Subs and Tuned port

[efm80 10+ year member]

If your box is tuned at 33hz does that mean your subs will not hit anything lower than that frequency???? Also what does tuning the box at a certain frequency really mean and do????!!!!

eddie

Click to read more...

 

[Immacomputer 10+ year member]

The tuning frequency is not the lowest frequency that the system is capable of reproducing. But, the "loudness" will start to decrease pretty rapidly below tuning and playing too far below tuning will make the sub appear to be in a free air environment (bad for output and for mechanical power handling).

Ported enclosures work very much like blowing over the top of a coke bottle. In a bottle, there is a radiating surface area (the air on the surface of the bottle opening and radiating means that it is moving in a wave like pattern) that goes from the top down through the throat area. When the bottle opens up to the body, that is another volume of air. We'll call it the net volume (like the net volume of an enclosure). The other air mass is the port. In case that isn't clear, look at this pretty picture:

When you blow over the top, you push down the port air mass and it pushes against the net volume of air in the bottle. That net volume of air acts like a spring and pushes back on the port air mass. Then the air from your mouth pushes the port air mass back down and the process repeats. The radiating air mass of the port works just like a speaker and creates sound. That sound will be based off of the frequency at which the air mass moves up and down. That is the tuning frequency of the system.

Have you noticed that when you drink more coke that the frequency gets lower? That's the same idea with a ported enclosure. Both ported enclosures and coke bottles are Helmholtz resonators. The speaker acts like the wind did when you blow over the top of the bottle and it compresses/expands the air mass in the enclosure to push/pull on the port air mass. The more net volume of air with the port air mass being the same means that the tuning frequency gets lower. This is because the spring force of the larger volume of air is less and so the restoring force of the air mass gets delayed making for a lower frequency. Making the air mass smaller by shortening the length will make the tuning frequency higher. Reducing surface area while keeping the same length and net volume will cause a lowering of the tuning frequency. Each of those three parts will determine the tuning frequency.

As your speaker starts to play frequencies approaching the tuning frequency of the enclosure, the speaker will begin to see a high acoustical impedance and the cone movement becomes less and less. While the cone begins to move less, the port air mass begins to move more and more reaching a peak when you're playing the tuning frequency and the speaker movement is at a minimum. The cone is pretty well controlled about 1/2 octave above and below the tuning frequency. So if the tuning frequency is 33hz, the cone will have reduced movement and the port will be excited from about 25hz to 41hz (8hz above and below because 1 octave is roughly 17 so 1/2 octave means about 8.5 or 8).

Now, not all speakers will sound the same in the same enclosure, even if they are tuned and sized exactly the same. This is because every speaker has different electrical and mechanical parameters. These parameters will decide how the sub/enclosure system will sound. Some combinations may be able to play very well right at the tuning frequency and some combinations may not be able to to perform well at or around the tuning of the enclosure.

 

[TheCarFather 10+ year member]

^^sick post //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif