subwoofers with dual opposed drivers

[TimVG]

well the effect on the cabinet is because of the acoustic waves within the cabinet. At least that how i understood it.

It's simply about the mechanical aspect, two equal opposing forces cancelling eachother out. In the past I had four of these dual-opposed subs, it was quite fun playing them at full force and being still able to balance a small coin on it.

 

[abdo123]

It's simply about the mechanical aspect, two equal opposing forces cancelling eachother out. In the past I had four of these dual-opposed subs, it was quite fun playing them at full force and being still able to balance a small coin on it.

so cabinet resonances are caused by the mechanical movement of the woofer? not by the sound waves produced by the woofer?

 

[gnarly]

so cabinet resonances are caused by the mechanical movement of the woofer? not by the sound waves produced by the woofer?

Correct. Like TimVG said.

The moving mass...T/S spec, Mms.
It's about 1/2 lb for the 18" i use (bms 18n862). A lot to shake back and forth, huh?

 

[TimVG]

so cabinet resonances are caused by the mechanical movement of the woofer? not by the sound waves produced by the woofer?

There's more to the discussion but I assumed (didn't read through the whole thread) that "by the effect on the cabinet" you meant the effect on the cabinet as a whole (which can cause cabinets to physically move) and not panel resonances. Panel resonances are usually not a problem in subwoofers as they occur far outside the passband of the subwoofer anyhow.

 

[abdo123]

not panel resonances.

I'm actually interested in eliminating the phenomenon in which the energy hitting the inside of the enclosure (or individual panels) causes them to vibrate and produce sound outside the cabinets.

Can dual opposed help with eliminating that phenomenon? or does it just stop the cabinet from physically moving because the woofer is moving?

 

[TimVG]

I'm actually interested in eliminating the phenomenon in which the energy hitting the inside of the enclosure (or individual panels) causes them to vibrate and produce sound outside the cabinets.

Look up "constrained layer damping"

 

[NTK]

Correct. Like TimVG said.

The moving mass...T/S spec, Mms.
It's about 1/2 lb for the 18" i use (bms 18n862). A lot to shake back and forth, huh?

Quick calculation.

Mass = 0.266 kg (Mms, SB Acoustics SB34SWPL-76-4)
Displacement amplitude (one-sided) = 10 mm = 0.01 m
Frequency = 30 Hz

acceleration of cone = displacement * omega^2 = 0.01 * (2 π 30)^2 = 355 m/s^2
reaction force = mass * acceleration = 0.266 * 355 = 94.5 N = 9.6 kgf (kilogram force)

So the reaction force amplitude in this case is almost 10 kgf and will be transferred to the floor (if the subwoofer does not move relative to the floor, otherwise there is a spring-mass-damper equation to solve to account for for the stiffness of the feet/vibration mount).

 

[NTK]

Can dual opposed help with eliminating that phenomenon? or does it just stop the cabinet from physically moving because the woofer is moving?

Dual opposing driver as in a dipole doesn't help (or is twice as bad compared to a single driver of the same size) because the cones are actually moving in the same direction with each other.

 

[abdo123]

Dual opposing driver doesn't help (or is twice as bad compared to a single driver of the same size) because the cones are actually moving in the same direction with each other.

yes but they're double opposed, shouldn't they cancel each other as the two sound waves 'slam' into one another?

 

[NTK]

yes but they're double opposed, shouldn't they cancel each other as the two sound waves 'slam' into one another?

Pressure waves yes. But not mechanical reaction.

 

[abdo123]

Pressure waves yes. But not mechanical reaction.

I'm so confused! would the inside of enclosure be dead silent with double opposed or not?

What do you mean by mechanical reaction?

 

[NTK]

Newton's third law. You need to apply a force to move (accelerate) the cone, and there has to be a corresponding reaction force from it.

 

[abdo123]

Newton's third law. You need to apply a force to move (accelerate) the cone, and there has to be a corresponding reaction force from it.

I understand this in a vaccum, but i cannot comprehend it with context of everything that has already been discussed.

my questions are (briefly):

1) is the inside of the cabinet quiet? are the panels being bombarded with sound pressure / sound waves? Can you eliminate cabinet resonances with a dual opposed config?

2) if not, then what is it doing and in what cases is it advantageous.

 

[NTK]

I understand this in a vaccum, but i cannot comprehend it with context of everything that has already been discussed.

my questions are (briefly):

1) is the inside of the cabinet quiet? are the panels being bombarded with sound pressure / sound waves? Can you eliminate cabinet resonances with a dual opposed config?

2) if not, then what is it doing and in what cases is it advantageous.

With opposing drivers, there is no (net) change in the internal cabinet volume. Thus, there is no compression and no pressure fluctuation from compression. The air inside the cabinet will slosh back and forth with the cone movements. The effects from this air movement are tiny compared to the compression effects from a single driver. Therefore the pressure loading on the cabinet walls is hugely reduced with opposing drivers.

We are talking about 2 very different effects.

 

[Chromatischism]

Have you ever looked at a running petrol or diesel engine and seen it vibrating? Have you ever sat in a car and started the engine and felt the vibration? It's the same effect.

 

[abdo123]

With opposing drivers, there is no (net) change in the internal cabinet volume. Thus, there is no compression and no pressure fluctuation from compression. The air inside the cabinet will slosh back and forth with the cone movements. The effects from this air movement are tiny compared to the compression effects from a single driver. Therefore the pressure loading on the cabinet walls is hugely reduced with opposing drivers.

We are talking about 2 very different effects.

My takeaway message is that with dual opposed you can make the cabinet from practically anything that is air tight.

But it does nothing to stop the cabinet from resonating.

 

[SigbergAudio]

With opposing drivers, there is no (net) change in the internal cabinet volume. Thus, there is no compression and no pressure fluctuation from compression. The air inside the cabinet will slosh back and forth with the cone movements. The effects from this air movement are tiny compared to the compression effects from a single driver. Therefore the pressure loading on the cabinet walls is hugely reduced with opposing drivers.

We are talking about 2 very different effects.

I may be misunderstanding what you are saying here, but not sure I get it. A typical dual opposing subwoofer is wired with the same polarity for both drivers. So they both move out and they both move in at the same time. So when they move out, the internal cabinet volume will increase, and when they move in, the internal cabinet volume will decrease, This is similar to a single driver subwoofer, only the change is twice as big.

So the cancellation of vibrations or movement is due to the fact that they are moving in opposite directions and cancelling out the push/pull of the other driver.

 

[sarumbear]

I may be misunderstanding what you are saying here, but not sure I get it. A typical dual opposing subwoofer is wired with the same polarity for both drivers. So they both move out and they both move in at the same time. So when they move out, the internal cabinet volume will increase, and when they move in, the internal cabinet volume will decrease, This is similar to a single driver subwoofer, only the change is twice as big.

So the cancellation of vibrations or movement is due to the fact that they are moving in opposite directions and cancelling out the push/pull of the other driver.

As seen here: https://uk.kef.com/pages/uni-core

 

[sarumbear]

With opposing drivers, there is no (net) change in the internal cabinet volume. Thus, there is no compression and no pressure fluctuation from compression.

If there is no pressure change how can sound is generated.

 

[NTK]

If there is no pressure change how can sound is generated.

That is referring to the hydrodynamic pressure inside the cabinet, not acoustic pressure (elastic wave pressure).