why ported and why sealed for a given driver?

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my question is not about any of the characteristics about box and how it sounds but the driver.

why is it that some sub woofer drivers are suited mainly for ported enclosures while others are mainly for sealed enclosures. i understand that some parameters are the reason but thats not enough for me.

1, why is it that some speakers are suited for ported or sealed? what construction/material mean sealed or ported? why do these things equal one box or the other?

2, how would it effect the sound if a speaker designed for sealed box was placed in a ported box and vice versa? how would it behave? how would it sound differently to what its designed for?

just been thinking about it and every time i try and google the answer all i find is the sealed vs ported debate; which i dont care about.

go!
 
In my opinion the TSP are only to be seen as a tool to simulate a possible good working cabinet. You also can try different types of cabinets and decide by your hearing or measurements. Don' take these parameters and resulting cabinets too strict/serious!
They are only meant as an iteration...
And it is much easier to calculate with them than building several enclosures ;)
 
my question is not about any of the characteristics about box and how it sounds but the driver.

why is it that some sub woofer drivers are suited mainly for ported enclosures while others are mainly for sealed enclosures. i understand that some parameters are the reason but thats not enough for me.

1, why is it that some speakers are suited for ported or sealed? what construction/material mean sealed or ported? why do these things equal one box or the other?

2, how would it effect the sound if a speaker designed for sealed box was placed in a ported box and vice versa? how would it behave? how would it sound differently to what its designed for?

just been thinking about it and every time i try and google the answer all i find is the sealed vs ported debate; which i dont care about.

go!

Hi ubza'

I think the greatest tool used to help me to understand some of these things was the reading. I was a young (now old), "up and coming" fender bass player back in the seventies/eighties... and needed to better understand the problems associated with "Live bass reproduction" e.g., my guitar player uses a 50 watt "Fender Twin Reverb" amp w/two 12 inch speakers, and I'm using a 120 watt "Ampeg BT-25L" w/TWO, 2x15 inch (Altec Lansing) cabinets, and I'm being drowned out!!

So I started to read everything available (no internet) on the subject of bass reproduction. Imho, it's not enough to understand why one driver will work and not the other but rather, what are the rules that all drivers must follow, giving you a more through understanding. PS: you seemed really determined to understand these things (much like myself), hence the little personal background; no offense :eek: I hope...

Here's a really good "Pro-sound audio" book that I studied from a while back from Hal Leonard publishing; only $25 bucks: Live Sound Reinforcement (Mix Pro Audio Series): Scott Hunter Stark: 9780918371072: Amazon.com: Books An "EXCELLENT" read, and must have in any audio aficionado's library, enjoy!

rigtec, best regards
 
Hi,

There are speakers that can only be sensibly used sealed.
There are speaker that can be used for either option.
There are speakers that are sensibly only used vented.

All can be used sealed, if fact all can be stuck in whatever
you like, but you may not like the result in a poor choice.

Any driver with a Qts of 0.6 or more = sealed.

Speakers of around Qts = 0.4 can be used for either.

A speaker of Qts = 0.5 is more suited to sealed.

A speaker of Qts = 0.3 is more suited to vented.

A speaker of Qts = 0.2 for bass really needs vented.

A speaker that should be used sealed vented ? A one note peaking boom box.
(For some that are marginal you end up needing huge boxes for good sound.)

A speaker that should be used vented sealed ? Very tight but very drooping bass.
(You can make the bass flat in a very small box, but you lose all the low bass.)

rgds, sreten.
 
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awesome, thanks guys so far, really helping me out and I really appreciate it. definitely going to check out that book rigtec. sounds interesting.

sreten,

you are so close to answering my question, but I feel I really need to understand TSP before understand WHY the qts dictates what type of box it would perform best in. I sorta get the various qt x parameters but not enough to see the relation to box design and general performance.

any where I can go to get some easy to understand explanations on thielle small parameters? allot I've read just give 1 sentence answers just aren't quite enough for me..
 
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WHY the qts dictates what type of box it would perform best in. I sorta get the various qt x parameters but not enough to see the relation to box design and general performance..

You have to think of (1) what a Qts is (2) what performance expected (3) how things correlate.

