Sound Design Waves And Sound 5 — Questions and Answers
Question 1: What is the speed of sound in dry air at approximately 20°C (68°F)?
- 1,125 ft/s (343 m/s) (Correct answer)
- 768 ft/s (234 m/s)
- 1,480 ft/s (451 m/s)
- 500 ft/s (152 m/s)
Correct answer: 1,125 ft/s (343 m/s)
Sound travels at approximately 343 m/s (about 1,125 ft/s) in dry air at 20°C, which is a key reference for delay and timing calculations.
Question 2: Which type of boundary condition exists at the closed end of an acoustic pipe?
- Antinode (pressure minimum)
- Node (displacement node / pressure antinode) (Correct answer)
- Neither node nor antinode
- Free boundary
Correct answer: Node (displacement node / pressure antinode)
At a closed end, air cannot move freely, creating a displacement node and a pressure antinode — maximum pressure variation occurs there.
Question 3: Refraction of sound waves occurs when they pass between two media of different:
- Amplitudes
- Wave speeds (Correct answer)
- Wavelengths only
- Frequencies
Correct answer: Wave speeds
Sound refracts (changes direction) when it crosses a boundary where wave speed differs, such as between air layers of different temperatures.
Question 4: A sound wave's amplitude directly determines which perceptual attribute?
- Pitch
- Timbre
- Loudness (Correct answer)
- Duration
Correct answer: Loudness
Amplitude corresponds to the physical magnitude of pressure variation, which we perceive as loudness.
Question 5: In a room, the 'critical distance' refers to the point where:
- Direct sound and reverberant field have equal intensity (Correct answer)
- The reverberation time reaches zero
- High frequencies are completely absorbed
- Bass frequencies become inaudible
Correct answer: Direct sound and reverberant field have equal intensity
The critical distance is where the direct sound level equals the reverberant sound level; beyond it, the room sound dominates over the direct source.
Question 6: What does RT60 measure in acoustics?
- The frequency of the 60th room mode
- The time for sound to decay by 60 dB after the source stops (Correct answer)
- The ratio of absorption at 60 Hz
- The speed of sound at 60°F
Correct answer: The time for sound to decay by 60 dB after the source stops
RT60 (reverberation time) is the time in seconds required for a room's sound level to fall 60 dB below its initial level after the source ceases.
Question 7: Which psychoacoustic effect causes the perceived loudness of a pure tone to increase when a narrow band of noise is added nearby in frequency?
- Temporal masking
- Auditory fatigue
- Spectral spreading (Correct answer)
- The missing fundamental effect
Correct answer: Spectral spreading
Spectral spreading (or auditory unmasking) can occur when nearby noise energizes the critical band, altering loudness perception in complex ways.
Sound Design Waves And Sound 5
What is the speed of sound in dry air at approximately 20°C (68°F)?