Advance SPL Topics - port volume

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Jan 31, 2011 | 07:25 AM

#31

MTA

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Quote:

Originally Posted by AB FTW

ok bro wich part of testin did you forget to do yourself cus yore scoer lookz like you shoudda done more of it

ouch! thats gotta hurt

no matter what anyone says... you still need TS to design a good starter box... it will help save you time and money

and remember TUCK( not duct) tape is king

Jan 31, 2011 | 08:19 AM

#32

Father Yuli

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why only easterners ever reply to these topics?

Jan 31, 2011 | 09:19 AM

#33

SP Canada

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It doesn't hurt considering the person saying it is hiding behind a username used by like 7 ppl solely to start . But I think 155.4 is good for 1 18. And it was all done in testing with very little work and no ts params!

Feb 1, 2011 | 01:38 PM

#34

Haunz

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Quote:

Originally Posted by Dukk

Pressure is a confusing term for a lot of people. People often use pressure and the force exerted by a high velocity fluid as the same thing. Think of a garden hose dispensing a stream of water. If the water shoots out a long way people generally say it has a lot of pressure. Reality is there is no pressure and it is all velocity.

Fluid dynamics tells us that a fluid flowing though a pipe/hose/duct has two components, velocity and pressure, and the two vary inversly to each other. If you make the pipe/hose/duct larger the velocity goes down and the pressure goes up. So by making a vent as large as possible to make the velocity though it as low as possible, yes the pressure within the duct will be as high as possible, but that pressure is gone once it leaves the vent.

I'm not 100% sure I'm clear on what you are trying to say in your post? Anyway I don't think trying to compare the flow of a fluid and its absolute pressure & velocity to a longitudinal pressure wave is really the best analogy.

To illustrate my point, how do you increase the sound pressure coming off a speaker cone? You increase the cone’s velocity! Indeed, sound pressure is equal to the product of RMS particle velocity and acoustical impedance.

At this point my previous comments can be seen as being analogous to comparing a high xmax 10” sub to a low xmax 18” sub with an equal displacement… clever positioning of the 10” sub may yield better SPL results depending on how the vehicle loads and the relative placement of the driver and mic...

On the other hand, a larger port area will always yield fewer losses due to drag within the port, which also translates to a greater Q factor for the entire system. This I imagine is why a larger port area is generally preferred.

Feb 1, 2011 | 03:27 PM

#35

Dukk

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See now therein lies the difference - I don't imagine. Trying to embiggen your response using terms like 'absolute pressure & velocity' and 'longitudinal pressure wave' was amusing though. Thank you.

Since you purport to be comfortable with the concept of acoustic impedance, I am surprised that you appear to ignore how it relates to the area of the vent relative to the space in the cabin of the car. The comment about drag is interesting too - if there is less drag within the mass of air (which must be what you mean as the larger vent will certainly have more wall drag) is will be directly due to the slower velocity within the vent, which supports my statement. Thanks again.

Regarding this point:

Quote:

At this point my previous comments can be seen as being analogous to comparing a high xmax 10” sub to a low xmax 18” sub with an equal displacement… clever positioning of the 10” sub may yield better SPL results depending on how the vehicle loads and the relative placement of the driver and mic...

What if you "cleverly position" the 18" woofer?
What if you aren't so clever with the positioning of the 10" woofer?
What exactly was your point here as it relates to the topic of port volume?

Let's slow it down a bit. Big port = good.

Feb 1, 2011 | 04:13 PM

#36

Haunz

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Dukk, I was in no means trying to 'embiggen' my response... the fact is you are trying to compare a direct flow of fluid to a pressure wave of high and low pressure feilds above and below ambient pressure - which are driven by kinetic energy... the greater the average particle velocity the greater the compression and refraction - get it ?

I'm not sure what else you are going on about, but I'm sure you can understand from my simple analogy a 10" can fit places an 18" can't; and the only point being a high velocity port in a certain position may load better under certain circumstances...

Don't read anything more into it...

I'm not sure what else to add except that a larger area port with lower velocity certainly has less drag - you really don't agree with me on that??

Just my $.02... and everyone can take it with a grain of salt if they like...

Last edited by Haunz; Feb 1, 2011 at 04:18 PM.

Feb 2, 2011 | 10:32 AM

#37

Haunz

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I just want to add that I think I've really digressed off on a tangent on this one...

Dukk is 100% right in his analogy with a lower velocity and a higher pressure inside the port.. but I don't believe that is why a larger port will generally be louder..

In both cases the same volume of air is still being moved.. the real difference being that a larger port will have a less turbulent flow and less drag on the walls, making it more efficient...