Sealed Box Subwoofer Recommendations

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My quest for the elusive subwoofer design that delivers excellent musicality, good bass extension and reasonably small enclosure continues. To avoid having my design parameters become altogether mutually exclusive I've ditched the 'low cost' part long ago. Also, custom electronic EQ is unlikley unless I can find someone to build if for me (what can I say, I am a software guy).

The current leading contender is a sealed design for good transient performance and smooth integration with room gain (this is the musical part) and isobarically loaded drivers for the half size enclosure and reduced harmonic distortion.

Any problems so far?

But what driver can deliver real low bass in a sealed box???

The only two that I've found that come close are the NHT 1259 yielding an F3 of about 27Hz in a 4.5 ft^3 box, and the ACI SV-12 at about 32Hz for 4 ft^3.

Any input gratefully appreciated.
I recently stumbled upon the "Brahman" series of subwoofer drivers on Adire Audio's website. They seem to be designed specifically for small sealed boxes. The inevitable downside is that they simply don't exist yet, although they are pre-ordering currently.

As far as electronic EQ... I think Rod Elliot has some pretty idiot-proof plans/PCBs on his website, Soldering and arithmetic (easier than designing a xover!) are the only skills you need to build this thing. The parts are cheap, and it shouldn't take too long to build. Rod himself is easy to deal with over e-mail, in case you run into any problems.

I, too, am a software-guy. I have found that electronics, while seemingly mysterious, is pretty easy to deal with in small enough chunks. It is probably a bit unrealistic to not have to make the traditional small-enclosure/bass-extension tradeoff without extra power/EQ.

diyAudio Moderator Emeritus
Joined 2001
To each his own, of course, but I am having difficulty understanding the attraction of the sealed subwoofer in a box as big as 4 cu. ft.

In a 1 or 1.5 cu. ft box, the port can be a large percentage of the box size, but not in a 4 cubic foot box.

It took awhile for loudspeaker manufacturers to figure out how to build ported boxes, but those problems have been solved.

Yes, I have seen the step responses, and the sealed box is better.

But in the sonic depths you can plumb with a 4 cu. ft subwoofer, slight distortions become unhearable. Studies have shown that distortion, phase response, etc are less detectable the lower you go under 100 Hz.

That slight increase in distortion is counterbalanced by a big jump in output. You are not listening to Stradivarii on ths subwoofer. Run the plots for a blueprint 1503 in a 4 cu. ft. sealed, and a blueprint 1503 in a 4 cu. ft box tuned to 16 Hz, and see how much more bass output you get down to 16 Hz-even if you reduce the effective volume of the ported box because of the vent.

I might point out that ported systems, for the same output, produce less intermodualtion distortion than sealed systems. This is because the cone moves less to produce the same output. The more a cone moves, the more it muddies the sound output of slightly higher bass notes in it's range.. And "mud" is what intermodulation distortion sounds like. It is sum-and-difference sounds that are not on the original program material-the less the better, obviously.

This is merely my opinion, of course, but it seems clear to me that in a 4 cu. ft box, ported is the way to go.
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Joined 2001
group delay

I usually prefer ported box, albiet very damped alignments. But a sealed 0.7 or lower QTC sealed box will sound superior to the same subwoofer in the most critically damped ported enclosure, if by 'superior' you are referring to more percievably defind sound. A port induces group delay, which is basicly in this circumstance is an increasing delay of the port output referenced with the cones output as you get closer to the port tuning frequency. This results in a very audible smear in even the best designed ported enclosures. Don't belive me? Take any woofer that works halfway decent in a ported AND sealed a ported box with good damping characteristics(SBB4 allignment, etc.) and then put it in a 0.5-0.6 QTC alignment sealed box. Use a sinewave generator and listen to various low freqencies near and half an octave above the tuning frequency of the ported enclosure and play the same audio tones with the sealed alignment. Even without test equipment the differences are obvious and discernable by the human ear. I am not referring to SPL or extension capability. Adjust the gain/level to match the vented box if nesecarry to get a better comparison. And note that this is with simple sine waves. When reproducing instruments such as pipe organs and acoustic bass it is even more apparent. It is true that the human ear is less sensitive to IM and harmonic distortions at these frequences, but it relatively easy to discern non linear sound reproduction.

However, IMO in the real world a ported enclosure tuned to a EBS alingment to sound better 'overall' to a sealed box given the increase in efficiency and significant lower frequency extension. I only used sealed enclosures for midrange cavities and car subwoofers.

