L09 & L10 Flashcards
Resonance - Where?
How is it strengthend?
Resonance:
Vibration of a system with maximum Amplitude as effect on an applied frequency.
Response Amplitude is @ w/WC = 10^0 highest
- Depends on damping Factor (<=1)
- smaller damping - higher response of system
- with phase shift pi the highest damping
- velocity:
Normalized power is spread more over the spectrum with higher damping - damping has an effect on power distribution
Which Resistances dies an acoustical transducer have?
Mechanical & acoustical impedance
How does the piezoelectric principle work?
Electrodes apply a voltage in material, eg. Ceramic and increase its thickness.
The strain let’s the surface force compression and movement of the neighbouring medium.
How are electrodynamic loudspeaker built?
- Diaphragm or cone
- lightweight mat.
- m9ves air to produce sound - Voicecoil
- Coil of wire attached to the diaphragm
- Current passes through coil,
- coil interacts with magnetic field
- coil& diaphragm move - Magnets
- create electric field for motion of coil - Suspension
- supports free motion of the voice coil
How is the power of an electrodynamic Loudspeaker distributed?
Higher frequency: w = 10^-1 - 10^3
-> velocity^2 decreases from 10^0
(0dB - -60 dB)
-> mechanical Resistance of the loudspeaker is increased (-60>0dB)
-> plateau region develops btw. w 10^0 > 10^2
How does an accelerometer work?
Glued on a surface, it behaves like a forced Harmonic oscillator
(seismic mass mounted to a piezoelectric material with stiffness k)
Operating underneath the resonance frequency, it Hava sensitivity of 0dB.
What’s the difference btw a coherent and an incoherent source?
Coherent sources have fixed phase differences between the waves.
Incoherent vary in phase difference and the interference pattern is complex.
Here we don’t add a Interference term and the average Intensity is sign. smaller.
How does spatial Correlation in noose fields look like?
There is isotropic noise (& surface noise,… )
The Coherence depends (sinc fct.) on the distancing:
- small distances (<lambda/2):
high correlation
- lambda/2: min. Correlation (-0.2)
- varies with higher d
In which case is ocean bient noise uncorrelated?
When the distance is high comp. to the wavelength.
Why is it necessary to measure uncorrelated noise?
The array gain is less than the Directivity gain, we have a clearer signal observed.
What is the array gain?
10log10( SNR_array / SNR_element )
The SNR of the array is calculated by the H^2 factor to the signal- and noise power in the integral (ratio)
The maximum array gain reachable is the Directivity Index.
Which effect does scattering have?
Incident wave fronts are scattered by objects eg.
An acoustical shadow zone appears directly behind the object as a result of the interference of scattered waves.
How does the directivity pattern of a rigid sphere vary with ka?
Higher wavenumber (freq.) The directivity gets up and at 180° we have the scattering located
The target strength depends on the expansion of the objects?
Yes, small objects cause lower backscattering.
When do we have Raleigh scattering?
For ka«1 the scattering function is constant, while otherwise the geometrical level varies.
