Sound
Don’t miss out on the latest A1 notes and tips from top Leaving Cert performers!
Every source of sound is a vibrating object.
Sound is a longitudinal mechanical wave.
We know this because sound shows all of the properties of waves: sound undergoes reflection, refraction, di fraction and interference. However, sound cannot be polarised as it is a longitudinal wave.
The frequency of a sound wave is the same as that of the vibrating source producing it.
Sound requires a medium (`clock-in-jar’ experiment).
The speed of sound in a medium depends on the elastic properties and density of that medium.
denser medium = faster speed
hotter medium = faster speed
Overtones:
frequencies which are multiples of a certain frequency are called overtones
of that frequency.
If f is a given frequency:
2f is its first overtone
3f is its second overtone
Characteristics of Notes:
1. Loudness: of a note depends on the amplitude of the sound wave. The greater the
amplitude the greater the loudness. [The loudness of a sound also depends on its
frequency as the human ear is more sensitive to sounds of certain frequencies.]
2. Pitch: of a note depends on the frequency of the sound wave. The higher the
frequency the higher the pitch.
3. Quality/Timbre: of a note depends on the number of overtones present in the note
and the relative strengths of the diff erent overtones present
The Frequency Limits of Audibility:
are the highest and lowest frequencies that can be heard by a normal human ear. The range is 20Hz to 20KHz.
Sounds with frequencies above 20KHz are said to be ultrasonic.
Resonance: if the frequency of a periodic force applied to a body is the same as or very near to its natural frequency, then that body will vibrate with a very large amplitude.
This phenomenon is called resonance.
Example: pushing a swing.
NB Resonance is the response of a body to its own natural frequency of vibration.
Examples of Resonance:
1. Barton’s pendulums;
2. High-pitched notes can shatter glass due to resonance;
3. A column of air in a tube can be made to resonate with a tuning fork;
4. Buildings in an earthquake;
5. Your voice sounds louder and deeper to yourself than it does to others (resonance
in the jaw bone is transferred to the cochlea via vibrations, thus altering your
perception of your voice);
6. Tacoma bridge disaster;
7. To avoid resonance, soldiers marching in step usually break out of step when crossing
a bridge.
The human ear is most sensitive between 2 KHz and 4 KHz due to resonance.
Sound Intensity at a Point: is the rate at which sound energy is passing through unit
area at right angles to the direction in which the sound is travelling at that point.
It is a scalar quantity.
Symbol: I
Unit: Wm^-?2
Threshold of Hearing:
is the smallest sound intensity detectable by the average human ear at a frequency of 1 KHz. Its value is 1 x 10^-?12Wm^-?2
Sound Intensity Level:
this de nition is not on the Leaving Cert. course (it require logs, oh the horror!) but you do need to know its units.
Unit: decibels – dB
NB Sound intensity and sound intensity level are two diff erent things – don’t confuse them!
When sound intensity in Wm?2 doubles, the sound intensity level increases by 3dB. The sound level meter: is a device which measures sound intensity level in decibels. It uses a scale called the dBA scale or the decibel adapted scale or frequency weighted scale.
This scale mimics the human ear in that frequencies which are most sensitive to human ears are given a greater weighting.
High noise levels (> 120 dB) can damage the ear permanently.
Vibrations on a String Stretched Between Two Points
The mode of vibration of a string is the way in which the string vibrates.
The mode of vibration of a string depends on where it is plucked and where it is held fi xed.
As the ends of the string are xed they cannot undergo any displacement, and hence must be nodes.
Fundamental Frequency of a String: a string vibrating with an anti-node at its centre and a node at each end (and no other nodes or anti-nodes) is said to be vibrating at its fundamental frequency.
If the string is plucked at various points along its length various modes of vibration
can be set up, with di erent numbers of nodes and anti-nodes between the endpoint
nodes.
The fundamental frequency of a string is inversely proportional to its length.
This fact is known due to empirical observation and can be veri fied with a sonometer.
Harmonics on a string:
Instead of using overtones, the same situation can be described in terms of harmonics.
Harmonics: frequencies which are multiples of a certain frequency f are called harmonics.
f is called the fi rst harmonic (or the fundamental frequency).
If f is the first harmonic:
2f is the second harmonic,
3f is the third harmonic,
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