Small vent hole in sealed subwoofer cabinet ?.

[alex30]

Hi,
I am hoping that someone can give me some accurate advice that is backed up by the science of sealed subwoofer cabinet construction.
I am building a small (one cubic foot volume) sealed subwoofer cabinet and as it is my first project I have been reading around a lot.
Most of what I read says to ensure that the box is totally sealed but I have read also that a very small vent hole of less than 1mm diameter should be made in the cabinet to allow for equalisation of air pressures inside and outside of the cabinet so that the driver is not permanently displaced by uneven pressures. The small nature of the vent is said not to adversely affect performance but aids it by keeping the driver in the correct resting position and so allow proper displacement when it is working.
I am confused by the differences in opinion ( not new in audio related matters ) and wonder if the respected builders of subwoofers , such as SVS and the like, make any vent holes in their sealed box designs ?
Any help with this confusion would be much appreciated.

 

[alex30]

alex2507 said :

Welcome. Vented subs are vented. Sealed subs are sealed. A vent in a sealed sub is a leak.

Hi Alex,
Thanks for the response and the welcome. For some reason your response isn't showing when I open the thread so I have copied and pasted from the email notification.
I suppose what I am trying to find out is if there are any real benefits to making a small vent hole and if conversely there are any negatives.
If I was to find that it was a practice followed by the respected sub makers then it would be something I would follow but if they do not do it then it would be something I would avoid.
There is a lot of conflicting opinion out there but , as is often the case in audio, not many are putting any science forward to support their view. I assume someone must have done the measurements to prove or disprove the idea but I can't find any papers relating to it.
You obviously would not vent but what are you basing your decision on ? Is it a gut feeling or something more?
Any ideas would be most welcome because it is starting to niggle me now.
If I don't find an answer soon I will finish the build without a vent with the proviso that it can always be added at a later date.
Once again , thanks for the input.

 

[TLS Guy]

It all depends in the design of the driver. There MUST be allowance for changes in atmospheric pressure. If not the cone will be pushed in and out with the changes in atmospheric pressure. A rise will push the cone in and a drop will make the cone come out. The movement will actually be large. There will only be one barometer reading were the cone is in the neutral position. That will be the pressure at the time the enclosure was sealed. A small whole will not change the instantaneous pressures in the cabinet, due to cone movement from program.

Now a lot drivers have what are known as QL losses. This is the loss of air from the VC gap. In this case changes of atmospheric pressure will be dealt with.

You can tell the difference, as if there is no perforation around the dust cap, or a phase plug, the the VC gap is sealed, and you need to make a hole to allow for changes in atmospheric pressure.

You want a sub and not a barometer, so very likely you do need a hole.

If you are uncertain, leave a hole anyway, as the hole you describe will be much less than the air losses around the VC.

As usual this is all elementary physics, and this one a simple concept to understand.

 

[alex30]

Hi TLS Guy,
Many thanks for the very full and useful answer, it was what I was searching for.
I haven't yet bought my driver but am planning on a Peerless 835017 XXLS.
You wouldn't happen to know how this should be treated would you ?

 

[TLS Guy]

Hi TLS Guy,
Many thanks for the very full and useful answer, it was what I was searching for.
I haven't yet bought my driver but am planning on a Peerless 835017 XXLS.
You wouldn't happen to know how this should be treated would you ?

That has a sealed dust cap for sure, and you need to make allowance for changes in the weather! So yes, you need a hole.

 

[alex30]

TLS Guy said :
That has a sealed dust cap for sure, and you need to make allowance for changes in the weather! So yes, you need a hole.

Hi TLS Guy,
Once again my thanks for the valuable info.
I will put a small hole (less than 1mm if possible) in the back of the cabinet.
Regards Alex.

 

[TheWarrior]

That has a sealed dust cap for sure, and you need to make allowance for changes in the weather! So yes, you need a hole.

Would you please elaborate? I have never heard this before!

 

[TLS Guy]

TLS Guy said :
That has a sealed dust cap for sure, and you need to make allowance for changes in the weather! So yes, you need a hole.

Hi TLS Guy,
Once again my thanks for the valuable info.
I will put a small hole (less than 1mm if possible) in the back of the cabinet.
Regards Alex.

Well I checked the spec sheet and the dust cap is vented!

However this driver sealed will make a pretty puny sub.

Beginners usually think a sealed sub is easier. It is not! It is much harder and far more expensive.

Although the Fs of that driver is 20 Hz, sealed alignment push the F3 high. The F3 point of that driver in a sealed enclosure is 45 Hz. So you have to be able to add equalization at 12 db per octave stating at 50 Hz. For every 3 db increase you double the power. So the power required is huge. In addition you have to add a high pass filter around 25 Hz to prevent driver damage.

Now the next issue is that as power goes up, driver excursion goes up. Although they brag this up, the linear driver excursion is 12.5 mm. For a sealed sub that is actually very low, and will NOT allow for the required Eq. Cone excursion in sealed alignments is much higher than in vented. A loudspeaker cone is a very inefficient acoustic interface to the room. A port is a much better one.

