Audio reproduction lesson #2

Discussion in 'OT Technology' started by 04, Nov 27, 2002.

  1. 04

    04 Guest

    In our second lesson about audio reproduction, we are going to cover loudspeaker enclosures.

    I see so many posts on OT about people asking what kind of box for their car would be, sealed or ported, or even bandpass. And the answer to that question is, it depends! There is no such thing as a perfect box.

    There are two major catagories for enclosure design, building housed, or automobile housed. Most of you guys build boxes for cars, so i will pretty much cover those. For you home audio lovers, don't fret, I will cover home audio aspects tommorow!

    Typically there are two different crowds. One crowd wants loud bass, and the other wants accurate music reproduction. And this is fine, different people have different tastes and preferences.

    First we will cover the Loudness crowd. For you guys, you would probably be best off with a vented(ported) design. The reason for this is obvious. The vent allows an additional 3dB of output, so it's like getting output for free. However there is a downside to this. What happens is that after you get to what is called the "system resonance" the output starts dropping like a rock, and you typically need to insert a type of filter to make sure the woofer does not overexcurr (move too much) at these low frequencies. With a vented box, you can go with either a design that extends super super low in frequency, or get one that peaks at a certain frequency such as your car's resonant frequency, for applications such as sound offs. Most people go for a little bit in between. It has a bit of a peak, but still goes pretty low, and gets pretty loud! Also note that boundry reinforcement is your friend for SPL. It is covered in the Sound quality section, so if you want more info on it look there. But know that the boundry reinforcement is what allows you to hit such astronomical SPL numbers. In an open field, there would be no way you would hit that high of a score.


    Next, we will cover the Sound Quality crowd. In this I am refering to the guys who like bass that is smooth, not overexaggerated, and matches up with the entire musical spectrum. For you all, the best enclosure would probably be sealed. In a car this works out extremely well. The cabin of the car being a constrained space allows the many surfaces in the car to provide what is called "boundry reinforcement" Boundry reinforcement basically means that surfaces will make a system 3dB more efficient for each boundry you have because it loads the acoustical output and only allows it to radiate in half the space as before. So, 2x the "sound" in one space, means double the SPL, and guess what? You get a 3dB gain for each boundry! In most typical cars, this gain starts at 60 or 70hz and gives an approximate increase of 12dB an octave. Which coincidentally is the same rate of falloff for a "max-flat" sealed enclosure with a system damping of .707. So lets say you have a woofer that has an f3 of 60hz in an enclosure with a system damping of .707. Lets also say your car is a bit larger sized sedan with cabin gain kicking in at around 60hz at 12dB an octave. Theoretically, you would have accurate, flat bass all the way down to 20hz or lower!

    With a smaller sealed enclosure, you will get several different things. First, the system will become less efficient. Second, the system damping will decrease. Here I will explain system damping, and how crucial it is to sound quality. If you have a highly damped system, the enclosure has good control of the woofer. Meaning, if you suddenly turned the music off, the woofer would stop almost instantly, and would not keep "ringing" for the lack of a better term. With smaller enclosures this problem becomes worse. The damping is increased, and the enclosure does not have as much control on the woofer, and the ringing is more pronounced. This is why I find it so odd that people think that small boxes make for "tight" bass. Indeed the exact oposite occurs, the larger the box, the tighter the bass. This also may show why many people think large diameter loudspekers such as 15" and 18" sound Sloppy! The large woofer is put in an enclosure that is too small for it, and it causes excess ringing, or sloppiness.

    So This is not to say that low low Q enclosures are the best (the large sealed ones) for sound quality. you have to match it for your vehicle. If you have a small hatchback, your boost would start even higher in frequency, meaning that you would probably not want to make a huge box, as the low low frequencies would probably get exaggerated. On the other end of the spectrum, if you have a large SUV like an Expedition, it would probably not be a wise idea to go with a small sealed, high q enclosure, or your woofer will not extend as low as you might like.

