Pitch of Sound:
We hear different sounds as high-pitched or low-pitched. Pitch is the characteristic of sound that helps us distinguish between a shrill sound and a deep sound.
Pitch depends mainly on the frequency of vibration of the source.
- A higher frequency means more vibrations are produced per second, so the sound has a higher pitch.
- A lower frequency means fewer vibrations are produced per second, so the sound has a lower pitch.
For example, a whistle usually produces a high-pitched sound, while the sound of a large drum is generally lower-pitched.
Frequency and Pitch:
Frequency is the number of complete vibrations produced in one second. Its SI unit is hertz (\(Hz\)).
Therefore,
Higher frequency \(\to\) Higher pitch
Lower frequency \(\to\) Lower pitch
Lower frequency \(\to\) Lower pitch
Pitch and loudness should not be confused. A sound can have a high pitch without being very loud.
Why do Human voices differ?
People do not all have exactly the same voice because the sound produced by our vocal organs is shaped differently.
When we speak, vibrations produced by the vocal cords travel through the throat, mouth and nasal passages. These parts modify and shape the sound before it reaches the listener.
The shape and size of these structures differ from person to person. Therefore, even when two people produce sounds with similar frequencies, their voices can still sound different.
Voice changes during Adolescence:
During adolescence, the voice-producing organs undergo changes. The vocal cords become longer and thicker, particularly in boys. This generally causes the voice to become deeper, because the vibration characteristics of the vocal cords change.
However, a change in voice should not be described simply as the sound becoming "louder." Pitch is related to frequency, whereas loudness is mainly related to amplitude.
Exploring Frequency using a Sound App:
A sound-generating app can be used to investigate how frequency affects what we hear.
Suppose a sound is produced at \(100\ Hz\) and its frequency is gradually increased.
As the frequency increases, the sound is perceived as becoming higher in pitch.
If the frequency is reduced towards \(20\ Hz\), the sound becomes very low-pitched and eventually may no longer be heard by a human listener.
This activity demonstrates an important relationship:
Change in frequency \(\to\) Change in pitch
It also shows that humans cannot hear every possible frequency of sound.
Audible Range of Human Hearing:
The human ear can detect only a limited range of frequencies. For a typical human ear, the approximate audible range is: \(20\ Hz\) to \(20\ kHz\) (\(20,000\ Hz\))
Sounds within this range can normally be heard by humans, provided their intensity is sufficient.
Sounds outside this range are classified according to their frequency.
Infrasonic Sound:
Sound with a frequency below \(20\ Hz\) is called infrasonic sound.
Humans normally cannot hear infrasonic sounds.
Some animals can detect very low-frequency sounds. Such sounds may be useful for communication or sensing events occurring over large distances.
Ultrasonic Sound:
Sound with a frequency above \(20\ kHz\) is called ultrasonic sound.
Humans normally cannot hear ultrasonic sounds.
Some animals, such as dogs and bats, can detect frequencies beyond the normal human hearing range. This means that a sound that is inaudible to humans may still be detectable by certain animals.
Therefore, whether a sound can be heard depends partly on the frequency range that the listener's ear can detect.
Loudness of Sound:
Loudness tells us how strong or weak a sound appears to our ears.Loudness is mainly associated with the amplitude of vibration.
- Greater amplitude \(\to\) sound is generally perceived as louder.
- Smaller amplitude \(\to\) sound is generally perceived as softer.
For example, striking a drum harder produces vibrations with greater amplitude, so the sound is generally louder. It is important to distinguish:
Frequency \(\to\) mainly determines pitch
Amplitude \(\to\) mainly affects loudness
Amplitude \(\to\) mainly affects loudness
Thus, increasing frequency does not necessarily make a sound louder.
Measuring Sound and Decibel:
The level of sound is commonly expressed in decibels (\(dB\)).
Very loud sounds can be harmful to our ears, especially when we are exposed to them for a long period.
Examples of sources of loud sound include:
- Very loud music
- Vehicle horns
- Firecrackers
- Loudspeakers
- Industrial machinery
- Heavy traffic
Continuous exposure to excessive sound can contribute to hearing problems and hearing loss.
Noise Pollution:
Unwanted or excessive sound is called noise. When unwanted sound becomes excessive and causes disturbance or harm, it is referred to as noise pollution.
Noise pollution can affect our ability to concentrate, communicate and rest. Prolonged exposure to very loud sounds can damage structures involved in hearing.
Preventing Noise Pollution:
Noise pollution can be reduced by:
- Avoiding unnecessarily loud sounds.
- Keeping the volume of music and electronic devices at a safe level.