A musical signal will push the cone and this cone should not just jump out but there should be spring mechanism to hold it in place. Mechanics such as spider contribute to Qms, magnet flux holding the coil I think to Qes (I might be wrong, not essential). Total Qms and Qes is Qts. Now the spring mechanism is not built from Qts only but also the air behind the cone or inside the box. Push the cone in a sealed box, there will be air pressure holding the cone and after you release your hand the cone will go back with certain "energy". Compare this to a sealed box of different volume, and compare also to vented box.

The quality of bass box is not only lowest frequency to be produced, but it must be flat etc. We want sufficiently "damped" system, not under not over. If the driver originally under damped (due to small magnet for example), we can use air behind the cone (hence sealed) to increase or make it sufficiently damped.
 
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"...any where I can go to get some easy to understand explanations on thielle small parameters? allot I've read just give 1 sentence answers just aren't quite enough for me..

ubza'

Takes years to begin to understand acoustic theory/engineering/physics. This is why no "one answered question" will satisfy your quest for this knowledge. Would anyone go to school for, four, five, six, or ten years, just to learn about some speakers if they could figure it out themselves... hahahaha! And then there's the tuition $$$$$! Think about it...

This is not to say that "I have arrived" (nor has anyone else that I know??!) But this is part of the fun, there is always more to learn; even Einstein was still learning when he died. ubza... take your time(!) what's the rush(?), we are all here still learning, so relax, read, think, :idea: enjoy!!!!!

riggy
 
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I hope this helps. To add to what the others have said, the Q of the driver is the inability to return to rest. The higher the number, the more it will flop back and forth before stopping. Therefore a high Q driver likes to be in a sealed box as the air inside acts as a spring and being sealed it helps to damp the woofer and bring it to rest.
A low Q woofer comes to rest sooner and therefore, the venting of the box allows for the sound inside to be used outside of the box. If it wasn't vented it would seem like the bass it too 'tight', or simply not enough.
This is very simplistic and doesn't tell the whole story but hopefully it helps a little.
 
Maybe another way to look at it is that some drivers are optimized for a box that will aid in the damping of the cone (sealed box) and other drivers don't need much if any damping to work well (ported box). It's about cone compliance, if I'm not mistaken.

Personally, I prefer closed box woofers because the box can be smaller, and it works much better when you use active EQ (ahead of the poweramps) to force the woofer to be acoustically flat down to 20 or 30 HZ (which I do). Closed box is slightly less efficient, so pretty rare in large scale PA systems where efficiency is a higher priority than low bass extension. With active EQ and a sealed box, I suspect that you could use a driver with any Qts and get very good results. Using a high Qts driver in a ported box on the other hand might be pretty bad (ringy).
 
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heck yeah! I get it! basically, some speakers rely on the box to center the speaker, and only a sealed box will do as the internal pressures are high then a vented box.

some speakers are stiff enough to to more or less center them selves and you can get away with a ported box.

does that sound about right?

thanks everyone, especially rigtec who I thinks a bit like me, and Cal Weldon for using words like "flop" that really helped me out. XD

does that mean that in general a high qms (I know we where talking about was before but.. ) would mean a more sensitive driver?
 
Have a google for the term Efficiency Bandwidth Product, EBP = fs/Qes. A high (>100) EBP mandates use in a vented box and a low EBP (<50) implies the use of a sealed box.

The idea is that you make up for the inherent limitations of the speaker with the box design and if you use the 'wrong' box, then the dynamics of the box will work against the speaker and you get at least one of poor frequency response, poor efficiency or poor power handling.

Edit: it's not about "centreing" the speaker, it's about how the speaker is supported (air: springiness or resonance, or speaker-mechanical) and how the speaker dissipates energy (the various Q) in order to stop vibrating when the signal stops. You can dissipate the energy electrically (low Qes, high EBP) or mechanically (into the air, using the box dynamics). If you use the "wrong" box, it will either dissipate energy too quickly to produce low frequencies well, or it will not dissipate energy fast enough therefore to prevent the box from booming you need to change the size or tuning in a way that will hurt the frequency response.
 
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awesome, thanks guys so far, really helping me out and I really appreciate it. definitely going to check out that book rigtec. sounds interesting.

sreten,

you are so close to answering my question, but I feel I really need to understand TSP before understand WHY the qts dictates what type of box it would perform best in. I sorta get the various qt x parameters but not enough to see the relation to box design and general performance.

any where I can go to get some easy to understand explanations on thielle small parameters? allot I've read just give 1 sentence answers just aren't quite enough for me..


