My ported subwoofer

[t.antony]

This sub is being made to compliment my MTM speakers. The primary use is for Home Theater. Since the MTMs roll off around 45hz, this sub is intended to be used only for very low bass frequencies between 18hz to about 45hz. There are some advantages and disadvantages of designing a sub for these frequencies.

Advantages

• Due to the non-directional nature of the sound at these frequencies, there is more freedom to position the sub in the room.
• It can be used for music without affecting the sound quality because it wont be producing sound if music does not have content in the lowest octaves, and will contribute to the spectrum when music has the lowest octaves (Subcontra - B0 and B1) which is in the vicinity of 30hz. In other words, the sub will blend in and become invisible sonically.

Disadvantages

• The enclosure size need to be relatively big to handle these frequencies efficiently.
• Big driver with low Fs is needed to provide sufficient output at those levels.
• Active xover have to be custom built to provide good integration with the MTM, also amplifier need to be capable of reproducing the lowest octaves. This increases the complexity of the project.

The choice of a ported enclosure is due to the ease of construction and bracing, the port will be firing through the front of the enclosure as this will allow the speaker to be kept closer to the back wall useful for small rooms.

I will be using a 12 inch sub woofer made by Peerless India for this project. This driver has an Fs close to 24Hz which can be easily tuned lower by a suitable ported enclosure. The power requirements seems to be modest for this driver, and I think I can power it with a bridged gainclone amp.

The project is already kicked off and the enclosures will be ready soon probably by this weekend I will update the thread with some pictures later.

 

[Audiodoc]

Thats great. Even I Have a subwoofer enclosure made years ago will try to purchase a driver for the same. My DIY amp and SUB woofer circuit board are also ready.

 

[grubyhalo]

You don't waste much time do you? All the best! Just curious, do you think it would help the BS perform better if you let it play only till around 70Hz and let the sub handle the rest?

 

[santhosh]

You don't waste much time do you? All the best!

You echo my thoughts mate. Just completed the MTM and already onto the next. Way to go buddy. Keep us updated on the progress and also pics pls.

TIA

 

[sonicbliss]

A down firing port (or side firing, if you need to show the port) would be better than a front firing one. It helps in masking the port noise, if it ever is an issue.

To better integrate with the MTMs may I suggest the following :-

1) choose a shallower slope for both the MTMs as well as the sub. Now, you already have MTMs that are ported at about 45hz and response will drop too fast below the tuning, so for better integration alone, the MTMs must be sealed (to get a natural shallow roll-off and NOT cross them at all) and the sub must be crossed higher with some overlap between the MTM and sub. This gives better integration as compared to ported mains with a lower crossover as is being currently planned. Port can be stuffed by socks (cork) etc so that the change to sealed box is reversible. If the MTMs are needed to be used alone without the sub anytime in future then the port stuffing can be easily removed.
This is not textbook method, but neither is a typical room a text book room (as if something like that ever existed ).

2) have a variable crossover/phase for the sub since its much easier to blend them with the mains.

Hope it helps.

Sonicbliss

 

[t.antony]

Just curious, do you think it would help the BS perform better if you let it play only till around 70Hz and let the sub handle the rest?

For the MTM, roll-off @45Hz is influenced by the box dimensions and port tuning. In other words, there is a predictable acoustical roll-off below 45Hz. If the sub is crossed above this roll-off frequency, the phase response and/or the peak(s) at the crossover could be difficult to tame, also considering room gain, it will be tricky to get a linear response.

The MTM's are capable of providing enough SPL for a domestic environment from 45-50Hz up. While I agree that the load will be less when the low freq extension is reduced, it's not really needed here. Also from 50Hz upward the directional nature of the sound is more perceivable to the ears and for true stereo reproduction I presume two subs might be required and their positioning could also become critical.

My intention behind this sub is to compliment the MTMs and to provide linear response from 20hz up with the -3db point somewhere near 18Hz. Easiest way to achieve is to cross the sub where the acoustic roll-off of the MTM occures.

 

[t.antony]

A down firing port (or side firing, if you need to show the port) would be better than a front firing one. It helps in masking the port noise, if it ever is an issue.

To better integrate with the MTMs may I suggest the following :-

1) choose a shallower slope for both the MTMs as well as the sub. Now, you already have MTMs that are ported at about 45hz and response will drop too fast below the tuning, so for better integration alone, the MTMs must be sealed (to get a natural shallow roll-off and NOT cross them at all) and the sub must be crossed higher with some overlap between the MTM and sub. This gives better integration as compared to ported mains with a lower crossover as is being currently planned. Port can be stuffed by socks (cork) etc so that the change to sealed box is reversible. If the MTMs are needed to be used alone without the sub anytime in future then the port stuffing can be easily removed.
This is not textbook method, but neither is a typical room a text book room (as if something like that ever existed ).

2) have a variable crossover/phase for the sub since its much easier to blend them with the mains.

Hope it helps.

Sonicbliss

A ported sub can give much more SPL than it's sealed cousin and that's needed for getting response linear enough to mate with the MTM's. Also considering the narrow band in which this sub is going to be used, ported will handle the sudden rush of low frequency passages of HT more efficiently than the sealed sub.

