Wednesday, April 7, 2010

What is Sound? The Physics of Sound.

Before I write this, please realize, that I am not an expert. I am just a music nut who studies all parts of sound, and then write about everything I learn to help my brain remember everything. I did not go to school for this, but have been self studying for years. This is just a basic scientific understanding of sound, explained with graphs from all over the internet I have found to make my points stand out. Without people putting free info on the internet for me to study, I would never know any of, I felt like I should give back by making a nice understanding with better graphs and in my own words. This is to help me, and if it helps you, great!

What is Sound?

A sound is a vibration. No vibration, no sound. When a source vibrates, it moves particles beside it and vibrates in a chain reaction. Usually, this means air particles, but it can also be water, gas, and other kinds of particles. But, here, we will talk about air particles because that is what we usually deal with in music. When a source vibrates, and then vibrates particles, it comes from a source location (where it is the strongest), and then moves outward in wave fashion until it reaches and vibrates our eardrums. The sound does not travel in bulk, but rather in a sound wave coming from the source, and then disturbs the surrounding area...slowly getting weaker. This is simple energy displacement. Sound requires a medium (air particles), therefor, sound cannot travel in a vacuum. It reacts the same way as if you threw a rock into a still lake...the water ripples will form, and then disappear...returning to its original form.

Now, once this displacement of particles has occurred, we are left with 2 regions. 1 region is of high particle density, and the other is of low particle density (or high pressure, low pressure).

2 keys words to remember

Compressions (Condensation) - Regions of high particle density
Rarefactions - Regions of low particle density.
Rarefactions and Compressions both move in the same direction that the wave travels.

Now, the particles of air don't actually MOVE (physically) in the direction of the wave movement, but instead move within their normal positions in a cycle fashion. 1 cycle is when a particle moves from its STARTING POSITION, to the maximum displacement distance in one direction, back to the starting point, and then to the maximum displacement distance in the other direction.

Now this is just one cycle. Particles can vibrate THOUSANDS of times per second in this fashion. The number of cycles completed by a wave in one second is considered the FREQUENCY of vibration. One way to easily tell how many cycles a wave is making is by listening to the pitch. The pitch is very much affected by the number of cycles occurring. (more cycles at higher pitches)

In music terms, 1 cycle = 1Hz. (Hertz), 1000Hz = 1kHz.

The human ear can hear (depending on the person of course) anywhere from 16Hz - 20000Hz
(normal being around 20Hz - 18,000)

A normal piano range is in the graph below -

But just because a wave is at a certain frequency (on an instrument), does not mean it is going to SOUND the same. This is because of TIMBRE of the sound. If you play C4 on a piano, it will sound different from a C4 on a flute or a tuning fork.

To understand this, you must understand PURE TONES and OVER TONES.

A pure tone is one that is created by a perfect wave (like a tuning fork, or a tone generator). These contain a ONE TONE frequency, meaning, it only creates one tone on the frequency spectrum. A note played on a piano or any other instrument will have OVER TONES, which are other tones within the spectrum accenting the fundamental it a unique sound. The tone with the lowest frequency is called the FUNDAMENTAL TONE. All other tones are OVER TONES.

This is where we get into harmonics. If an over tone has a frequency value that is a multiple of the fundamental tone ( x2, x3, x4, up to x14), these tones are called HARMONICS. It is the difference of each instruments over tones and fundamental tones that make up the sounds TIMBRE. Therefor, even though an instrument may produce a fundamental tone at say 4kHz, it has overtones at much higher frequency values. This is why most DAWs and other recording gear must be able to handle a much higher frequency than can actually be produced by the fundamental tone of an instrument. There will be MUCH loss in the dynamics of the sound if it is not able to be captured. Some instruments like cymbals or dog whistles produce frequency levels much higher than the human ear can hear, but they effect the whole sounds TIMBRE if not captured correctly. This is why many analog purist stay analog, and not digital...where most frequencies are cut at 20,000Hz. Below is is a graph of a general saw wave and its harmonic overtones.

