their group delay.
I don't get your plots, they seem to leave out the port output. This is what a comparable SB woofer does in a 24l enclosure. Red line gives SPL at 1m half (2π) space. App is Basta!
If you look at the top left corner of the plots, there is a legends area. The driver cone output, port output, leaks output, are all drawn and marked separately. So, no, the total doesn't leave out the port output.
Unibox doesn't show it. Will step response be a good proxy for it?
Unibox doesn't show it. Will step response be a good proxy for it?
Step response is one of those things. Often it makes for pretty pictures but it doesn't present this information in the way people tend to talk about with audibility.
I saw that. But it's quite useless if you don't show the sum of driver and port. Should be easy (if relative phase info is available).
It is. That's the dark blue line. System response (combined driver + box) is dark blue. Driver alone is grey / black. Box alone is light teal. Leakage is light blue. Nominal excursion-limited SPL for inputted Xmax and power handling is red. All listed in the legends. Simple. 😉 Unibox is actually a very good & accurate piece of software. It doesn't include quite as much as, say, WB&CD, but it covers enough for most purposes.
To be honest, the chances of you putting 60w into the driver are usually fairly slim; as you've seen you'll likely run out of excursion long before that and you're unlikely to be cranking that much current into the thing, unless you're at a high average & suddenly faced with a big LF dynamic peak below Fb. The nominal ratings provided by the manufacturer are usually (usually) just one or several of the thermal limits of the coil. So it that sense, plotting it like this is a bit misleading -no direct harm, but worth keeping the practicalities in mind.
Which alignment is preferred really depends on what your objects / preferences are, so there's no one answer; I probably wouldn't pick any of those shown, though without the data to hand & purely going by what's shown above, I'd likely be inclined as a reasonable compromise toward something like 19 - 20 litres, tuned to the mid-high 30Hz regions for a modestly damped response with decent power-handling. As noted though -this really depends on what it is you're trying to achieve (remember to factor wire loop resistance & amplifier output impedance into the equation too, since both will raise the effective Q & the alignment should ideally be designed with this incorporated).
To be honest, the chances of you putting 60w into the driver are usually fairly slim; as you've seen you'll likely run out of excursion long before that and you're unlikely to be cranking that much current into the thing, unless you're at a high average & suddenly faced with a big LF dynamic peak below Fb. The nominal ratings provided by the manufacturer are usually (usually) just one or several of the thermal limits of the coil. So it that sense, plotting it like this is a bit misleading -no direct harm, but worth keeping the practicalities in mind.
Which alignment is preferred really depends on what your objects / preferences are, so there's no one answer; I probably wouldn't pick any of those shown, though without the data to hand & purely going by what's shown above, I'd likely be inclined as a reasonable compromise toward something like 19 - 20 litres, tuned to the mid-high 30Hz regions for a modestly damped response with decent power-handling. As noted though -this really depends on what it is you're trying to achieve (remember to factor wire loop resistance & amplifier output impedance into the equation too, since both will raise the effective Q & the alignment should ideally be designed with this incorporated).
Last edited:
Hi, its probably two way box? I would optimize it for mid range which means smallest box possible for lower diffraction and easy construction. Perhaps closed instead of reflex to avoid bad sound from the port. Bass is dominated by room anyway and would need much bigger solution for good performance. If bass ends up lean when optimizing mid performance, then I would just add another box for bass. Optimizing for mid might make a peak for the bass response though, then I'd use some other driver.
Perhaps not what you asked for 😀 But that's what I'd use without knowing more context what you are targetting at. Sound quality increased with more cost and complexity, fine compromise in my opinion but of course might be exactly opposite what you are targetting.
Weekend!🙂
Thanks Scottmoose. It's possible that I am not clear "what my objectives are". I think my objectives are to get deep bass extension without any peaking or one-note bass, at modest volumes, for music, not HT. So, the signal content will almost never go below 35Hz (I'm not designing for EDM) and when there are deep+loud bits, they'll be very brief peaks.
In that case, purely assuming whatever conditions / data you used to generate the above (note caveat 😉 ) I'd be looking toward that 19litre Vb, with an Fb lurking around 35Hz. Again assuming those conditions, that should provide a lightly / modestly damped alignment with a reasonable balance between that, power-handling & GD / ringing. Everything's a compromise of course, but there are worse than that -especially if it's anywhere near boundaries.
By the way -I don't know if you have the drivers or not yet -if not, based on prior experience, I'd be inclined toward the 4ohm version.
