Boundary Effects on Sound Frequency Distortion

Flashcards cover boundary definitions, direct/reflected paths, interference conditions (constructive/destructive), phase shift upon reflection depending on impedance, frequency-dependent effects, and a worked example from the notes.

What is a boundary in acoustics?

An interface between two media with significantly different acoustic impedances.

What are direct paths and reflected paths in boundary acoustics?

Direct path: sound travels straight from sender to receiver. Reflected path: sound reaches the receiver after reflecting off the boundary.

What determines whether direct and reflected waves interfere constructively or destructively at the receiver?

The phase difference between the two waves, which depends on the wavelength and the extra distance the reflected wave travels.

Destructive interference condition for boundary reflections?

When the reflected path is an odd multiple of half wavelengths longer than the direct path.

Constructive interference condition for boundary reflections?

When the reflected path is a whole number of wavelengths longer than the direct path (including zero).

What happens to the reflected path when the sender and receiver are far from the boundary?

It attenuates significantly and has little effect; the direct path dominates.

What effect does being near the boundary have on interference?

Direct and reflected paths have more similar travel distances, increasing the potential for destructive or constructive interference.

Phase shift on reflection when the first medium has higher acoustic impedance than the second?

A phase shift occurs in the reflected wave (typically 180 degrees).

Phase behavior on reflection when the first medium has lower impedance than the second (e.g., air to water)?

No phase shift occurs upon reflection.

Why are very low frequencies typically not experiencing destructive interference as boundary effects?

Their long wavelengths mean the path difference is small relative to the wavelength, and without a phase shift they tend to interfere constructively.

Which frequencies are most likely to experience cancellation near a boundary and why?

High frequencies, because shorter wavelengths make it easier to achieve an odd half-wavelength path difference, leading to destructive interference.

In the example of two birds on flat ground with a 0.04 m path difference, what is the lowest frequency that will show total destructive interference?

4,250 Hz.

What are the next frequencies predicted to show complete destructive interference after 4,250 Hz in the same example?

12,750 Hz and 21,250 Hz.

How can you predict which frequencies will be amplified or attenuated by boundary reflections in a flat-ground scenario?

Compute the path difference using geometry (Pythagorean theorem) and apply the half-wavelength and whole-wavelength conditions to identify destructive/constructive frequencies.

What happens to low frequencies when reflecting off a boundary with no phase shift (first medium impedance less than second)?

Low frequencies tend to interfere constructively with the direct wave because the phase shift is absent and wavelengths are long.