A sound is a wave that is both a `mechanical wave`_ and a `longitudinal wave`_.

Sound waves are often graphically represented as `sine waves`_, where the low points on a sine wave correspond to rarefactions and the high points correspond to compressions.

Sound waves cause changes in `air pressure`_. We can't see these ripples, but our ears can hear them. When sound reaches our ears, the air pressure goes up and down and this make our ear drums go in and out at the same rate. Our brain analyzes these signals and interprets them as sound.


1   Properties

1.1   Speed

The speed of sound through air is 343 m/s. This is affected by altitude, humidity, and temperature of the air. Sound travels faster through liquids and solids because the molecules are closer together. The speed of sound is 1482 m/s through water and 6420 m/s through aluminum.

Note, the speed of sound is how fast the disturbance moves, which is different than the speed of the air.

2   Perception


Wave shave determines timbre, not pitch.

Sound wave have four main qualities, which determine our perception of them.

  1. Frequency, which determines pitch
  2. Amplitude, which determines volume
  3. Wave shape, which determines timbre (For example, sine waves vs digital waves. Altering wave shapes is done extensively in `electronic music`_.)
  4. Phase, which determines how waves interact

Animals and different auditory fields. Humans can only hear sound waves as distinct pitches when the frequency lies between about 20 Hz and 20,000 Hz. Dogs can hear up to 40,000 Hz and whales can hear up to 175,000 Hz. Children have a larger auditory range than adults. The lowest key on a standard piano is the note A and is 27.5 Hz. The highest key on the piano is the note C and is 4,186 Hz.

A dog whistle uses ultrasonic sound which we cannot hear.

2.1   Loudness

The loudness of a sound is determined by the intensity of the sound wave.

Below 1 picowatt per square meter, sounds are too soft for us to detect them. Although we will hear sounds above a watt per square meter, they tend to hurt our ears.

Sound waves must have 10 times the intensity to sound twice as loud to us. The intensity of sound is measured in decibels which are based on bels (a decibel is one tenth of a bel). Bels convert the intensity of a sound wave to a logarithmic scale, where every notch on the scale is ten times higher than the previous one. 1 picowatts per square meter corresponds to 0 bels.

Sound pressure level equivalents [1]
SPL-dB Common example
140 Irreparable damage
130 Jet aircraft taking off
120 Threshold of pain, Thunder
110 Threshold of discomfort
100 Dirt bike, riveter
90 Start of unsafe levels
80 Average factory
70 Kids at play
60 Normal conversation
50 Elevator music
40 Normal home background
30 Studio background
20 Rustling leaves, quiet whisper
10 Butterfly swoop
0 Threshold of hearing

Commonly sound wave decibels (dBA) are rated against the softest sound that a human can hear (0 dBA).

120 dBA is 1012 times the intensity of the quietest sound you can hear (0 dBA), which means our hearing has something like a 1,000,000,000,000 to 1 dynamic range. Also, the loudest possible sound without distortion in normal earth atmospheric conditions is about 194 dB SPL because the de-compression side of the sound wave is a vacuum at that point, so you can't have sounds louder than that without distortion.

  • What is the difference between dB, dBA, and dBm?

    But don’t certain sounds still have a set number of decibels they make? So couldn’t have we have “gunshot”, “airplane engine”, “crying baby” as the volume options?

    This is just bad media interpretation of the system.

    A gunshot next to your ear isn't the same as a gunshot 30ft away. There is an assumption of distance from the source of the sound when they make those examples.

2.2   Pitch

Pitch and be high or low. High pitch corresponds to high frequency. The shorter the wavelength, the higher the frequency, and the higher the pitch of the sound.

When you play Middle C on a piano, the frequency is 262 Hz. When you play Middle A above Middle C, the frequency is 440 Hz.

Pitch is determined by the frequency of the sound waves. When you speed up a playback, it shortens the duration between the sound waves, resulting in a higher frequency and thus a higher pitched sound.

To be more accurate, the higher pitch is not because you're shortening the duration between the waves but rather because you are shortening the waves themselves (actually more like squeezing them)

the accurate way of saying what you're both mangling is that sounds occur via frequency and speeding up sound literally means you're increasing frequency

Like loudness, the pitch of a sound changes based on the distance of the source. This called the Doppler effect. As a source moves toward you, the pitch of as sound increases, and it moves away, the pitch decreases. This is because as a source moves toward you, the peaks that hit your eardrums are closer together, and you get hit by them more often, which means you hear a higher-pitched sound. The opposite happens as a source moves away. This is also how sonic booms are created.

2.3   Notes & Noise

Both noise and notes are mixtures of sound waves at different frequencies.