By the way -I don't know if you have the drivers or not yet -if not, based on prior experience, I'd be inclined toward the 4ohm version.
What do you mean by "conditions/data" with that very strange wink? I'm feeling quite nervous. 😳 I just entered the T/S parameters into Unibox based on the published spec sheet. When I buy the drivers, I'll do a T/S params double-check with my DATS but I have usually found the published spec of SB Acoustics drivers fairly accurate.
And I think I'll go with what you are broadly hinting at. Do you see something specific about 19 litres which makes you pick it, or is it just a broad ballpark of box size? I have only shown 25 litres and 15 litres, so I was wondering.
And about 8 Ohm vs 4 Ohm, please tell me more -- why do you prefer the 4 Ohm? I've not bought anything. I was going to choose from between 4 and 8 depending on whether I want to do just one TM (in which case 4 Ohm of midbass matches 4 Ohm of typical tweeter) or an MTM (in which case I'll probably do two 8 Ohms in parallel) or WWMT (in which case the two W will either be two 8 Ohms in parallel or two 4 Ohm in series to match the sensitivity of the midrange I choose). Am I making sense?
And I think I'll go with what you are broadly hinting at. Do you see something specific about 19 litres which makes you pick it, or is it just a broad ballpark of box size? I have only shown 25 litres and 15 litres, so I was wondering.
And about 8 Ohm vs 4 Ohm, please tell me more -- why do you prefer the 4 Ohm? I've not bought anything. I was going to choose from between 4 and 8 depending on whether I want to do just one TM (in which case 4 Ohm of midbass matches 4 Ohm of typical tweeter) or an MTM (in which case I'll probably do two 8 Ohms in parallel) or WWMT (in which case the two W will either be two 8 Ohms in parallel or two 4 Ohm in series to match the sensitivity of the midrange I choose). Am I making sense?
Zipping coffee so a bit sideline thoughts:
Those are vastly different systems, TM, MTM, WWMT and need to be optimized differently, each is different set of compromises. Optimizing for bass cannot be done without knowing what happens to mid range. Why not WMT? why box at all? IF money and size is not concern as it seems as you consider these vastly different systems, and if the box is strictly for bass use crossover say below 300Hz or something, less than 1kHz anyway, then you'd probably benefit from the biggest box to get most bass output with given power. Perhaps you consider the bass box to be a stand for the MT? Or perhaps make them all into same enclosure? If its a MT or MTM then the mid range needs to be considered, you probably want to avoid diffraction, modes inside the box, ports, problems on the mids.
It's impossible to give good answer currently in my opinion before you know what kind of system you are building 🙂 Because bass wavelengths are so long room dominates. You should definitely optimize the speaker for highs and mids first, then just add enough bass boxes to get some control for low frequencies from the room.
Here is fun thought experiment: you are tuning output around 40Hz with the examples, around drivers resonance. Does your listening spot have a null or a peak at 40Hz? If on a null the alignment doesn't matter much as you are not going to hear it anyway. If you are on a peak then 40Hz is probably overwhelming and the smallest box might be best. Which one to choose, which one to advice on? If we consider this that the alignment doesn't matter too much it would make more sense to select the box size with any of the other criteria like cost or looks or mid performance and so on.
As general rule of thumb, always make biggest bass box possible, use biggest cone / multiple drivers as possible as there is rarely enough bass. Restrictions on size, cost, looks or mid range performance can demand something else than biggest box, and you have to consider which one to optimize for, whats your main thing you do not want to compromise on? These get answered if you think the system backwards, from what sound you like, how hearing system works, from room acoustics of yours, and figure out what kind of a system would make the pressure variations in room so that maximum enjoyment ensures. The speaker system gets designed around that. If you start by randomly selecting random parts of a speaker system and haphazardly put it together you probably not meet expectations. Which wouldn't matter if you didn't have one to begin with 😉
sorry about the philosophy, most important thing with the hobby is to have fun and yours might be different than mine and thats fine!😀 I hope this helps you select correct size box.
Those are vastly different systems, TM, MTM, WWMT and need to be optimized differently, each is different set of compromises. Optimizing for bass cannot be done without knowing what happens to mid range. Why not WMT? why box at all? IF money and size is not concern as it seems as you consider these vastly different systems, and if the box is strictly for bass use crossover say below 300Hz or something, less than 1kHz anyway, then you'd probably benefit from the biggest box to get most bass output with given power. Perhaps you consider the bass box to be a stand for the MT? Or perhaps make them all into same enclosure? If its a MT or MTM then the mid range needs to be considered, you probably want to avoid diffraction, modes inside the box, ports, problems on the mids.
