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Old 4th March 2014, 02:32 AM   #11
laplace is offline laplace  Australia
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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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Last edited by laplace; 4th March 2014 at 02:42 AM.
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Old 4th March 2014, 02:35 AM   #12
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by ubza1234 View Post
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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Last edited by sreten; 4th March 2014 at 02:37 AM.
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Old 4th March 2014, 02:54 AM   #13
GeneZ is offline GeneZ  United States
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Originally Posted by Bob Richards View Post
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
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Old 4th March 2014, 03:02 AM   #14
laplace is offline laplace  Australia
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Quote:
Originally Posted by GeneZ View Post
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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Last edited by laplace; 4th March 2014 at 03:10 AM.
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Old 4th March 2014, 03:03 AM   #15
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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?
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Old 4th March 2014, 03:21 AM   #16
rigtec is offline rigtec  United States
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Originally Posted by Cal Weldon View Post
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

rigtec, best regards
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Old 4th March 2014, 03:24 AM   #17
laplace is offline laplace  Australia
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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.
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Old 4th March 2014, 03:37 AM   #18
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cheers laplace, that makes allot of sense. I can visualize how that would work.

so the dampening is predominantly from the inertia of the Air? that's why it's velocity is an important quality for dampening?
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Old 4th March 2014, 03:57 AM   #19
laplace is offline laplace  Australia
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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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Last edited by laplace; 4th March 2014 at 03:59 AM.
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Old 4th March 2014, 04:10 AM   #20
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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.


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Last edited by Richard Ellis; 4th March 2014 at 04:16 AM. Reason: typo
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