Porting Test

[gpw5150]

So, I had a little time today and decided to see how my Ashdown 408 cab sounded ported.  The easiest way to do this was to unscrew and remove the speaker input plate and leave it hanging out. 

So, as a 4x8 sealed cab using very middy TF0818 drivers, the bass is there, but farts out fairly easily - the mids are more pronounced and accurate as you would expect.  The drivers only have an XMAX of 2.5mm.....

Undo the speaker plate and a different journey begins

The bass appears more pronounced and the mids die back.  You can physically see the drivers really being pushed harder for the same input values and the cab farts out even easier on the low notes - I guess the driver is maxing its excursion.

Not terribly scientific, but I certainly learned a thing or to in terms of my cab refurb!

[ahpook]

Always good to tinker

[Twincam]

The reason the cab farted out earlier was because there was now just a hole in it which acts very different from a tuned port. If you put your hand over the hole the air will move quickly in and out. On a tuned port that doesn't happen. 

While if there was a properly tuned port it would sound much better and there would be more bass/power handling. With the same drivers so likely you could make a proper porting and use the current drivers. But as you know from the modelling software better drivers and porting will be better. The cheapest option is to try self porting first making sure the port velocity is ok.

[Chienmortbb]

4 hours ago, Twincam said:

The reason the cab farted out earlier was because there was now just a hole in it which acts very different from a tuned port. If you put your hand over the hole the air will move quickly in and out. On a tuned port that doesn't happen. 

While if there was a properly tuned port it would sound much better and there would be more bass/power handling. With the same drivers so likely you could make a proper porting and use the current drivers. But as you know from the modelling software better drivers and porting will be better. The cheapest option is to try self porting first making sure the port velocity is ok.

Air moves out out rapidly from a tuned port as the back of my leg can testify. A hole in the cab could be a tuned port bit it is unlikely to work that we as the port length is only a few mm

[Bill Fitzmaurice]

6 hours ago, gpw5150 said:

 You can physically see the drivers really being pushed harder for the same input values and the cab farts out even easier on the low notes -

With a properly tuned port cone excursion will be less, not more.

[Twincam]

2 hours ago, Chienmortbb said:

Air moves out out rapidly from a tuned port as the back of my leg can testify. A hole in the cab could be a tuned port bit it is unlikely to work that we as the port length is only a few mm

I think this on the barefaced site explains it nicely. 

https://barefacedbass.com/technical-information/the-mysteries-of-ports.htm

What you might feel from a cab is soundwave presure but not air rushing in or out of a port. 

[gpw5150]

21 minutes ago, Twincam said:

I think this on the barefaced site explains it nicely. 

https://barefacedbass.com/technical-information/the-mysteries-of-ports.htm

What you might feel from a cab is soundwave presure but not air rushing in or out of a port. 

Isn’t soundwave pressure...air?

[Twincam]

12 hours ago, gpw5150 said:

Isn’t soundwave pressure...air?

Not exactly it is a wave form/vibration. Sound travels through air but it's not air itself. That's also why you can hear under water. Or why you can hear sounds, in say a block of wood being tapped with your ear held against it. 

Click your finger close to your face, you won't feel that much air from it. Then blow on your hand. Which is louder and which do you feel more?

The sound of a port is not air rushing in and out. Just like the drivers themselves don't blow out air.

[Phil Starr]

A ported cab works as a resonator. A Helmholtz resonator to be precise. Blow across the mouth of a bottle and it makes a note, half fill it with water and it'll make a higher note, find another bottle the same size with a different size neck and it will make a different note again. The air in the neck is bouncing on the air in the bottle and that is what makes the noise. In a ported cab the air in the port bounces on the air in the cab at the resonant frequency. At that frequency the port will make a sound, above that frequency the air won't move and the port won't do much and below that frequency the port is just a hole letting air in and out. The trick is to tune the cab so that it gives a little boost in output just as the speaker is starting to fade in it's bass so you get a bit of 'free extra bass' just as the speaker needs it. (a little more complex than this of course but this is the basic idea) Another  advantage of all that resonance is that it creates a back pressure on the cone and stops it moving much so it won't move into the distortion and damage area as easily.

In the case of your speaker it was almost certainly tuned too high, so you got the bass boost in the wrong place, lot's of uncontrolled extra high bass which you heard. Below that the hole meant the air was passing through the hole and that is why it was distorting and the cone was flapping around.