Is (L/D)max achieved at a specific speed or AOA? I'm little bit confused about the (L/D)max.

So I've read this blog: https://joeclarksblog.com/?p=4155
and the author shows this graph: 



So if I understand it correctly the (L/D)max is at the intersection of parasitical drag and lift-induced drag. Thus, at the point which has the minimum total amount of drag. Which is achieved at a specific speed. 

Skybrary states the following: 


  The maximum lift/drag ratio occurs at one specific CL (Lift
  Coefficient) and AOA (Angle of Attack (AOA))


https://www.skybrary.aero/index.php/AP4ATCO_-_Lift/Drag_Ratio,_Forces_Interaction_and_Use

Here is is the lift coefficient: 



Source: https://en.wikipedia.org/wiki/Lift_coefficient

The graph shows that the airfoil is the most efficient at a specific AOA. Thus in the ranges between 15° and 18°. 

sciencedirect.com states the following: 


  The L/D is not constant for a given wing, however, but changes with
  the angle of attack. Researchers thus normally use the maximum
  lift-to-drag ratio, L/Dmax, to characterize a given wing.


https://www.sciencedirect.com/topics/earth-and-planetary-sciences/angle-of-attack



I'm confused because one source tells me that (L/D)max is achieved at a specific speed. The other source tells me that (L/D)max is influenced by the AOA. 

Can you clarify this thought twist?
I'm confused because one source tells me that (L/D)max is achieved at a specific speed. The other source tells me that (L/D)max is influenced by the AOA.


The speed is only correct for a specific wing loading and straight flight at 1g. Only then will the optimum lift coefficient be reached in level flight at that speed.

The angle of attack version is the more generic and better way. Actually, optimum L/D is achieved at a specific lift coefficient, which is closely related to the angle of attack via the lift curve slope.


  The graph shows that the airfoil is the most efficient at a specific AOA. Thus in the ranges between 15° and 18°.


No, it only achieves the highest lift coefficient in that range. This is right before stall and certainly not the position of optimum L/D. Drag will have grown disproportionally over the previous angle of attack range. Without a drag polar it is impossible to say where that specific airfoil has its best L/D.