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small vs. large port, same tuning frequency


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luckydevil 
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Posted: September 14, 2006 at 11:23 AM / IP Logged  
What is the difference (other than size) between a small port and a large port that are both tuned to the same frequency?
For example, say we have a 1ft^3 enclosure that we want tuned to 35Hz. WinISD shows that we can use a 4"IDx24.6"L port or we could make it much smaller and use something like a 1"IDx1"L port.
Now I know sound velocity is going to play a part in this, but how do you determine when a port is too small or too large? What is the ideal sound velocity?
Thanks guys.
*EDIT: Well, after modeling a few ports in WinISD I can't get the sound velocity to change.
haemphyst 
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Posted: September 14, 2006 at 11:30 AM / IP Logged  
As per the engineers at Harman, whenever the speed of the air in the port EXCEEDS Mach .1 (one tenth the speed of sound), the port is too small.
In the car, especially in a trunk that is separate from the passenger compartment, I have designed for UP TO Mach .2, althought I'd NEVER use that in a home sub, or where the chuffing coises will be noticeable.
Smaller ports mandate faster air speed WITHIN the port, so a larger port is always better than a smaller port.
It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
luckydevil 
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Posted: September 14, 2006 at 11:33 AM / IP Logged  
Good info, thanks haemphyst.
How do you go about determining what is too small though? Does WinISD show air speed for the port anywhere?
haemphyst 
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Posted: September 14, 2006 at 2:41 PM / IP Logged  
Sorry... not being familiar with WinISD (I use JBL Speakershop) I wouldn't be able to answer that question for you. I've used it, but not nearly as much as Speakershop. Perhaps one of the other members, significantly more familiar with it, could answer that for you.
It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
hemanjoyman 
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Posted: September 14, 2006 at 5:06 PM / IP Logged  
Yes, WINISD will show you the air speed. You have to change to the graph called "Rear Port-Air Velocity" and there you can see the speed of the air. This is in WINISD Pro, I don't know if the alpha version does it or not.
stevdart 
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Posted: September 14, 2006 at 7:00 PM / IP Logged  

I'll add to this as well.  Refer to the sticky subject on WinISD Pro:

  • Input power as applicable.

"Use the 'Rear port air velocity' chart that I mentioned in the beginning to see what port noise looks like.

I have my chart shown set to feet/second and specified to show RMS, not peak, output.  This pic shows a port noise of 55 ft/sec, which is great for car audio because it is 5% of the speed of sound.  You should always keep it at 10% (110 ft/sec) or less.  The vent opening area will determine this noise, so making the vent smaller in diameter will make more noise, which might become unacceptable.  If you are working along with this guide, you will have to give your sub some power in order to get a port noise response.  Select the 'Signal' tab on your working box and input the sub's RMS, or the power that it will be getting, in that block labelled 'Input power'.  Now your port noise chart will jump to attention."

So to answer your question about best port diameter to use, the port air speed must be top priority.  Other factors that will influence port opening size are:

  • Available space on the box for the port
  • Room inside the box for the port length

For car audio sub boxes, the slot port is the easiest to design and build, and adds to the structural integrity of the box.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
luckydevil 
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Posted: September 14, 2006 at 11:15 PM / IP Logged  
Much appreciated guys.

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