- Using silencers and noise-control measures in vehicles and machines.
- Avoiding unnecessary honking.
- Using hearing protection in very noisy environments.
Protecting our ears is important because damage to the hearing system can sometimes be permanent.
How do we hear Sound?
The human ear is the organ responsible for hearing.
When a sound reaches the ear, it passes through the outer ear and reaches a thin membrane called the eardrum.
Sequence of Hearing:
Sound enters the ear \(\to\) Eardrum vibrates \(\to\) Tiny bones amplify the vibrations \(\to\) Cochlea converts the vibrations into electrical signals \(\to\) Signals travel to the brain \(\to\) Brain perceives the sound
Eardrum:
The eardrum is a thin membrane that vibrates when sound waves reach it.
Tiny Bones:
The vibrations of the eardrum are transmitted to tiny bones in the middle ear. These bones help amplify the vibrations.
Cochlea:
The vibrations eventually reach the cochlea, a spiral-shaped structure in the inner ear. The cochlea converts the vibrations into electrical signals.
Brain:
These electrical signals travel through nerves to the brain. The brain processes these signals, allowing us to perceive and recognise sound.
Therefore, hearing is not simply the ear receiving sound. The ear converts sound vibrations into signals that the brain can interpret.
How do we locate the direction of a Sound?
Having two ears helps us determine where a sound is coming from.
When a sound reaches a person from one side, it generally reaches the nearer ear slightly earlier than the farther ear.
The brain compares the small differences in the arrival of sound at the two ears.
Using this information, the brain can estimate the direction from which the sound originated.
This is called sound localisation.
For example, if someone calls your name from your left side, the sound reaches your left ear slightly before your right ear. Your brain uses this tiny difference to help identify that the sound came from the left.
Hearing Loss and Hearing Aids
Exposure to excessively loud sounds for long periods can damage the structures involved in hearing.
A person with hearing loss may have difficulty detecting or understanding sounds.
Depending on the type and cause of hearing loss, a hearing aid may help some people hear more clearly. A hearing aid receives sound, processes or amplifies it, and delivers the amplified sound to the ear.
However, preventing damage is better than exposing the ears repeatedly to harmful levels of sound.
Tone and Musical Note:
Not every sound is considered musical. A tone is a sound associated with a regular vibration of a particular frequency. A musical sound may contain a fundamental frequency along with additional frequencies.
A musical note is generally produced by a periodic sound and has a definite pitch. The sound produced by a musical instrument is often more complex than a single pure frequency.
To understand why different instruments can produce the same note but still sound different, we need to understand fundamental frequency and overtones.
Fundamental Frequency and Overtones:
The fundamental frequency is the lowest frequency associated with the vibration of a musical sound.
Along with the fundamental frequency, a vibrating source can produce additional frequencies called overtones.
The particular combination and relative strengths of these frequencies affect the character of the sound.
For example, a violin and a flute can produce the same musical note. Their fundamental frequencies may be the same, so they have approximately the same pitch. However, their overtones are different.
Therefore, we can identify the instrument even when the pitch is the same.
Timbre — The Quality of Sound:
The characteristic quality that allows us to distinguish between two sounds having the same pitch and similar loudness is called timbre.
Timbre depends on the combination of the fundamental frequency and the overtones present in the sound and their relative strengths.
For example, a flute and a violin may play the same note at approximately the same loudness. Yet we can usually tell which instrument is playing.
This is because their sounds have different timbres.
Therefore:
Pitch \(\to\) mainly related to frequency
Loudness \(\to\) mainly related to amplitude
Timbre \(\to\) related to the characteristic combination of fundamental frequency and overtones
Loudness \(\to\) mainly related to amplitude
Timbre \(\to\) related to the characteristic combination of fundamental frequency and overtones
Octave:
An octave describes an interval between two frequencies in which the higher frequency is twice the lower frequency.
For example:
- \(200\ Hz\) and \(400\ Hz\) form an octave.
- \(500\ Hz\) and \(1000\ Hz\) form an octave.
Although the frequencies are different, notes separated by an octave are perceived as closely related in pitch.
The idea of the octave is important in understanding musical scales and the relationship between musical notes.
Tabla and Syaahi:
The tabla is a percussion instrument that produces musical sounds when its stretched membrane is struck. The black circular region on the membrane is called syaahi.
The presence and arrangement of the syaahi affect the vibration of the membrane and help the tabla produce a distinctive musical sound. The resulting sound contains a characteristic combination of frequencies, which contributes to its timbre.
Thus, the construction of a musical instrument influences the vibrations it produces and, consequently, the quality of the sound we hear.