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by John L. Murphy

Subject: Diffraction Loss Compensation
(posted 25Jun99 to Bass List)


> I've been researching the idea of adding a baffle step compensation circuit
> to my Marchand XM9 active crossover.  The only thing that seems hard to do
> is calculate the amount of boost actually needed as this is room dependent. 
> The theory, see John Murphy's article at www.trueaudio.com, suggests 6db
> boost.  An article at the TL web page suggests 3db is more likely with room
> reinforcement.  Has anyone looked at this. 

The 6 dB loss is correct for a speaker enclosure in free space.  When the enclosure is placed in a room it will encounter various effects due to the room (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.

I suggest the 6 dB diffraction loss correction as a correction for the diffraction loss alone.  I don't suggest that it will neutralize all the effects due to a unique listening environment.  Others suggest you "deal with diffraction" in the crossover, usually by just lowering the tweeter level a bit.

In some situations  3 or 4 dB of diffraction loss correction may result in an overall response that is closer to neutral (flat).  But the most correct way to compensate the room would be to do it separately from any diffraction loss correction.  Room compensation might take the form of several notch filters tuned to the worst peaks resulting from room modes.  Next you might want to tilt the treble up a smidge to compensate for reverberation that has significant treble loss.  Dark room reverb will make the playback sound a little darker. Bright reverb . . .  bright.  Next, depending on the size of the room and speaker response, you might need to compensate for the cavity effect.  In larger rooms cavity effect can be ignored but in vehicle cabins it is a major effect. 

Diffraction loss compensation is only part of the job of precisely compensating for the difference between a theoretical half-space acoustic load and what happens when we place an enclosure in a real world listening room.  Reducing the degree of diffraction loss compensation MAY reduce the coloration from the room effects as these effects largely tend to "boost the bass" but such an adjustment is imprecise at best. 

If we can systematically identify each source of color between our half space model and our particular listening room then we can then take steps to precisely neutralize the response in our own listening room.  Spherical diffraction compensation is one effect we can correct with a high degree of precision.  As we move toward a better understanding and modeling of our listening rooms I'm sure we will work out more practical and precise ways to compensate our rooms. 

Comments and critique are welcome.      :-)



John L. Murphy
Physicist/Audio Engineer
True Audio
Check out my new book "Introduction to Loudspeaker Design" at Amazon.com


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