Thursday, March 1, 2012

First Reflections

I remember the time I listened to my first echo.  I was sitting in church about knee high to a grasshopper fiddling with something on a wooden pew when my brain alerted me to something that was peculiar. The huge old organ in the loft had just finished playing some sustained chord and as I listened to the music trail off in the ever-diminishing reverberations, I glanced up at the multi-faceted ceiling looming a thousand feet overhead (well, since I was pretty young it appeared that way, maybe 80 or 100 feet would be more accurate :). I could almost see the sound bounce back and forth from one wall and then the next as the sound decayed into the stillness. I was totally mesmerized by this effect and lost all concept of whatever else was happening around me. I did not understand at all what was going on but I observed that sound bounced. I never forgot it.

A Grande Cathedral Ceiling

One day, I went to church and something was incredibly different. Someone had the bright idea to spray acoustic treatment (popcorn) on the ceiling to give it a more contemporary appearance. It totally destroyed the reverberation time and the character of the acoustic experience. I was depressed and had a huge feeling of loss knowing that my memories of this wonderful phenomenon is all that survive.

As I traveled to places later in life, anytime I had the opportunity to hear this phenomenon, I either yelled aloud in canyons or honked my horn in tunnels. I just loved listening to the effect such acoustic reflections had on the way the sound changed. Sometimes I could hear exactly what I said and other times it was delightfully jumbled; that's why I did it because I never knew what the result would be.

Unfortunately, while entertaining in the outdoors, the single most destructive force in the clarity of a high-quality playback system occurs from what is called the first reflection, a form of an echo. When sound strikes a surface, its behavior is much like that of billiard balls on a pool table; the sound is reflected at the same angle it strikes the surface. The reflection in itself is harmless but what happens after the reflection is not.

As the reflected sound bounces back toward the source, constructive and destructive interference patterns rise literally altering the subsequent sound coming from the speaker. The closest object to a speaker capable of creating a reflection is the source of the first reflection. In speakers, this may be the screws that attach the driver to the baffle board, the edge of the grill, the baffle board recessed cutout for the driver, or the closest object to the speaker.

Speakers Designed With and Without Concern for First Reflections

The speaker on the left of the above two pictures has smooth surfaces everywhere on the baffle board and gentle rounded edges on the corners to avoid any first reflection issues. The speaker on the right, on the other hand, has ignored the importance of the first reflection problem. Other examples of avoidable first reflections are non-recessed drivers, protruding mounting screws, overly recessed holes for mounting screws, and the like.  The point is that there should be literally nothing nearby a speaker driver that could potentially cause a first reflection.

The desirable effect I observed in that old church where echoes bounced around the cieling is totally undesirable in a high quality playback system. If you have any way to eliminate built-in first reflections on your existing speakers, you should try to remove them.

If you have a removable grill, see how you system sounds without the grill off. If you have protruding screw heads, change them out for flat head screws. If you have over-recessed holes in speaker drivers, fill them flat with wood putty or spackling (the stuff used to fill nail holes in walls). Although you may be tempted to fill the gap between the speaker and the baffle board cutout with wood putty, do not do this; it creates more problems than it solves.

I have also used a trick to eliminate first reflections on the backs of my speakers where the speaker wires connect to the terminals and you can too. Cut out a piece of thick polyster batting (the stuff used to make quilts) to fit neatly behind the speaker and tape it to the back panel.

Batting Applied to the Rear of the Speaker

What you will hear as these first reflections are eliminated is the vacuum cleaner effect where things that appeared to be garbled are no longer so. Complex nuances in instruments come forward to your attention instead of hiding in the background. The size of the sound stage expands and the position of instruments in three-dimensional psycho-acoustic space stabilizes. And the front-to-back depth improves, sometimes in a startling fashion.

While echoes are appealing in cathedrals, caves, and canyons, first reflections should be well controlled in your playback system. Taking the time to investigate and eliminate them can be very rewarding.

Yours for higher fidelity,
Philip Rastocny

I do not use ads in this blog to help support my efforts. If you like what you are reading, please remember to reciprocate, My newest title is called Where, oh Where did the Star of Bethlehem Go? It’s an astronomer’s look at what this celestial object may have been, who the "Wise Men" were, and where they came from. Written in an investigative journalism style, it targets one star that has never been considered before and builds a solid case for its candidacy.

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