The Volume of the Wave

1
The Volume of the Wave

• The Amplitude is a measure of volume
• The wave pink is softer than the blue wave.
• It represents both air pressure variations, and
  the displacements of the source, the air
  particles, and our ear drums, enabling us to hear

2
The Pitch of a sound wave

• The pitch is determined by the frequency, or how
  often (frequently) a full wave reaches our ears
• The higher the frequency, the higher the pitch
• The pink wave below has a higher pitch than the
  blue wave (by an octave - discussed later)

3
The Guitar - Pitch Control

• Factors affecting the pitch included-
• 1. The Length - placing fingers on the fretboard
  shortens the string, and increases the pitch
• 2. The Mass / Unit Length (or thickness) - low
  bass strings are thicker
• 3. The Tension - tuning using vibrato and
  bends

4
Standing Waves - Strings
Key Results Harmonics form a series with
frequencies given by f, 2f, 3f,4f... i.e. All
Harmonics are present. (f First Harmonic freq.)
5
The Tone or Timbre

• Most instruments (and voices) have a combination
  of possible vibrations occurring at the one time.
  
• The next few slides attempt to show how this
  results in a characteristic wave shape for each
  instrument, which determines its tone (timbre)
• This helps us distinguish between different
  instruments playing the same pitch and volume

6
The Fundamental Frequency

• This is the first harmonic.
• It determines the pitch of the note we hear
• It is the same for all different instruments
  playing the same pitch

7
Standing Waves - Open Pipe
Key Results Harrmonics form a series with
frequencies given by f, 2f, 3f,4f... i.e. All
Harmonics are present. (f First Harmonic freq.)
8
The Flute - First 2 Harmonics

• The fundamental (first harmonic) is shown in blue
• The second harmonic is shown in pink
• The black line shows the sum using superposition

2
1.5
1
0.5
0
-0.5
-1
-1.5
-2
9
Harmonic Spectrum - Flute

• As the flute is a pipe which is open at both
  ends, all possible harmonics are present, but
  with different amounts of each. The Harmonic
  Spectrum shows how much of each is present.

1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
5
6
7
8
10
The Flute - first 8 harmonics

• The graph shows the resulting wave from adding
  the first 8 Harmonics in the correct amounts,
  using the superposition principle.

11
Harmonic Spectrum - Violin

• The harmonic spectrum of the violin displays a
  relatively high amount of the upper harmonics,
  resulting in a bright tone

12
Violin Wave Shape - 10 Harmonics
13
Closed Pipes -The Clarinet

• The clarinet is a good example of an instrument
  which is effectively a pipe closed at one end
• The result is a wave made from only the odd
  numbered harmonics
• The Harmonic series is then
  f , 3f , 5f , 7f, etc

14
Clarinet Wave - 1st 4 odd harmonics

• Note how the absence of even harmonics changes
  the overall form to be more like a square wave

15
dB and Equipment Ratings

• To provide a noticeable 3 dB increase in Volume
  of a Stereo, the power of the system must be
  doubled if it was running before at max. output.
• The frequency response of equipment is usually
  quoted as a frequency range which can be
  reproduced /- 3dB. (e.g. Microphones, Speakers,
  etc.)

Output in dB
3dB
Frequency Response eg 25 Hz - 18 kHz
Frequency
16
Graphic Equalisers and dB

• A Graphic Equaliser changes the shape of the
  Frequency response curve by dividing it into
  sections
• Each slider adds or subtracts to the dB level of
  each frequency band

17
The Dynamic Microphone

• Dynamic mics have a coil attached to a diaphragm
  suspended in the magnetic field of a magnet
• Pressure changes cause the diaphragm to vibrate,
  changing the amount of magnetic flux threading
  the coil inducing a current in the coil to
  produce a signal

18
The Condensor Microphone

• Condensor mics have two charged plates, the front
  one of which is very thin, and usually covered in
  a very fine gold layer
• Pressure changes cause the front plate vibrate,
  changing the distance between the plates. This
  changes the capacitance of the system
• A Voltage is supplied to plates, and thus the
  amount of charge on the plates varies, causing a
  current to produce the signal

19
Microphone Responses

• Condensor mics tend to have a much smoother
  frequency response, and are often used for
  recording
• Dynamics have peaks designed to gain clarity with
  stage vocals

20
Speaker Design

• Resonance of a speaker has a critical effect on
  its frequency response
• Designs employing closed cabinets with ports to
  minimise resonance work well
• The sound of a system results from a combination
  of physics factors!