# QRD near/far field issue.



## MarkusBonk (Jun 5, 2010)

One of the limitations of the QRD design is that this assumes a far field whereas most of the listening is done in the near field. One solution to this proposed by Prof. Schroeder, and slightly modified by Prof. Cox and Dr. D'Antonio was to modulate the well depths by the locus of a concave parabolic mirror. i.e to bring the far field into the near field.

For the DIY builders this is not great problem - the question that needs to be answered is where to put the focal point of the mirror?

If I assume that a flat wall is in fact a concave mirror focused at infinity, then choosing a focal point anywhere nearer than that, say twice the room length, should be an improvement, as this effectively maps everything between the QRD and infinity into the space between the QRD and the focal point. Well, that's how I imagine it.

There are other focal points which I imagine might be interesting: the speaker positions for example.

Do we have an expert who knows the answer to this?

Markus


----------



## bpape (Sep 14, 2006)

I'm not sure your initial premise is completely accurate for a couple of reasons.

How you listen and how far you sit from the diffusers is 2 very different things. I can listen in a nearfield environment yet still be 10' or more from the wall behind me where I could have diffusion.

The distance from diffusion that it takes to be effective is based purely on the wavelengths of the frequencies where the diffuser operates. If a QRD is only designed to work from say 2kHz to say 8kHz, like many thinner commercial ones, then sitting closer will allow proper functioning and diffusion than say one that has a lower limit of 500Hz and an upper limit of 3kHz.

I wish I could address your question directly but I'm not a diffusion designer and have not read the paper you're referring to so as to better understand the purpose behind this.

Bryan


----------



## MarkusBonk (Jun 5, 2010)

Lets forget the diffusers for a very short moment. 
If I where to shape the front of the room as a parabolic mirror with the focal point at the listening position, would I not be effectively creating as reflection free zone - because the sources are between the focal point the the mirror?


----------



## bpape (Sep 14, 2006)

Not really. Any forward firing waves from the speakers will still have interactions with the side walls. By the time you got to the point where the side wall reflections were in the parabola, they'd no longer be in the focusing range. 

Now, at lower frequencies where waves act more spherically and not like a ray, then it might help - though those are not the frequencies we normally think of diffusing due to the very long wavelengths and the unrealistic sizes of diffusers for those frequencies.

Bryan


----------