A note is a regular and evenly spaced wave; a `standing wave`_. This can be generated by a repeating string. Notes may be complicated or not; the essential feature is that they repeat. Anything that vibrates and disturbs the air in a regular pattern will produce a note.

A noise is made up of individual ripples which have no relationship and do not repeat.

Some repeating waves are so complex that our ears cannot detect the pattern and we perceive them as noise.

3   Substance

Sounds are usually described by three characteristics - pitch, loudness, and quality (or "timbre"). Pitch is determined by frequency and describes how high or low a note is. Everything else is generally categorized as "timbre," which is what allows you to tell the difference between a piano and violin playing a note at the same pitch and loudness.

Quite a bit goes into timbre such as harmonic content, attack and decay, and vibrato. I don't really know enough about those to explain them further other than to say they are characteristics of sound waves that affect how you hear, but don't change the pitch or loudness.

The timbre, or the kind of sound, is determined by the type of waveform. A guitar, for instance, has a sine wave, as can be easily seen in slow motion. A violin, on the other hand, has a sawtooth wave, due to the string constantly catching the bow as it vibrates. Percussion has a scattered waveform that doesn't resonate well, hence why most people don't think of it having "pitch" in the same way other instruments do. What we think of as electronic sounds are usually a square waveform.

4   Instruments

The vibrating string causes the wood to vibrate at the same frequency, thus amplifying the sound. You can see this by hitting a tuning fork. By itself, the fork is quiet. But if you put the fork on a piece of wood, the volume will increase. This demonstrates that we are hearing the vibration of the wood, not the tuning fork. (You can also put it on your head.)

5   Study

Acoustics is a branch of phonetics that studies the physical transmission of speech sounds from speaker to the listener.

6   History

Sort of a real answer: Microphones used to be real shitty, so you had to speak loudly (in early days, you pretty much had to straight up yell) and that lent itself to a certain manner of speaking, to mitigate the fact that you're IRL having to shout-speak.

Listeners wouldn't know this at home (radio only days), and they would just assume you were yelling at them if you did it wrong.

This ties into why so many people seemed to have a Trans-Atlantic accent in the old days of media: You simply had to use a specific dictation method to make your voice come across loud enough and understandable enough to be blasted into everyone's homes and have it be anything other than totally inaudible "person speaking" noise layered between CHHHHHHCHHHSHSCHHHHH (radio noise), which tended to be way more of an issue, although I am not an audio engineer, so maybe someone who knows stuff can take it from here.

7   References

[1]1983. Rane Corporation. Understanding headphone power requirements.

Sound travels 5x faster in water. Above-ground noises are significantly attenuated when they cross the air-water barrier. So every sound originating above water is muted. Underwater sources aren't affected by this. However, making noise underwater requires more energy per dB, because water is 1000x as dense. You need to vibrate 1000x the mass, which is 30dB 60dB (darn convention changes) of attenuation. Our ears also judge sound logarithmically, where each 10dB sounds about 2x as loud. So equal sound sources at equal distances underwater sound about 32x 64x as quiet.

Additionally, you'll kind of feel muffled, not just from the drop in volume, but because the sound doesn't seem to come from any direction. Our brains are very well trained to find the direction of a sound source by the difference in time of arrival between our ears. That tells us left, right, or center, and the shape of our ears and face blocking sound from certain directions helps us judge forward/backward and up/down by subtle differences in volume. Sound traveling 5x as fast makes the time delay only 1/5 as long. And since the volume is already significantly attenuated, we have trouble judging forward/backward from the small difference in volume caused by the shape of our ears. So everything sounds like it's coming from right in front of us, or on top of us.

A speaker may include a "tweeter" above its main disc(?). The tweeter is devoted high frequencies. You get better sound quality by separating frequencies in the same way having an instrument for each part of an orchestra is a more nuanced sound than a small quarter trying to play the same piece.

Super-sonic airplanes never became popular because of the issues they cause.

Highways have sound barriers.

You hear your own voice as it is transmitted to your middle ear through bones. Other sounds, however, travel through the ear canal, the process by which others hear you. Sound waves transmitted through bone have a different frequency than those transmitted through air, and so your own voice sounds different to you than to anyone else.

-- Popular Science. September 1999. Why does my voice sound different when it is recorded and played back?

You can visualize sound by placing a mirror on top of speaker (or back-to-front drum) and shining a narrow beam of light onto it. As the speaker vibrates, it will cause the light to move up down against a wall in response to the sound.

NRR (Noise Reduction Rating) is. Simply put, this is a standard value set by the United States Department of Environmental Protection for how much noise can be blocked . It makes the sound quieter by the decibel of this number.

If the quality is good, this number will exceed 30.

MOLDEX Goin'Green ear plugs are NRR33.