The choice of a sealed 4ft box (about 115 litres) seems one of the optimal choices for NHT woofer. This driver has a Qt too high for good use with reflex enclosures.

The other one you mentioned has a QT of .395, but is a bit on the edge for use in reflex (you should stay on drivers with Qt of 0.2-0.3 if you want a good reflex box)

One possible improvement for the NHT driver is to try a 4th order bandpass alignment : you'll achieve higher MOL and a 'natural' high freq rolloff (i.e. you will be able to use a lower slope crossover filter ) at the cost of a more complicate box.

I've attached a zipped word file with screenshots of the closed and bandpass alignments as calculated by the most reliable software I have, unfortunately is only in Italian but you can easily figure out how to translate it. If not and need assistance just ask me.

Sealed box is considered with acoustic filler on walls
Bandpass project is with sealed box completely filled and reflex box on walls.
Qa Ql and Qf are assumed as averages for a well built box.

Also, if you want to get rid of the long vent duct and you have drone drivers available, let me know their parameters and we'll tune the bandpass with passive drivers.


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To each his own, of course, but I am having difficulty understanding the attraction of the sealed subwoofer in a box as big as 4 cu. ft.
In a 1 or 1.5 cu. ft box, the port can be a large percentage of the box size, but not in a 4 cubic foot box.
I should have clarified. I would be using 2 drivers Isobarically for a 2ft^3 box. Also, the issue for me is not the percentage of space consumed by the port per se, but rather how to create highest fidelity sub with a 2ft^3 volume ceiling and a low to mid 20s F3.

All things considered ( IM and harmonic distortion, group delay, transient response etc. etc.....) do you feel that a good ported design's increase in efficiency more than makes up for any sonic advantage of a sealed box?

I think your take on this is that an ported EBS design would sound best - true? How big would that be? And would I need anything more that a good 250W plate amp?

You vote for a bandpass or PR design? Thanks for the files.

More that one way to skin the proverbial cat eh? Anyone else want to weigh in ??
OK, I better clarify my opinion (pontification in audio is a free of charge and harmless sport, so let me jump right in :) !)

1 - Sealed box around 110 litres (4ft) is a very good choice, I would stay with it considering all parameters (performances, sound type, building simplicity)
The only weak point is a reduction in max output level (MOL) at very low frequencies, due to the typical excursion curve of the sealed box. If it was for my use, I think this limitation is acceptable.

2 - If we want to pull the max performances out of the NHT in a box of about 4ft I would go with the bandpass tuned with a passive driver (better than the duct because of a) lenght & dimensions b) a PD filters out the typical 400Hz resonance of bandpass) . Bandpass allows a better MOL because we have a drop in the excursion curve around the tuning frequency of the vented box.

3 - I would NOT use this driver in a vented enclosure (or a 6th order bandpass) due to his parameters. I think reflex work very well until you stay on QB3 alignements, and (even playing with the 'compliance shift' trick) you need a Qt around 0.3 for that.
fwiw, i started out planning to build a sealed 1259, given it's f3 in the mid 20's, but changed my mind after i looked at the max spl of the nht. as fine a woofer as it is, it won't give a very high spl before reaching it's displacement limit (all considerations about anechoic v in room response set aside).

now, i suppose it depends on how loud and what you're using it for. if you don't mind a lower spl, the nht is a good choice. if this might be an issue, you're going to make it worse with an isobarik.

if you want a small enclosure, sealed, an f3 in the teens, and a high spl, you're asking alot. probably the only way to do this is with a long throw woofer, small sealed box, and active eq. (and a hugh amplifier)

if you're thinking of spending money on an additional woofer in an isobarik, why not just spring for one of marchand's bassis units, which is a semi-adjustable LT. you can buy a 1 channel kit for under $100. (yes, you can build it yourself for probably $20-30, from the above mentioned plans, but you've got to scrounge around for all the parts, order the pcb, etc-i think i'd rather just send $69 to marchand)

chris's idea of an ebs alignment is a good one, though not what i'd call small.
Hummmm.... this Marchand idea sounds good. I had shied away from the active EQ route due to my total lack of electronics background.

Besides an appropriate power supply and a case of some kind what else would I need? Can I buy stock items from someone like Partsexpress to do this? Also it would be nice to build it into a case with the plate amp and have the whole package on the shelf with the receiver. Does this sound feasible?
Noticed that Elliot Sound Products has a ELF EQed sub project where he makes a big point of warning not to use it with ported main speakers due to unpredictable phase response.

Wouldn't this be the case with any sealed sub and ported mains? My main speakers will defiantly be ported - should I stick with a ported sub design??
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