Topic Nine - Acoustic Impedance Flashcards
What is volume velocity?*
The amount of air that is moved by an acoustic source (e.g., loudspeaker).
It is measured in m³/s
It is calculated by multiplying the particle velocity by the surface area
What is the relationship between sound pressure, impedance and volume velocity?*
They all interact with eachother.
When you have a high impedance, you will need a higher sound pressure level to get the same volume velocity from a low impedance system.
Sound pressure = volume velocity x impedance
What the definition of impedance?*
The opposition to the transfer of acoustic energy through a medium
What components contribute to impedance*?
Impedance is made up of resistance and reactance.
Reactance is further divided into:
- Mass reactance
- Compliant reactance
Of the components that contribute to impedance, which are frequency dependent and which are not?*
Both mass reactance and compliant reactance ARE frequency dependent.
The mass reactance is directly proportional to frequency (every double of frequency, a double of mass reactance).
The compliant reactance is inversely proportional to frequency (high frequency = low compliant reactance, low frequency = high compliant reactance).
Resistance IS NOT frequency dependent but it is proportional to velocity.
What is it called when the mass reactance and the compliant reactance cancel each other out?*
When the net reactance is 0 (mass reactance - compliant reactance) this is the resonant frequency.
The resonant frequency is the natural frequency the system is most likely to resonate at.
At the resonant frequency, any motion is limited only by resistance or friction.
What are compliant reactance, mass reactance and resistance in phase with?*
- Compliant reactance is in phase with displacement
- Mass reactance is in phase with acceleration
- Resistance is in phase with velocity
What is the inverse of impedance?*
Admittance
What is the inverse of resistance?*
Conductance
What is the inverse of reactance?*
Susceptance
how a tympanometer measure only the impedance of the middle ear and not the ear canal?
As the probe is situated in the external ear canal, there needs to be a way in which the impedance of the middle ear can be separated from that of the ear canal.
It does this by changing the pressure of the middle ear to both extreme positive and extreme negative. At the pressure extremes, the admittance of the ear drum is at zero and we therefore know any admittance is purely due to the ear canal volume. To then find the static compliance measurement, the admittance from the ear canal is taken off the admittance of the whole system.
What does acousitc compliance mean?*
It is the opposite to impedance. It is how easily a acoustic signal is transferred.
Acoustic compliance is the ability of a volume of air (or a structure) to stretch or compress in response to a sound pressure.
What does inertance mean?*
The ratio of sound pressure to the rate of change of volume velocity.
The heavier the air mass, the more it resists being moved quickly.
More resistance at high frequencies, because they try to speed up and slow down the air more rapidly
What effect do inertance and compliance have on the phase relationship between pressure and volume velocity?*
If a system is stiffness dominated, there will be a pressure phase lag
If a system is dominated by mass, there will be a pressure phase lead.
WHY
