Flared vs. Non-flared ports - An experiment

[Triticum Agricolam]

So I think a lot of ported boxes are built with undersized ports and this is why I'm always trying to figure out just how much port do you need to have? In the past I've talked about Small's formula that uses driver displacement and tuning frequency to determine port area needed to keep you under the dreaded 17 m/s of port velocity. My big question is what effect does flaring the ports ends (or aero ports) have? I know it allows you to have higher velocities, but just how much? This is what I wanted to find out.

To find out here is my testing apparatus:

I used an old 1.5 cu ft box I had built before I knew what I was doing, a cheap MCM 8" driver, and flared and unflared round and slot ports I can attach to the outside of the box.

Here are what my round ports look like. Its kinda hard to tell in the picture but the one on the left has a 3/4" round over on both ends giving it a nice flare.

I purposefully undersized my ports by a significant amount. I didn't want to have to work hard to get port noise, and during the test I certainly didn't. All the ports are tuned to 35 Hz and they all have pretty much the same area. The flared ports are a little longer to take into account the flare and still keep the tuning the same. To do the test I played a 35 Hz tone through the driver and slowly turned up the gain until I could just start to hear chuffing. I then recorded the voltage going to the driver. I also turned up the gain until the port noise was fairly noticeable from a couple feet away and recorded the voltage. Here is what my setup looks like during a test:

So what were the results? The non-flared slot port was the first to start making port noise, which comes as no surprise with its sharp edges and corners. The non-flared round port took a little more power but not much. The flared ports both took quite a bit more power before they started making noise. Turns out you can put about 2.5 times as much power into a flared port vs a non-flared port before you get any port noise.

So 2.5 fold increase of power will increase your port velocity by about 37%, so port that would start making noise at 17 m/sec can now flow a little over 23 m/sec. Knowing this I should be able to reduce port size by 37% if I use well flared ports. Now we are getting somewhere!

While I definitely learned some interesting info from this test, it also left me with more questions. While I can measure how much power increase it takes to make a flared port chuff I don't have a way of knowing how many actual watts it took or what my actual port velocities were and I really wish I did. I know that when it comes to fluid dynamics things don't scale linearly with size, just because a 2" port chuffs at 17 m/sec doesn't mean a 4" or 6" port will. This is why I think Small's formula, while certainly a good starting point, gives us a little too simple of an answer. Really just how much velocity can I push through a 75 sq. in. 4.5" wide flared port before I get a noticeable amount of port noise? I suspect its a lot more than 23 m/sec. I guess finding a way to figure that out will be my next goal.

Anway, I hope you folks at least found this interesting, if not useful in some way.

[HHR Ed]

Very nice. Thank you.

It may also be useful to show how this effects frequency response on an RTA and also SPL.

Do larger ports actually make a woofer louder?

[Souldrop]

Awesome post! I'm also curious as to whether or not dimpled flares help,

[mrd6]

Interesting test...did you know you can buy relatively inexpensive meters that can measure air velocity?

I don't know how well they would work at such a high alternating rate of wind direction but the right meter could be a very cool addition to you testing.

[Triticum Agricolam]

Very nice. Thank you.

It may also be useful to show how this effects frequency response on an RTA and also SPL.

Do larger ports actually make a woofer louder?

A calibrated mic is at the top of my wishlist. I'll be able to test a lot of this stuff then. If a port is undersized I can definitely see where making it larger would get you more output, but once its big enough to do the job properly I don't think any more size is going to get you any more sound volume.

[Triticum Agricolam]

Interesting test...did you know you can buy relatively inexpensive meters that can measure air velocity?

I don't know how well they would work at such a high alternating rate of wind direction but the right meter could be a very cool addition to you testing.

Yeah I actually have a cheap wind meter, but since the air in the port is changing direction 70 times a second I don't think it would work.

[Souldrop]

I'm curious why you chose to do the experiment with respect to power? Maybe I'm overcomplicating the relationship between power and excursion. Also I would imagine you could do a dirty calc of port velocity from simple fluid equations. A_subV_sub=A_portV_port. It is a bit tricky figuring out the max velocity of the sub, but I'll think on this while I go to the grocery store.

[alaskanzx5]

Interesting.

Another question is how much of a flare do you need to stop port noise. I know you did a 3/4 round over but would a bigger flare improve it more?

Also this testing can help determine if a flared square port will or will not be as effective a flared round port.

I've seen a lot of people saying round Ports.with flares are more efficient then square Ports so they need less port area.

[Triticum Agricolam]

I'm curious why you chose to do the experiment with respect to power? Maybe I'm overcomplicating the relationship between power and excursion. Also I would imagine you could do a dirty calc of port velocity from simple fluid equations. A_subV_sub=A_portV_port. It is a bit tricky figuring out the max velocity of the sub, but I'll think on this while I go to the grocery store.

I chose to do my measurement based on power because it was an easy thing for me to measure. I would really like to know what the port velocities were though. If you can come up with a way for me to figure that out I'll go back out to the shop and re-test!

[Souldrop]

Nvm on why you chose power to measure the effectiveness of the different ports. I see now that it would be a huge PITA to do it any other way. .