It's impossible to give good answer currently in my opinion before you know what kind of system you are building 🙂 Because bass wavelengths are so long room dominates. You should definitely optimize the speaker for highs and mids first, then just add enough bass boxes to get some control for low frequencies from the room.
Here is fun thought experiment: you are tuning output around 40Hz with the examples, around drivers resonance. Does your listening spot have a null or a peak at 40Hz? If on a null the alignment doesn't matter much as you are not going to hear it anyway. If you are on a peak then 40Hz is probably overwhelming and the smallest box might be best. Which one to choose, which one to advice on? If we consider this that the alignment doesn't matter too much it would make more sense to select the box size with any of the other criteria like cost or looks or mid performance and so on.
As general rule of thumb, always make biggest bass box possible, use biggest cone / multiple drivers as possible as there is rarely enough bass. Restrictions on size, cost, looks or mid range performance can demand something else than biggest box, and you have to consider which one to optimize for, whats your main thing you do not want to compromise on? These get answered if you think the system backwards, from what sound you like, how hearing system works, from room acoustics of yours, and figure out what kind of a system would make the pressure variations in room so that maximum enjoyment ensures. The speaker system gets designed around that. If you start by randomly selecting random parts of a speaker system and haphazardly put it together you probably not meet expectations. Which wouldn't matter if you didn't have one to begin with 😉
sorry about the philosophy, most important thing with the hobby is to have fun and yours might be different than mine and thats fine!😀 I hope this helps you select correct size box.
Last edited:
Oh, I know what I'm building -- I was trying to respond to your suggestion of 4 Ohm. I said that I am curious to know why you recommend the 4-Ohm version, and I choose the Ohm based on the application of the driver.
Currently, when I'm looking at these bass reflex alignments, I'm thinking of either a TM standmount or an MTM floorstander. A 3-way is quite a different animal, and I'm not looking at these SB17NBAC drivers in a bass reflex arrangement for a 3-way. And about the room nulls --- I agree with what you're saying, but then I'd rather not worry too much about the room modes when designing the box -- I'll worry about room modes, placement, nulls, later, so that I don't make a speaker too specific for one placement.
Currently, when I'm looking at these bass reflex alignments, I'm thinking of either a TM standmount or an MTM floorstander. A 3-way is quite a different animal, and I'm not looking at these SB17NBAC drivers in a bass reflex arrangement for a 3-way. And about the room nulls --- I agree with what you're saying, but then I'd rather not worry too much about the room modes when designing the box -- I'll worry about room modes, placement, nulls, later, so that I don't make a speaker too specific for one placement.
Ah it was someone else who commented on the ohmage 🙂
If you use this driver and consider TM or MTM I'd choose the MTM as it would have more output capability.
Another thought experiment, if its a TM and you chose the box for good mid range performance which probably means lean bass, then you could find a placement in room where the bass is balanced. Now you have both optimized 😉
I wish you good time with the project! jolly weekend
If you use this driver and consider TM or MTM I'd choose the MTM as it would have more output capability.
Another thought experiment, if its a TM and you chose the box for good mid range performance which probably means lean bass, then you could find a placement in room where the bass is balanced. Now you have both optimized 😉
I wish you good time with the project! jolly weekend
It means that anything I said above was purely based on whatever data entry you used, which wasn't shown so necessarily 'your business rather than mine'. 😉 That said though, and for whatever it's worth -I'd always suggest making sure you account for amplifier output impedance, wire loop, connection & as relevant any other DCR in circuit as it raises Q, and the box alignment ideally should account for this.
As for SB -I haven't always, especially in this size, which is why I suggested the 4ohm model, as one thing I did consistently find was that they tended to be both closer to advertised spec. after suspension break-in, and better matched in general than their 8ohm stablemates. Just calling it as I've found it.
Not as such; it isn't a stock alignment, just a Vb and Fb that will give what I would call (for the conditions you mention) a reasonable compromise between damping around Fb, power-handling and GD, as well as being relatively flexible in terms of positioning near boundaries if necessary. Others will have their own views -that's just mine & I don't pretend it to be any more than that.
