How to Measure Port Length For and AeroPort?

[B1GR3DDD]

I feel like I know the answer to this already, but I just wanted to make sure. When cutting an Aeroport down to match your calculated port length, what are you actually measuring?

http://i1111.photobucket.com/albums/h468/lolcows69/image.jpg1.jpg

Click to read more...

 

[audioholic 5,000+ posts]

Measurement A is the correct way to measure. The ends of the port are flared, to increase port diameter and decrease air-speed (and round what would otherwise be sharp edges) in order to minimize port noise. But those flared portions of the port still are part of the port itself, and must be included in "port length".

 

[Umbra]

I did a lot of research over the summer when I did my first aeroport build and I found a great site that covered this. It took a bit of google-fu to find it again, but here it is: Port Flares

They suggest you split the difference:

"A flared port shares part of the air in the flare with the surrounding air, meaning that the port appears shorter. The generally accepted adjustment is to subtract half the flare radius from the physical length to obtain the effective length. If both ends of the port are flared, the adjustment is done for each flare."

 

[audioholic 5,000+ posts]

I have read the same info, that flared ports 'share' the airspace outside the port. But even a non flared port shares *some* airspace outside the port (remember that ports move the air back and forth, not just out). I have never seen an explanation of why a 3" port flared out to 4" acts different than a non-flared 4" port (for example).

 

[Umbra]

I have read the same info, that flared ports 'share' the airspace outside the port. But even a non flared port shares *some* airspace outside the port (remember that ports move the air back and forth, not just out). I have never seen an explanation of why a 3" port flared out to 4" acts different than a non-flared 4" port (for example).

I found this on the same site:

"Subwoofer ports have to flow a lot more air than their conventional speaker counterparts. Since increasing the area of a port quickly produces very long ports, the most effective means of moving more air is to increase the air velocity.

Ports operating below about 10 m/sec generally have no problems with turbulence and compression. As velocity is increased beyond this, turbulence occurs as air exiting the port is forced to slow too quickly as it encounters the surrounding still air.

Flared ports cause the airflow to expand and loose speed in a controlled fashion, allowing higher speeds without turbulence. This method targets the air in the "boundary layer", which is close to the walls of the port.

Increasing velocity even further, the air in the "core" of the port becomes turbulent. Flares are unable to help with this problem, which represents the limiting velocity for the port. By this stage the port is beginning to present a different load to the system, resulting in de-tuning and subsequent loss of output, known as compression. "

I've never stopped to consider the "why", I've just flared my ports because I knew there was such as thing as port noise and there must be a reason all the pro audio companies do it on their speakers.

Take it with a grain of salt, by all means, but these guys have a ****-ton of data to back up their claims and have done 100x experimentation than I'd ever care to.

Either way OP measures it, it's not going to make an earth shattering amount of difference anyway.

 

[audioholic 5,000+ posts]

I found this on the same site:
"Subwoofer ports have to flow a lot more air than their conventional speaker counterparts. Since increasing the area of a port quickly produces very long ports, the most effective means of moving more air is to increase the air velocity.

Ports operating below about 10 m/sec generally have no problems with turbulence and compression. As velocity is increased beyond this, turbulence occurs as air exiting the port is forced to slow too quickly as it encounters the surrounding still air.

Flared ports cause the airflow to expand and loose speed in a controlled fashion, allowing higher speeds without turbulence. This method targets the air in the "boundary layer", which is close to the walls of the port.

Increasing velocity even further, the air in the "core" of the port becomes turbulent. Flares are unable to help with this problem, which represents the limiting velocity for the port. By this stage the port is beginning to present a different load to the system, resulting in de-tuning and subsequent loss of output, known as compression. "

I've never stopped to consider the "why", I've just flared my ports because I knew there was such as thing as port noise and there must be a reason all the pro audio companies do it on their speakers.

Take it with a grain of salt, by all means, but these guys have a ****-ton of data to back up their claims and have done 100x experimentation than I'd ever care to.

Either way OP measures it, it's not going to make an earth shattering amount of difference anyway.

Ultimately, you and I agree. Its not going to make an earth shattering difference.

A flared port decreases port noise (turbulence) in 2 ways. One, it decreases air velocity right at the port mouth. And two, it softens the angle at which that air must flow over.