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Building Blocks 20: Mic Technique 3

Mic Technique pt 3: What is Sound?

What we hear as sound is actually a form of energy called acoustical energy, which is a form of physical kinetic energy.  It is the fluctuation of waves of pressure in a physical medium.  In our case this is usually air, but aquatic creatures prefer sound waves to be transmitted through water.  As a wave, sound has 2 distinct and indirect quantifiable attributes: frequency and wave length.

Frequency is the rate at which the fluctuations occur and is represented by the unit Hertz (Hz) which indicates how many cycles occur per second.  The full range of human hearing is 20 Hz to 20,000 Hz.  A speaking voice is between 100 Hz and 6 kHz (kiloHertz – 1 kHz = 1000Hz) but about 80% of the energy of a speaking voice is below 500 Hz.  Lower sounds, as in the bass end, are lower in frequency while the high end of sound (treble if you are playing with your car stereo) are higher frequency sounds.

For those that are musically inclined, octaves are a special musical interval between 2 sounds and are a ratio of 2:1.  This means that if one sound is a single octave higher then it will have a frequency of exactly double the lower octave sound.  A octave above 100 Hz is 200 Hz while an octave above 2 kHz is 4 kHz, but to the human ear the difference of these 2 examples will be musically similar.

Wavelength is the amount of distance a wave travels during 1 cycle.  Because the speed of sound is a constant and known quality (1130 ft/sec or 344 m/sec at sea level at 59 F, 15 C) that is independent of frequency, we can easily calculate wavelength = speed of sound / frequency.  This equation shows us the indirect relationship between the 2, as frequency increases wavelength decreases.

Have you ever pulled up alongside someone at a stop light and noticed that their music is exceptionally loud and that it sounds like they turned the bass up?  While they may indeed have, you are in fact experiencing a function of wavelength.  A wave with a larger wavelength (and shorter frequency) will pass through physical medium easier than a wave with a shorter wavelength (and higher frequency).  High wavelength sounds also are more omnidirectional (that is their sounds go in all directions).  This is why you can hear low end sounds from farther away and why things like fog horns use low end sounds.  In sound we generally ignore wavelength because it gives us little relevant information and is inherently implicit with frequency.

As a wave, sound also has what we call “phase.”  This is the time relationship to a known reference.  This can be an arbitrary fixed moment or it can be another wave and is usually references in terms of degrees.  If 2 waves are “in phase” then they will add together creating a larger wave that is the summation of both waves.  If 2 waves are 180 degrees “out of phase” then they will cancel each other out. (If you’ve ever had to add together 2 sine waves in graph form that is exactly what I’m talking about here).

Waves also have the attribute of amplitude.  This refers to how “high” the wave is and is independent of the previous 3 parameters.  You can also think of this as the displacement of the wave in relationship to the absence of a wave.  If you are observing an ocean wave, the amplitude is literally how high (or low) the wave gets in relation to a calm/smooth surface.  In sound amplitude is descriptive of volume or loudness.

Sound levels are usually measured in decibels (dB). This is always a ratio of 2 quantities and decibels are logarithmic.  While this makes the math more complicated, it allows sound levels to be expressed in values that are smaller than they otherwise would be.  Because decibels are a ratio, they do not have an absolute value and they can in fact be used to quantitate multiple qualities of sound and the related power.  Most commonly it is used to quantitate dB SPL (sound pressure level).  SPL is the level of energy measured at a specific point relative to the sound source.  We arbitrarily define 0 dB SPL as the threshold of hearing of an undamaged (ie young) ear between 1 kHz and 4 kHz (the human ear’s most sensitive range).  A difference of 3 dB SPL would barely be perceptible as being louder while a difference of 10 dB SPL would be perceived as twice as loud.  20 dB SPL would be 4 times as loud, that is twice as loud as twice as loud (this is where the logarithmic operation comes into play).  Here’s some relative values as a (rough) benchmark (all values in dB SPL).

65 – average conversation from 3 ft

80 – aircraft cabin at cruising

120 – threshold of pain

130 – 50 HP siren from 100 ft

140 – .45 Colt pistol from 25 ft

If you’d like to read more on these subjects I recommend Sound Reinforcement Handbook by Gary Davis & Ralph Jones.  Much of the content here is covered in Chapters 1, 2, and 3

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