Chapter 15
description
Transcript of Chapter 15
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Chapter 15
Successive Tones: Reverberations, Melodic Relationships,
and Musical Scales
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Audibility of DecayingSounds in a Room
The first of the tone we hear is the directly propagated wave. Because of the precedence effect, the
direct wave will combine with the most direct reflections (within 30 to 50 milliseconds) and be perceived as one.
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Picture of a Clearly Heard Tone
Attack – heard as one because of Precedence Effect
Decay – similar to the attack
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Reverberation Time
The time required for the sound to decay to 1/1000th of the initial SPL
Audibility Time Use a stopwatch to measure how long
the sound is audible after the source is cut off
Agrees well with reverberation time It is constant, independent of frequency,
and unaffected by background noise
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Why does Audibility Time Work?
Threshold of hearing temporarily shifted to 60 dB below a loud tone? 60 dB is 1000 times in SPL which then
matches the definition of Reverberation Time
Measurements show that this happens, but only for a few tenths of a second Not long enough to make audibility time
work
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Why does Audibility Time Work?
The ear is responding to the rate of change of loudness? Look at example on next slide
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Advantages of Audibility Time
Only simple equipment required Many sound level meters can only
measure a decay of 40-50 dB, not the 60 dB required by the definition
Instruments assume uniform decay of the sound, which may not be the case
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Device to Study Successive Tones
Tone Generator 2
Tone Generator 1 21
SwitchAmplifier Speaker
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Notes on Tone Switcher
Tone generators produce fundamental plus a few harmonics to simulate real instruments
Switching cannot be heard Reverberation time at least ⅓ sec.
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Experiment
Start with TG1 on C4
Switch to TG2 and adjust
At certain frequencies the decaying TG1 will form beats with the partials or heterodyne components of TG2 The beats will be most audible when the
amplitudes are equal.
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Using Reverberation
These experiments show that we can use reverberation as an aid in performing It is easier to perform in a live room
(shower) Noise can mask the decaying partials
and make pitch recognition more difficult
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Conclusions We can set intervals easily for successive
tones (even in dead rooms) so long as the tones are sounded close in time.
Setting intervals for pure sinusoids (no partials) is difficult if the loudness is small enough to avoid exciting room modes.
At high loudness levels there are enough harmonics generated in the room and ear to permit good interval setting.
Intervals set at low loudness with large gaps between the tones tend to be too wide in frequency.
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The Beat-Free Chromatic(or Just) Scale
We will use the Tone Switcher to help find intervals that produce beat-free relationships to the fundamental. The fact that the frequency generators
contain harmonics makes this possible Notice that the octave is a doubling of
the frequency and the next octave would be four times the frequency of the fundamental
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First Important Relationship
Three times the fundamental less an octave 3f/2 or an interval of 3/2 or a fifth Fundamental will have harmonics that
contain the fifth Five such relationships can be found in
the first octave
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Just Intervals (with respect to C4)
Chromatic ScalesListed Interval Interval Computed Cent
Frequency Name Ratio Frequency Difference
(equal-tempered) (beat-free)
C 261.63
E 329.63 3rd 5/4 327.04 14
F 349.23 4th 4/3 348.84 2
G 392.00 5th 3/2 392.45 -2
A 440.00 Major 6th 5/3 436.05 16
C 523.25 octave 2/1 523.26 0
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Relationships Among Five Principles
NoteFrequency
(equal-tempered)IntervalRatio
IntervalName
ResultingFrequency
Note
F 349.23 3/2 5th 523.85 C
E 329.63 4/3 4th 439.51 A
G 392.00 4/3 4th 522.67 C
F 349.23 5/4 3rd 436.54 A
E 329.63 6/5 Minor 3rd 395.52 G
A 440.00 6/5 Minor 3rd 528.00 C
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Finding the Missing Steps
Notice the B and D are not harmonically related to C
Finding B A fifth (3/2) above E gives 490.56 Hz A third (5/4) above G gives 490.00 Hz Difference is 2 cents – sensibly equal
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The Trouble with D
A Fourth (4/3) below G gives 294.34 Hz
A Fifth (3/2) below A gives 290.70 Hz Difference is 22 cents or 1¼%
Sounded together these “D’s” give clear beats
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Intervals with B and D
5th
CGDC E F A B
4th
5th
3rd
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Filling in the Scale
3rd
3rd
3rd 3rd
4th
Minor 6
G CDC E F A B
Notice that C#, Eb, and Bb come into the scheme, but Ab/G# is another problem.
