Atlas MCAT Course
Physical Foundations → Sound & Acoustics
Sound & Acoustics
Pressure waves, energy, and perception.
01The idea
Sound is not a “thing.” It is a mechanical disturbance that propagates through matter.
No medium → no sound.
THREAD · Gradients and flow
02Build the model
ASound as a longitudinal wave
In sound waves, particles oscillate parallel to the direction of propagation. Regions alternate between compressions (high pressure) and rarefactions (low pressure). What travels is the pattern, not the particles themselves.
BWave speed of sound
The speed of sound depends on the medium, not the source. The general relationship:
Qualitative consequences: a stiffer medium gives faster sound; a less dense medium gives faster sound. Thus, sound is fastest in solids, then liquids, then gases.
CFrequency, wavelength, speed (revisited)
For sound, frequency sets pitch, amplitude sets loudness, and speed is a property of the medium. Changing the source frequency changes wavelength, but does not change speed (in the same medium).
DSound intensity: energy transport
Sound intensity (I) measures power transmitted per unit area.
Spreading out decreases intensity, and doubling distance makes intensity drop sharply.
EThe decibel scale: logarithmic perception
Human hearing spans an enormous intensity range. To manage this, sound level is defined logarithmically:
Here I₀ is the reference intensity (threshold of hearing). Interpretation: +10 dB means 10× intensity; +20 dB means 100× intensity. Perception increases more slowly than physical intensity.
FLoudness vs. intensity
Intensity is a physical quantity; loudness is a psychological perception. They are related but not identical. This mirrors stimulus vs perception (psych) and pressure vs sensation (physiology).
GStanding sound waves: air columns
Sound can form standing waves in pipes, tubes, and resonant cavities. Boundaries determine allowed wavelengths.
| Pipe | Fundamental | Harmonics |
|---|---|---|
| Open (both ends) | λ₁ = 2L | λₙ = 2L/n, n = 1, 2, 3, … (all) |
| Closed (one end) | λ₁ = 4L | λₙ = 4L/n, n = 1, 3, 5, … (odd only) |
Why boundary conditions matter: an open end is a pressure node, and a closed end is a pressure antinode. These constraints enforce discrete resonant frequencies.
HDoppler effect: relative motion changes frequency
When source and observer move relative to each other, the observed frequency changes while wave speed in the medium stays constant. Approaching → higher frequency; receding → lower frequency.
ISound energy and biological context
These physics principles explain pitch discrimination, loudness perception, hearing damage thresholds, and cochlear frequency mapping. Physics provides the input constraints for physiology.
03See the connections
The lesson names these links itself. Read the map left to right.
Wave
- Longitudinal; needs a medium
- v set by stiffness and density
Energy
- I = P/A
- Decibels: logarithmic
Patterns
- Open pipe: all harmonics
- Closed pipe: odd only
- Doppler shifts frequency
Next: light and optics.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
A sound goes from 40 dB to 60 dB. How has its intensity changed?
Think first.
Which pipe can produce only odd harmonics?
Think first.
An ambulance approaches you. What changes about its siren as you hear it?
Think first.
Can sound travel through a vacuum?
05MCAT lens
Equation-level checks
Louder ≠ higher pitch
Changing source frequency ≠ changing wave speed
Decibels are logarithmic, not linear
Sound cannot travel in vacuum
06Common traps
Trap 1: Louder means higher pitch
Misconception
Loudness and pitch are separate.
Correct model
Amplitude sets loudness; frequency sets pitch.
Trap 2: Changing the source frequency changes wave speed
Misconception
Speed is set by the medium.
Correct model
Wavelength adjusts instead.
Trap 3: Decibels are linear
Misconception
They are logarithmic.
Correct model
+10 dB = 10× intensity.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
How sound speed depends on medium
Check against 02 · B: stiffness and density.
How frequency and wavelength relate
Check against 02 · C: v = fλ in a fixed medium.
How intensity and decibels work
Check against 02 · D and E: I = P/A; β = 10 log(I/I₀).
Why standing waves form in air columns
Check against 02 · G: boundary conditions at open and closed ends.
If something feels unclear, the missing idea is usually thisSound is pressure energy propagating through matter under physical constraints.
08Remember
REMEMBER · if you keep only a few things
- Sound is a longitudinal pressure wave; no medium, no sound.
- Fastest in solids, slowest in gases.
- Frequency → pitch; amplitude → loudness.
- +10 dB = 10× intensity.
- Open pipe: all harmonics. Closed pipe: odd only.
- Doppler changes frequency, not speed.
09Forward hook
We now transition from mechanical waves to electromagnetic waves.
How does wave physics work for light, which needs no medium?
That leads to light and optics, the formal physics layer.
Next lessonLight & Optics→