Pinpoint imaging and life-like sound stages are something taken for granted by those of us graced with modern, high quality speakers and appropriately tamed rooms. But it was not always this way, and is still difficult to obtain for many people. Here we extract the basics of imaging straight from the notebook of Art Noxon, PE, and learn about an essential element of accurate imaging: a good loudspeaker box.
Old vs New
Modern speaker boxes are very smooth and rounded over. Old school classic boxes have square corners and worse yet grill covers with edge sticks to hold the fabric flat. Is this just styling or is there more to it?
Musicality vs Imaging
In the beginning of HiFi, the goal was “high fidelity,” in other words, musical-sounding output from the loudspeaker. Once the fidelity was figured out, a new concept came in vogue-imaging. Why is it old school speakers don’t image as well as modern speakers? Edge diffraction effects.
The speaker grill reflected higher frequency wavelengths, 3k and above. The speaker produced a strong direct wavefront followed by weak delayed wavefronts.This created an unclear or diffused image. The listener didn’t know exactly where the sound came from.
Remove your grills?
The grill frames were next removed from HiFi speakers but that didn’t improve things much. Now we saw edge diffraction, which amounted to a phase reversed wavefront being emitted from the square edges of the speaker box.
In addition to the direct signal, classic speakers delivered a time delayed signal off the edge of the box. It was either a reflection off the grill frame or a diffraction off the sharp edge.
Time Delay = Confused Imaging
Square Edges = Time Delayed Signal
Fixing the Problem
And so…modern speaker boxes do not have grills with frames or sharp front corners. Because they do not create secondary reflections, these boxes have the potential to create crystal clear imaging.
Taking advantage of this clarity…requires a room with good acoustics. But you already knew that!