The Physics of Sound

Learn the attributes of sound, how we hear, measure and record sound.

Sound is an experience that many filmmakers take for granted, yet it is one that completes the world created on screen. That audible world can be as true-to-life or as bizarre as you can imagine, so long as it is internally consistent. Being able to create the sound experience for audiences comes from creativity wrapped in an understanding of how sound works.

What is Sound?
Sound is the disturbance of molecules in a medium, whether that medium is air, a brick wall, or the thin membrane of your ear drum. The sound “waves” move and vibrate that medium causing a resonance we pick up as sound. This is why there is no sound in space – there is no medium to be vibrated, so therefor there is no sound.

On a microscopic scale, sound is the rattling of molecules. When one molecule rattles, it rattles the one next to it and so on. The denser the molecules, the faster this disturbance will be moved, hence denser materials like the iron of a train track will transmit sound waves much faster than in a more spacious medium like air.

Although sound is measured at traveling 770 MPH, the actual speed is dependent on the temperature and climate.PhysicsofSound01

Components of Sound (see chart at right)

Since sound waves are a disturbance of the medium through which they're traveling, we can measure the components of that wave. The attributes of the wave gives sound its sound. If we were to look at a pond as an analogy, that pond represents the medium through which the sound waves travel. A stone represents the sound source, disturbing the medium. By throwing a stone into a pond we can measure the resulting waves in in three ways – the height of the waves, which translates into the loudness of the sound wave; the frequency, which is how tight or wide each individual wave is; and the velocity which is how for the sound wave travels. The frequency translates to the pitch or tone of the sound.

The sound wave is incredibly variable, with a measurable distance ranging from ¾” all the way to 56 feet in length.

Frequency
Frequency is simply the number of vibrations per second, and this is measured in the unit Hertz, or Hz for short. Low frequency sounds which have fewer vibrations per second sound lower to the human ear and tend to be more omnidirectional in the way they radiate outward. This is why you can hear the thumping from a car subwoofer, but can’t determine the direction o the sound source. Conversely, high frequency sounds have more vibrations per second and sound higher to us. The higher the frequency, the more directional the sounds become, making is easier for us to pinpoint the sound’s origin.

We can hear frequencies from 20 Hertz all the way up to 20,000 Hertz (or 20 kHz). Anything sounds lower than 20 Hz cannot be heard, but can be felt. The distant rumble of thunder of falls in this infrasonic range, and although you cannot hear the sound, it is still very powerful. So powerful, that low frequency vibrations around 12 Hz can rattle your internal organs and make you sick. Typically a good movie theater sound system is capable of 20-25Hz, so the audience will not only hear, but feel the low rumble of explosions, a score or other low frequency sound effects.

You may have heard the term "presence" used to describe a sound. Usually a sound with strong presence is easy to hear and understand - such as dialogue. There are certain frequencies that resonate well in our ears, and it is important to know when recording and mixing audio.

Interestingly, you will not get the full impact of a sound unless you give the sound wave a chance to propagate. Propagation means that you have allowed one complete sound wave to form before you can experience the entirety of that sound. This is evident with low frequency sounds where the waves can be 40 feet in length. For example, if you were standing 10 feet in front of a bass guitar amp, you would not be able to hear the low frequency sounds as loudly as the audience 50 feet away because the sound wave wouldn’t have propagated until 40 feet away from the speaker.
Higher frequency sounds, which are more directional, are heard with more clarity. The typical adult male voice is a range of 85 – 180 Hz, while the typical adult female voice falls within the 165-255 Hz range.

We can describe frequency in many ways, namely pitch and tone. The pitch of a sound wave can be easily demonstrated through the notes played on a piano. The lowest note on a piano is 27 Hz, while the highest is 4185 Hz. Middle C is 261 Hz, and middle A is 440 hz.

Ultimately, the tone of a sound can be broken into three ranges. Low frequency sounds provide the impact, mid range provide the presence, while high frequency sounds give the sizzle.

The Human Ear
Our brains are able to interpret the frequency of the sound waves into tone or pitch but what is more amazing is how our ear senses the sound wave. The outside of the ear is designed to guide the sound into your ear, focusing it into your ear canal. Your ear drum is a thin membrane that vibrates and is attached to three little bones – the hammer, the anvil and the stirrup. These bones work are connected to the Cochlea, which are full of tiny little hairs. These hairs pick up the vibrations from the sound waves, which are then translated into an electric signal that is sent to your brain.

Our ears and brains are not only able to interpret sound waves into tone and pitch, but also the direction of the sound. The size of our head is designed to hear mid-range frequencies and we change the way we hear depending on the frequency. At low frequencies, we depend on the intraoral time difference of the time the sounds reaches each of our ears to localize the direction of the sound source. At high frequencies the wavelengths are so short and if we move our head just a little bit our ability to perceive direction would be incredibly confused, so for high frequencies we use the intraoral time differences – meaning our brains can tell the difference in the amount of time it takes for the same sound to reach each ear independently, allowing us to triangulate the source of the sound.

Our brains are so finely tuned to this intraoral time difference that we can perceive a difference as little as ten milliseconds.

Unfortunately, our ears are not perfect and our ability to hear degrades over time. As we get older the little hairs on the cochlea start to die, and we start to lose the ability to hear higher Hz, which is why older people do not hear as well as younger.

Compressing and Transmitting Sound
An uncompressed recording of a sound, which captures all frequencies can be a pretty big file, making it impractical to transmit through limited-bandwidth systems like telephones. As such, the high and low frequencies of the sound are clipped to reduce the size of the data being transmitted. While quality and sonic information is lost, the listener can still understand what the speaker is saying because the frequencies conducive to human speech are kept. This compression comes at a price – while many higher and lower frequencies only add to the presence of the sound and can be affordable lost, certainly high-sibilant tones like “s’s” and “f’s” are lost, making them nearly indistinguishable.

Amplitude
The volume – or loudness – of a sound is dictated by the amplitude, or height of the sound wave. This amplitude is measured by the baseline – which is the middle of the sound wave – we are able to hear any sound above the baseline.

Some of our perception of loudness is based on the frequency of the sound. High frequencies at a lower amplitude sound louder than high amplitude, low frequency sounds. As such, our ears don't perceive the change in volume equally across all frequencies, especially since it’s harder to hear lower frequencies – we tend to feel them, so a comparable increase in volume of a lower frequency will be as noticeable as a change in a higher frequency.

We can measure the volume of sound through a logarithmic scale measured in decibels, or DB. Much like the way a Richter Scale is used to measure the strength of an earthquake, each step in the decibel scale increases by a power of 10.

A general rule of thumb is that by doubling the volume of a sound will yield an increase of 3 dB on the scale.

Conclusion
So in review, sound waves ripple the medium through which they're traveling, and have two components: Amplitude, which equates to the volume, and Frequency, which equates to pitch or tone.

Understanding how sound functions is the key to understanding how to record it. As is the is the case with any artist working in any medium, the more you understand your tools the better you will be at creating a compelling sonic experience for your movie.