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Old 02-09-2013   #26
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Default Re: does xmax matter in ported boxes,

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Originally Posted by William_White View Post
now that its been established how power and Xmax affect ported and sealed enclosures, how do they affect bandpass boxes?
Every single type of enclosure works the same way in this respect. (Geddes has argued that there's one exception - the acoustic lever - but I don't know enough about that enclosure type to speak about it.)

For every type of enclosure type, output at the low end of the box is dictated by bore and stroke. (aka cone area and xmax)

There's only two real exceptions to this rule:

1) If you can restrict the radiation, efficiency will start to go up. This is how horns and waveguides work. In a car, the cabin itself does this, because the wavelengths exceed the dimensions of the car itself. This is why SPL cars invest a lot of effort to keep the car itself from flexing.

2) Horns and ports stop the cone from moving at certain frequencies. For instance, in a ported box tuned to 30hz, the cone is not moving. So voice coil thermal limits become a big factor at that frequency.


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Old 02-09-2013   #27
 
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Default Re: does xmax matter in ported boxes,

So they work like the rear enclosure; Xmax is less crucial in a 6th order/ported as long it can reach the tuning frequency before exceeding it and power handling is less crucial in a 4th order/sealed as long as xmax is acheived without exceeding it. Am I understanding this right?
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Old 02-09-2013   #28
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Default Re: does xmax matter in ported boxes,

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Originally Posted by William_White View Post
So they work like the rear enclosure; Xmax is less crucial in a 6th order/ported as long it can reach the tuning frequency before exceeding it and power handling is less crucial in a 4th order/sealed as long as xmax is acheived without exceeding it. Am I understanding this right?
6th order BP acts like a "dual vented enclosure". You will have 2 different tuning frequencies, excursion will be low at both frequencies. Group delay ain't pretty either, but they have the capability to get quite loud. Have fun building one, they are hard as crap to get right even if you model them

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Old 02-09-2013   #29
 
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Default Re: does xmax matter in ported boxes,

I don't plan on building a 6th order unless some one gives me an 8" driver and a challenge.
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Old 02-11-2013   #30
 
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Default Re: does xmax matter in ported boxes,

There are a few things that can be visualized a little better with pictures. First lets get a few basics down. Patrick has covered most of it but just wanted to clarify a little. Some things to understand.

SPL is always equal to the amount of air being moved or displaced. The diameter of drivers, efficiency, etc plays no part in this, only the overall amount displaced.

The more air displaced with the least amount of power is better. A woofer that can move X amount of air with 300W is far better than another woofer that requires 2000W to move that same amount of air. There seems to be some kind of misconception in the car audio world that it is better to have a woofer that can "handle" tons of power than one that moves the same amount of air with far less power. The less power to get a given SPL the better the results. This keeps thermal failure and thermal distortions lower. Keeping less current in the VC also lowers the effects of eddy currents in the motor and as a result lowers distortion. The only benefit to a crazy inefficient woofer that can take tons of power is the ability for it to help heat your car in the winter. A 2000W electric heater can help warm up a vehicle quite quickly.

Every time you drop one octave, it takes 4x as much displacement to get the same SPL level. As an example 40hz takes 4x as much displacement to get the same SPL as at 80hz.

Power handling of a woofer has nothing to do with excursion. Power handling is a thermal rating based on the amount of power the VC can safely take. It is very possible that you can damage a driver due to over excursion with far less than the rated power of the driver. The amount of power required to get to excursion limits is determined by the enclosure. People can build multiple size and enclosure types. As a result, a manufacture will never have a way to rate the amount of power a woofer requires to reach it's physical limits. This is something the installer needs to be aware of and pay attention to.

In a sealed enclosure only the front wave of the woofer is being used. The back wave is contained and not used. All displacement comes from this front wave of the woofer only.

In a vented enclosure, the back wave of the woofer is being used to sum with the front wave through the use of a port. As a result the woofer and port have an effect where they add to achieve extra output. Also, at tuning, most all the output is coming from the port, greatly reducing the excursion of the woofer. For this reason, vented enclosures can have a lot more output for two separate reasons. Not only because of the additional output from the port, but also because more power can be applied before reaching excursion limits of the driver. There is often a misconception that a sealed enclosure keeps a woofer from moving as far. That as you can see is incorrect.

Lets look at a PB12-4ohm in a 2 cubic foot enclosure. You can see in the sealed enclosure(purple curve) below it is reaching the 14mm Xmax at 35hz with about 700W input. Output is 111dB at that point. Once the vent is put in and tuned at 35hz(white curve), there is significantly more output. The level is already at 116dB at 35hz, a gain of 5dB already with no more power applied. Of most interest though is the decrease in excursion at 35hz. The woofer barely moves at that point. This is where the port is doing most of the work.