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Putting numbers to the Ab/G# Problem
From at Interval Ratio Giving
E 327.04 Third 5/4 408.80
C 523.26 Third 5/4 418.61
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The Problem with F#
3rd
3rd
min3
CDC E F A BG
Other discrepancies exist but these highlight the problem.
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Saving the Day
As the speed increases discrepancies in pitch are more difficult to detect.
The sound level is greater at the player’s ear than the audience. He can make small adjustments. He is always better tuned than the audience demands.
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Working Toward Equal Temperament
The chromatic (Just) scale uses intervals which are whole number ratios of the frequency. Scales have unequal intervals
E 327.04 F 348.84 1.0666 16/15
B 490.5 C 523.26 1.0667 16/15
but
C# 279.07 D1 290.7 1.0417
F 348.84 F#1 363.38 1.0417
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Making the Interval Equal An octave represents a doubling of the
frequency and we recognize 12 intervals in the octave.
Make the interval 1.059463 212
Using equal intervals makes the cents division more meaningful
The following table uses
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Breaking Up One IntervalInterval in Cents Frequency Ratio Frequency Note
0 1.00000 261.63 C4
10 1.00579 263.15
20 1.01162 264.67
30 1.01748 266.20
40 1.02337 267.75
50 1.02930 269.30
60 1.03526 270.86
70 1.04126 272.43
80 1.04729 274.00
90 1.05336 275.59
100 1.05946 277.19 D4
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ComparisonFrequency
RatioMusical Interval Cents
(Just)Cents
(Equal-Tempered)
1/1 Unison 000 000
2/1 Octave 1200 1200
3/2 Fifth 702 700
4/3 Fourth 498 500
5/3 Major sixth 884 900
5/4 Major third 386 400
6/5 Minor third 316 300
8/5 Minor sixth 814 800
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Pitch Discrepancy Groups When pitch discrepancies exist in a
scale, the cent difference from the equal-tempered interval cluster into three groups
Low Group Middle Group High Group
12 cents lowEqual-tempered
frequency12 cents high
Each group has a range of about 7 cents If a player is asked to sharp/flat a tone, (s)he
invariably goes up/down about 10 cents, moving from one group to another.
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Complete Scale ComparisonInterval
Ratio to TonicJust Scale
Ratio to TonicEqual Temperament
Unison 1.0000 1.0000
Minor Second 25/24 = 1.0417 1.05946
Major Second 9/8 = 1.1250 1.12246
Minor Third 6/5 = 1.2000 1.18921
Major Third 5/4 = 1.2500 1.25992
Fourth 4/3 = 1.3333 1.33483
Diminished Fifth 45/32 = 1.4063 1.41421
Fifth 3/2 = 1.5000 1.49831
Minor Sixth 8/5 = 1.6000 1.58740
Major Sixth 5/3 = 1.6667 1.68179
Minor Seventh 9/5 = 1.8000 1.78180
Major Seventh 15/8 = 1.8750 1.88775
Octave 2.0000 2.0000
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Indian Music Comparisons
Indian sa re ga ma pa dha ni sa
Western do re mi fa sol la ti do
Letter C D E F G A B C
Indian music uses a generalize seven note scale like the do re mi of Western music.
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The Reference Raga
The rag is the most important concept of Indian music. The Hindi/Urdu word "rag" is derived
from the Sanskrit "raga" which means "color, or passion". It is linked to the Sanskrit word "ranj" which means "to color".
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The Alap
An Indian piece will usually open with an alap, notes going up and down the scale to establish position and relationship. They will play around a tone, the tone
evasion becoming very elaborate. It becomes a game between the player
and the listeners. Jazz has similar variations.
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Indian Modes
Play Bilawal
Play Kafi
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Pitch Variations
In Western music we have similar pitch wanderings (vibrato, for example) that the Indian musician would find strange.
We almost always make abrupt transitions from one note to the next without the slides of Indian music.