# Tuning a ported enclosure below the driver's Fs?



## TREETOP

I've heard differing opinions on this over the years, but without much science to back up those opinions. 
It seems the general consensus is that tuning an enclosure below the sub's resonant frequency is dangerous to the sub because it can unload below the Fs, but the more I think about it, it seems like Fs is measured in a freeair environment, right? So I'm not sure how much Fs really matters in choosing a frequency to tune a port to. 

Thoughts, ideas, comments?


The specs on a Cerwin Vega Stroker 12 triggered my thought train on this, it has an Fs in the high 30s/allmost 40 but their recommended enclosure is tuned to 34hz and down 3db at 26hz. 
http://www.cerwinvegamobile.com/manuals/stroker_subwoofer_manual-2009.pdf


----------



## chad

the driver's FS is moot when it goes in an enclosure for the most part, you can tune below FS.


----------



## TREETOP

I tend to agree with you.

What's throwing me off is I was talking with one of the national reps from Hertz about a week and a half ago. He was adamant about _never_ tuning below Fs with any subwoofer. I didn't debate this with him (or a couple other things I disagreed with) because my time was limited and I didn't want to act like a know-it-all in front of the people I was with, but I kinda bert-stared when he said that.


----------



## fish

What was his reasoning? Unloading? Also, curious to know what other topics you disagreed on?


----------



## TREETOP

fish said:


> What was his reasoning? Unloading? Also, curious to know what other topics you disagreed on?


From what I gathered, yeah. 

As far as other stuff, he seemed like a knowledgeable guy but he spoke in absolutes rather than admitting there were a lot of variables that he was leaving out of his information. He had a lot of comments about how _all_ mainstream car audio equipment is overpriced, how one should _never_ use a class D amp for front speakers that cost over $300, how subs should _only_ play below 40-50hz and an enclosure should _always_ be ported, how a midbass with a larger magnet will sound better, etc. 
He categorized the Hertz and Audison lines by dollar amounts to compare them with the competition rather than comparing by features or specs, which may make sense from a retail sales perspective but not to someone with more of a DIY/active approach.


----------



## Oliver

Gonig back and forth through the resonant frequency introduces a lot of distortion.

Buy a speaker with an FS above where you'll be playing it and then tune it to play beneath FS.

So... get a subwoofer with an FS above 80 hertz and then play it below that.


----------



## TREETOP

a$$hole said:


> Gonig back and forth through the resonant frequency introduces a lot of distortion.
> 
> Buy a speaker with an FS above where you'll be playing it and then tune it to play beneath FS.
> 
> *So... get a subwoofer with an FS above 80 hertz and then play it below that.*



Um, no? :laugh:


----------



## Oliver

If you are listening to *Rap or HipHop *... I'd play right through the FS.

with an FS of say 40-45 Hz ... I'd tune it at *32Hz and play it up to 100 Hz.*

*You'll never notice on anything but excellently recorded material.*


----------



## TREETOP

I normally tune an enclosure in the low to mid 30s for music, but most of my experience is with subs with an Fs in the 20s (or lower). 
I guess my concern is with linearity and musicality down low, with a sub that has a freeair resonance in the 40s. Typically I'd automatically choose a different sub with a lower Fs, just out of reflex, but I'm wondering now if I've been unknowingly rejecting valid candidates.


----------



## VP Electricity

Back when I used to do box-shaped enclosures regularly, I regularly used a porting freq lower than the Fs of the driver, and I never noticed any pattern of SQ issues in doing so. That's not to say everything sounded good all the time, but I detected no pattern.


----------



## Ronmeister

I feel it goes out the door when the driver is placed in the enclosure. Fs is free air resonance of course, if an enclosure is tuned to control and keep the driver from unloading really low, it will do that.
I had a friend who used did 12" eD SPV woofers tuned to 18Hz in a hatch with 1kW on each, dang thing did a 143 @ 18hz and sounded beautiful. If I'm not mistaken the SPVs fs is somewhere in the mid to upper 30s.


----------



## Oliver

Ronmeister said:


> tuned to control and *keep the driver from unloading really low, it will do that.*


Actually , *you cannot tune an enclosure to not unload* below the tuned frequency 

You can use a subsonic filter to keep signals from being played below the frequency you have tuned it to.

