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Re: What Are The Advantages To A Quadrupole Sub?

Posted by andy19191 on October 30, 2006 at 16:10:29

In Reply to: What Are The Advantages To A Quadrupole Sub? posted by thetubeguy1954 on October 30, 2006 at 14:25:57:

A monopole acoustic source is a scalar quantity and radiates sound equally in all directions. In nature this tends to be caused by mass injection or something like a pulsating balloon displacing volume equally in all directions. A loudspeaker in a cabinet approximates this for frequencies with wavelengths well above that of the cabinet.

A dipole acoustic source is a vector quantity which has a figure of 8 directivity: +ve in the forward direction, -ve in the rearward direction and zero sidewards. It can be represented by two monopoles of opposite signs a small distance apart, the line between the two monopoles determining the axis. It is less efficient than a monopole because of the strong cancellation. In nature fluctuating forces tend to create dipoles like a fluctuating piston or turbulence impinging on a surface. A free loudspeaker driver behaves like a dipole with +ve pressure in front of the driver as it moves forward, -ve behind and zero off to the sides.

A quadrupole acoustic source is a second order tensor quantity. It has 9 scalar components (the vector dipole has 3 and the scalar monopole 1) although symmetry for the rate of deformation for typical fluids would result in 3 pairs being the same and only 6 components being indepedent (OK I cannot explain this simply because of the need to explain deformation, Newtonian fluids and such but if you look at quoted values of quadrupoles you will only see 6 values and not 9). A quadrupole is less efficient than a dipole and can be constructed from two opposing dipoles (or 4 monopoles arranged in 2 pairs of + and -). Your diagram shows two orientations of opposing dipoles if you look at the open and filled circles in the centres. There are 4 other orientations if you rearranging the pairs of monopoles in 3D. (This is where the 6 rather than 9 comes in.) In nature, quadrupoles are typical of turbulent motion away from surfaces. Your loudspeaker example is created by bringing two dipoles close together leading to them largely cancelling and leaving a particular quadrupole radiation. I leave it as an exercise to identity which of your diagrams applies to the quadrupole loudspeaker shown.

> What is advantageous about using this Emerald Physics linear
> Quadrupole sub as opposed to other dipole or bipole subs?

A disadvantage will be caused by it being more inefficient than a dipole because of the extra cancellation. The advantage will be... well it is different. It will drive the room modes in a slightly different manner but it is not clear that it will better or worse than a dipole. Anyone?

I have just looked up emerald physics and the speaker was described as a dipole, the manufacurer commented a couple of times and did not correct the description. My guess is that it is arranged as a dipole.

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