26 replies to this topic

[~thematt~]

Its basic physics.

When the box is too small, there is more air resisting the woofers movement then the motor/suspension was designed to allow for. To really understand the impact though, you need to move your thinking into the space of the first derivative as well (as spring force resists motion, dampening resists velocity)

In the time domain, a small box produces a response similar to thwacking a ruler on the edge of a desk. It vibrates up and down quite substantially before the system regains control. The ability for the cone to return to rest is very quick, but the system has built up so much energy getting it there, it overshoots. Like a see-saw. This is all due to the properties of air.

Performing a Laplace transformation, and you get into the frequency domain. Here, you're system will result in a high-energy 'hump' (due to the excess energy needed to overcome the resistance to motion the air presented) and its response can seem very fast (naturally, due to the systems ability to return to the 'rest' point quickly. However, the ability for the motor to travel is compromised. It is moving so fast around the rest point, it simply does not have the energy to fully-exert its motion to the limits. This compromises the low-end. You need the motion to get the low-end from a system, so when you limit this, you can no longer go low, loudly.

The ringing produced by small box systems actually contributes to the distortion levels of the woofer. So small box systems have measurably higher distortion.

Since the motor system of a woofer is designed to dissipate ~99% of all the input energy (anywhere between 10 and 1000Watts..) as heat (so long as the coil stays within the field), small boxes actually contribute to a longer life for the woofer. This is due to the lower travel, and therefore less time out of the field for the coil, as well as the inability for a motor to reach its mechanical limits (or at least its much much harder). This is the reason why manufacturers always recommend boxes that are smaller then they should be.

[~Samuel~]

^ Damn good post... End of Thread

[krayzie]

is there a general guide or size of sub to volume of the box it should be?

[syd-monster]

krayzie, on Nov 4 2009, 11:56 PM, said:

is there a general guide or size of sub to volume of the box it should be?

well... its like asking is there a general size shoe? There's reasons why people who are really into this trend things on a computer, try and see what space they have and what are they willing to give up etc. and also may build several boxes to try and get the best sound in the actual car. Then there's other issues like which way is it mounted (driver displ) etc etc...

however, in car audio for each driver sealed, from trends I have seen lately.

8" = 0.5 to 0.7 cubic foot
10" = 0.8 to 1.2 cf
12"/13" = 1.1 to 1.5
15" = 1.5 to 2

I'm shooting myself for even posting that, but meh...

[data_mine]

syd-monster, on Nov 5 2009, 12:44 AM, said:

I'm shooting myself for even posting that, but meh...

You're right.


Subs come with a specs sheet (or should), they often give some manufacturer recommended box sizes. Theses aren't usually the best options, but instead are a compromise between sound and box size. Most ideal boxes are heaps bigger than the recommended. And every sub is different.

[~Samuel~]

Yeah id agree with that Syd

When you go ported thats when it becomes much more complicated and driver dependant

[syd-monster]

SPL Samuel, on Nov 5 2009, 10:51 PM, said:

When you go ported thats when it becomes much more complicated and driver dependant

Can I ask why?? its the same building techniques and there is a mass of programs out there to make calculations easy...
I think a modern vented box is really good! provided you can combine it with a sub of similar pedigree...


*ufff, all this sub talk.

[~Samuel~]

syd-monster, on Nov 5 2009, 10:26 PM, said:

Can I ask why?? its the same building techniques and there is a mass of programs out there to make calculations easy...
I think a modern vented box is really good! provided you can combine it with a sub of similar pedigree...


*ufff, all this sub talk.

Becasue sealed is more general, most 10 subs will perform reasonably well in .8cf sealed, even if it too small the sub will not sound too bad

With ported if the box is too small, or tunned to high then it will start to sound really bad, think listening to music through a burp box that is really larger and tunned at 48hz...

I hate sealed boxes and always build ported but i thought you were correct in your general sizes for sealed boxes thats all

[~thematt~]

syd-monster, on Nov 5 2009, 07:56 PM, said:

Can I ask why?? its the same building techniques and there is a mass of programs out there to make calculations easy...
I think a modern vented box is really good! provided you can combine it with a sub of similar pedigree...

The programs are based on calculations using T/S specs. I've measured the T/S specs of my drivers before, and the levels are really really really low.

The difference between low level signals and high level signals is current. Moreso the result of the current, or heat. This heat causes changes to the performance characteristics of the driver that cannot be predicted from low level signals. The changes are common referred to as 'Power Compression'.

A sealed box has a direct and simple relationship. I've explained it above. This relationship is the same at low levels as it is at high levels, save for power compression. However, there is no substantial changes due to anything else besides power compression to the relationship. Otherwise put, there arent any other substantial variables.

In a ported box, you have another relationship. Thats the port. Helmholtz resonators are fairly simple devices, but their impact changes depending on the agitation to the overall system. When you have another variable entered into the mix, the relationship starts to become more complex.

Think Sealed - Y=ax, but ported y=ax+bz. Another variable.

We can calculate, because we understand the relationship, a ported box fairly well. But all the calculations are based on low level signals. The interaction at higher levels changes, and this is less understood.

As you begin to add more variables, the results become more complex.
- Sealed Box
- Ported Box. Sealed box plus single port.
- Bandpass. Ported Box plus sealed box.
- 4th order BP. Standard BP plus port in the sealed section (two ports).
- Front horn loaded.
- Rear horn loaded.
- Transmission Lines
- Tapped Horns.

[syd-monster]

^^^ understood (before so) but not what I meant. My point was the given the access of programs & information we have. It's no harder to box a sub be it sealed or vented in intended design. Perhaps I didn't make that clear in my question, I just think it would be a waste for someone to turn away from constructing a vented box (or more advanced) because we have pidgeon holed it as "too hard".

[joms]

what if the box gets bigger? what are its advantage?

like a JL12w7 on a recommended 1.25cu ft. What if you make it 1.5 / 1.75 / etc ?

[data_mine]

You'll lower the tuning frequency, and eventually it'll be so big it'll act the same as an infinite baffle setup.