Baffle diffraction loss

Diffraction loss is transition between a half-space acoustic load and freespace. The 6 dB loss is correct for a speaker enclosure in free space.  When the enclosure is placed in a real life room it will not exhibit 6dB loss, it will be aroud 3 or 4 dB, since room is not free space and will encounter additional effects(reverb, standing waves, boundary effect, cavity effect . . .). 

Diffraction loss and room effects are independent and completely different effects.  The diffraction loss is nicely predictable whereas the effects of the room are highly variable, not only from room to room but also with speaker placement and room furnishing.  This typically means that each listening environment will be unique and will require unique compensation.

Spherical diffraction compensation is easily corrected with a high degree of precision, while other enclosures are a little bit less predictable and demand large amount of equalisation.

Regardless of where it is placed in the room, a typical hi-fi speaker system sees a half space load at frequencies from the midrange up as a result of the drivers placement on the
At high frequencies the speaker is radiating into "half space" i.e. it is only radiating into the forward hemisphere.  Starting in the midrange (depending on the baffle size) the system shifts from radiation into half space to radiation in full space at lower frequencies.  This transition from half space loading to full space loading results in what is commonly called the "6 dB baffle step",
At even lower frequencies, say from 100 Hz down, the wavelength of the radiated sound is such that the walls and cavity of the listening room begin to load the system in a way that results in a complex load that is less than half space and results in increased output from the system.  This effect in the low bass is called variously "room gain", "boundary effects", "room resonance", "frequency dependent radiation
impedance", etc.

The smaller the baffle the higher the transition frequency.

The shape of the diffraction loss frequency response curve depends on the size and shape of the enclosure. All enclosure shapes exhibit a basic 6 dB transition (or "step") in the response with the bass ending up 6 dB below the treble.  A spherical enclosure exhibits this transition clearly with a very smooth diffraction loss curve.  More "angular" enclosures exhibit the underlying 6 dB step along with a series of response ripples that are dependent on the placement of the speaker with respect to the baffle edges.  The worst case appears to be placing the driver at the center of a circular baffle so that it is the same distance from all diffracting edges.  Placing the driver on the baffle so that it is a different distance from each edge tends to minimize the response ripples and make the diffraction loss look more like the smooth loss of the sphere.  Because the spherical diffraction loss is a common element for the diffraction of all enclosure shapes and the response ripples are much more difficult to predict (and can be minimized anyway) it makes sense to approximate the diffraction loss of a loudspeaker as the diffraction loss of the equivalent sphere.