If the subwoofer plays through the natural resonating frequency , it will cause your music to sound ****ty ... other than that you are fine doing it...* ENJOY !!*


----------



## Ronmeister

a$$hole said:


> Actually , *you cannot tune an enclosure to not unload* below the tuned frequency
> 
> You can use a subsonic filter to keep signals from being played below the frequency you have tuned it to.
> 
> If the subwoofer plays through the natural resonating frequency , it will cause your music to sound ****ty ... other than that you are fine doing it...* ENJOY !!*


You misinterpreted what I said.
That is common knowledge 
I was stating that if an enclosure is tuned very low, it will keep the driver from unloading when playing very low, that is all.. Like any ported enclosure.


----------



## pjhabit

FS does not exist when you put the driver in an enclosure, it is a free air measurement, there is no Fs to "play right through". In a ported box, the Fs (or resonant frequency) becomes the boxes tuning frequency or its Fb. In a sealed box, Fc is the systems resonant frequency. 

Tuning below Fs is completely fine...it's done all the time. The only issues (or non-issues) I can think of would be the suspension limits & quicker unloading below tuning (less motor control to keep it in line). As long as you don't exceed Xmax to get to your desired tuning & have an appropriately set SSF, all should be fine.


----------



## chad

motor control above and below the FS of a driver is fairly consistent.. it's just that one point where it resonates. but the enclosure will control that.


----------



## Oliver

Ronmeister said:


> You misinterpreted what I said.
> That is common knowledge
> I was stating that if an enclosure is *tuned very low*, it will keep the driver *from unloading when playing very low*, that is all.. Like any ported enclosure.


*With-in reason*

If U send it a signal below where it is tuned , it will play that signal [ if U have no control from the enclosure { the box works above the frequency you've tuned it to, as far as having control over the speaker} then the load put on the speaker by the box will be nonexistant for the most part and *the speaker will move up and down controlled only by the spider  and surround ]*.[* unloaded by the box*

If you tune an enclosure low enough the sound out put will suffer [ less bang for your buck ].

P*robably why most people just use a subsonic filter to keep the low notes attenuated.*


----------



## Sulley

a$$hole said:


> If you tune an enclosure low enough the sound out put will suffer [ less bang for your buck ].


So how does one determine when the tuning freq is low enough to effect output? trial and error? or is there data somewhere... I see a lot of people running low 30's, some people in the high 20's and some down as far 17-18hz. I understand its driver specific but generally speaking, does output loss become a factor when say tuning in the subsonic frequencies? or even mid 20's? 
I have so many questions about vented enclosures


----------



## Oliver

stockley.rod said:


> So how does one determine when the tuning freq is low enough to effect output? trial and error? or is there data somewhere... I see a lot of people running low 30's, some people in the high 20's and some down as far 17-18hz. I understand its driver specific but generally speaking, does output loss become a factor when say tuning in the subsonic frequencies? or even mid 20's?
> I have so many questions about vented enclosures


You need to acquire a box building program [ there are many to choose from ].

Input the T/S parameters and know how to interpret the data, you are now on your way to being able to design and build enclosures for speakers


----------



## Sulley

a$$hole said:


> You need to acquire a box building program [ there are many to choose from ].
> 
> Input the T/S parameters and know how to interpret the data, you are now on your way to being able to design and build enclosures for speakers


perfect, why didn't that occur to me first. Thanks a$$hole.


----------



## sqshoestring

It can drop spl below Fs, but with the power and subs available you can do near anything you want anyway. I used to run those old efficient subs in large boxes (2cf per 10) then stuff and port them high just to unload them on purpose, but only had 75rms each. It made them sound much like IB subs and they got lower that way. On the other hand you don't need a long port in an oversize box, so I really don't know what the port was tuned to. It was something like 3x3 or shorter for two tens in 4cf. Had to do it by ear and that sounded best at around 30Hz.

Get a box program or winISD, you can change the box size and port tuning and see what it does to the response. Some of the lower Q subs can have response ported you didn't think they would, model that 10 on sale at PE that looks sort of like an audiobahn with a higher Fs. It can look pretty tasty ported.


----------



## Salad Fingers

TREETOP said:


> I tend to agree with you.
> 
> What's throwing me off is I was* talking with one of the national reps from Hertz* about a week and a half ago. He was adamant about _never_ tuning below Fs with any subwoofer. I didn't debate this with him (or a couple other things I disagreed with) because my time was limited and I didn't want to act like a know-it-all in front of the people I was with, but I kinda bert-stared when he said that.