Decent sealed subs need a through and through brute force approach. It requires a driver with the right T/S specs, that has a motor system that can take oodles of power, and a suspension giving 20 mm or more linear travel. That is expensive. Then you need a very powerful amp, with Eq for the high F3 point and a high pass filter to protect the driver. So it is complicated and expensive.

Now that driver gets to about 30 Hz ported, not very good. The spec sheet says to use Eq. The problem is that you can not Eq a ported sub, as the driver rapidly decouples from the box below tuning, and cone excursion rapidly increases and becomes useless, with high distortion.

 

[TLS Guy]

Would you please elaborate? I have never heard this before!

I already did. That driver I found does have a built in vent. A speaker in a totally air tight box is a barometer and only useful for predicting the weather, and not a speaker for obvious reasons.

 

[alex30]

TLS Guy said ; However this driver sealed will make a pretty puny sub.

Hi,
I may be mistaken here but I believe SVS used this driver in the sub I own, the SB 12 NSD. It certainly seems so from what I read on the SAB Hall Of Fame review of that sub.
SVS no longer make this sub so if I want another I will probably have to try and copy it.
I don't think I would call that sub puny although I concede that it will not deliver the punch that a bigger ported design will. As I listen to a lot of music the quality of the bass is more important to me than the absolute quantity. The SVS certainly delivers in that respect.
I take your point about degrees of difficulty but I need to explore if my plan is at least feasible because I would be very reluctant to give up the SVS I currently own.

 

[TLS Guy]

TLS Guy said ; However this driver sealed will make a pretty puny sub.

Hi,
I may be mistaken here but I believe SVS used this driver in the sub I own, the SB 12 NSD. It certainly seems so from what I read on the SAB Hall Of Fame review of that sub.
SVS no longer make this sub so if I want another I will probably have to try and copy it.
I don't think I would call that sub puny although I concede that it will not deliver the punch that a bigger ported design will. As I listen to a lot of music the quality of the bass is more important to me than the absolute quantity. The SVS certainly delivers in that respect.
I take your point about degrees of difficulty but I need to explore if my plan is at least feasible because I would be very reluctant to give up the SVS I currently own.

If you want limited output for a small room you will be OK. You will still need to provide the 12 db per octave boost below 50 Hz, and install a high pass filter 12 db per octave at 25 Hz. SVS include this type of Eq in their amps.

By the way it is not true that a sealed unit has inherently better quality than ported one or some other loading methods. The trouble is that DIYers and above all manufacturers go for extended bass alignments, to game the spec sheet, and therein lies the problem.

 

[alex30]

TLS Guy said :
If you want limited output for a small room you will be OK. You will still need to provide the 12 db per octave boost below 50 Hz, and install a high pass filter 12 db per octave at 25 Hz. SVS include this type of Eq in their amps.

By the way it is not true that a sealed unit has inherently better quality than ported one or some other loading methods. The trouble is that DIYers and above all manufacturers go for extended bass alignments, to game the spec sheet, and therein lies the problem.

Hi TLS Guy,
Thanks yet again for the valuable advice.
I find that at the moment my bass is superb for music and decent for films, where I do notice some uneven response in the room when two seats are being used. For music only one seat is ever in use, the sweet seat.
I figured that another sub will , if set up correctly, help to even things out and also give a bit more oomph to the movie soundtrack.
I understand what you say about ported v sealed but for positioning I needed to keep the size of the unit down as I am in a small room. Otherwise I would have been tempted by the SVS PB 12 NSD. Unfortunately it was just a little too tall for me to be sure of having multiple placement options and as it turned out it would not have gone where I have placed the one I went for. Of course it may have needed to be in a different place but options are limited.
Once again thanks for the input. I now know that a lot of work is needed to build that sub but I am determined to try.

 

[TLS Guy]

TLS Guy said :
If you want limited output for a small room you will be OK. You will still need to provide the 12 db per octave boost below 50 Hz, and install a high pass filter 12 db per octave at 25 Hz. SVS include this type of Eq in their amps.

By the way it is not true that a sealed unit has inherently better quality than ported one or some other loading methods. The trouble is that DIYers and above all manufacturers go for extended bass alignments, to game the spec sheet, and therein lies the problem.

Hi TLS Guy,
Thanks yet again for the valuable advice.
I find that at the moment my bass is superb for music and decent for films, where I do notice some uneven response in the room when two seats are being used. For music only one seat is ever in use, the sweet seat.
I figured that another sub will , if set up correctly, help to even things out and also give a bit more oomph to the movie soundtrack.
I understand what you say about ported v sealed but for positioning I needed to keep the size of the unit down as I am in a small room. Otherwise I would have been tempted by the SVS PB 12 NSD. Unfortunately it was just a little too tall for me to be sure of having multiple placement options and as it turned out it would not have gone where I have placed the one I went for. Of course it may have needed to be in a different place but options are limited.
Once again thanks for the input. I now know that a lot of work is needed to build that sub but I am determined to try.

