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Divergent Tunnel

Divergentes Tunnel

Tunnel divergente

It is, as the name states, a divergent tunnel, not a pipe. And the section of this tunnel is increasing to the exit
- reduce the speed of the air in the tunnel
- have higher pressure at the exit
- increase the coupling surface to the external air
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There is a relationship between section, pressure and speed of an incompressible fluid flowing in a divergent or convergent duct. This relationship is known as Venturi-effect: if we consider a volume constant flow, the bigger the section, the smaller the speed and higher the pressure inside the fluid.

The surface at the beginning of the bass reflex or the horn is smaller than the surface at the end of it. This brings a change in the speed and pressure of the flow between entrance and exit. It is a paradox: at the exit is slower and does have more pressure.
Therefore it is clear that this duct is very important for the sound of the loudspeaker.

But. Air is compressible. Therefore the laws and parameters describing the theory of the behavior of the fluid get more and more complex. Just very complex mathematic simulations can show us what the air makes inside the flow. The parameters are, for example, density, viscosity, linear or turbulent behavior of the gas. Then again the form of the exit of the duct influences the turbulence inside the duct and its surfaces define the linearity of the flow or the boundary behavior. The mathematics behind these phenomena are the Euler-equations and the continuity-equation: these are the fundaments behind sound propagation…

If this is not enough, we have to consider that the average volume flow on the exit of a loudspeaker is zero: the loudspeaker does not blow up or out. It is not a constant but an oscillating flow: air is pushed out and then sucked back into the loudspeaker with high strength and high accelerations. Therefore extreme pressure variations occur: as example inside a subwoofer of a dancing floor we have found some tennis balls sucked inside it by the subwoofer itself.

In conclusion, the standard components can not translate the whole experience Blumenhofer Acoustics collected on this matter during the years: therefore it is understandable the high effort in the development of tunnels and horns.
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Venturi-Effect, Wikipedia