This week we dig deeper into the operation of the “Acoustic Zoom Lens” that placed a magnified Tina Turner in our faces for that memorable playback of Proud Mary. While this is a very brief discussion, we heartily recommend Fritz Winckel’s classic book, Music, Sound, and Sensation: A Modern Exposition for a real primer in psychoacoustics.
Before addressing the Acoustic Zoom Lens directly, let’s quickly look at the mechanisms our mind uses to locate sound sources.
Time and Loudness: It’s All About the Differences
Imagine you are the listener, at the bottom of the sketch. Assume we are in a reflection-free environment for the sake of this discussion, let’s say a flat grassy field. Time delays and loudness differences establish position B to be distinct from position A.
Person A is clearly front and center as both ears receive the same loudness at the same time. In a good listening environment, this also holds true as any reflections will affect sounds reaching each ear equally.
Person B is speaking from behind and to the side. The loudness of B is less than A due to the greater distance. Your right ear hears B louder than your left ear through acoustic shadowing. Your right ear hears B a half millisecond sooner than your left ear because of the width of your head.
Though none of these differences may be perceptible on their own, the combination allows the human mind to triangulate the sound source location. Amazing!
Stereo Speaker Arrangement: Classic
HiFi loudspeakers in traditional room setups have well-controlled early reflections so that the center image fits into the crescent footprint. We discussed this a few weeks back, and it provides a sound stage most people are familiar and comfortable with.
Stereo Speaker Arrangement: Enhanced Center Image
Add a low level, time delayed reflection between the speakers and the central image steps forward and seemingly into the sonic spotlight while the rest of the band stays in place within the crescent.
Why is it Called a Zoom Lens?
Think of a telescope or binoculars with variable zoom. You can maintain a wide field of view or you can enhance your perception of a small portion of the image. To make this change, you adjust the focal length relative to the objective lens. The objective lens is a bit like the sound source (the speakers) and the focal length is a bit like the path length of the reflections coming off the center image Trap.
Move the early low level central reflection forward even more, and the center image gets closer, larger, and brighter as the time delay decreases, loudness increases, and high frequency loss decreases. Hence: Acoustic Zoom Lens.
Albert von Schweikert 1945-2020
We are very sad to hear that Albert von Schweikert passed away a few days ago on May 29. He was a visionary and pioneer in creating big HiFi loudspeakers. He saw the room acoustic as a natural component in the audio chain that had to be accounted for in delivering the best sound possible to his listeners.