If you want to build that sub use an external amp. This unit from Mini DSP will be your freind combined with REW.

On the other hand if you want to make tings easier for yourself, this small ported sub will do what you want to do and be musical.

I should add, that that sub will be much better than your SVS sub. It is deigned for a nice driver of SEAS, that was designed in conjunction with the great Saul Linkwitz. This is a low Q driver and I have designed for a low Q alignment, and I bet it is lower than your SVS sealed sub. This sub will be tight, tight, tight and very musical and extend to 24 Hz, and no bothering with EQ. You will just need and amp.

By the way SEAS drivers are much better quality than Peerless.

I guarantee that this sub will be better in every parameter than what you propose, and far easier to build. In fact you can order in supplies today.

 

[alex30]

TLS Guy Wrote :
If you want to build that sub use an external amp. This unit from Mini DSP will be your freind combined with REW.

On the other hand if you want to make tings easier for yourself, this small ported sub will do what you want to do and be musical.

That idea, about an external amp and the miniDSP to provide the customisation I need, is superb. I must say I hadn't considered that but understand exactly where you are coming from. The only downside is that I want to copy the SVS as closely as possible to aid integration and not having an amp in the box will alter it's effective internal volume. I imagine I could compensate for this if I could find out the volume of the Sledge 400 amp that SVS use.
I did consider a different sub but everything I have read pointed me in the direction of the same model to ensure easier integration with my system.

 

[TLS Guy]

TLS Guy Wrote :
If you want to build that sub use an external amp. This unit from Mini DSP will be your freind combined with REW.

On the other hand if you want to make tings easier for yourself, this small ported sub will do what you want to do and be musical.

That idea, about an external amp and the miniDSP to provide the customisation I need, is superb. I must say I hadn't considered that but understand exactly where you are coming from. The only downside is that I want to copy the SVS as closely as possible to aid integration and not having an amp in the box will alter it's effective internal volume. I imagine I could compensate for this if I could find out the volume of the Sledge 400 amp that SVS use.
I did consider a different sub but everything I have read pointed me in the direction of the same model to ensure easier integration with my system.

I can tell you that SEAS sub will be far superior to the SVS sub, which I bet you will sell if you build the SEAS sub, and likely you will only need one and not two. You see that design I did is very low Q and potent. It will couple to the room better than a sealed design.

Don't make life difficult. Simplicity is always best. I understand your SVS sub does not do anything offensive, but you can do better, much better.

By the way the amp volume will be insignificant.

 

[alex30]

Hi TLS Guy,
O.K. this has got me interested but I am not sure what we are dealing with.
Does this sub come as a kit with box design included or is it more DIY than that ?
Is there a sales site that I can peruse and any reviews of the finished article ?
The problem I see with just one sub is that room modes will still be present or am I missing something here ?

 

[TLS Guy]

Hi TLS Guy,
O.K. this has got me interested but I am not sure what we are dealing with.
Does this sub come as a kit with box design included or is it more DIY than that ?
Is there a sales site that I can peruse and any reviews of the finished article ?
The problem I see with just one sub is that room modes will still be present or am I missing something here ?

No, you have to trust that I know what I am doing.

I have designed quite a number of subs for people on these forums over the last 10 years, and the have all been extremely pleased with them.

This is a sub I actually will be building this year. It is the sub I have designed for the HT system I'm designing from scratch for one of my sons who is just finishing off a major remodel of his home.

The other issue, is that one good sub beats two poorer ones. There are no exceptions to that rule. If you feel you need two subs after building the first you can build another. I doubt in that room you will need to, but you might.

I will talk you thought the build, like bracing and filling, although that is on the pdf. The volume of the port, driver and bracing have to be added to the design to get Vt.

This is the driver the design requires.



If you want to save the volume and trouble of the port, they have a matching ABR.

If you desire the design can easily be tweaked for this passive radiator.

 

[TheWarrior]

No, you have to trust that I know what I am doing.

I have designed quite a number of subs for people on these forums over the last 10 years, and the have all been extremely pleased with them.

This is a sub I actually will be building this year. It is the sub I have designed for the HT system I'm designing from scratch for one of my sons who is just finishing off a major remodel of his home.

The other issue, is that one good sub beats two poorer ones. There are no exceptions to that rule. If you feel you need two subs after building the first you can build another. I doubt in that room you will need to, but you might.

I will talk you thought the build, like bracing and filling, although that is on the pdf. The volume of the port, driver and bracing have to be added to the design to get Vt.

This is the driver the design requires.



If you want to save the volume and trouble of the port, they have a matching ABR.

If you desire the design can easily be tweaked for this passive radiator.

I just did some quick math for a dual driver variant, two isolated boxes, one baffle front firing, front firing vents placed top and bottom flanking centered drivers.... I've got some 3/4" plywood drops that would be perfect, I'll do a test fit with pocket hole screws this weekend, if I like the sound I'll give it a nice thick mdf baffle. Thanks for the idea!

Was about to tighten up some of the cuts on the OB sub this week, as it sounds pretty good corner loaded! But since I already have the material....