Transducers for Music Dr Ian Drumm

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Transducers for MusicDr Ian Drumm

• Aims
• Discuss Enclosures and Directivity
• Learning Outcomes
• Stiffness of Enclosure
• Directivity and Directivity Index
• Practical Implications

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Why Loudspeaker Enclosures?

• No acoustic considerations
• Fashion
• Packaging
• Convenience
• Ruggedisation
• Transport
• Mounting
• Stacking

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• Un-enclosed loudspeaker
• Acoustic "dipole
• Phase cancellation
• (especially low frequencies)
• Highly directional

• Enclosed
• Effectively an infinite baffle
• (sort of)
• Becomes monopole source

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What about air in box?

• Stiffness
• Increases with surface area S of speaker cone
• Decreases with volume V of box

c..speed of sound
..density of air
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Example
Radius a0.06m
Surface area S0.0113m2
In addition to suspension stiffness
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Increases Resonant Frequencyreduces bass response
Pressure Response

• Options to compensate
• Increase cabinet size
• Increase moving mass
• Add absorbent material

Frequency
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Consider Omni directional Source
Intensity I of a loudspeaker radiating omni
directionally
W is power R distance from source
Hence
To give sound pressure level .
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plots shows different frequencies ka 2pfa where
a is speaker radius
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Directivity
Directivity Q(intensity) / (intensity of omni
directional source)
Directivity Index
Acoustic Power Level
DI
LW
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Directivity Index
A loudspeaker with directivity index of DI
generates a sound pressure level which is DI dB
greater than an omni-directional source radiating
the same power.

• The directivity (and so directivity index)
  influenced by
• frequency
• angle (of elevation and azimuth)
• environment (example - operating a loudspeaker
  next to a wall immediately doubles its low
  frequency directivity)

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In a studio

• Beware
• reflection of low frequencies (room gain)
• phase cancellation
• high frequency splash giving phantom stereo
  images
• Recommendations
• Head height
• On axis
• Away from boundaries
• Equilateral triangle
• Not too loud or for too long
• Comfortable quite environment

Follow manufacturers recommendations