SCE 3105 Kuliah 9

Fizik dalam muzik - Gelombang bunyi

Membincangkan topik topik berikut:

• Gelombang bunyi dan gelombang membujur.

• Keamatan dan kenyaringan• Frekuensi and kenyaringan• Resonan• Pengesanan gelombang tekanan• Kualiti bunyi• Membuat muzik

Sumber sumber bunyi?

Bunyi dihasilkan oleh suatu sistem bergetar seperti getaran tala bunyi, getaran tali gitar, atau getaran kon suatu pembesar suara.

Gelombang bunyi dan gelombang membujur

Gelombang membujur

• Gerakan zarah-zarah individu medium adalah dalam arah selari dengan arah pengangkutan tenaga.

• Molekul2 udara (zarah-zarah medium) bergerak dalam arah yang selari dengan arah gerakan gelombang.

• Maka gelombang bunyi dirujuk sebagai gelombang membujur.

• Hasil getaran membujur seperti itu adalah disebabkan oleh mampatan dan regangan udara.

Penghasilan mampatan dan regangan dalam udara.

Gelombang bunyi mempunyai amplitud, semakin nyaring bunyi itu, semakin besar keupayaan molekul udara untuk berada pada titik tertinggi gelombang.

Keamatan dan kenyaringan

Amplitud gelombang adalah perubahan tekanan bila gelombang bunyi melaluinya.

Membesarkan amplitud bunyi, membuat ia

nyaring. Mengecilkan amplitud, bunyi akan menjadi perlahan.

Amplitud gelombang berkait dengan jumlah tenaga yang dibawanya. Gelombang dgn amplitud yang tinggi membawa jumlah tenaga yang besar. Gelombang dgn amplitud yang kecil membawa jumlah tenaga yang kecil.

• Purata jumlah tenaga yang melalui satu unit luas per unit masa khususnya di panggil keamatan gelombang.

• Keamatan = tenaga/masa = Kuasa = P luas luas

4πr2

• Bilamana amplitud gelombang bunyi meningkat, keamatan bunyi juga meningkat.

• Bunyi dengan keamatan yang tinggi adalah lebih nyaring.

• Kedua-dua keamatan pada jarak yang berbeza daripada sumber kuasa yang tetap boleh dibandingkan sebagai nisbah:

I2 = P/4πr 22

I1 P/4πr1 2

or I2 = r 22

I1 r1 2

• Keamatan bunyi relatif diberi dalam unit bel (B). • Skala keamatan yang lebih kecil didapati

dengan menggunakan unit yang lebih kecil, decibels (dB).

Frekuensi and kelangsingan

• Jika anda meningkatkan frekuensi bunyi (bilangan kitaran dalam 1 saat), anda akan dapat bunyi yang lebih langsing

• Langsing merujuk kepada frekuensi gelombang bunyi.

• Langsing kadang2 juga dipanggil frekuensi perceived.

• Ciri-ciri had pendengaran bunyi. • Hanya gelombang bunyi antara 20 Hz to

20 kHz menginitiate impuls saraf yang diterjemahkan oleh otak manusia sebagai bunyi.

• Julat ini dipanggil kawasan boleh dengar (audible region) spektrum frekuensi bunyi.

• f lebih rendah lower than 20 Hz are in the infrasonic region (e.g wave generated by earthquakes, wind and weather patterns) where some animals like elephants and cattle can hear and may give us early warnings of weather disturbances for example

• Above 20 kHz is the ultrasonic region and this ultrasonic waves can be detected by animals like dogs (extends to about 45 kHz, cats 70 kHz, bats 100kHz)

If you increase the frequency of sound, you get a higher pitched sound. When you decrease the frequency, you get a lower pitched sound.

Resonancemeans to "resound" - to sound out together with a loud sound.

This bridge was designed with a natural frequency equal to that created by the wind through the Narrows. As a result, whenever the wind blew, the bridge began to roll and sway and fold, resonating to greater and greater convulsions due to this natural frequency. 2 months after construction in the late 1940's, this motion caused the bridge to fail in a dramatic fashion.

Tacoma Narrows Bridge in Washington

• A guitar string has a number of frequencies at which it will naturally vibrate.

• These natural frequencies are known as the harmonics of the guitar string.

• The natural frequency at which an object vibrates at depends upon the tension of the string, the linear density of the string and the length of the string.

• Each of these natural frequencies or harmonics are associated with a standing wave pattern.

• In sound applications, a resonant frequency is a natural frequency of vibration determined by the physical parameters of the vibrating object.

• Most vibrating objects have multiple resonant frequencies.

• A vibrating object will pick out its resonant frequencies from a complex excitation and vibrate at those frequencies, essentially "filtering out" other frequencies present in the excitation.

• If you just whack a mass on a spring with a stick, the initial motion may be complex, but the main response will be to bob up and down at its natural frequency.

Resonance only occurs when the first object is vibrating at the natural frequency of the second object.

If the frequency at which the tuning fork vibrates is not identical to one of the natural frequencies of the air column inside the resonance tube, resonance will not occur and the two objects will not sound out together with a loud sound.

• The location of the water level can be altered by raising and lowering a reservoir of water, thus decreasing or increasing the length of the air column,

• An increase in the length of a vibrational system (the air in the tube) increases the wavelength and decreases the natural frequency of that system.

• So by raising and lowering the water level, the natural frequency of the air in the tube could be matched to the frequency at which the tuning fork vibrates.

• When the match is achieved, the tuning fork forces the air column inside of the resonance tube to vibrate at its own natural frequency and resonance is achieved.

• The result of resonance is always a big vibration - that is, a loud sound.