Hi,

For a driver you also have Fs and Vas as well as Qts.

If you put the driver in a box = Vas, then Fb and Qb = x 1.4
If you put the driver in a box = 1/3 Vas, then Fb and Qb = x 2
If you put the driver in a box = 1/7 Vas, then Fb and Qb = x 2.8
f you put the driver in a box = 1/15 Vas, then Fb and Qb = x 4

Qb for a sealed box can be anything . For a driver it can never
be less than Qts. So if your target is Qb = 0.7, then a Qts =
0.35 driver needs a 1/3 Vas box, whilst a Qts = 0.5 driver
needs a box = Vas and a Qts = 0.6 needs about 3 x Vas.

Understanding a driver is understanding Fs, Vas and Qts
in relation to the drivers stated size, sensitivity, impedance
and excursion compared to other drivers.

The above very much equates to cone mass, suspension
stiffness, the size of the magnet and length of the coil.

Sealed boxes boxes can be aligned to any Q above
driver Qts, and seldom work well with Vbox > Vas ,
unless your looking at small high Qts drivers.

Vented boxes typically are twice as big as Q=0.7 sealed.
Good vented boxes if sealed would have a Qbox of ~ 0.5,
but many use ~ 0.6 and 0.7 and the worst examples higher.

The vagaries of sealed and vented alignments I'm not
going to go into. But I'll give a typical example say this :

H1659-08 U22REX/P-SL

Qts is ~ 0.3, Vas is ~ 100L and Fs is ~ 30Hz.

Vbox = 22L for Qbox = 0.71 and Fb and F3 = 71Hz.

Sealed you could use it in Vbox 11L (Qbox =0.95) to
anything between say 60L (Qbox=0.5) to 1110L (0.40),
though not much to gain nearly doubling internal volume.

Vented you can use 35L tuned to 40Hz, or 50L/30Hz or 70L/26Hz.
The last looks good, a nice flat taper down to 30Hz - 6dB.
Very different tuning to the vented 35L box, which
like the 35L sealed box is down 13dB at 30Hz.

rgds, sreten.
 
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Maybe another way to look at it is that some drivers are optimized for a box that will aid in the damping of the cone (sealed box) and other drivers don't need much if any damping to work well (ported box). It's about cone compliance, if I'm not mistaken.

Personally, I prefer closed box woofers because the box can be smaller, and it works much better when you use active EQ (ahead of the poweramps) to force the woofer to be acoustically flat down to 20 or 30 HZ (which I do). Closed box is slightly less efficient, so pretty rare in large scale PA systems where efficiency is a higher priority than low bass extension. With active EQ and a sealed box, I suspect that you could use a driver with any Qts and get very good results. Using a high Qts driver in a ported box on the other hand might be pretty bad (ringy).

I want to make sure I got this right. I was told by speaker salesman that an acoustic suspension speaker EQ'd to go lower than what its measured range is, could cause the woofer to distort. You are not having that problem.. So, I wonder if what you are doing would not always be applicable. I wish it were. You can get tighter deeper bass that way.

GeneZ
 
I want to make sure I got this right. I was told by speaker salesman that an acoustic suspension speaker EQ'd to go lower than what its measured range is, could cause the woofer to distort. You are not having that problem.. So, I wonder if what you are doing would not always be applicable. I wish it were. You can get tighter deeper bass that way.

GeneZ

Note "could". It depends on the power-handling capability of the specific driver, i.e. both the thermal limit and the Xmax value. Doing low-frequency boost requires lots more amplifier power and therefore lots of cone excursion, which is OK if the driver is designed for that purpose. So you have a -3dB point of 40Hz on driver in a sealed box, it will be -15dB at 20Hz. If you want to EQ it for a flat (anechoic) response to 20Hz, you need 15dB more amplifier power, i.e. 30 times! To make the same sound level at 20Hz as you can at 80Hz will require a 3000W amplifier instead of 100W. And the cone will need to move 4x as far as it does at 40Hz.

If you take a driver suitable for a ported box, put it in a sealed box and EQ it, you will either set fire to it or cause the suspension to bottom out.

Some more reading.
 
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so qts is not about centering, but shock absorbing both electronically and mechanically? a sealed box is not used to help push the speaker back to rest, but to absorb the acoustic energy of the speaker to dampen resonances? is that right? or does the sealed box dampen the speaker by re-enforcing the rest position?
 