Since the MTM rolls off steeply below f3, the low pass filter of the sub need to be steeper to avoid humps in the overall response.

Port noise is not an issue in this case since the calculated port-air velocity is within limits. I am inclined towards front firing ports since they are more accommodating and predictable in room positioning.

Active crossover and amplifier will be DIY as well, need to do some testing with the sub before I can finalize the design of those components.

 

[t.antony]

You echo my thoughts mate. Just completed the MTM and already onto the next. Way to go buddy. Keep us updated on the progress and also pics pls.

TIA

I suspect the DIY bug had bitten me multipple times and it's side-effects didnot wear out yet... :lol:

In fact this project started off sometime back and was progressing on the designboard.

 

[t.antony]

Pics of the 12" driver

 

[quad]

The driver looks nice. Do let us know more details - t/s parameters,
price and where you bought it.

 

[GeorgeO]

Amp?

Unless the driver is designed to be used face down it is foolish to use it that way-the surround will start to sag.

 

[GeorgeO]

Amp?

Unless the driver is designed to be used face down it is foolish to use it that way-the surround will start to sag.

 

[sonicbliss]

My suggestions are all based on Dr. Earl Geddes's sub setup. I dont have exact figures. You may want to check each one below. The figures are ballpark numbers.

A ported sub can give much more SPL than it's sealed cousin and that's needed for getting response linear enough to mate with the MTM's.

Only MTM need to be sealed, not the sub. Ported sub is fine and a good choice.

With Earl's sub setup, the mains need to have a shallower roll-off till the first room mode. He ran his mains ported initially and then moved to sealed mains.

Its not about more SPL from the MTMs. Its about overlap of sub with mains in frequency, a very non textbook approach. The sub IS intended to handle the SPL at the sub low frequencies, no doubt about it. Its more about how the room blends with the mains L/R and sub. Given a good quality mains L/R and an equaly good sub (you have the former already and will get to the latter soon ), the room dominates the blend. Its about tackling this room dominance. The room blend does NOT work with the textbook crossover method as per Earl. Lets make it even more clear. The first room modes start somewhere about 40-50hz. We know that below this frequency, sub alone is what is needed given the huge SPL requirement in this region (below the first room mode). Sealed MTM will not play in this reagion so no SPL problems for them. Now what happens above the first mode (frequency 50-150hz) is interesting. The room response has lots of peaks and nulls, near the first room mode the peaks/nulls are wide apart then as we go higher in freq, the peaks/nulls start coming closer and closer to each other till the shroeder frequency (did i spell it right?) This modal region from first room mode to shroeder frequency is exactly where multiple sources are needed to tackle the room dominance issue. Lets now assume that the mains are not crossed at all and are sealed. By going sealed, the excursion of the MTM is not a problem because the sealed alignment controls it very effectively. Sealed gives a slow acoustic rollof, from some where about 120-150hz or so to down 50-60hz or so. Now lets cross the sub higher, (exact freq can only be found by measurements at the listening area, of course). Do note that in this modal region there are multiple sources now, two mains and one sub. This is exactly what helps in better integration as per Earls setup guidelines.

BTW, did you note that even Linkwitz recommends a monopole sub below the first room mode @ ~40hz. Its because a dipole does not have this room dominance problem in the modal region (from first room mode to schroeder frequency), at least not as much as monopoles do.

Also considering the narrow band in which this sub is going to be used, ported will handle the sudden rush of low frequency passages of HT more efficiently than the sealed sub.

As in my above explanation, sub handles region below first room mode alone where the SPL demands are easily met by the sub, the MTM never operates in this region due to sealed alignment. The region 50-200 is handled by 3 sources

Since the MTM rolls off steeply below f3, the low pass filter of the sub need to be steeper to avoid humps in the overall response.

This is again a textbook method. Its does not deal with room dominance at all. This is where Earls guidelines really shine in practice. Shallow acoustic roll off works much better along with sub overlap.

Port noise is not an issue in this case since the calculated port-air velocity is within limits. I am inclined towards front firing ports since they are more accommodating and predictable in room positioning.

I did nt explain this point clearly. Minimum Port velocity (before port starts creating compression problems) is variable with frequency. A peak velocity of 30m/s may be fine at ~35hz but same velocity is too high at lower frequency say ~25hz. If you have a big dia port then no issues at all. All is well. The drawback is a longer port, a bigger box, more bracing and so more complications. What I suggested was to make the port just appropriate and assume that port artefacts WILL be there at peak SPLs. This way we know we get smaller box overall. The artefacts can be masked by port placement, ie away from the listener. Its all about tradeoff with competing factors. I do understand that the trade offs may not exactly suit your application.

Hope it helps.
Sonicbliss

 

[t.antony]

Only MTM need to be sealed, not the sub. Ported sub is fine and a good choice

The purpose of this sub is to compliment the MTM, not to make any fundamental changes in the latter's design. MTM's are designed as ported, and it's ideal to leave them as is.

With Earl's sub setup, the mains need to have a shallower roll-off till the first room mode. He ran his mains ported initially and then moved to sealed mains.

