I'm going to make a pair of sealed boxes for mid-bass to mid-range drivers (they'll run from 100 Hz to ~ 3 kHz) and I'm looking for choices for dampening.
The first thing I've been advised for is to make a box with no parallel walls to reduce the formation of standing waves, but you still need to add dampening to reduce the total Q.
So I thought: what if I get more dampening in the driver instead?
I thought of making a shorted aluminum voice coil (instead of having the slit) to have greater eddy currents and thus a stronger dampening.
That'll sure heat the voice coil more, but I'm going to work at low power (< 40 W RMS), so I think this is manageable.
What do you think? Could that serve as dampening?
The first thing I've been advised for is to make a box with no parallel walls to reduce the formation of standing waves, but you still need to add dampening to reduce the total Q.
So I thought: what if I get more dampening in the driver instead?
I thought of making a shorted aluminum voice coil (instead of having the slit) to have greater eddy currents and thus a stronger dampening.
That'll sure heat the voice coil more, but I'm going to work at low power (< 40 W RMS), so I think this is manageable.
What do you think? Could that serve as dampening?
It would depend on how much muffling of the sound you can tolerate. Just about anything that limits the motion of a driver cone is going to affect both clarity and output.
I would stay with box damping. Add some bracing if you notice vibration on the outside. Even consider Constrained Layer Damping, which I've used in the past.
I would stay with box damping. Add some bracing if you notice vibration on the outside. Even consider Constrained Layer Damping, which I've used in the past.
That will not help. It will only make it more difficult to predict at what frequencies those standing waves will be.
Damping material inside a box can serve different purposes:
1. Remove the 'hollow' sound
2. Dampen the bass alignment
3. Dampen cabinet wall vibrations
Damping properties of the driver do not affect purpose 3. They do affect purpose 2, which is easy to take into account: do not take it into account. 🙂 Many box simulation tools assume no damping material inside the box.
Purpose 1 is the most interesting. Essentially it means: how much is sound attenuated, when it travels from inside the box, though the cone, to the outside? I would guess that cone mass is the most important parameter.
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But isn't acoustical dampening have a similar result in the end?
Specially considering it's going to be a small sealed box, where cone is strongly coupled to everything inside?
For that purpose I was thiking of making the enclosure in a way such that the cone itself made up a large portion of the largest wall of the enclosure, so it would get to dampen the waves inside the box by itself efficiently.
I thought that this added to not having parallel walls could help, because in that way nearly every mode would include reflections upon the wall where the driver is.
But you're right about dampening cabinet vibrations, that it surely won't do.
But if anything, if one can't "replace" the other, do you think it could at least help?
Say, if I would need too much acoustical dampening and started to affect the box tunning too much (changing the effective volume), do you think more dampening in the driver could at realistically reduce the amount of acoustical dampening needed?
Say, if I would need too much acoustical dampening and started to affect the box tunning too much (changing the effective volume), do you think more dampening in the driver could at realistically reduce the amount of acoustical dampening needed?
Yes there are acoustic repercussions linked to box shape, volume and material. These, however, are far more manageable than tampering with the driver itself.
Inside the box the pressure is a function of cone movement and position (in or out, along it's axis). In a perfect world this relationship would be fixed ... cone position X gives box pressure Y, each time. But in reality the travel time of the pressure waves inside the box does tend to modify cone motion somewhat. Damping materials tend to slow the wave motion inside the box, producing an average pressure rather than an instantaneous one, allowing the cone to position itself more accurately.
About the only way I know to get exact cone positioning for a given input, is to use motion feedback correction, with a sensor on the cone to modify the waveform coming from the amplifier.
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Need, not necessarily, but your driver Q and your box size are also variables in this.
While this may change things, and sometimes a change works out, it doesn't actually fix anything. Better to go with the absorbing material.
Interesting. I'd rather go with the other options but if you do this, consider keeping the gap and having it feed a resistor.
Even if he does this --and feeding a resistor with an unused voice coil will improve the damping factor-- he's still going to have to tame the cabinets. You don't fix your washing machine by changing the oil in your car.
Video Here
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That hollow sound is literally the reverberation coming from inside of the box, and can be in some way monitored by the level of the peak in the impedance graph and decay related charts
Dampening is dampening I believe. Pressure, electrical and mechanical Q,
A unique way to dampen is the PPSL or just plain slot loading. The pressure created in the slot, dampens the driver. When modelling you can witness the decrease in the impedance chart peak. I'm not sure, but as long as you don't loose spl, in comparison to the free air response, things should be ok.
I think it's important to put some space between fundamental box action and modal propagation. If not because they have unique properties, then at least because one is generally wanted while the other is generally not, in a conventional box.
"Dampening is dampening" - a distinction needs to be made between Driver Q & Box Q, the OP is seeking to replace one with the other - won't work for the reasons given above
You should be thinking on the following lines:
The speaker enclosure will be constructed from stiff and dense material, in an effort to prevent any rear radiation from escaping into the room, particularly to prevent radiation of cavity resonances.
But there's an acoustically transparent hole in the front of the speaker - the driver's cone. Cabinet walls may be 1 inch thick, but the cone will be maybe 1mm thick. No matter how firmly the voice coil controls the centre of the cone, this will have no effect on the overall acoustic transparency.