I hope this helps. To add to what the others have said, the Q of the driver is the inability to return to rest. The higher the number, the more it will flop back and forth before stopping. Therefore a high Q driver likes to be in a sealed box as the air inside acts as a spring and being sealed it helps to damp the woofer and bring it to rest.
A low Q woofer comes to rest sooner and therefore, the venting of the box allows for the sound inside to be used outside of the box. If it wasn't vented it would seem like the bass it too 'tight', or simply not enough.
This is very simplistic and doesn't tell the whole story but hopefully it helps a little.

Hi Cal,

Now This... is one of the most satisfying explanations of the "Q" that I've ever heard (and I ain't even the one who asked the question hahaha). "That was righteous!" thanks for bestowing that reply upon us brother :worship:

rigtec, best regards
 
Q in a resonant system is the ratio of energy kept to energy lost per cycle of operation. High Q (underdamped) means it will ring, low Q means it will not. Q is about resonant efficiency and how quickly (and where) the energy dissipates.

There are many sources of Q in a speaker: electrical (Qes), mechanical (Qms), box (Qb, i.e. flexibility in the walls, leakages), etc., all of which combine to form the total Q.

The air in a sealed box acts as both spring and resistive damper: it pushes the cone back both as a function of its displacement from centre (spring) and its velocity (damping). As you make the box smaller, you get more spring than damping and the Q increases. If you have a large sealed box, it provides very little spring support; if you have an extremely small sealed box it acts like a tight spring and may make the driver resonate/ring. You design your box size to have an appropriate (to your tastes) Q, typically between 0.5 and 0.8.

If you have a ported box, the support from the cone comes both from springiness of the air in the box, but also from the inertia of the air in the port. The port acts like a resonant moving mass (piston) of air. The dimensions of the port will define the frequency above which it can support the cone - below the tunes frequency there is no support and it's just an open box.
 
Yes, damping effectively comes from inertia of the air. For a linear system (therefore no distortions so this is a bit of a simplification, but the whole Thiele-Small approach is based on linear-system assumptions), there are only two restoring forces on things in motion:
- spring-like force, which is proportional to position, and
- resistive force, which is proportional to velocity.

If you have all spring and no resistive (think of a mass on the end of a spring), it has infinite Q and will bounce up and down forever at its resonant frequency. If you have only resistive force (viscous drag) and no spring, you have zero Q and no resonant frequency: the item will slow down but never quite come to a halt, Zeno's paradox-like. If you have mostly-spring, it will oscillate for a few cycles then come to a halt (underdamped). If you have mostly-resistance, it will slowly move towards the centre but not quite get there in any finite time (overdamped). If you balance them just right (Q=0.5) then it returns as quickly as possible to the centre without overshooting (critically damped).

Air suspension provides both forces, in a ratio defined by the volume of air available. That ratio defines your damping factor and therefore Q of the box, which in conjunction with the Q of the driver gives you the Q of the whole loudspeaker.

Ported is more complicated because there is a restoring force that is kind of spring-like, but its magnitude and phase depends on the frequency. It's still a linear system, but it now has multiple interacting resonant components.

If you're familiar with AC circuit analysis (L, C and R systems using laplace transforms), it turns out that that is how speakers are modelled, because the exact same mathematics applies to LCR systems as it does to masses, springs and friction. So you can use circuit analysis software like Spice to analyse your speakers and their boxes! Thiele-Small parameters are an intermediate step between the mechanics of the speaker and some equivalent values required to perform such modeling.
 
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It is the compressibility of air that controls, damps out cone movement, not all but just the right amount.
To give you some insight perhaps, a story. I had a pair of Minimus Seven loudspeakers, a small Aluminum enclosure maybe 190mm by 105mm by 98mm in size. They are a two-way sealed "Acoustic suspension" set-up. One day when I was playing music thru them, the music was clean un-distorted, decent bass output. I proceeded to un-screw the small woofer driver AS it was playing, bad idea! When the woofer got un-sealed from the enclosure, (all the air inside stopped damping effect to the woofer).........the cone started rattling from being driven forward & back to its mechanical limits, a god-awful buzzing/rattling sound.....now I didn't up the volume or anything, it was just being denied that enclosed air to "calm it down". It shocked me just how much awful noise it made & I almost dropped the whole thing before I realized what was happening & I plopped it back down in its rightful location...& screwed it back down.


__________________________________________________Rick...........
 
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