Now there are 2 types of sound waves (we will break these 2 kinds down even more in a minute)

One is a wave with a definite pitch (we call a note). Another kind is one with no definite pitch (we call a noise). Music has both almost all of the time. The difference is definitely audible, but what is the scientific difference? A note contains regular vibrations (periodic motion), and a noise contains irregular vibrations(non-periodic motion).

Now if we break down the periodic motion vibrations, some of the common vibrations are below -
Most of these waves are created by synthetic sounds. Traditional instruments function in a sine wave form.

In music, we divide these sound sections into groups called OCTAVES. 7 different notes A-G, & the 8th note being the start of the next octave. Each note named the same letter sounds very similar to the others named the same letter because they are in a multiple of the others frequency. (x. 220Hz, 440Hz, 880Hz). This means the actual frequency rate (or cycles) is 2ce the speed of the previous. These notes are said to differ in pitch by ONE octave.

Now lets talk a little about amplitude. A loud sound is produced by more violent vibrations than that of a weak sound. The amplitude is the maximum displacement of the air particles from their original place. As the sound wave continues to displace particles in a wave fashion, it is displacing energy (energy loss). That is why the sound will get weaker and weaker. The energy is displaced in the form of HEAT, and the sound will get weaker as it travels away from its source. To understand graphically, the farther away from the center line on the graph that the wave gets, the higher in amplitude it is.

Now scientifically, the amount of energy loss follows the "Inverse Square Law". (but keep in mind, this is not what the human ear actually hears...because of our ears compensation system).

Scientifically, if the distance from the source is TWICE the distance, the sound drops to one quarter. If the distance is 4 times the distance, it drops to one-sixteenth.

Now, if you think about this....this doesn't make much sense. Because if you are at a club, listening to some speakers, and you move twice your the distance away from the speakers, the audio does not seem to drop in these proportions. This is because of the compensation system in our ears due to the 3 pivot system in place. It helps your ear create leverage on desired frequencies. This system lets your ears hear weaker sounds at maximum strength, while louder sounds are reduced to prevent damage to your ears. That means, that the scientific measurable amplitude differs from what is heard, and what is actually happening. We refer to this phenomenon as "LOUDNESS". Loudness is what we use to measure what our EAR hears in change, and not the scientific term of amplitude. Loudness is also determined by how sensitive your ears are, and differs person to person. It is measured in decibels. (db)

Now, what happens when 2 sound sources meet at the same time. If 2 waves arrive at the same place at the same time, at the same high pressure and low pressure displacement areas...we will get what is called "Reinforcement". (in-phase) This will result in a greater wave intensity. If 2 waves arrive, and one is a half of a cycle late, the high pressure of the one wave, and the low pressure of the other wave will cancel each other out. This is what is called "cancellation". (out of phase)

In the photo, graph 1 is in phase, graph 2 is out of phase.

If we were to set up 2 speakers producing perfect waves at 1/2 the cycle speed difference, this is how it would sound to your ear (cancelled) -

Now, how do we measure all of this. Well, the equation for "Speed of Sound" is rather simple. I liked this tattoo to help explain -

V = speed of sound
f = frequency
λ = wavelength (lambda in greek)

A wave front is a surface on which all particles are in the same phase of vibration. The distance between the 2 wave fronts is called the wavelength. (or the distance between 2 separate compressions, or 2 separate rarefactions)

Now obviously, the speed of sound is affected by other things. This equation only works in a perfect environment with no obstacles. In an everyday situation, we have air pressure, and physical objects that can obstruct a waves path.

When a sound wave is made in air thats temperature changes with altitude, REFRACTIONS occur. This is why sound can travel at different speeds depending on air pressure and particle density.

So obviously the equation for the speed of sound has variables.

Well that's it for my small piece about sound. If you have a better understanding of how sound works, you can begin to change it the way you want rather than just randomly.




Anonymous said...

very informative thanks

Descoindia said...

I had visited your website which was really good Physics Sound

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