excursion above Fb

[Beatin' 10+ year member]

I understand that subs put in overly large boxes will lose output above Fb. And if the box is too small, you will gain output above Fb.

Doesn't the excursion INCREASE as the box gets SMALLER, and the excursion DECREASES as the box gets LARGER?

From what I understand, if the box is too large, the woofer relies on mechanical suspension to control the cone. The motor will struggle to get the cone moving in the opposite direction because of the contending momentum. This leads to the signal being fed into the voice coil and the woofers position being out of phase somewhat, which leads to less excursion.

If the box is too small, the air spring controls the woofers momentum better than the mechanical suspension or motor force. This way, the motor does not have to struggle slowing down the cone to change its direction of movement. As soon as the signal phase changes, the cone has already been deaccelerated by the air spring, and therefore would be more in phase in movement with the signal, giving the cone larger excursion.

Discuss......

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[ciaonzo 5,000+ posts]

Let me ask you this - With an arbitrarily chosen amount of power, say 250w at a static frequency of 20Hz, will a speaker with high compliance move further in a tiny enclosure or in free air?

 

[Beatin' 10+ year member]

Let me ask you this - With an arbitrarily chosen amount of power, say 250w at a static frequency of 20Hz, will a speaker with high compliance move further in a tiny enclosure or in free air?

I'm not talking about 20hz. I'm talking about the notes around 60-100hz, which are routinely above Fb, which is the resonant frequency of a sub in a box.

I already know small boxes choke output below Fb.

 

[ciaonzo 5,000+ posts]

Doesn't the excursion INCREASE as the box gets SMALLER' date=' and the excursion DECREASES as the box gets LARGER?
[/quote']This notion is incorrect. ^^^

This is why I asked you that question.

 

[Beatin' 10+ year member]

This notion is incorrect. ^^^
This is why I asked you that question.

so why does output decrease ABOVE Fb when the box is made larger than recommended, and increase when the box is made smaller?

It's no surprise that the air spring pressure has greatly to do with it. The real question is whether phasing between the cone position and signal causes more or less excursion.

 

[bbeljefe]

What do you mean when you say output decreases above Fb? More often than not, output is flat above Fb.

So let's clarify things before we start debating with different definitions.

If the reference is flat, then output above flat is an increase and output below flat is a decrease.

In most ported enclosures output at and just around Fb is increased and the further above Fb you go, the closer it gets back to flat until it is again flat. Consequently, output below Fb tends to fall off more quickly and always decreases dramatically below flat.

With that said, output always falls above Fb in a vented enclosure (flat responding designs notwithstanding). If it decreases above Fb (below flat) then the box is poorly designed.

Also, how do you define "recommend"? I ask because that is an entirely subjective term. Recommended by whom and for what purpose? Manufacturers usually recommend a box that blends multiple concerns... mostly the safety (power handling) of the sub, overall sound quality (or pressure) in a variety of environments and of course, space constraints... in that order of importance. Myself, on the other hand, might tend to recommend a box that takes the most advantage of sound quality to one person but to another, I might recommend a box that takes the most advantage of sound pressure. Still to other customers, I might recommend a box that takes the most advantage of power handling... all depending on the user's, level of competence/knowledge, their goals and the amount of space they have to work with... and again, in that order of importance.

Regarding excursion, more volume equates to more excursion and less volume to less. Small volumes can restrain excursion that could be beneficial (think sealed box) and large volumes can rob the sub of much needed acoustical excursion control (think cheap subs). All of this happens to one degree or another at all frequencies above, at and below Fb and all of it is dependent on the sub, the alignment, the volume and the amount of power per application.

 

[Beatin' 10+ year member]

when I say "recommended" I really meant a QTC figure of 0.707 or lower.

And I'm talking about subs put in sealed boxes.

 

[bbeljefe]

when I say "recommended" I really meant a QTC figure of 0.707 or lower.
And I'm talking about subs put in sealed boxes.

Okay. Will you answer the first clarification?

 

[ciaonzo 5,000+ posts]

so why does output decrease ABOVE Fb when the box is made larger than recommended' date=' and increase when the box is made smaller?
It's no surprise that the air spring pressure has greatly to do with it. The real question is whether phasing between the cone position and signal causes more or less excursion.[/quote']It's relative. Output above Fc (not Fb) does not decrease when enclosure volume is increased, the lower frequencies become more efficient.

 

[Beatin' 10+ year member]

It's relative. Output above Fc (not Fb) does not decrease when enclosure volume is increased, the lower frequencies become more efficient.

Then how come when subs are put into an ultra small box, they become peaky in the mid bass region? The subs gain more in that frequency range of 80hz+. What causes that increase, more or less excursion?

 

[ciaonzo 5,000+ posts]

Then how come when subs are put into an ultra small box' date=' they become peaky in the mid bass region? The subs gain more in that frequency range of 80hz+. What causes that increase, more or less excursion?[/quote']You are simultaneously choking the low end and increasing (narrowing) the overall Q of the system (driver and enclosure). In other words, you are placing the center of the system resonance at a higher frequency and reducing bandwidth. It only makes sense that it would sound like that.

 

[ciaonzo 5,000+ posts]

Then how come when subs are put into an ultra small box' date=' they become peaky in the mid bass region? The subs gain more in that frequency range of 80hz+. What causes that increase, more or less excursion?[/quote']But this would also depend on the Vas and Qts paramters. A driver with very low Qes figure (very damped) and a tiny Vas figure (low compliance, again very damped) will want to see that tiny enclosure and may still end up being .577 Qtc as a system. Not peaky at all.

 

[T3mpest 10+ year member]

Then how come when subs are put into an ultra small box' date=' they become peaky in the mid bass region? The subs gain more in that frequency range of 80hz+. What causes that increase, more or less excursion?[/quote']
Output in a sealed box is totally dependent upon it's excursion. If you used a in a open field for example and know it's SPL at a given frequency and distance away, you can calculate how far the speaker must be moving to produce that SPL. Without a port there are no other variables, excursion=output.

What your seeing is the effect of FB of a sealed enclosure. Once the speaker goes into the enclosure it's the mix of the suspension and air suspension of the box that dictates the systems FB. Using a very small box pushes that resonance up high enough that you see a peak response as a speaker is more effecient at tuning, just like it would be with a ported tuning. Also, just as a ported box tends to get louder the higher you tune since you gain more output at tuning as you tune higher, same is true sealed. Once the FB of the sealed enclosure drops low enough, it provides basically no increase in SPL at it's tuning. As the box size apporaches infinity, the overall Q of the system drops to the overall Q of just the speaker as well as the systems FB approaches the fs (as fs is tuning in free air). The general upward slope is 6db/octave rising up around fb.

Regardless of this, the main thing to rememeber is this hump is never in the subbass frequencies. A sub's fs along with the slow gain along with the reduced output at the speakers FS as the box gets smaller makes this gain very subtle. A sealex box rolls off 12db/octave down low so a 6db rise as you head upwards when your already outside the subbass region doesn't do much. The main effect of too small a box for a subwoofer is simply reduced output down low as the box is less effecient. However for midbasses you can use this effect to your advantage somewhat if you need a peak, I know my last car had a null in the midbass and using a very small box for my midbass allowed me to cater the response to fill in the gap using the gain due to a very high Q and FB.