Who was it?


----------



## TREETOP

Salad Fingers said:


> Who was it?


PM sent..


----------



## stills

i have a MTX t5000 10 in a .95ft sealed box w/ @ 100w
my fs is 33hz

this is an old school type sub. that wants 2ft.
mtx suggests porting in the 40s
according to win isd if i port high i run out of excursion in the mid- 20 hz range
if i port closer to 20hz i run out of travel at a few ticks below 20hz.

would i be better of going for the lower freq. tuning in this situation?


----------



## VP Electricity

stills said:


> i have a MTX t5000 10 in a .95ft sealed box w/ @ 100w
> my fs is 33hz
> 
> this is an old school type sub. that wants 2ft.
> mtx suggests porting in the 40s
> according to win isd if i port high i run out of excursion in the mid- 20 hz range
> if i port closer to 20hz i run out of travel at a few ticks below 20hz.
> 
> would i be better of going for the lower freq. tuning in this situation?


 Nope. Might want a 25-30 Hz subsonic, but probably don't need that.


----------



## Ronmeister

stills said:


> i have a MTX t5000 10 in a .95ft sealed box w/ @ 100w
> my fs is 33hz
> 
> this is an old school type sub. that wants 2ft.
> mtx suggests porting in the 40s
> according to win isd if i port high i run out of excursion in the mid- 20 hz range
> if i port closer to 20hz i run out of travel at a few ticks below 20hz.
> 
> would i be better of going for the lower freq. tuning in this situation?


What exactly are your goals?
I'd maybe do 2 cubes then, @36-38Hz and call it a day.


----------



## sqshoestring

With a 'normal' single 10 you are not going to make the dB down at 20, it gets pointless to try. Better off trying for a strong 30Hz if that and call it a day. I have a pair of 15s, had quad 12s, trying to get 20. Right now I can just get 23, and at 25 they are doing fairly good. I have the subsonic turned off and they are IB.


----------



## The real Subzero

So, My question to this is this: Can you control the sub more if you tune to a lower FS, and allow less port velocity by cutting the down the sq port area?????


----------



## The real Subzero

cajunner said:


> it's all relative.
> 
> port velocity is higher with smaller diameter, or less cross-sectional area.
> 
> smaller diameter, is easier to install because the lengths are shorter.
> 
> you can tune to a lower Fs, by increasing port length, or decreasing port diameter.
> 
> if your question is why doesn't everyone just tune every sub to 20 hz, because that gives you the widest frequency response then my answer is that kind of tuning means you give up most of the benefits of porting, which is higher output in the range of frequencies most music is recorded.
> 
> you try modeling sub boxes for a while, and you will see the relationship between port sizing and enclosure sizing, and increased output with vented alignments being subject to a scale that is determined mostly by the excursion limits of your particular subwoofer.


My subs have an FS of 39.55, My box is tuned to 40hz, and my car peak is 47hz. It sounds good, but I want the low lows sometimes.


----------



## sqshoestring

When you tune a port it resonates against the sub. It gets active at that frequency and actually restricts the movement of the sub....it 'bounces against the sub'. So you can actually push more power without hitting xmax there. In response you get a peak there from the port output. So if you look at a response curve of the box with no port the typical thing to do for better sound not spl, is to put that bump in output from the port right where the bottom is rolling off...output is decreasing as frequency goes lower. Lets say cabin gain is going to help the lows some so at 30Hz you can have less output than 50 in a model and it still sounds good, when you port at 30 it is lower spl than 50....but in the car it might come out great with cabin gain pushing 30 up more yet so it ends up somewhat flat from 50+ down to 30, then it unloads under that.

If you tune at 40Hz it will be 'louder' because about any sub is more capable up there. Myself I would not like a 40 tune at all I prefer 30-35 peak if any. But its all what you want from it, if you want lower it is typical to tune a port to the lowest you need to avoid unloading but on the other hand if you have small subs they will not have much output low it gets pointless to try beyond a certain dB of rolloff.


----------



## XtremeRevolution

There is one way to get more output in the sub 40hz range without needing a larger box than what you'd use in a sealed alignment. Passive radiator.


