Perfect cube subwoofer enclosure

[yusuf]

I started bu8ilding box for sealed subwoofer. Do you see issue with perfect cube subwoofer enclosure. I am aware of standing wave issue about perfect cube but I assume that does not apply to subwoofer, is that correct assumption?

[rcavictim]

No problem with standing waves inside a subwoofer box as the wavelength is extremely larger than the box dimensions.

[Svante]

The cube shape will move the three first box resonances (1,0,0), (0,1,0) and (0,0,1) to the same frequency. This will lead to a powerful resonance at that frequency, which is bad. On the other hand, this shape pushes the frequencies as far upwards as possible, which is good. And then again, the frequency can typically be made so high that it doesn't matter at all anyway, given that the crossover is sufficiently steep. Some stuffing in the box also greatly reduces the "problem".

[yusuf]

Agree. However, does it apply to subwoofer for operating frequency range < 100 Hz? Shouldn't box size > 3.4 meters (wavelength) for any such resonance?

Can you elaborate more on stuffing part too

Thanks

[Svante]

Originally posted by yusuf
Agree. However, does it apply to subwoofer for operating frequency range < 100 Hz? Shouldn't box size > 3.4 meters (wavelength) for any such resonance?

Can you elaborate more on stuffing part too

Thanks

Ok, assuming a 100 litre box, the side would be (0.1)^(1/3)=46 cm. That would mean that this huge resonance peak would occur at 345/(2*0.46)=372 Hz, which is slightly below 400 Hz. If the sub has a 2nd order (12 dB/octave) lowpass filter @ 100 Hz, it would have attenuated the signal by ~12*2= 24 dB, since 400 Hz is two octaves above 100 Hz. This might not be enough if the resonance peak is large. There are at least two ways to solve the problem, one would be a steeper filter (and possibly a lower cutoff), or to attenuate the resonances by adding fibreglass, wool or similar inside the box. Or both.

[Collo]

The driver to rear wall resonance will be half the frequency of the wall to wall figure. ( Velocity node to pressure node vs pressure node to pressure node)

For a cube, this resonance would be less intense than the box resonances which coincide.

You can see your resonances with "boxnotes".
http://www.subwoofer-builder.com/freesoft.htm

To model a sealed box, set number of ports = 0

[Svante]

Originally posted by Collo
The driver to rear wall resonance will be half the frequency of the wall to wall figure. ( Velocity node to pressure node vs pressure node to pressure node)

I disagree...

If there was nothing there at all (free air), that would be true. However, the loudspeaker membrane has an impedance much higher than the air, so waves will be reflected from it very much like if it was a wall that was completely still. There will be a slight difference from a wall due to the mobility of the cone, but not much.

[Collo]

Interesting.


My understanding is based on this line of reasoning...

Start of cycle
- cone at mid travel moving backward:
- The air behind the cone is moving at maximum backward velocity

@1/4 cycle
- cone at maximum extension backwards
energy reflected at back of box (maximum pressure on back wall)

@1/2 cycle
Energy returns to the front just in time to meet the cone travelling forwards at maximum velocity

@3/4 cycle
- Cone at maximum extension forward. (stopped)
- energy reflected towards the rear of the box

This has the air behind the cone undergoing maximum displacement and velocity.

I've measured a peak corresponding to this line of thought on this page:
http://www.subwoofer-builder.com/speaker-stuffing.htm

[Svante]

Yes, what you describe is the classic open pipe resonance. The fact that the cone actually moves gives some support to this line of reasoning. Actually there is a continuum between 1/2 wave resonance and 1/4 wave resonance for a pipe if one of the walls has an increasing degree of flexing.

I might have been a bit categoric in my disagreement, if you have measurements that support the 1/4 wave resonance in a real system, then that is the case in that particular system. However, this also depends om the geometry of the box; if the driver covers the entire wall, the mass per surface if the loudspeaker cone etc. I have not seen this 1/4 wave behaviour in real life, though.

One last thing, I think it is good to think of the resonances as properties of the system, regardless of what excites them. This line of thinking makes it easier to see the loudspeaker cone as part of the wall, possibly a bit flexing, and resulting in an imaginary velocity zero slightly beyond (outside) the cone.

[Collo]

The overall response may well be a combination of both types of resonance.


You're treating the distance between the cone and the back wall as a half wavelength.

This means that each time the energy is reflected off the cone, the position of the cone is the same.

This could give rise to a range of resonances corresponding to the change in the cone-to-rear-wall distance as the driver moves from fully extended to fully withdrawn


I'm treating the distance between the cone and the back wall as a quarter wavelength.

The energy arriving at the cone is basically captured as the cone moves forward and then launched back as the cone reverses and accelerates towards the back wall.

I suggest that this mechanism will couple to the air much more strongly, resulting in a stronger peak.


I have no problem with a half wave resonance between the front and rear walls - I would expect this component to be present in any measurement. This would tend to "muddy the waters" of any measured response.

The definitive test would be something like a sonosub with the driver occupying most of the area of the endcap.