Back EMF - Sealed Enclosures

[62AustinHealey]

I have heard that Subs last longer in a sealed box due to the acustic suspension effects that limit the cone excursion.

However, doesn't cone excursion cause a "back EMF" - a "voltage" that the system sees to resist the current being supplied by the amp? Won't a sealed box be easier on the speakers, but harder on the amp becuase of the reduced EMF produced by the restricted cone travel? Won't the amp run hotter with a sealed box?

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[ILLCOMM]

the increased dampening of a sealed enclosure will not make life harder on an amp. sealed enclosure actually have the highest power handling characteristics.

Here how I understand back EMF. This could very well be a misinterpertation:

The only real way for the speaker to create its own current is for it to move on its own. If a musical signal requires that a sub stop excurting (word?) at +10mm but the enclosure isn't appropriate and it underdampens the driver then it is going to move further than 10mm creating a back EMF. Thus, a poorly desinged box, whether ported, sealed, or anything else, can cause this to happen.

 

[bigbassman Premium Member]

back emf does a good bit to sealed box woofers.. but nothing to damage an amp.. lets jump into it a lil //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

well as you know, sealed woofer systems have a huge impedence peak at resonance.. yet their actual response maintains a rather flat signature.. here's why...

Now you understand that a subwoofer motor is a generator, meaning it has a conducting voice coil moving in magnetic field which generates voltage. This voltage is in series with amplifier voltage but the polarity does not have to be the same. In fact whenever the speaker cone is moving in the same direction in which the amp is forcing it to move the voltages are of opposite polarities and try to cancel out each other. This is the case at resonance. but at resonance the amount of cone motion is also very high, this means that voltage produced by amp and that produced by speaker motor (generator) almost totally cancel each other out. The result of diminished voltage is a small current...

Since impedance is just voltage divided by current, you have a large voltage divided by small current = very high impedance as "seen" by the amp. This impedance peak in a sealed system would exactly counteract the efficiency peak produced by resonance.

So back EMF is a major player in a sealed box.. but it's not abusive to the amplifier... Most amplifiers with a decent damping factor are perfectly adept at controling whatever the sub sends back to it.. Well.. except for vacuum tube amps.. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

now Im gonna go relax my fingers.. //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

Loyd L.