This drop in excursion is really the "power band" where the most power can be applied without physically driving the woofer to excursion limits. This band is fairly narrow but when tuned properly and placed where it is most needed, allows for far more power to be input. In an SPL vehicle where only one small range needs to be played, this is a key factor. In this case from 49hz down it is affecting the excursion of the driver. The peak in excursion happens at 1/2 octave above tuning. This excursion peak is not going to be higher than the excursion would be from an equivalently sized a sealed box. It is however this peak that primarily determines the max input power before physically damaging the woofer due to over excursion. It is also critical to apply a high-pass filter to keep excursion under control below tuning.

A 6th order bandpass has the benefit of a second chamber being tuned. This gives a second point where excursion is brought down. When these tuning frequencies are close enough together, they can come together and bring down the overall excursion at all points between the two tuning frequencies. This gives the ability to put in more power over that entire operating range.



The biggest challenge with 6th order BP boxes is the change in parameters with respect to excursion. Of most importance is the change in BL. This is illustrated in the following graph. At rest and full BL value, you may see a fairly nice flat curve across the pass band as seen in the green curve. As Bl drops, that changes. A typical overhung woofer has a rated Xmax where BL has dropped to 70% of the rest value. In a sealed or vented enclosure, this has only a minor affect on changing the response curve. In a 6th order BP box though, it starts to change things more significantly. In the middle of the passband, there is already a 2dB drop in the output and peaks at the tuning points begin to get sharper (orange curve). If driven farther past this point to where the BL drops to 50% of it's rest it has a 5dB dip in the middle of the pass band and very pointed peaks above and below (red curve).



This is why 6th order bandpass boxes can get so boomy at two points. The other issue is that the woofer doesn't stay at that reduced BL through it's entire stroke. As the woofer moves from rest, and comes forward, you see the change in the response curve. As it comes back to rest it changes again and then changes another time as it hits the excursion limits the other way. The result is a continually varying response curve with excursion. This is a huge factor in distortion. Inductance changing with respect to excursion also has an effect on changing the upper end of the response curve in a bandpass enclosure. Typical drivers have an S shaped inductance curve where inductance goes up on the inward stroke and down on the outward stroke. This curve is somewhat exaggerated, but illustrates the point. Green is the rest point. As inductance goes down on the outward stroke, the orange curve slightly increases at the upper end. As the coil goes inward and inductance goes up, the red curve drops on the top end.



Add up both of these non-linearities causing the response curves to change, and it is easy to see why even a bandpass box that looks good in a model can seem to sound bad. Linear BL and linear inductance are more critical in a bandpass than any other box. Many cheap bandpass car boxes use woofers with short Xmax and no inductance control that are continually driven beyond their magnetic limits.

Regarding isobarik alignments, in the following you can see what happens. BL itself doesn't change. Motor strength however does double. Motor strength is measured by using Bl^2/Re. Because 2 drivers are now used, Re is cut in half and Bl stays the same. Thus doubling Bl^2/Re for 2x the motor strength as expected. The other parameters also make sense. Cms is cut in half because it is 2x the suspension stiffness. As a result Vas is 1/2 also. Mms is doubled. The 1w/1m SPL is deceased 3dB but the 2.83V SPL is essentially the same due to the Re being cut in half.



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Old 02-11-2013   #31
 
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Default Re: does xmax matter in ported boxes,

That is a lot of information. You mention 6th order BP boxes but what about 4th order BP boxes?
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Old 02-11-2013   #32
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Default Re: does xmax matter in ported boxes,

Quote:
Add up both of these non-linearities causing the response curves to change, and it is easy to see why even a bandpass box that looks good in a model can seem to sound bad. Linear BL and linear inductance are more critical in a bandpass than any other box. Many cheap bandpass car boxes use woofers with short Xmax and no inductance control that are continually driven beyond their magnetic limits.
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Old 02-11-2013   #33
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Default Re: does xmax matter in ported boxes,

Told you guys BL doesn't change...

The acoustic lever arrangement...it's a bandpass enclosure of sorts. You have an active driver in one chamber that is firing into another chamber that has 2 connected passive radiators- one small on the inside and a larger one on the outside (the one you see and hear).
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Old 02-11-2013   #34
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Default Re: does xmax matter in ported boxes,



Wow I haven't noticed posts from John Janowitz in ages. Great info there - this is a great thread to bookmark.

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