The negative of music
March 7, 2011 7:51 AM   Subscribe

What would you hear if you played the negative image of an LP?

I was talking to a friend this weekend about the few LPs we both still have. He mentioned that he had learned a surprising trick: molten wax can be used to clean the dust and microgrit off of them by putting a generous layer on, letting it dry, then popping the wax off with all the scuzz embedded in it (if you're trying this at home, he mentions you need a lot: a thin layer just crumbles and you do more damage picking off the individual remnants).

He further mentioned that when he removes the sheet of wax, he is left with the negative image of the LP, and wondered what would happen if he tried to play that on a turntable. Notwithstanding the question of whether or not the needle would just dig into the wax instead of riding along it, what would he hear? I suspect it would just be white noise: the stylus would be riding in a groove which is the negative of the ridge separating the grooves on the LP. He wonders if you might get some weird, out of phase, backwards ghost of the music through the noise. What does the hive mind think?
posted by ricochet biscuit to Media & Arts (10 answers total) 5 users marked this as a favorite
Here is a picture of a record groove. The groove is V-shaped, and the stylus will travel through the deepest point. The 'antigroove' is U shaped, so the exact path is uncertain, but it will stay between the left side of groove 1, and the right side of groove 2. I suspect the sound will be a 50-50 mix of both grooves, with a lot of high-frequency noise added because the stylus can wiggle between those to origional grooves.
posted by Psychnic at 8:00 AM on March 7, 2011 [1 favorite]

All other real-world variables excepted, what you might hear would be the left channel of one section of music and the right channel from the part of the music that plays on the next rotation of the LP.

It wouldn't be backwards, and it would no longer be in the correct relation to the other side of the stereo image, so phase wouldn't be an issue either.
posted by Aquaman at 8:02 AM on March 7, 2011 [1 favorite]

It wouldn't be backwards, but you would need to rig the turntable to spin backwards. The start of the groove is still the start of the groove, the rotation direction would just have to change.
posted by AzraelBrown at 8:06 AM on March 7, 2011 [2 favorites]

Oh no! Here is a record which will only result in noise. The length of a soundwave in the groove is very small compared to the width of the groove. also: origionaloriginal
posted by Psychnic at 8:08 AM on March 7, 2011 [1 favorite]

This is a very interesting question. I suspect Aquaman is right; with a standard needle, it would ride in the valley between adjacent ridges and you'd get a roughly 50/50 mix... I wonder what you'd get if you had a V-shaped needle meant to ride along the ridge?
posted by xedrik at 8:17 AM on March 7, 2011

I ruined a record doing your friend's trick (accidentally) when a candle fell on my record during a DJ gig.
posted by mkb at 8:31 AM on March 7, 2011

As others have pointed out, the wax impression doesn't really make a negative of the sound because the needle won't ride on the wax peaks that come out of the vinyl grooves. However, if what you really mean is a mirror image of the wave in which the needle now goes up where before it went down and left in place of right, then you wouldn't hear any difference. You can test this by just reversing the two wires connected to a speaker. That is effectively the "negative" you are talking about. The speaker cone goes out where before it goes in. You won't hear any difference.

(Nice one, tomswift.)
posted by JackFlash at 8:42 AM on March 7, 2011

Actually, the mirror copy's spiral would go the other direction. If your turntable kept turning clockwise, the mirror copy would play starting from the center, going out.
posted by Chocolate Pickle at 9:49 AM on March 7, 2011

To add to JackFlash's comment...if you reverse the phase of one speaker in a stereo pair, you *will* hear a difference. Any common (center-located) sounds will be canceled by the out-of-phase alignment. Reversing the phase on both speakers is indistinguishable from correct phase on both.
posted by rocket88 at 9:50 AM on March 7, 2011

if you reverse the phase of one speaker in a stereo pair, you *will* hear a difference. Any common (center-located) sounds will be canceled by the out-of-phase alignment.

Theoretically, yes. But given that the wavelength of sound above 1000 Hz is less than a foot, just moving your head slightly back in forth between the speakers will produce the equivalent phase shift.
posted by JackFlash at 6:45 PM on March 7, 2011

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