Natural sound

By Nick Kovarik 

Acoustic or natural sound is the propagation of vibrating energy through a medium; typically air. When an object vibrates, the air molecules near it compress and stretch, radiating outwards. This process is called compression and rarefaction. 

There are 5 major properties of acoustic sound. For simple digital recordings, only the first two are fundamental. Recall them with the mnemonic, Find A Very Wide Parachute.

  • Frequency:   

    1. The number of compression and rarefaction cycles that a vibration completes in one second, measured in hertz (Hz.)
    2. Most people are able to hear between 20 hz (ultra low frequencies) and up to 20 khz (20,000 hz, which are ultra high frequencies.)
    3. So we could say that the maximum range of human hearing is composed of approximately 10 octaves. For reference, a grand piano with 88 keys
      1. 20-40 hz, 40-80 hz, 80-160 hz, 160-320 hz, 320-640 hz, 640-1.28 Khz, 1.28-2.56 Khz, 2.56-5.12Khz, 5.12-10.24 Khz, and 10.24 Khz up to the maximum range of hearing.
  • Amplitude:
    1. Amplitude is the power of sound waves, experienced as loudness. If we think of Frequency as an X-axis (representing time,) Amplitude would be the corresponding Y axis (representing intensity.)
    2. Loudness in audio is measured in decibels (dB) which has no specifically defined physical quantity. The decibel is a logarithmic function that is used to compare the ratio of acoustic energy between two sources; typically between acoustic energy (Sound Pressure Level / SPL) and an electrical signal.
    3. The reason the decibel is a logarithmic measurement is to make managing loudness simpler, because the human ear is capable of hearing acoustic energy at a ratio of  1:10,000,000 and greater. If our hands could measure weight the way our ears measure loudness, we could accurately weigh both a feather and an elephant.
    4. In general, an increase of 6dB is a doubling of perceived loudness, and a change in 1dB is about 12% of the signal. Small numbers in this case can make large differences.
    5. In terms of acoustic energy, 0 dB-SPL is the threshold of hearing, 60 dB-SPL is the loudness of an average conversation, 120 dB-SPL is known as the threshold of feeling, and 140 dB-SPL is the threshold of pain.
    6. Lastly, human beings are not sensitive to all frequencies equally. We are much more sensitive to midrange frequencies (about 500hz – 5khz) than we are high or low frequencies. Therefore, the frequency response of audio can perceptively change when we change its volume.
  • Velocity:
    1. The speed of sound, which is approximately 1,130 feet per second at sea level at a temperature of 70° F.  This is crucial for live sound in open air environments, but it  varies wildly based on a myriad of factors, and is not usually a component of small scale recordings.
  • Wavelength:
    1. The physical length of sound,  which has an inverse relationship to the frequency; high frequencies have much shorter wavelengths than low frequencies. This is useful when considering professional or large scale acoustics.
  • Phase:
    1. The time relationship between two or more sound waves at a given point in their compression and rarefaction cycles. 
      1. Two identical sound waves, taken 180° out of phase, will cancel out into a null value and produces no sound. This is destructive interference
      2. Two identical sound waves, perfectly in phase, will increase the amplitude of that soundwave. This is known as constructive interference.
    2. If you are recording a single source, such as one voice, phase should not be an issue.
    3. If you are recording multiple sources simultaneously, simply follow the 3:1 ratio rule, which is that the distance between microphones should be 3 times the distance to their sources. 
      1. Example: Two microphones are both 10 centimeters from their vocalists. Therefore, the distance between the two microphones should be at least 30 centimeters to avoid phasing issues. 

It is important to remember that in reality, every sound we hear is a mixture of different frequencies at different amplitudes in different environments. Therefore, any recording of natural sound will be composed of a wide range of frequencies and harmonics that create the timbre of that sound. This is how we can detect the difference between a violin and piano, even if they are playing the same note simultaneously. 

More

Analog audio — Electronics tutorials