periodic functions and phase shifts

I'm looking for input from those of you that are more experienced with enclosure design here.

I was recently looking at sine functions [don't forget, I am an Algebra II teacher] and was thinking about discussions regarding 1/4 waves, 1/2 waves and how it might apply to port tuning using the Fs spec of a sub. It appears that most discussion for enclosure design centers on a 1/4 wave length. Is this because of a 45 degree phase shift would move amplitude to its highest on a sine wave? Just wondering if that is the case or if 1/4 waves are used because 1/2 waves would be extremely difficult in most cars/trucks for even the best "port folder".

For example, consider a sub has a manufacturer's spec, or even better, tested, Fs of 28 Hz. Using 1124 ft/s as the speed of sound (this was from memory of a physics class I had 20 years ago so let me know if I'm way off here since I was too lazy to look it up before I started typing), the full wave would be approx 40.14 ft, 1/2 wave at 20.07 ft, and 1/4 wave at 10.035 ft. In this case, the quarter wave length would be tricky, but if space were NOT an issue, would the 1/2 or full wave be louder all else being the same?

My apologies if it looks like I'm just being a math dork with this one.

The way I always understood it was that you do not want the wave to crash back into itself. If that happened then the two wave would be fighting each other instead of helping each other. The measurement of the vehicle allows us to know how long the wave is. Example if a car is ten feet long, with your calculations you determine that the first frequency that fits in the car is say 100 hz. Half that and the next frequency that fits in the car is 50, then your quarter wave would be 25. Those would be the three approximate frequency's that the car will "like", because they will be less likely to clash.

I probably need to clarify some. I understand what Rob is saying about what the car likes, but I was looking at more of a hypothetical situation where there car size was not a factor.

So if the sub and port were coplanar and I wanted to use the rear wave to reinforce the front wave, would 1/4 wavelength or 1/2 wavelength be best?

In this hypothetical, the enclosure could be designed, tuned, and built and then [i]installed in the car at the proper distance from the "sweet spot".

Do some reading on transmission line enclosure. This is the exact theory they are built on.

Full > 1/2 > 1/4 .......I bet you understand that lol.

my head hurts. time for a beer

AMI CUSTOMS wrote:

Full > 1/2 > 1/4 .......I bet you understand that lol.

this. and right now my car is actually peaking at the 1/8 wave of the freq length from my sub face to kick panel. but gain again at my 1/4 wave length but isnt as strong because of my low tuning, once i tune in the 50s my 1/4 wave will jump ahead.

Does a peak on a 1/8 wave make follow theory or experiment? I know that physics theories don't always work out in real-world applications so I was wondering if this is a case where theory says the 1/8 wave should work or a situation where experimenting shows that it just does.

Again, just looking at a sine wave, it looks like the 1/8 wave would not be a big enough phase shift (only 22.5 degrees, if I'm looking at the graph correctly)to really reinforce as well as a 1/4 wave.

In regard to t-lines, what if the "enclosure" used a 1/2 wave and the baffle was placed a 1/4 wave from the metering point? Should there be cancelation issues or would there be reinforcement?

theory is simple look at a sine or cosine wave it has peaks around a center line one minimum and one maximum. you will want your wave peaks to match as close as possible. the combination of your waves will create a standing wave. which is what happens when your front waves clash after hitting the window in your car. the theory im stating is just my overview of it. so for basics try your best to get your wave lengths from your backwave to be a full wave length for full combination but if you get to 1/2 your lose alot of sound. so i suggest on the enclosure stay cclose to the 1/4 wave length or over your 3/4 wave length.

craigjones806 wrote:

Does a peak on a 1/8 wave make follow theory or experiment? I know that physics theories don't always work out in real-world applications so I was wondering if this is a case where theory says the 1/8 wave should work or a situation where experimenting shows that it just does.

Again, just looking at a sine wave, it looks like the 1/8 wave would not be a big enough phase shift (only 22.5 degrees, if I'm looking at the graph correctly)to really reinforce as well as a 1/4 wave.

In regard to t-lines, what if the "enclosure" used a 1/2 wave and the baffle was placed a 1/4 wave from the metering point? Should there be cancelation issues or would there be reinforcement?

its there, but your right, its not as strong as the 1/4 wave

I skipped that math class haha.

wow I had to sober up a bit before I even had a chance of understanding that first post.....

There are a couple of things in this thread that helped out yesterday. I know a 144 is not super loud, but for a single$ 70 sub, Lanzar power and completely stock electrical, I was pleased.

Now if I could get that race quad and enclosed trailer sold, things would change.