Class:
E
Name:
(
13c
Musical notes
)
Date:
13c
Apparatus
You will study the pitch, loudness and quality of musical notes by
observing their waveforms on an oscilloscope.
• 1 signal
generator
• 1 loudspeaker
• 1 microphone
• 1 oscilloscope
(CRO)
• tuning forks on
resonance box
• some musical
instruments, e.g.
harmonica, flute,
violin
Note: CRO shows the
amplitude and the
frequency of the signal.
The signal generator has
2 outputs, a high and a
low impedance outputs.
Connect the loudspeaker
to the low impedance
output of the signal
generator and the CRO to
the high impedance
output.
Procedure
Pitch and loudness
1
(a) Connect a signal generator to a loudspeaker and a CRO
(Fig 13c-1).
(b) Starting with 20 Hz, gradually increase the frequency of the
signal generator to 20 kHz or more.
(c) Hear the note emitted by the loudspeaker and watch the
waveform on the CRO.
(d) Sketch the waveform in Figure 13c-2.
oscilloscope
loudspeaker
signal
generator
Fig 13c-1
Fig 13c-2
In some signal generators,
the amplitude also
changes with the
frequency. Ts may need to
adjust the amplitude to
obtain a more or less
constant amplitude output
over the frequency range.
© Oxford University Press 2006
On increasing the frequency, the pitch of the sound
increases
(increases/decreases/remains the same).
On increasing the frequency, the number of waveform displayed on
the CRO
increases
(increases/decreases/remains the same).
New Physics at Work (Second Edition)
65
13c
Class:
2
Name:
(
)
Date:
(a) Set the frequency at 300 Hz and increase the loudness of the
sound.
(b) Observe any change in the waveform.
(c) Sketch the waveform in Figure 13c-3.
Fig 13c-3
On increasing the loudness, the
increases.
amplitude
of the waveform
Quality
Prolong the note as long
as possible and keep it
steady, otherwise the
trace on the CRO will
become unsteady after a
short while. This is due to
the problem with triggering
when the signal becomes
too weak.
3
(a) Connect a microphone to a CRO.
(b) Strike a tuning fork and place it near the microphone
(Fig 13c-4).
(c) Observe the waveform on the CRO.
(d) Also whistle into the microphone and look at the waveform
produced.
(e) Sketch the waveform in Figure 13c-5.
Some electronic musical
instruments can emulate
the notes of different
instruments, e.g. piano,
violin, trumpet, etc. They
can produce notes of
steady intensity for a long
period of time and are
suitable for monitoring
waveforms of musical
notes on the CRO.
tuning forks on resonance box
hammer
Fig 13c-4
4
Fig 13c-5
(a) Sing doh, re, me, fah, soh, la, ti and compare their waveforms on
the CRO (Fig 13c-6).
(b) Play several musical instruments and compare their waveforms.
(c) Sketch one of the waveforms in Figure 13c-7.
66
New Physics at Work (Second Edition)
© Oxford University Press 2006
Class:
Name:
(
)
Date:
Fig 13c-6
13c
Fig 13c-7
For the same note, e.g. the middle C (261 Hz), the waveforms of
different instruments differ. How do they differ?
The notes do not sound alike. They have different qualities (they have different
numbers and amplitudes of the overtones accompanying the fundamental frequency).
Discussion
Use a musical instrument to produce the middle C (doh) and high C
(doh′ ) note. How do the two notes compare?
The doh ′ sounds one octave above doh. The waveform of doh ′ shows that its
fundamental frequency (the large variation of the waveform) is about twice that of doh.
1 The higher the pitch, the higher the
frequency
of the
amplitude
of the
sound waves.
2 The louder the note, the greater the
sound waves.
3 Notes of the same pitch and same loudness from different musical
instruments do not sound alike — they have different qualities. The
quality of a note depends on the number and amplitude of the
overtones
© Oxford University Press 2006
which accompany the fundamental note.
New Physics at Work (Second Edition)
67
13c
Class:
Name:
(
)
Date:
Related question in HKCEE
1996 Paper 1 Q4(b)
Figure a shows the traces on a CRO of three notes produced by different
musical instruments. (Note: The settings of the CRO remain unchanged.)
Note X
Note Y
Note Z
Fig a
(i) Which of the notes is produced by a tuning fork?
*(ii) Compare the pitch and loudness of the three notes.
Explain briefly.
(1 mark)
(5 marks)
Some candidates had difficulties in identifying the note produced by a tuning fork from the
waveforms displayed on the CRO. Some confused the period of the trace with its wavelength.
Some did not know how the pitch and loudness of a note are related to its frequency and
amplitude.
68
New Physics at Work (Second Edition)
© Oxford University Press 2006