The unwanted sounds emanating from the cone will be a combination of time-delayed rear radiation due to reflections from the cabinet internal wall surfaces, plus the really objectionable resonances due to standing waves within the cavity. Very non-parallel walls will certainly help here.
The other method is to damp down any standing waves. Now, if you're familiar with guitars, you will know that you can damp the strings by a relatively firm finger or palm pressure at the bridge. Alternatively, if you're trying to isolate the 2nd harmonic, you need only the gentlest of finger pressure at the 12th fret. Well, it's similar in the case of acoustic standing waves.
So, the cabinet internal walls need to be lined with some fairly robust, and reasonably thick treatment - I'd suggest thick felt. This should not only soften the hard surface, but also will provide damping where acoustic pressure is high, and velocities low. And towards the centre of the box, the damping will need to be rather less robust, to suit the lower acoustic pressures and higher velocities.
Fibreglass has a very good reputation as a damping material, but it's nasty stuff, and steps must be taken to ensure bits don't drop into the driver's motor. And for a ported enclosure, it's probably ruled out - in any case, steps must be taken to keep any material well clear of the port.
Hope the above helps - and stay safe!
The speaker enclosure will be constructed from stiff and dense material, in an effort to prevent any rear radiation from escaping into the room, particularly to prevent radiation of cavity resonances.
But there's an acoustically transparent hole in the front of the speaker - the driver's cone. Cabinet walls may be 1 inch thick, but the cone will be maybe 1mm thick. No matter how firmly the voice coil controls the centre of the cone, this will have no effect on the overall acoustic transparency.
The unwanted sounds emanating from the cone will be a combination of time-delayed rear radiation due to reflections from the cabinet internal wall surfaces, plus the really objectionable resonances due to standing waves within the cavity. Very non-parallel walls will certainly help here.
The other method is to damp down any standing waves. Now, if you're familiar with guitars, you will know that you can damp the strings by a relatively firm finger or palm pressure at the bridge. Alternatively, if you're trying to isolate the 2nd harmonic, you need only the gentlest of finger pressure at the 12th fret. Well, it's similar in the case of acoustic standing waves.
So, the cabinet internal walls need to be lined with some fairly robust, and reasonably thick treatment - I'd suggest thick felt. This should not only soften the hard surface, but also will provide damping where acoustic pressure is high, and velocities low. And towards the centre of the box, the damping will need to be rather less robust, to suit the lower acoustic pressures and higher velocities.
Fibreglass has a very good reputation as a damping material, but it's nasty stuff, and steps must be taken to ensure bits don't drop into the driver's motor. And for a ported enclosure, it's probably ruled out - in any case, steps must be taken to keep any material well clear of the port.
Hope the above helps - and stay safe!
Replacing box compliance with driver compliance....you could have a tight driver but a loose box, or a loose driver and tight box. One is probably more noticeable than the other. I think you probably could replace one with the other to a certain extent but as a whole the resonances are going to be unique to where if the driver or box was perfectly behaved but the other was not, a unique coloring would be added to the signal depending on which part of the system was least dampened. So in the end you'd want to control both. This is looking at modal resonance and the fundamental. The fundamental is there to create the frequency response, still, it is sloppy and its decay needs tailoring via tuning choice and damping material.
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for closed boxes i prefer oversized enclosures and lots of mineral wool inside, anything smaller is a compromise 
Getting a sealed box to perform up to its potential is not easy. Some of what is needed is pretty obvious and well known. SOP. However, there are other design issues that are often overlooked or just ignored. Group delay vs QTC is one. Usually a sealed enclosure is sized to react with the driver free air Q to provide the lowest possible - practicable FB. In most woofer applications that make sense. However your application is mid bass and mid range. Assume you will have a combination of High Pass and Low pass (active or passive) filtration for your overall build.
A couple of thoughts. If possible for your 100 Hz low end target you want your box FB to be at least one octave below (50 HZ) if possible. This keeps the box resonant peak out of your pass band. Next, you want the largest driver possible to minimize required cone travel. With your upper cut off at 3Khz a 6" to 7" driver would be a typical upper limit. And you want your final box/driver QTC to be around 0.9 so you have the best possible group delay - transient response. Then there are the usual enclosure issues. Air tight. Damped - heavy MDF or equal. Non parallel sides. Hard wood Dowel(s) lateral and vertical bracing. Heavy internal spray on sound deadening. And most important, proper stuffing. Doing these and similar things are way more productive than altering a correct for your application raw driver.
A couple of thoughts. If possible for your 100 Hz low end target you want your box FB to be at least one octave below (50 HZ) if possible. This keeps the box resonant peak out of your pass band. Next, you want the largest driver possible to minimize required cone travel. With your upper cut off at 3Khz a 6" to 7" driver would be a typical upper limit. And you want your final box/driver QTC to be around 0.9 so you have the best possible group delay - transient response. Then there are the usual enclosure issues. Air tight. Damped - heavy MDF or equal. Non parallel sides. Hard wood Dowel(s) lateral and vertical bracing. Heavy internal spray on sound deadening. And most important, proper stuffing. Doing these and similar things are way more productive than altering a correct for your application raw driver.