Radiation from an Infinite baffle

1
Radiation from an Infinite baffle

• Transducers for Music
• Dr Ian Drumm
• www.aee.salford.ac.uk Student Area
  Undergraduate
• Username Students Password Module

• Aims
• To consider radiation from an infinite baffle and
  so get reasonable values from speaker model
• Objectives / Learning Outcomes
• Radiation load
• Low frequency mass loading
• Radiation at 1 meter

2
Loudspeaker in free space

• NOT plane wave radiation directional response
• Complicated relationship between pressure and
  velocity

3
Infinite Baffle

• large plane rigid surface (doesn't have to be
  literally infinite)
• loudspeaker mounted in a hole
• front and back of the diaphragm are acoustically
  isolated.
• Stops front signal interfering with out of phase
  signal from back

4
Radiation Load

• Radiation Load Pressure/Velocity
• Pressure - caused by the loudspeaker
• Velocity - that caused the pressure
• Is function of frequency and size of speaker cone

e.g. for 120mm diameter speaker f tiny 100 Hz 1kHz
415 familiar?
Plot ZRAD w.r.t. frequency
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New Pressure Response
Given velocity response
New mechanical impedance
Plot Pressure Response w.r.t. Frequency
Plane Wave
Realistic ZRAD
Radiation load smaller for below 1kHz than plane
wave case
6
Velocity Response
Plot Velocity Response w.r.t. Frequency
Realistic ZRAD
Plane Wave

• Almost same as in vacuum
• Hence front load doesnt influence velocity of
  diaphragm

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Loudspeaker Characteristics
 

Pressure response radiation load velocity
response
8
Low Frequency Mass Loading
At low frequencies ZRAD is mass dominated Hence
can consider as a lump The mass of lump given
empirically by
(a is the diaphragm radius)
10 total moving mass
9
Pressure Sensitivity _at_ 1m
Mid point -18dB ref 1
76 dB SPL ref 2010-6 Hence Sensitivity 7620
log(2.83)85dB