biglaz Posted February 10, 2010 Share Posted February 10, 2010 What does it mean when individuals talk about a driver being underdamped or overdamped and how do you identify each. Quote Link to comment Share on other sites More sharing options...
WMcD Posted February 13, 2010 Share Posted February 13, 2010 Actually it is speaker in a box (usually sealed) which is underdamped or underdamped. We usually want it perfectly damped. The damping comes from the voice coil and magnet. They are a motor to the extent the amp puts current into them and creates a force to move the diaphragm. But when the diaphragm moves the coil in the magnetic field, they become an electrical generator creating voltage and thereby forcing current back into the amplifier (which is near zero ohm load -- a very heavy load electrically). So, the stronger the magnet and the more wire in the coil, the stronger the motor. But also the stronger generator. The stronger the generator, the harder it is to crank if it has a zero ohm load if we had a rotary generator -- but the same with the speaker. The figure of merit is the Qes of the driver. The lower the Qes (Q of electrical system) the stronger the motor-generator and this provides a force which damps out oscillation. - - - - The above is an important point. We may think that we want a strong motor. However that also creates more damping. You can't have one without the other. - - - - The driver in a box oscillates because of the mass of the diaphragm and the spring (caused by the suspension and air compression in the box). This can be put to good use because as we go down in frequency toward resonance, the diaphragm becomes easier to move -- if we had no damping at all, the mechanical system (spring and mass) will ring or oscillate like a tuning fork. This is good because the diaphragm is becoming a poorer and poorer radiator as we go down in freq. In terms of wavelenght of the sound to be reproduced it is becoming smaller. So this provides a type of built in equalization allowing the diaphragm to move more and more. Sort of free bass peak spread out in a range. This is sort of like having a graphic equalizer hooked up to the amp. Suppose the speaker is rolling off at 70 Hz. But you have a slider to boost 50 Hz We know that slider causes a bump centered at 50 Hz and some boost on either side. So it would be nice to slide up the control just enough to compensate. for the roll-off. And not more. Otherwise we get a peak at 50 Hz. - - - - Looking at it in the frequency domain: There is just enough bass boost to make the overall response flat. But without the optimal damping (underdamping), there is bump at resonance. This causes one-note bass. If there is not enough bass boost from overdamping, we get roll off in bass earlier (going down in freq) earlier than needed. In the time domain: Suppose the diaphragm is displaced to near limits. The music (or drum hit) stops. Now we want the diaphragm to go back to its neutral rest position. If it is underdamped it goes back but then overshoots, and then comes back to neutral and overshoots the other way. So it traces out a sine wave at the resonance freq. Boomy bass. If it is overdamped the damping force prevents the diaphragm from getting to the neutral rest position as quickly as possible even though the resonance is indeed damped out. But we really do want it to return to neutral as fast as possible without overshoot. - - - - - Basically we are playing with damping to prevent mechanical oscillation -- but also put the system on the edge of oscillation so we get that boost. So: Underdamped is too much bass boost at the note (frequency) of oscillation. Optimally damped, the equalization works out just right. If overdamped, we are throwing away some boost we want. - - - There is a bit of a price to pay Once we move below the resonance, the diaphragm becomes harder to move in a sealed system. The electrical energy is used up trying to compress the air in the box. - - - Wm McD Quote Link to comment Share on other sites More sharing options...
biglaz Posted February 13, 2010 Author Share Posted February 13, 2010 Wow! Thanks for this response. I think I understand it now. Quote Link to comment Share on other sites More sharing options...
michael hurd Posted February 13, 2010 Share Posted February 13, 2010 It's sort of like a suspension on a car, you wouldn't want the car to keep bouncing after going over a bump, like it's got worn out shocks ( underdamped ) or the suspension to be so stiff that you don't have any compliance. Some other variables to throw in to the mix is that the suspension used on most woofers isn't exactly a perfectly linear device, as well as when the voice coil gets hot from high power input, the effective strength falls off, and an effectively 'overdamped' enclosure will become more 'critically damped'. Way back I linked an article about compression ( thermal ) in speakers, I will try and find it again, it's a good read. Quote Link to comment Share on other sites More sharing options...
JohnA Posted February 14, 2010 Share Posted February 14, 2010 I like the wind chime description to imagine the sound. Strike a wind chime and it rings; underdamped. Grip the chime with your hand and strike it again and it "tinks"; Overdamped. I don't know a physical analogy for optimally damped. Quote Link to comment Share on other sites More sharing options...
biglaz Posted February 14, 2010 Author Share Posted February 14, 2010 Great info. Thanks Quote Link to comment Share on other sites More sharing options...
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