Its not about more SPL from the MTMs. Its about overlap of sub with mains in frequency, a very non textbook approach. The sub IS intended to handle the SPL at the sub low frequencies, no doubt about it. Its more about how the room blends with the mains L/R and sub. Given a good quality mains L/R and an equaly good sub (you have the former already and will get to the latter soon ), the room dominates the blend. Its about tackling this room dominance. The room blend does NOT work with the textbook crossover method as per Earl. Lets make it even more clear. The first room modes start somewhere about 40-50hz. We know that below this frequency, sub alone is what is needed given the huge SPL requirement in this region (below the first room mode). Sealed MTM will not play in this reagion so no SPL problems for them. Now what happens above the first mode (frequency 50-150hz) is interesting. The room response has lots of peaks and nulls, near the first room mode the peaks/nulls are wide apart then as we go higher in freq, the peaks/nulls start coming closer and closer to each other till the shroeder frequency (did i spell it right?) This modal region from first room mode to shroeder frequency is exactly where multiple sources are needed to tackle the room dominance issue. Lets now assume that the mains are not crossed at all and are sealed. By going sealed, the excursion of the MTM is not a problem because the sealed alignment controls it very effectively. Sealed gives a slow acoustic rollof, from some where about 120-150hz or so to down 50-60hz or so. Now lets cross the sub higher, (exact freq can only be found by measurements at the listening area, of course). Do note that in this modal region there are multiple sources now, two mains and one sub. This is exactly what helps in better integration as per Earls setup guidelines.

BTW, did you note that even Linkwitz recommends a monopole sub below the first room mode @ ~40hz. Its because a dipole does not have this room dominance problem in the modal region (from first room mode to schroeder frequency), at least not as much as monopoles do.

Thanks for the details. I don't have much information regarding Geddes setup, since I cannot use his method(my mains are ported) may be someone can try it out and post their experience with the room modes.

This is again a textbook method. Its does not deal with room dominance at all. This is where Earls guidelines really shine in practice. Shallow acoustic roll off works much better along with sub overlap.

As indicated in the initial posts,this is more of a musical sub and designed to be operated in the lowest octaves only. The rationale behind choosing a higher slope is to restrict it to that frequency range and thus make it fairly non-directional.

Lower xover slope and it's effect on room modes is something I need to experiment with. I will test it out when the enclosure is ready.

 

[t.antony]

The driver looks nice. Do let us know more details - t/s parameters,
price and where you bought it.

I got this from Mumbai some time back, don't remember the exact price but I think it was around Rs.3300. I think you can still get these drivers from Dev Electronics at Lamigton Road.

Here are the TS specs from the manufacture's data sheet

Fs : 24.94 Hz
Re : 3.68 ohms
Qms : 1.17
Qes : 0.45
Qts : 0.32
Le : 0.53 mH
Vas : 256.35 liters
Mms : 59.71 gms
Cms : 678
BL : 8.80
SPL : 94.7 dB
Sd : 518.75 cm2

 

[kapvin]

I got this from Mumbai some time back, don't remember the exact price but I think it was around Rs.3300. I think you can still get these drivers from Dev Electronics at Lamigton Road.

Here are the TS specs from the manufacture's data sheet

Fs : 24.94 Hz
Re : 3.68 ohms
Qms : 1.17
Qes : 0.45
Qts : 0.32
Le : 0.53 mH
Vas : 256.35 liters
Mms : 59.71 gms
Cms : 678
BL : 8.80
SPL : 94.7 dB
Sd : 518.75 cm2

Hi

what about the xmax figures?

this driver wouldn't necessarily meet your goals in a sealed enclosure without boost; so a ported enclosure is a good idea. but have you give thought to a TL?

also where there other sub drivers available at Dev when you purchased this?

 

[t.antony]

Hi

what about the xmax figures?

this driver wouldn't necessarily meet your goals in a sealed enclosure without boost; so a ported enclosure is a good idea. but have you give thought to a TL?

also where there other sub drivers available at Dev when you purchased this?

No plans for TL due to size constraints and also WAF . I am not sure about the xmax, I think it will be around 8mm.

Someone else picked it for me from Mumbai, so I don't know about the other drivers at the shop.

 

[kapvin]

No plans for TL due to size constraints and also WAF . I am not sure about the xmax, I think it will be around 8mm.

Someone else picked it for me from Mumbai, so I don't know about the other drivers at the shop.

don't reject it yet.. there are ways of managing WAF (coz of size) in a TL design

I'll send you a pm with some links in a bit...

btw 8mm one way would be more than adequate for any music duties.

 

[t.antony]

don't reject it yet.. there are ways of managing WAF (coz of size) in a TL design

I'll send you a pm with some links in a bit...

btw 8mm one way would be more than adequate for any music duties.

Thanks Kapvin, I have replied your PM.

This sub is designed with Martin's MLTL worksheet, so in theory it is a mass loaded transmission line. But due to the relatively small size, the effect of quarter wave loading will be minimal and I think it will behave more like a ported enclosure.

I collected the enclosure from the carpenter this morning, will post a pic soon.

 

[t.antony]

A pic of the enclosure: unfinished but good for some early listening tests.