Art's Blog: More On Early Reflections

Comment to Audiophile Review, Roger Scoff, 6/19/14, Room Acoustics,  “What gets you more, $10k …”

The diagram showing the direct, 1st reflection and reverberation as being something that comes from the back corner reflection is misleading and should be corrected. The first side wall reflections only widens the apparent stage. The most destructive reflection, the second reflection, is the crosstalk reflection where signal from the left speaker hits the right wall and returns to the right ear. Right signal info delivered to the left ear ruins the L-R stereo experience.   

The ear brain system is sensitive to much more than the first few reflections. It blends all the “early reflections” with the direct into one composite sound. Early reflections are those that arrive in general within the first 25ms following the arrival of the direct signal. If you sit 8’ back from the speaker, it takes the direct about 7ms to arrive. The first early reflection is usually the floor bounce which takes about 9.5ms to arrive from the speaker. It is 1.5ms delayed from the direct signal.

The “last” early reflection leaves the speaker and travel about for about 7 + 25 = 32ms before it hits the listener. This is a total path length of about 30’. If your room is 8’ tall, an upwards moving sound wave could hit the ceiling, the floor, back up to the ceiling again before it finally hit the listening position and still be an early reflection. If the room is 15’ wide, sound from the left speaker can hit the right wall, bounce across the room in front of you, hit the left wall and get reflected towards the listening position, also an early reflection. 

It’s not just the first reflection that matters to the listener, it’s all the early reflections that fuse with the direct signal to form one composite “direct” signal. They distort the crystal clear information being delivered by the direct by overlaying it with a multitude of reflections that sound just like the direct except that they are time delayed and come from all different directions. The careful time alignment of the speaker is obscured by the delays of the early reflections. Precision stereophonic effects are blurred over by all the different directions of sound in the early reflection group.

All those early reflections add up to a sound level that exceeds the strength of the direct signal by about 7 dB. For example 20 reflections that are each on average 6 dB lower than the direct signal deliver a late reflection package that is 7 dB louder than the direct alone. Add the direct to that early reflection package and you get a combined signal that is 8 dB louder than the direct.

Or, move one speaker outside and leave the other one inside. Play music through both and leave the volume balanced. Listen to and measure sound level 8’ away from the outdoor speaker and compare to what you hear and measure inside the house. Early reflections are sometimes called room gain, they distort the loudness, musicality and imaging in the playback music.  

Late reflections, those after 25ms, mask the dynamic properties of the music, they should be absorbed or converted from hard distinct reflections into multiple low level diffusive reflections.  

Art Noxon is a fully accredited Acoustical Engineer with Master of Science degrees in Mechanical Engineering/Acoustics and Physics. A Professional Engineer since 1982, he is licensed in Oregon to practice engineering in the public domain with the specialty area of acoustics. A prolific inventor, he developed and patented the iconic TubeTrap, the original corner-loaded bass trap/treble diffuser, 150 other acoustic devices, and counting. Lecturer, writer, and teacher of acoustics, he has presented 7 AES papers, numerous magazine articles, white papers and blogs. He is president of Acoustic Sciences Corporation, the company he founded in 1984.

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