n00b: What is Tuning?

[frikk 10+ year member]

I have been designin my box via manufacturers specifications. My box says that I will have a -3dB dropoff at 35Hz, and a Tuning Frequency of 36 Hz. I understand the -3dB, but what is the Tuning Frequency? Also my sub has a resonant frequency (Fb) of 37.4 Hz. I understand that this is the frequency where the sub resonates in free air. But how do I use this? Do I try to get the box tuning frequency as close to the resonant frequency as possible? This seems like it would tear everything apart. I also see that according to the calculators out there, I can adjust the port length and change my tuning frequency. Cool, but why?

Also - what is tuning exactly? I am basically just looking to find out what I am supposed to do once I have my box, port length, etc. configured so that it is at a specific tuning frequency, and how this relates to my sub's resonant frequency.

Thans in advance,

Blaine

//content.invisioncic.com/y282845/emoticons/fro.gif.c695f1f814b01c4ad99fe7f8cccadd29.gif

Click to read more...

 

[Immacomputer 10+ year member]

Fb is the tuning frequency of an enclosure. Fs is the tuning frequency of a driver in free-air. The resonant frequency of a driver is going to change when it gets put into an enclosure. The Fs of a driver is the highest point of impedance seen while playing free-air. The tuning frequency of an enclosure is the point where the impedance of the enclosure is at it's highest. That does not mean your amp will see the highest impedance from the sub at the tuning frequency of the enclosure. It is the point where the sub will see the highest impedance though.

At and close to the tuning frequency, the enclosure begins to resonate like a helmholtz resonator. This phenomenon is just like when you blow over the top of a coke bottle. There are three points of interest here. One is the enclosure net volume of air. This volume of air is like the wide and bottom end of the bottle. The other point of interest is the neck of the bottle that holds a column of air. The third point is the area of this neck. When you blow over the top of the bottle, you are creating a change in pressure that causes the air in the neck to be pushed into the air that is in the bottle. This air in the bottle compresses like a spring and starts exerting a force back against the air in the neck. The air in the neck then begins to travel back up the neck and is then met by you blowing over the top of it. This makes it oscillate, which creates sound. The tone you hear is the tuning frequency. If you make the air in the bottle larger (by drinking more coke) you will lower the tuning frequency (make the note lower) because the air spring in the bottle is not as strong and therefore it takes longer to compress. If you increase the length of the neck, you increase the air mass traveling over the same area and it can compress the air in the bottle farther, which will also lower the note. If you increased the area on the neck of the bottle, you would raise the tuning frequency due to the force of the air in the neck being distributed over a larger area of air in the bottle.

So now that you know how a resonator works, it's easy to apply it to a ported enclosure. Your sub now becomes your mouth blowing air over the port. Only this time, you're compressing the air in the box with the sub and that compression and decompression is moving the air mass in the port. This is what gives you your output. When you increase the frequency that the sub is playing, the port becomes less and less effective as the enclosure is moving out of resonance. Now the sub will see a large impedance in the box around tuning. This will cause the cone to be controlled and excursion will not be very high. The sub will have a new resonant frequency now that it is in the box. It will usually be a little bit higher than the tuning frequency of the enclosure but not always.

You should design the enclosure (tuning, port area, port length, size) and sub setup around your vehicle, your desired frequency response, and musical tastes.

 

[frikk 10+ year member]

oh my gosh man - thank you so much for that explanation. Thats beautiful. Can I give rep? I would if it were available!

Thanks again!

 

[appleyard 10+ year member]

ok noob question

Why do you need more xmax to produced lower freq (at same Db) compared to higher ones? Does it mean you need to compress more air to produce low freq?

 

[frikk 10+ year member]

If i have a 4" straight port vs a 4" tapered port - whats the difference?

Blaine