----------



## NOTORIOUS97200

Many manufacturers tell you to tune the ported box between 28 and 34 Hz, for music listening. 
A 40 Hz tuning may be good for spl burps, but you will loose the lower notes on music.


----------



## BowDown

Mmm.. high Fs one note wonder subs.


----------



## Chaos

Although the topic is from last year, I'm going to comment on the original sub in question anyway. I built an enclosure for a Cerwin-Vega Stroker 12 the other day which appeared to model reasonably well in 2.2 cu.ft. @ 30 Hz (39 sq.in. vent). It should have played cleanly down to an F3 of 25hz, but instead it sounded terrible. Probably the single worst box I have ever designed.

Even the manufacturer specs indicate Fb @ 35 Hz, which is still a few Db below Fs. Does the fact that is was tuned so far below Fs have anything to do with the result? I honestly don't know. As many times as I have heard of this "rule of thumb" I have never had a problem tuning below Fs before now. 

I'm actually about to try it in a sealed box built to spec for comparison, and then try a few other subs with Fs higher or lower than the tuned frequency of the box just to see what happens.


*edit: Believe it or not, it didn't sound much better in a sealed enclosure either. That one was built to spec (1.25 net @ .7 Qtc) and the sub still didn't have any impact. I don't think I've ever been so disappointed in a subwoofer.


----------



## Danometal

My JBL P1224 has a Fs of 27 hz, but I still built a box tuned to 23 hz. It loves it. You can feel the lows in your lungs.


----------



## ATOMICTECH62

The main reason some of these boxes sound bad is the decreased overall output combined with very long group delay.
Tuning a box lower and lower always makes the group delay longer.
I have seen some upwards of 60 milliseconds which then makes the sound from the box sound like its coming from 70 feet away.
There is a point at which making the box larger will not give you any lower Fb and trying to tune it lower only increases group delay.
Tuning the box lower than the speakers Fs really is not a problem as most box designs I have done with BassBoxPro will suggest.


----------



## sqshoestring

Chaos said:


> Although the topic is from last year, I'm going to comment on the original sub in question anyway. I built an enclosure for a Cerwin-Vega Stroker 12 the other day which appeared to model reasonably well in 2.2 cu.ft. @ 30 Hz (39 sq.in. vent). It should have played cleanly down to an F3 of 25hz, but instead it sounded terrible. Probably the single worst box I have ever designed.
> 
> Even the manufacturer specs indicate Fb @ 35 Hz, which is still a few Db below Fs. Does the fact that is was tuned so far below Fs have anything to do with the result? I honestly don't know. As many times as I have heard of this "rule of thumb" I have never had a problem tuning below Fs before now.
> 
> I'm actually about to try it in a sealed box built to spec for comparison, and then try a few other subs with Fs higher or lower than the tuned frequency of the box just to see what happens.
> 
> 
> *edit: Believe it or not, it didn't sound much better in a sealed enclosure either. That one was built to spec (1.25 net @ .7 Qtc) and the sub still didn't have any impact. I don't think I've ever been so disappointed in a subwoofer.


Maybe the factory specs are bs.


----------



## captainscarlett

So if I've read this thread correctly so far:

1) FS figures are recorded in free air not in any type of enclosure?

2) Will sub software help not to exceed xmax? (i run a mac and i'm still looking for sub software). 

I was hoping to rip my DD 8 and 10" out of their enclosures and tune them to around 30-32hz like the DDLE312, as the DDLE308 is tuned to 38hz and the DDLE310 39 Hz???

http://www.ddaudio.com/mobile-audio/woofers/500-series/specs.aspx


----------



## sqshoestring

Yes and yes, but it is still a model not fact. You need to move air to make low bass, the lower you go the more you need to move. PR and vents do help, but it can get to a point where the smaller sub is just not going to give much output going really low. It is a tradeoff, if you want SQ and don't care about loud it may work fine, it just has to work harder to make that low frequency and so the output will suffer.

The enclosure can modify the response of the sub, Fs will turn into Fb. It is helpful to look up Q and understand what that means, its pretty simple, the Q rises as the box gets smaller and 'humps up' the response you get. 0.707 is the flattest response. Also the Qts of the sub determines how it responds around Fs, how it will respond to changes in the box as well. This is how some speakers are made for boxes and some are not, or some small and some need large boxes. If you figure out how those few things work you will understand what the model is telling you a lot better, and what sub you need to get what you want.


----------