My $0.2 is this driver's Fs, Qts specs hark back to the pioneer's days of low Fs mid bass drivers coupled to high output impedance amps in big boxes for plenty of tuning capability, so Vb = Vas, Fb = Fs tuned with at least a 2.5" dia. vent yields a modest EBS alignment that combined with a bit of room and/or boundary gain will fill it out and if too 'one note'/'boomy' down around tuning, then critically damp it to 'taste' same as the pioneers, DIYers did way back when.
Click Test with 'stretched' as in 'TIGHT AS A DRUM' stretched:
Attachments
I would try to mimic the max exercusion red line as it is smooth and BR with smooth low end are often more liked. The unknown is the honey combed low end response of the room gain. Do not focus on -F3 but maybe more -F6 , -F10. And what also Allen B and Tmuiku say : try to trade off between the mid magnitude and the group delay as it is a all to do driver here !
If you look for instance that driver Faital 12PR320 and three designs around it, all the designers chose BR to extend the low end according the T&S of theiir unit (measure yours, SB Acoustics datasheet are not so precise about the Vas, so be aware the sim migth be false but you can certainly find third party serious measurements). But they all finished with different average spl and magnitude level and F3 with more or less different volume box ! The trade off is how much you lower the midrange magnitude to shape the magnitude of the bass area and extend the low end : the flatter, the less spl magnitude butt the more extense : so depend of your room, the size of the box relative to the T&S, etc
Open Source Monkey Coffin (recalled OSMBox for wokism reason by some) : https://github.com/mbrennwa/osmcdoc/blob/master/osmc_paper.pdf , there is the thread here that is very good as a learning purpose to see progression of a devolpment, I learned a lot here 🙂 thanks to the good brain storming attitude. Around 80L box at the end. A bigger floorstanding had its thread but more average datas as it is finally somewhat changed datas (internal resonances for instance, fine tuning of the port length... Around 107 L box, no measurements in free air for that bigger veersion but sims)
Troels Gravsen Faital 3WC : http://www.troelsgravesen.dk/Faital-3WC.htm : around 77 L ... with optimistic room gain partially chosen (the guy design and tune for his 27 square meters room iirc)
Humble Home Made hifi : https://www.humblehomemadehifi.com/Nova-Andromeda.html around 118 L (the guy seems to love huge space and his design is going lower, group delay seems good below 100 hz, which would give non boxy sound)
Max magnitude in the high bass low midrange is 90.5 db/2.83V to 93 dB according the design, passive filter all differents.
Good luck 😉
If you look for instance that driver Faital 12PR320 and three designs around it, all the designers chose BR to extend the low end according the T&S of theiir unit (measure yours, SB Acoustics datasheet are not so precise about the Vas, so be aware the sim migth be false but you can certainly find third party serious measurements). But they all finished with different average spl and magnitude level and F3 with more or less different volume box ! The trade off is how much you lower the midrange magnitude to shape the magnitude of the bass area and extend the low end : the flatter, the less spl magnitude butt the more extense : so depend of your room, the size of the box relative to the T&S, etc
Open Source Monkey Coffin (recalled OSMBox for wokism reason by some) : https://github.com/mbrennwa/osmcdoc/blob/master/osmc_paper.pdf , there is the thread here that is very good as a learning purpose to see progression of a devolpment, I learned a lot here 🙂 thanks to the good brain storming attitude. Around 80L box at the end. A bigger floorstanding had its thread but more average datas as it is finally somewhat changed datas (internal resonances for instance, fine tuning of the port length... Around 107 L box, no measurements in free air for that bigger veersion but sims)
Troels Gravsen Faital 3WC : http://www.troelsgravesen.dk/Faital-3WC.htm : around 77 L ... with optimistic room gain partially chosen (the guy design and tune for his 27 square meters room iirc)
Humble Home Made hifi : https://www.humblehomemadehifi.com/Nova-Andromeda.html around 118 L (the guy seems to love huge space and his design is going lower, group delay seems good below 100 hz, which would give non boxy sound)
Max magnitude in the high bass low midrange is 90.5 db/2.83V to 93 dB according the design, passive filter all differents.
Good luck 😉
Last edited:
That was interesting, thanks. The different Vb for the same driver, different alignments. I guess one can decide to make the box as large as needed with those high power pro drivers because they have high power handling, but with my 6.5" driver, I need to be more conservative?
Also, it was interesting to see Troels Gravesen's measurements of in-room vs just raw speaker measurements, and see how much the bass extension stretches with in-room gain.
Also, it was interesting to see Troels Gravesen's measurements of in-room vs just raw speaker measurements, and see how much the bass extension stretches with in-room gain.