L04 - Sound and Acoustics

Why can sound not travel through a vacuum?

It is a mechanical wave, so it needs particles of a medium to carry the pressure changes.

What does the waveform recorded by a microphone actually represent?

How the local air pressure deviates from ambient pressure over time at the microphone.

Why is sound level usually expressed in decibels rather than pascals?

Hearing spans a huge pressure range, and a logarithmic scale compresses it into manageable numbers.

Sound pressure level is calculated from RMS pressure and a reference pressure as L_P = ____ dB.

20 log₁₀(p_rms / p_ref).

Why does the decibel formula use 10 log₁₀ for power ratios but 20 log₁₀ for amplitude ratios?

Power is proportional to amplitude squared, and the log of a square doubles the value.

By roughly how many dB does a sound's level change if its pressure amplitude doubles?

About +6 dB, because amplitude ratios use 20 log₁₀(2).

One sound's pressure amplitude is 20 dB above another's. By what factor is its amplitude larger?

10 times, since 20 log₁₀(10) = 20.

Why is a dB value meaningless unless you know what it is relative to?

dB expresses a ratio, so it only has meaning against a stated reference.

A sound repeats with a period of 5 ms. What is its frequency?

200 Hz, because f = 1/T.

What is the wavelength of a 343 Hz tone in air, where sound travels at 343 m/s?

1 m, because λ = v/f.

Why is the wavelength of the same tone longer in water than in air?

Sound travels faster in water, and λ = v/f with the frequency unchanged.

What does a stationary listener hear as a siren approaches and then moves away? [IMAGE: Slide 11, hide the three frequency labels above the listeners]

A higher frequency while approaching and a lower one while receding (the Doppler effect).

In a pure tone a = A sin(2πft + φ), the symbol φ represents the ____.

Phase: where in its cycle the wave starts.

What does the superposition principle say happens when sound waves meet?

The resulting pressure at each point is the sum of the individual waves.

What type of interference is shown in the bottom panel, and why is the sum flat?

Complete destructive interference: equal waves in opposite phase cancel out.

What is the difference between pitch and timbre?

Pitch relates to repetition rate (F0). Timbre relates to how energy is distributed across frequencies.

Two vowels are spoken at the same pitch but sound different. What differs acoustically?

Their spectral shape: energy is distributed differently across frequencies.

Why does a speech recording made in a room contain more than the speaker's voice?

Sound reflects off surfaces, so room acoustics, noise and device effects are added.

How does a noisy sound such as /s/ differ in its time pattern from voiced speech?

/s/ is aperiodic turbulence with no repeating pattern; voiced speech is quasi-periodic.

A plosive release is an ____ sound: a short burst of energy.

Impulsive.

What is the difference between resonance and anti-resonance?

Resonance: constructive interference emphasises some frequencies. Anti-resonance: destructive interference reduces energy at some frequencies.

In the neutral "uh" position, how is the adult male vocal tract modelled acoustically? [IMAGE: Slide 19, hide "17.5 cm", "Glottis" and "Lips"]

A tube about 17.5 cm long, closed at the glottis and open at the lips.

For a 17.5 cm tube closed at one end, roughly where is the lowest resonance (v = 343 m/s)?

About 490 Hz, because the lowest resonance of a closed-open tube is v/4L.

Why can the effect of vocal tract resonances on a source be described as filtering?

They enhance or reduce the source's energy at different frequencies, shaping its spectrum.