    Now, that isn't to say that a ported box won't work well in a sound quality situation, and a sealed box wont work well in a space limited SPL install. But it is a lot harder to get a vented box correct, and to match it to your car for a flat frequency response can be a bit tougher. BUT, it is typical that a ported box will sound better and is easier to integrate into a larger SUV type vehicle than a small automobile. It is also hard to get as much SPL out of a sealed box as it is a ported one. So keep these things in mind when picking out which box would be right for you.

    As for bandpass, it is much more complex to design, and I would not reccomend it for beginners at all. Until you realize the relationship between the speaker and the enclosure, I would not reccomend trying such an advanced design. This also goes for horn loaded, transmission line, aperodic bi-chamber, etc.

    Tommorow I will cover acoustics and how they work in your house!
     
    Last edited by a moderator: Oct 13, 2003
  2. poisonfist

    poisonfist Guest

    You are correct about the efficiency of the driver dropping in smaller boxes but I believe you are misinformed here. The QTC (total Q when a driver is enclosed) will decrease represented as a number in a smaller box but this means that the damping increases (transient response will increase as well, obviously). You have to think of the trapped air inside an acoustic suspension enclosure as a spring, and the driver installed as a weight suspended on this spring. A larger enclosure represents a longer spring with less control and less transient response as a smaller enclosure will represent a shorter one. A smaller enclosure will always result in a tighter bass.
     
  3. 04

    04 Guest

    Re: Re: Audio reproduction lesson #2

    I don't think I am following your basis for tight bass in a smaller enclosure?

    Better damping means that their is better control over the driver. Therefore, there is less "overhang" with the larger enclosure.

    You have less control of the woofer at resonance with a smaller box.

    Also, I am curious where you got your information from? I have never read anything that indicated anything above.

    Infact, it seems as if you have got the two mixed up. (I used to get them messed up as well). If you have a copy of the 'Loudspeaker Design Cookbook', check it, it will reveal that lower Q enclosures result in less "boom".

    Then again, it also depends on what your definition of "tight" is. I suppose a few people define "tight" as enhanced output in the kickdrum region....
     
  4. poisonfist

    poisonfist Guest

    Absolutely not, I am not confusing anything at all. I do not mean "tight" as in enhanced kickdrum region. I am educated in audio electronics/engineering and acoustics, from world reknown audio engineer David Moulton (you'll sometimes find articles by him or about him in professional recording publications). I am also an avid stereophile, and audio electronics enthusiast. I have built numerous speaker systems as well as tube and solid state circuitry.
     
  5. 04

    04 Guest

    :(

    Ok, but I still don't see why you are saying that the high QTC enclosure will sound tighter? I have never heard that before, at least not from anyone educated... What if it was excessively high, like 1.2 or more?
     
  6. 04

    04 Guest

    Can you provide any information proving your point? Like I said, I have never seen anyone or anything say that a higher Q enclosure would have tighter bass.... TIA
     
  7. poisonfist

    poisonfist Guest

    No no, the Q decreases in smaller boxes, Q represented as lower numbers = more damping factor. Q of 0.6 (smaller boxes) is a higher damping factor than Q of 1.0 (larger box).
     
  8. poisonfist

    poisonfist Guest

    I think what you're confused with is you think that Q rises in smaller boxes, when it's the contrary.
     
  9. poisonfist

    poisonfist Guest

    I must apologize, I went over my equations and I must say I am the one that was getting confused. You are absolutely right, a smaller box will result in a higher Q with less damping factor than a larger box. I haven't gone over these equations myself in a while and was only thinking from logic (which was obviously flawed). It is also true though, that a driver becomes closer and closer to an infinite baffle setting (no damping) when the enclosure becomes overly large. Again, I will apologize for the chaos and confusion I created here in this thread. I guess I'll have to go back and read all the books again, I am getting a bit rusty.
     
  10. 04

    04 Guest

    NP, heh, I was beginning to think for a second there that I was wrong somehow.... :o
     

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