Port compression and optimization

[DubNDodge]

tuned

[Triticum Agricolam]

Well I was able to get outside and do some testing today. The wind came up after a while, so I wasn't able to get as much done as I would have liked, but I did make some progress. First off here is a shot of the test setup:

The measurement mic was about 6' away from the test box. That was as close as I could get it without clipping the mic on the higher power sweeps.

So on to the results, I did 20-150 Hz sweeps with each of the three ports at 3, 4.5, 6.3, 8.9, 12.6, 17.9, 25.3, 35.8, & 50.6 volts of input power. That works out to 2.5 - 640 watts at 4 ohms. After measuring I offset the graphs in 3 db increments (since my voltage increase increments were 3 db each). This brings the lines so that they are pretty much at the same level at 50 hz. The differences in the lines below 50 hz is due to port compression. Here are the graphs:

Dual 3 in ports:

4 in port:

5 in port:

I measured the output levels at 34 hz relative to the 2.5 watt line (which shouldn't have any compression) to try to measure how much output was lost. I also used WinISD to get a rough estimate of what the port velocity would have been at the various output levels. Here is all of that info:

Conclusions:

So looking at this data the first thing that jumps out at me is port compression can really hurt output. The 4" port was showing losses of over 7 db. I also noticed that the amount of port compression was somewhat similar between the ports when comparing levels where the port velocity was similar. For example, with port velocities around 32-36 m/sec there was about 3 db of compression losses, that's a pretty big hit and is like cutting your input power/efficiency in half. I know there are a lot of boxes out there with port velocities over 30 m/sec too.

Looking over the data it appears that port compression begins at a pretty low port velocity. Almost immediately really. It seems to scale in a fairly linear fashion as port velocity increases. At around 18 m/sec, which was when I could first notice port noise, there is probably already a little over 1 db of port compression taking place. By the time you get to around 25 m/sec of port velocity compression is around 2 db.

Next steps:

I would like to test the three different ports at whatever voltages they need to give me 40 m/sec of port velocity through all three. I would like to know if the amount of compression is similar at similar velocities despite the difference in port diameter. I'd also like to get some impedance sweeps at various input power levels to see if I can detect port compression just from the impedance. So these two tests on next on my to-do list. Hopefully I'll get more cooperative weather next time too.

If there is anything any of you would like me to try out just let me know and I'll see what I can do.

[Patrick Bateman]

Great work.

A couple of random observations:

1) a bandpass box has some advantages over a ported box here. Although the efficiency of both boxes are equal, the bandpass has the advantage of being tuned higher. So that will reduce the required port size. (Since the higher a box is tuned, the less port you need.

2) I think one of the reasons that tapped horns work so well is that the entire box is basically a port (albeit one tuned to a quarter wave.)

[Triticum Agricolam]

Great work.

A couple of random observations:

1) a bandpass box has some advantages over a ported box here. Although the efficiency of both boxes are equal, the bandpass has the advantage of being tuned higher. So that will reduce the required port size. (Since the higher a box is tuned, the less port you need.

.....

You might want to recheck that part. The higher a box is tuned the MORE port area you need.

I agree with you on about tapped horns. The biggest limitation with them is their size though. Designing a tapped horn that will have greater or equal efficiency compared to a ported box of similar size at, say, 35 Hz is quite the challenge.

[Triticum Agricolam]

Well I got some time today to do some more testing. Here is what I found.

I tested the dual 3", single 4", and single 5" ports at the required power level to create 30 and 40 m/sec port velocities. I wanted to see if the different ports had different amount of compression despite having the same high port velocity.

The single 4" port had:

2.6 db of compression at 30 m/sec

3.3 db of compression at 40 m/sec

The dual 3" port had:

2.1 db of compression at 30 m/sec

2.9 db of compression at 40 m/sec

The single 5" port had:

1.7 db of compression at 30 m/sec

2.4 db of compression at 40 m/sec

So the trend seems to be as port area increases the amount of compression at a given port velocity goes down. This is good new since most boxes have port much larger than what I'm using in testing.

In another thread there was some discussion as to whether port area changes frequency response. I put together another port for the test box to see what happens. My single 4" port has the least amount of port area of my current ports (12.54 sq in). I build a port with two 5" round ports and tuned it to exactly the same as the single 4" port (32.5 Hz) As a side note, the 4" port was 33.5 Hz a couple weeks ago, its interesting that the tuning changed whole Hz from the last time I did testing, presumably due to a change in air density from temperature or barometric pressure. Anyway, the dual 5" port has just about 40 sq in of port area, which is over 3 times what the single 4" port has. Here are the results:

The red line is the 4" port and the green line is the dual 5" port. There was no smoothing applied to the graph, that's raw output. As you can see they are pretty much identical. Hopefully this lays to rest the misconceived notion that larger ports are more peaky and smaller ports are more "musical".

Next up - now that I have a pretty reliable way of testing for port compression I'd like to do some in-vehicle testing on a few systems. I'll be curious to see if I'm able to measure any, and just how much port compression there is with some real world examples.

[MrSevs]

Very interesting!!!! As I get more and more into this hobby, all these tests and observations help answer so many questions I start to have. So many think its as easy as putting a "speaker" in a "box" and then putting into your vehicle.

[bkolfo4]

Funny - I have been trying to tell people this for years.

Another unrelated but great test since you have everything setup would be to show people that tuning at 28 vs tuning at 40 does not cause you to lose output at 150 Hz. There continues to be this idea that tuning lower causes you to lose bass in the upper frequencies.

Of course if you tune high enough, the upper frequencies could become louder as they get into the peak of tuning higher, but the point really is that tuning lower does not cause the upper bass frequencies to magically roll off

[Triticum Agricolam]

Funny - I have been trying to tell people this for years.

Another unrelated but great test since you have everything setup would be to show people that tuning at 28 vs tuning at 40 does not cause you to lose output at 150 Hz. There continues to be this idea that tuning lower causes you to lose bass in the upper frequencies.

Of course if you tune high enough, the upper frequencies could become louder as they get into the peak of tuning higher, but the point really is that tuning lower does not cause the upper bass frequencies to magically roll off

You are totally right. To kind of expand on that, I don't think most people realize once you get about an octave or more above tuning ported boxes have the same output as a sealed box of equal size too.

If I get this test box out again I can test that too for folks if they are interested in seeing it.

[kirill007]

Hopefully a lot of people will see and learn from this thread.

Ports apply to general fluid/air dynamics rules, I assume you already know why the larger ports have a lower port velocity when using the same port area.

[bkolfo4]

Have a way to measure excursion?

Would be nice to show people this difference in excursion with different amounts of port area (above tuning of course).