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04-22-2010, 04:07 PM
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#81
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I, Vettezuki
Join Date: Feb 2006
Posts: 14,754
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Quote:
Originally Posted by enkeivette
. . .
You know what's really interesting, in the loud speaker world, when you hit the resonant frequency of a port, it will actually push air out as the speaker tries to suck air in, and it will suck air in as the speaker tries to push air out.
I wonder if this happens with exhaust gas when the resonant frequency of the pipe is hit?!
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I'm 99% sure this is not a function of resonant frequency. The purpose of the ports in cabs is precisely to allow woofer cones to move more freely. I think you could measure a constantly variable pressure across a port proportional to the displacement of the cone regardless of freq.
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04-22-2010, 04:36 PM
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#82
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Super Moderator
Join Date: Dec 2007
Posts: 6,850
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Quote:
Originally Posted by Vettezuki
I'm 99% sure this is not a function of resonant frequency. The purpose of the ports in cabs is precisely to allow woofer cones to move more freely. I think you could measure a constantly variable pressure across a port proportional to the displacement of the cone regardless of freq.
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Indulge your 1% My Dad is a loudspeaker engineer. Not to mention I worked for his company as a tech for a few years. I grew up cutting port lengths to find the resonant frequencies of boxes. He's explained it many times to me. I am 100% certain. Actually when you hit the resonant frequency, the driver almost completely stops moving. The air in the box becomes "excited" and pushes and sucks air like crazy. It feels like a high power fan and the sub is barely moving (when it should be going crazy at around 30hz), pretty cool to see. This can also be done with passive-radiators.
If the port was just to allow the speaker to move more freely, it would cause cancellation. Think about it. Why do we put speakers in boxes (baffles) in the first place? To prevent cancellation.
Acutually, below the port frequency, the frequency response drops off rapidly. If you don't have enough box volume to have a port of sufficient length, its better to have the box sealed. Porting a box is typically done at the 3db down point and gives you a bump up until that point, at which point the frequency response will fall off faster after that point than it would had the box remained sealed. |
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04-22-2010, 04:45 PM
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#83
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I, Vettezuki
Join Date: Feb 2006
Posts: 14,754
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Quote:
Originally Posted by enkeivette
Indulge your 1% My Dad is a loudspeaker engineer. Not to mention I worked for his company as a tech for a few years. I grew up cutting port lengths to find the resonant frequencies of boxes. He's explained it many times to me. I am 100% certain. Actually when you hit the resonant frequency, the driver almost completely stops moving. The air in the box becomes "excited" and pushes and sucks air like crazy. It feels like a high power fan and the sub is barely moving (when it should be going crazy at around 30hz), pretty cool to see. This can also be done with passive-radiators.
If the port was just to allow the speaker to move more freely, it would cause cancellation. Think about it. Why do we put speakers in boxes (baffles) in the first place? To prevent cancellation.
Acutually, below the port frequency, the frequency response drops off rapidly. If you don't have enough box volume to have a port of sufficient length, its better to have the box sealed. Porting a box is typically done at the 3db down point and gives you a bump up until that point, at which point the frequency response will fall off faster after that point than it would had the box remained sealed.
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Cool, I learned something. 
PS I'm not a speaker/cab designer.
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04-22-2010, 05:09 PM
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#84
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Super Moderator
Join Date: Dec 2007
Posts: 6,850
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Quote:
Originally Posted by Vettezuki
Cool, I learned something.
PS I'm not a speaker/cab designer. |
Are we not allowed to say ****** on this forum? |
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04-22-2010, 05:42 PM
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#85
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Neanderthal
Join Date: Mar 2008
Posts: 5,320
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Quote:
Originally Posted by 94cobra69ss396
So you still haven't said what you would call it. It is not just a resistance to flow because a resistance to flow wouldn't necessarily create back pressure. A 90° bend in the exhaust pipe would be a resistance to flow but may not create back pressure.
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Old school tech (late sixties early seventies) called it resistance to flow.
But because of the resistance to flow will cause pressure behind the resistance then I see the issue---not a problem.
If you want the tern "back pressure" to mean the same as resistance to flow, that's fine by me. I'm always easy to get along with----  
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04-22-2010, 06:02 PM
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#86
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I, Vettezuki
Join Date: Feb 2006
Posts: 14,754
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Quote:
Originally Posted by enkeivette
Are we not allowed to say ****** on this forum?
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No.
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04-22-2010, 06:07 PM
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#87
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Fast & Filthy
Join Date: Jun 2008
Posts: 3,840
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Quote:
Originally Posted by BRUTAL64
Old school tech (late sixties early seventies) called it resistance to flow.
But because of the resistance to flow will cause pressure behind the resistance then I see the issue---not a problem.
If you want the tern "back pressure" to mean the same as resistance to flow, that's fine by me. I'm always easy to get along with----
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But they are not the same thing. You can have a resistance to flow without having back pressure but you can't have back pressure without having a resistance to flow. Phil and I were talking to my dad about scavenging the other night (they were explaining the sound wave/tuning part to me) and even my old time hot rodder dad who's 59 called pressure in the exhaust back pressure and resistance in the exhaust resistance. |
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04-22-2010, 06:18 PM
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#88
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Fast & Filthy
Join Date: Jun 2008
Posts: 3,840
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I have to add more. You say that "because of the resistance to flow will cause pressure behind the resistance..." but this is not true. Just because there is resistance doesn't mean there is back pressure. Take a 90 degree bend in the pipe. That bend is a restriction or a resistance to flow. However, it may not cause pressure in the exhaust and so there wouldn't be any back pressure. Another example is my Suburban's exhaust. It has many bends that are restrictions as well as cats and a muffler. These all cause resistance to flow. But when I hooked up the back pressure gauge only the side of the exhaust that had the plugged cat had back pressure. The side with the good cat had 0psi. So it has resistance to flow without having back pressure.
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04-22-2010, 06:21 PM
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#89
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Super Moderator
Join Date: Dec 2007
Posts: 6,850
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Damn, I really need to get that high flow cat welded in my Neon. I miss my metal saw  |
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04-22-2010, 06:48 PM
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#90
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I, Vettezuki
Join Date: Feb 2006
Posts: 14,754
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Quote:
Originally Posted by 94cobra69ss396
I have to add more. You say that "because of the resistance to flow will cause pressure behind the resistance..." but this is not true. Just because there is resistance doesn't mean there is back pressure. Take a 90 degree bend in the pipe. That bend is a restriction or a resistance to flow. However, it may not cause pressure in the exhaust and so there wouldn't be any back pressure. Another example is my Suburban's exhaust. It has many bends that are restrictions as well as cats and a muffler. These all cause resistance to flow. But when I hooked up the back pressure gauge only the side of the exhaust that had the plugged cat had back pressure. The side with the good cat had 0psi. So it has resistance to flow without having back pressure.
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I think this was summed up with
- You can have resistance to flow without back pressure.
- You can not have back pressure without resistance to flow.
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