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I am not sure where to post this, but I hope it fits here.

If you have ever recorded music, and play the track backwards, you'll notice it sounds quite strange, almost demonic. I have always wondered, why doesn't music sound good when played in reverse? It just doesn't seem at all intuitive to me, if you had never heard classical music recorded and played backwards you might expect it to still sound ok. There is so much structure in the sound, so why does it seemingly disappear in reverse?

Why does the harmony of the notes break down, and become (what appears to sound) so chaotic. This doesn't work the same way if I just play the notes in reverse on a piano. Scales are a perfect example, and sound identical in either direction.

I hope this is relevant, it is something I have always found curious.

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The solo of Hendrix's "Castle made of sand" was pretty cool..doesn't sound bad at all. May be if you reverse something bad or random you'll end up with something cool sounding. –  KMC Feb 2 '12 at 8:54

5 Answers 5

Eric,

Classical music, and really, most kinds of music, sound odd when played backwards because the harmonic progression does not lead where you expect it to. Without going to deep into music theory, I'll outline the basics.

A typical cadence, or ending of a chord progression, is a IV chord to V to a I chord. (Roman numerals signify chords built out of a major scale, starting on that degree of a major scale, so V is the fifth scale degree.) This cadence is a very typical one, and you'll hear it everywhere. What that cadence does, is go from the chord with the most tension in the key… to the tonic, or home chord of the key. In this way, you have tension and relaxation, which is the foundation of harmony.

A more advanced chord progression might go I-vi-ii-IV-V-I. This progression sounds different, but it still ends with that characteristic IV-V-I, also called a Perfect Authentic Cadence.

Now getting into one of the reasons music played backwards sounds so odd: Our ears expect that Perfect Authentic Cadence. (Simplifying here, there are other pleasing cadences.) Sometimes, a composer will do a cadence like: IV-V-vi. This cadence is called a Deceptive Cadence, because your ear will expect the IV and V to inevitably lead to I. When it goes to the vi you get a denial of expectations. While I'm simplifying, these basic ideas of tension and relaxation are what gives music it's sound, feeling, attitude, soul, etc.

When these characteristic cadences and progressions get reversed, it essentially throws that whole system out the window. Now you're going I-V-IV-ii-vi-I and not only does that defy expectations, your brain likely is just going WTF?!? For lack of a better term…

To hear these cadences for yourself play this on the piano: enter image description here Courtesy this Wikipedia article:

To hear a recording of just these three chords, go here.

Hopefully this makes sense!

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The reason why music sounds so odd when played backwards has little to do with musical theory per se. It's all about the our psychological expectations of the physical sound phenomena.

Our brain expects instruments to sound a certain way, it expects a piano to begin with the attack, a quick rise in intensity, then stabilize somewhere along the way and then fade away smoothly. When we hear a reversed piano all these aspects are now reversed and it becomes a sound that begins quietly and ends with a powerful attack, something we're not familiar with because it's a rare thing to hear in nature (it might not even exist naturally).

The same goes for all instruments. Our brain uses the wave shape and variations in volume and other aspects of a sound to extract musical information. When it's listened in reverse the lack of familiarity with the sound structure makes it sound "odd", "scary", "demonic". This is even used as a compositional element, as you might be aware of artists recording reversed vocal tracks or instruments.

To further illustrate what I mean, listen to these sound samples generated with Timidity and the Unison soundfont. A little melody followed by some chords is played and in sequence it's played in reverse:

See how the piano sounds weird, but in turn the flute and strings are "passable"? We might notice there's something wrong but it isn't as much as the piano. Try opening these files in a program like audacity and observing the shape of the sound waves.

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+1 This is a good point as well. I'm not sure it alone can explain the phenomena, especially since some instruments have a more "symmetrical" sound than others, but it certainly has a big effect in many cases. I can't imagine playing a recording of my trombone playing backwards… my attacks and releases are occasionally shoddy enough as it is! –  Josh Fields Jan 30 '12 at 16:47
    
This makes sense to me, nothing in nature smoothly amplifies to reach a sharp noise and then stop altogether. Of the mandmade things in our society, all I can think of that would do this sound terrible anyway. (If you use powertools, I think certain cutting noises will do it, but I believe there is concensus that powertools sound really bad.) –  user1864 Jan 31 '12 at 10:58
    
I believe power tools have been used in music. –  horatio Feb 1 '12 at 18:14

The above are all good points. Another thing I want to point out is that many instruments are in a sense defined by their attack note. I hypothesize that by cutting off the attacks in a recording of many different instruments playing the same pitch, the tones are largely indistinguishable.
So, if the attack is really as important as I think, 'backwards-tape' playing of a song/piece will loose much of its characteristics, as the attacks do not define the note or pitch, but only briefly punctuate them. The strange-sound effect is much more overwhelming this way.

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Yes indeed, this seems to be the case many times. In fact the brain uses the clues given by the attack and the different volumes of the sound and its harmonics to perceive musical information. I know there's some papers analysing this somewhere but I'm not finding them now. –  lfzawacki Jan 31 '12 at 21:13

The synth people categorize music by four categories: attack, decay, sustain and release. Attack is the initial peak when the note starts, Decay is the drop from the attack's peak to sustain's steady state, and Release is how long the note stays around when you stop playing it.

Piano and guitar are known for pronounced attack, and as lfzawacki noted, piano sounds stranger reversed. Guitar is the canonical instrument for reversing. In general, you can strike a note, get a big note that comes down and fades, or you can blow a note, which stays fairly constant until you let go. Reverse the second type and it sounds about right. Reverse the first type and you reverse the way sound works.

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The answers herein all make great points, but chord progressions, ADSR envelopes, and expectations aside, the notion of music "sounding bad" is purely subjective to begin with. (Although the percentage of agreement may be higher on some pieces than others!)

Some compositions, particularly in the electronic music domain do not follow "traditional" ADSR envelope and spectral distribution expectations to begin with. There are many aesthetics of what makes a musical piece "sound good". Rhythm, spectral distribution, harmony, and balance are all still artistic tools that can be used and exploited in reverse. Don't be afraid to experiment. You may just find something new that you otherwise wouldn't have appreciated!

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