Tuesday, December 19, 2006

Depth Perception and the wave

Mike Jorgenson

I tried to do a little analysis of the frequencies in the waveforms in relationship to the sense of percieved distance/depth in the recording. First, a few disclaimers. 1) I'm not 100% sure I'm analyzing the frequencies right. 2) The sounds I've chosen are based on what I feel to be good (or poor also) examples of depth. That said, here we go.
I looked at the frequencies using the spectral frequency view in Adobe Audition. I took tests 03-01, 03-14, 08-3, and 10-11 to try to compare a few things. First, I took test 03-01 and 03-14. These clips are from the same test and the particular clicks I compared are the same click. In addition, I chose 03-14 because it's position was not too far off of the 03-01 position. So, in theory they should sound generally the same. However, they are from different rigs. I found a few things. First, I thought in general the 03-01 sounded like there was a little more distance to the clicks. From just hearing it I thought that one of the major differences was the middle and background noise that affected this depth perception. It seems that the brighter the middle and background tones the closer the object is perceived to be. Comparing the two waveforms this seemed to be confirmed. (This sound is the 03-01 rig click 2x, followed by the 03-14 rig click 2x). The 03-14 rig has much more green and orange in this picture, signifying higher energy sound, in the 4k-10k frequency range. In comparison to the other tests I looked at it seems that the tests that sounded closer (08-03 and 10-11) had more frequency response in this same range. The lower frequencies seemed to roll off in the decay whereas in the 03 tests they did not. They seemed to remain consistent from about 6khz down. In the 08 and 10 tests you can visually see this as the green dissipates bulges in the middle (looking vertically at the click) and then sucks in at the lower frequencies. However, there is also something else to it. Looking horizontally across a small range of frequencies (say roughly one 'spike' horizontally) there are different shapes produced by the high energy and low energy colors through the decay. For example, the 03-01 test has a fat, thin base on the left that spikes fairly quickly (see pic). Test 08-03 has a more bulbous base with only a small spike. And test 10-11 has a fairly consistent slope to it. I viewed test 08-03 as having better depth percieved than test 10-11 and worse then 03-01. So it seems that the more gradual the slope of the energy, the closer the sound appears to be.
The rigs that I have chosen to look at actually says something about this I think. Test 03-01 is one of the tests on the dowel with only a simple baffle. Test 08-03 is a double boundary. Test 10-11 is a triple boundary. It seems that the more boundaries you have, the closer the sound is percieved to be at. Like I said though, I'm no expert. But this is what I found to be true.

Stereo vs. Mono. Is it worth it? By Anna Krutzik

For my test of different variables I chose to see if I could hear any difference between the three tests I had chosen as a good representation of stereo when I put them in mono. To my ears, all of the original tests sounded basically the same, with only very minimal variations on each other. While I could hear the stereo effects in the localization of the dings, not much else seemed to really make it "stereo" sounding. And the dings were sort of tempermental, you could tell the direction of some, but not completely all of them.

So, to test my ears, I compared the same tests in both stereo and mono and noticed an immediate difference between the two. In creating a sense of space, the stereo versions win hands down, the mono versions sounding flat in comparison. Of course, hearing the sound surrounding your ears (in stereo) rather than being duplicated into both ears the same (in mono) creates the feeling of actually standing in the room because it mirrors how a person would actually hear, each ear picking up on different sounds based on their positioning and putting them together to create a composite of the space.

As for the localization of the dings in mono vs. stereo, I didn't hear as much of a difference as I would have expected. For whatever reason, I could consistently hear more difference in the location of the dings between 1-4 in the stereo recording. I'm not quite sure if this is my own mind just subconsciously interperating the difference between hearing the dings from the left to the right side as thinking that the right side dings are more localized. Or if there is a specific technical reason for this. So in the mono versions of the tests, I didn't notice that much of a difference in hearing the dings on the left side.

I thought that maybe listening to the recordings in mono but still viewing the animations which illustrate where the ding is coming from in the clock positioning would fool my mind into thinking that I heard it the same as in stereo. And while it did a little bit, there was still an obvious difference when watching the animation and listening to the stereo version.

All in all, I now appreciate much more fully the value of stereo sound recordings and their ability to recreate and place the listener in the space of the original recording.

I recommend listening to the mono versions first and then to their stereo counterparts immediately after to get the full effect that the stereo brings.

Mono Versions

Stereo Versions