What is sound?
- Change of pressure in time and space
- Variation in air pressure
- Propagated through space
- Air pressure -> number of air molecules
Things to note
- Molecules do not travel, they oscillate
- Energy moves
- Speed of sound: how fast the energy can propagate NOT how fast the molecules themselves move
- Sound speed depends on temperature and material properties
What is the amplitude in reference to air pressure?
- The maximum pressure at a given point in space
- The minimum pressure at a given point in space
- With respect to the stationary state (normal atmospheric pressure)
- The amplitude is the difference in pressure relative to the stationary state
What is intensity?
- Intensity is a log scale based on some reference pressure
- Intensity is measured in microbars which is a measure in pressure
- Used to measure amplitude in decibels
- The amplitude doubles every 6 decibels
What are some examples of different decibel levels?
0dB – barely audible sound (more later)
20dB – Whisper
40dB – Quiet office
60dB – Normal conversation
80dB – Hair dryer
100dB – Heavy traffic, pneumatic drill
120dB – Loud thunder, music concert
140dB – Jet aircraft at take off
Past this limit, we cannot detect any further increase in volume as it becomes deafening
How does the human ear perceive sound?
- The sound enters the ear and travels through the external auditory canal
- The sound hits the tympanic membrane (like a drum)
- The tympanic membrane vibrates which also vibrates the incus and malleus
- These oscillations move through the stapes and then the cochlea
- The high frequencies are detected at the start, and then the low frequencies as the different nerve hair cells move due to the oscillation to measure the vibrations
- The cochlea (hair cells) transmits electrical impulses to your brain to interpret the sound
What happens to your hearing as you grow older?
As you grow older, you become less sensitive to high frequencies due to the nerve hair cells degrading and becoming less sensitive.
What are different types of sound perception and how do they correlate?
Physical Measures
1. Intensity (amplitude)
2. Frequency
3. Spectrum (complexity)
Perceptual Sensations
1. Loudness
2. Pitch
3. Timbre
What is the relationship between intensity and loudness? How can we maintain the same ‘loudness’?
Sounds with the same intensity but different frequencies can different perceived loudness
Humans are uniquely sensitive to different pressures
If we want to perceive pressures at the same loudness, we need to modify different parts of the spectrum (need to increase/decrease sound pressure according to frequency)
Measured using phons
What is the relationship between frequency and pitch?
The greater the frequency, the higher the pitch.
1 octave -> doubling of the frequency
12 increments within each octave
f = 2^(n/12) * 440 Hz
What happens when we combine frequencies?
We can combine frequencies to create certain sounds, e.g. to create a perfect fifth or major triad which are perceived differently (major or minor chord)
What is the relationship between spectrum (complexity) and timbre?
Two signals can sound very different even if they have the same main peak frequencies due to having different spectrums (additional frequencies)
What is masking (perceptual irregularity)?
The rise in the detection threshold of one tone (test tone) due to the presence of a second tone (masker tone)
What is spectral masking?
If we have 2 frequencies, but one of them is dominant or the other is close to the dominant frequency and has a low amplitude, then we may no longer perceive the second frequency.
What is temporal masking?
If we have a low amplitude sound after a high amplitude sound, we will not be able to fully perceive the low amplitude sound. This is because our tympanic membrane disconnects slightly from our cochlea to protect our hearing and dampens any sounds immediately after the loud noise.
In this case, our perception depends on when the sound is played.
What is the difference between wind and sound?
- Wind is the displacement of air molecules
- Sound is the vibration/oscillation of air molecules to produce changes in pressure
If 2 sounds have frequencies of 300Hz and 3kHz, but are both 20 phons, which one is louder? Which one has a greater amplitude?
- Phons are already corrected for the irregular human perception in different frequencies, so they will sound approximately equally loud
- Therefore, they will both have the same loudness.
- To compensate for the less sensitive perception at lower frequencies, the 300Hz signal has to be louder
