Go HERE for the intro and previous chapters
CH. 3 I HATE DIGGING
One of the largest misconceptions in the car audio world is regarding bass. What you ask. It’s that free-air subs do NOT need any enclosures. I am sure several of you have heard this or believe this to be true.
If you were to take this fabulous free-air sub and hang it from a rope, run an amp to it and play it, there would be NO bass. WHAT? That’s correct, try it. The car audio industries term “free-air” is horribly misleading. To help explain this we need to understand exactly how speakers produce or generate bass. Here is where I wish I had a nice drawing program…..
Sound waves consist of two main parts, positive and negative. If you were to view sound waves through an oscilloscope it would have a straight horizontal line with an equal height wave going above and below it. Above the line is the positive area where below the line is the negative area.
With all sound waves there are 2 main parts, POSITIVE and NEGATIVE. When a subwoofer cone moves out, the POSITIVE part of the sound wave is made. When the cone moves in, the Negative part of the sound wave is made. Thos 2 parts complete what is called a cycle. If that cycle is repeated say 50 times in 1 second, you would hear a very high bass note. If it was repeated say 20 times in one second, you would hear a very low bass note. This process is called “cycles per second” or more commonly known as “Hz” (Hertz). For those wondering, the human ear can detect sound as low as 20 Hz and as high as 20 KHz (Kilo Hertz).
Take a look at the picture below. It shows the cycle process related to a speaker. The front is creating a positive wave front and the back is creating a negative wave front. Now remember this, BASS only happens if those two waves are kept apart. If they are allowed to collide, you will have what’s called “bass cancellation”.
Lets discuss this cancellation more with the help of our friend Carl. We are going to compare the cycle process to digging holes. Carl is going to help us dig the holes. We take Carl out back with the shovels and explain that we are going to “dig” a 20 cycle note. So Carl is tasked with digging 20 holes 30 feet apart in a straight line. Carl begins his task and I ask, “Are you digging yet Carl?” He responds with a quick yes. I then ask, “Are you positive?” Carl then sarcastically states, “I am very positive.” Carl here is digging holes which represent the wave fronts coming off the front of the sub, the positive half. If he gets all 20 holes dug he gets to hear a great big bass note!
Carl finishes the first hole and looks at us funny. "Aren't you going to do anything?" he exclaims. "Yes we are Carl. We also have to dig 20 holes, but we can't start until you finish!" That's the way the cycle works. "Go down there and get by the spot where you're going to dig the second hole... you can start digging again as soon as we're finished!" Carl begins walking off 30 feet. While he is walking we quickly fill Carl’s first hole back up. By now you can't even tell there was a hole! Carl is now marked off 30 feet and we walk over to where he is. Carl starts digging his second hole.
Carl finishes the second hole, and leaves to start the third one. We quickly fill the hole again while he is not looking. About an hour later Carl completes his last hole, and sits down on the grass. Ok, lets hear that awesome bass note Carl demanded. Sorry Carl, it only works when you have 20 holes straight in a line, and I only see one hole here. "What?" he shouts! Yea, take a look! We filled in the entire holes one at a time right after you dug them! This was due to us being the negative wave today, and what you've just experienced is called CANCELLATION! Carl obviously didn't find our explanation very funny.
You may read or hear someone throw out a technical term explaining cancellation like: "Your woofers are 180 degrees out of phase!" That means we (the negative waves) were 180 degrees out of phase with Carl. You see, I didn't mention that we were required to move the exact amount of dirt as Carl, and each time the same distance apart. If we had only filled each hole 1/2 way with dirt, and shoveled the rest onto the grass, we would have been 90 degrees out of phase with Carl. Carl would have heard his bass note only half as loud, since all his holes are half full. Makes since doesn’t it?????
If Carl would have put us in a box, he would be able to TRAP us and keep us from canceling his holes. Therefore the purpose for a box is to eliminate cancellation and thereby permit low frequency information (bass) to exist. The moral of this story is that for a speaker to have bass, it must be in a box, or in some kind of baffle that is large enough to delay the back- wave so as not to be out of phase with the front wave. Remember this part too: “Boxless" woofers do not exist. The term Free-air & Boxless was meant to mean you can mount woofers in a baffle board located against the back seat inside your trunk rather than in a box placed in your trunk. If done correctly, your trunk becomes the box and traps the back waves of the woofers, while the front waves come through your back seat.
The difference between a "Free-Air" woofer and any other kind is a rather gray area. In general a free-air woofer will be lower compliance than usual which means it is stiffer. Since it will not have a tightly sealed box to push against, the suspension system which controls how the cone moves in and out, must be tighter. If it were not, the speaker would act like a fish out of water every time you turned it up loud.
I would probably say that 7 out of 10 free-air installations that I have seen are done incorrectly. These installs use a board with two holes in it and screw it to the back seat from the trunk. Better known as a false wall so when you drop the back seats you see the subs. But, all around the board are gapping holes from which cancellation can occur. These need to be completely sealed or like Carl’s holes when they're 1/2 full of dirt and will only live up to half of its potential.
Stay tuned for Chapter 4.....
Last edited by SeRiousZX2; 01-08-2009 at 10:57 AM.
Driving a fast Honda is like coming out of the closet. Surprised at first but in the end you're still gay.
I read these at work lol and I must say I'm very much looking forward to chapter 4. I was going to go out this weekend to look for a new sound system but now maybe I should plan things out a little better haha :P thanks for taking the time to write this up. repp'd
lol, thanks. I have Chapters 4 and 5 already typed out. I just need to post them.
If you need help or pricing, let me know.
There are currently 1 users browsing this thread. (0 members and 1 guests)