Detection of pressure waves

If a detector, (human ear or a man-made instrument) is used to detect a sound wave, it would detect fluctuations in pressure as the sound wave impinges upon the detecting device

Since sound wave consists of a repeating pattern of high pressure and low pressure regions moving through a medium, it is sometimes referred to as a pressure wave.

• the sound signal is transformed from a pressure wave traveling through air to the mechanical vibrations of the bone structure (the hammer, anvil, stirrup) in the middle ear.

• These vibrations are then transmitted to the fluid of the inner ear where they are converted to electrical nerve impulses which are sent to the brain.

The continuous arrival of high and low pressure regions sets the eardrum into vibrational motion.

When a pressure wave reaches the ear, a series of high and low pressure regions impinge upon the eardrum.

• Sound pressure can be measured using a microphone in air and a hydrophone in water.

• The SI unit for sound pressure is the pascal (symbol: Pa).

• The instantaneous sound pressure is the deviation from the local ambient pressure p0 caused by a sound wave at a given location and given instant in time.

"Water Hammer" a tapping sound in a water pipe which may occur when a tap is suddenly closed, causing the water column to vibrate.

• When water flowing down a pipe is suddenly stopped at the downstream end, a pressure wave travels up the pipe.

• When this wave reaches a stationary body of water in the reservoir it is reflected back down the pipe to the valve end.

• A negative pressure wave is then generated which travels up the pipe again to the reservoir.

• Without friction this process would continue forever.

It was the first atomic bomb ever used as a weapon and was dropped three days before the "Fat Man" bomb was used against Nagasaki.

Little Boy, the atomic bomb which was dropped on Hiroshima, on 6/8/1945 by the 12-man crew of the B-29 Superfortress Enola Gay, piloted by Colonel Paul Tibbets in the 393d Bombardment Squadron, Heavy of the US Army Air Forces

The damage came from 3 main effects: blast, fire, and radiation

• The blast from a nuclear bomb is the result of x-ray-heated air (the fireball) sending a shock/pressure wave in all directions at the speed of sound

• analogous to thunder

generated by a bolt of lightning.

• Studies of Little Boy at Hiroshima have given us most of what we know about urban blast destruction from nuclear weapons.

Mushroom cloud from the nuclear explosion over Nagasaki rising 18 km (60,000 ft) into the air.

Tsunami Detection System

A tsunami wave in deep water creates a small but measurable change in pressure that will be maintained for as long as 20 minutes

By monitoring any such changes, subsea detectors can be used to trigger an alarm that sends a warning message to a receiver on the surface, which in turn, relays the message via a satellite data link to a control centre that can issue a warning to vulnerable communities.

Sound quality• defined as the degree of accuracy

with which a device records or emits the original sound waves

• Sounds may be generally characterized by pitch, loudness, and quality.

• Sound "quality" or "timbre" describes those characteristics of sound which allow the ear to distinguish sounds which have the same pitch and loudness.

• Timbre is then a general term for the distinguishable characteristics of a tone.

Pitch

• The perceived fundamental frequency of a sound. The actual fundamental frequency can be precisely determined through physical measurement, it may differ from the perceived pitch

• Pitch is the quality of sound which makes some sounds seem "higher" or "lower" than others. It is determined by the number of vibrations produced during a given time period, called frequency.

• A high pitch sound corresponds to a high frequency sound wave and a low pitch sound corresponds to a low frequency sound wave. The higher the frequency the higher the pitch.

• Frequency is often measured in units called Hertz (Hz). • The average person can hear sound from about 20 Hz to about

20,000 Hz. The upper frequency limit will drop with age.• measured by its perceived frequency

• Loudness is the amount or level of sound (the amplitude of the sound wave) that we hear.

• It is a measure of sound wave intensity (amount of energy which is transported past a given area of the medium per unit of time). The more intense sounds, will be perceived to be the louder sounds

• Intensity of a sound is a very objective quantity which can be measured with sensitive instrumentation, where as loudness of a sound is more of a subjective response which will vary with a number of factors.

• The same sound will not be perceived to have the same loudness to all individuals. Age is one factor which effects the human ear's response to a sound.

• Sound level is often measured in decibels (dB). Sound pressure level (SPL) is a decibel scale which uses the threshold of hearing as a zero reference point.

loudness

• describes those characteristics of sound which allow the ear to distinguish sounds which have the same pitch and loudness

• describes those characteristics of sound which allow the ear to distinguish sounds which have the same pitch and loudness

quality 3 different complex waveforms that all have the same period and amplitude

Making music

• Different musical instruments produce sounds in very different ways, but all of them take advantage of some of the fundamental properties of sound

• the physics of sound - to make a variety of interesting and pleasant sounds. You will find here a Strings Activity, Wind Instrument Activity and Percussion Activity,

The physics of sound

Making a variety of interesting and pleasant sounds.

• Strings Activity, • Wind Instrument Activity, • Percussion Activity, and

Strings Activity

• construct a simplified version of a stringed instrument, using rubber bands as strings, and will use the instrument to explore the effects of various string characteristics on frequency and amplitude

Wind Instruments Activity

• explore the effects of air column size (and shape) on the frequency and amplitude of standing waves in the air column, using empty glass bottles, and water if necessary to vary air column size.

Percussion Activity

• explore the effects of various object characteristics on frequency and amplitude.

Discussion Questions Does the size of the object seem to affect its pitch/frequency? Its loudness? Does the shape of the object seem to affect its pitch/frequency? Its loudness? Does the object's material seem to affect its pitch/frequency? Its loudness? Can you tell what effects the thickness of an object has on its sound?

• Observation of events and explain their functions eg how does a loudspeaker works?

• Why are the length of strings of a piano different?

• Why does a glass shatter when certain musical notes are produced?

• How are music and noise different? etc.