ME 513 Exam 1 Concept Questions

What is sound?

Small amplitude propagating disturbance in an elastic medium

Name the two equations that are combined to create the differential equation governing the oscillatory motion of a single-degree-of-freedom system (SDOF).

Equation of motion - Restoring force equation

Why are two independent initial conditions required to solve for the motion of a SDOF system?

The motion of an SDOF system is governed by a second-order differential equation,requiring two unknown constants; therefore, two conditions are needed

How can you find the natural frequency of a SDOF system?

We need to determine the frequency at which the input mechanical impedance becomeszero.

The response of a SDOF system when driven at two frequencies simultaneously is thesum of the responses to the two individual frequencies acting by themselves. This is an example of the principle of for linear systems.

superposition

What are the characteristics of a stiffness-like impedance and of a resistive impedance?

For stiffness-like impedance: Imaginary, negative and inversely proportional to frequency

For resistive impedance: Real and positive; independent of frequency

The wave number is the spatial equivalent of the temporal _______

frequency

When a pulse propagating along a tensioned string hits a fixed termination, the reflected pulse is ____________

upside down and backward

hen deriving the wave equation for sound propagation in air, it is usuallyassumed that the air is an "ideal" gas - that means we neglect the effects of________________ and ______________

viscosity;heat conduction

A two-dimensional sound field, in which kx2 + ky2 > k2, is described as a_____________

near-field

For a system to oscillate, it must possess two physical properties. They are

Stiffness and mass

How can the mechanical impedance of a vibrating system be used to identify thenatural frequencies of the system?

At the natural frequency, the imaginary part of the mechanical impedance is equal to zero.

When does resonance occur?

When a system is driven at one of its natural frequencies.

Why is the Principle of Superposition important when evaluating the forced response of a linear system?

Can treat inputs at different frequencies independently, and sum the response.

What is the difference between a "free" response and the "forced" response of asystem?

Free response is the system response to the initial conditions without external force applied, and the systemoscillates at its natural frequencies only. Forced response occurs only at the frequency of the driving force.

The wave number represents the rate of change of ______________ with__________________, and is sometimes referred to as the_______________________________ .

Phase, position, spatial frequency

When a wave traveling along a tensioned string reflects from a fixed termination,how does the wave appear to emerge from the boundary?______________________________________________________________

Upside down and backwards

The type of acoustics that we are studying in this course is referred to as "idealacoustics". What is assumed in that case?

No viscosity, adiabatic compression.

Consider a plane wave that satisfies the wave equation and which is propagatingin an arbitrary direction in a three-dimensional space. When kx2 + ky2 > k2, thewave number in the z-direction is purely ____________________________ .

Imaginary

What is the boundary condition at the free end of a tensioned string?

dy/dx = 0 (zero transverse force)

What are the typical initial conditions for an SDOF system?

Initial displacement and initial velocity

What are the characteristics of a mass-like impedance?

Positive imaginary, proportional to frequency

What physical property controls the forced response well above resonance?

Mass (inertia)

In e^jkx, what does k represent?

Rate of change of phase with position

What is the directon of rotation of e^-jwt in the complex plane?

Clockwise

Why is a spherically symmetric wave considered one-dimensional?

It depends only on radial position, not direction

What is the reference RMS sound pressure in air, and why?

20 uPa, approximately the reference threshold of human hearing