Understanding Sound: Properties and Phenomena
The Nature of Sound
Sound is a vibration that spreads through the air. It follows the same principles as when we throw a stone into a pond: the disturbance of the stone causes the water to stir in all directions until their amplitude is so small that it ceases to be perceived.
Frequency
The number of times that this vibration is produced per second is what is known as frequency. The frequency of sound is measured in Hertz (Hz). A person with healthy ears can hear frequencies from 20 to 20,000 Hz.
When these air vibrations reach our ears, they are turned into electrical signals that can be understood by our brain. A microphone acts similarly, transforming the acoustic vibrations into power so they can be stored, manipulated, and reproduced. This signal is called an analog signal.
The simplest waveform is called a sine wave, which is characterized by a constant frequency and amplitude. Such waves are commonly used for studies because any signal can be decomposed by Fourier Transform into this series.
Amplitude
The amplitude of a wave tells us about the strength or signal power. The unit of measure of this strength is the decibel (dB).
The ear needs a high percentage of variations in the strength of a sound to detect a slight change in the perceived intensity, which indicates that the sensitivity of the ear is logarithmic. To get an idea, an increase of just 3 dB doubles the loudness of a sound, but we only perceive a slight increase in this. We need an increase of 10 dB so that our ears perceive sound with twice the intensity.
Echo
Echo is the phenomenon by which, due to the reflection of sound waves, our ears can perceive a sound more than once.
To perceive the echo, there needs to be 17 meters or more between the sound source and the reflecting surface.
Reverb
Reverb is the phenomenon by which sounds are extended a bit after they are issued as a result of the superposition of sound waves, both incident and reflected. The time it lasts is called the reverberation time.
Resonance
Resonance is the phenomenon by which a body vibrates at the approach of a sound produced at a frequency equal to that which the body can produce.
The body that vibrates in resonance is called the resonator. The vibration that starts it is called the driver.
Sound Masking
When the ear is exposed to two or more simultaneous sounds, there is the possibility that one of them masks the others. To be more precise, it should be defined as an effect produced in the perception of sound when you hear two sounds of different intensity at the same time. As this happens, the weaker sound will be inaudible because the brain will only process the masking sound.
A high-level sound has a greater masking effect if the softer sound has a similar frequency.
Digital-Analog Conversion
In electronics, a digital-to-analog converter (DAC) is an input device that converts digital data (usually binary) to an analog signal (usually voltage or electric charge). Digital-to-analog converters are interfaces between the abstract digital world and real-life analog signals. The reverse operation is performed by an analog-to-digital converter (ADC).
This type of converter is used in audio players of all kinds because audio signals are now stored in digital form (e.g., MP3s and CDs). To be heard through speakers, the data must be converted to an analog signal. Digital-to-analog converters can also be found in CD players, digital music players, PC sound cards, etc.
Envelopes
Sound is not a static phenomenon. Quite the contrary, usually, every frequency component of a sound has its own life, in the sense that its behavior becomes independent (or nearly independent) in time. This means that each part runs along a particular path or surrounds its temporal evolution. In the time domain, the sound as a whole also has a performance envelope defined by a more generic amplitude called the ADSR envelope (Attack, Decay, Sustain, Release), for short.
The following describes each section of this envelope:
- Attack: The attack is the time it takes the signal to reach its peak.
- Decay: The decay is the time it takes the signal to stabilize.
- Sustain: The sustain is the duration of the stable signal.
- Release: The release is the time it takes the signal to fade.