Properties of Sound Neil Freebern

Sound Sound is produced when something vibrates. Vibrations disturb the air, creating variations in air pressure. Variation in air pressure are transmitted through the air in Sound Waves, much like throwing a rock into a pond. Ripple Tank

As the sound wave travels through the air, air molecules are pushed together or pulled apart creating alternating high and low pressure. The higher pressure of molecules pushed together is called compression. The lower pressure molecules pulled apart is called rarefaction.

Waves The wave is the disturbance (people jumping up and sitting back down), and it travels around the stadium. However, none of the individual people the stadium are carried around with the wave as it travels - they all remain at their seats It’s like this… Longitudinal sound waves in air behave in much the same way. As the wave passes through, the particles in the air oscillate back and forth about their equilibrium positions but it is the disturbance which travels, not the individual particles in the medium.

Three Components of Sound Pitch, Loudness and Timbre

Pitch #1

Pitch Each repetition of a waveform is called a cycle. The number of repetitions that occur per second is called the frequency. Frequency is measured as cps = cycles per second or in Hz called hertz. “Hz” is derived from the name of Heinrich Hertz, a German scientist who did pioneering work in electromagnetic waves during the 19th. Century.

Slow motion simulation of a propagating sound wave in air.

Those sounds with a high frequency are perceived as high pitched sounds. Those sounds with few cycles per second are called low in pitch.

When a frequency of a sound is doubled, we perceive it as being an octave higher in pitch.

Double the frequency...create an octave.

Normal range of hearing: 20 Hz to 20 kHz

Loudness Known as Volume expressed in Decibels, determined by Amplitude #2

Loudness The relative strength of the deviations in air pressure created by a vibrating object determines the loudness (or volume). The greater variations in air pressure, the louder we perceive the sound. These deviations are referred to as the amplitude of the waveform. Amplitude

To create vibrations, energy must be expended. The amount of energy expended is called power and is measured in watts.

Humans do not perceive sound intensity in a linear fashion; that is, for a sound to be perceived as twice as loud, the intensity must be 10 times as great. The intensity level of a sound is measured in a unit known as the decibel (or dB). A unit of measurement named after Alexander G. Bell, who invented the telephone. The use of the capital “B” in the abbreviation is in his honor. The softest sound that a person can hear-the threshold of hearing”- is defined as 0dB. The “threshold of pain” is about 120 dB.

What a human perceives as loud or soft depends on the frequency as well as the intensity of the sound. The graph below displays intensity levels compared with the frequencies for sounds of equal loudness for humans. The bottom line is the threshold of hearing. At a 1 kHz frequency, the hearing threshold is 0 dB, but at 60 Hz the decibel level is 50. Only one percent of all human beings can hear sounds this low, so, the lower line is mainly for those with very good hearing. The next line up is the hearing threshold for the majority of people. The top line is the pain threshold. Other than at one point, about 4 kHz, this line varies little. All of the other lines also dip down at 4 kHz. We can gather from this graph, then, that the human ear is most sensitive at about 4 kHz.

Timbre #3

Timbre (TAM-ber) The property of a sound that allows us to determine the difference between a saxophone and a flute is called timbre, or tone color. Different timbres occur because most sounds actually contain many frequencies. The predominant pitch is called the fundamental frequency of the sound. The other frequencies present occur in a mathematical series called the harmonic series, or overtone series.

Harmonics The frequency of each harmonic is a whole number multiple of the fundamental frequency. (i.e.; 2x,3x,4x, etc..)

Harmonic series In acoustics the term "harmonic" is a noun used to describe an overtone or partial whose frequency is an integer multiple of the fundamental frequency.partial frequencyfundamental The diagram illustrates a fundamental frequency and the first five harmonics with frequency ratios of 2:1, 3:1, 4:1, 5:1 and 6:1 in relation to the fundamental frequency.

Some musical sounds have overtones that are direct multiples of the fundamental frequency, and some do not. When the overtones are not whole number multiples of the fundamental, they sound out of tune and are called clangorous overtones. The number of harmonics present and the relative strength of each determines the timbre of the sound. Harmonics Applet

Sine Wave A sine wave consists of only the fundamental. No other harmonics are present.

Square Wave With a square wave only the odd harmonics are present.

Triangle Wave With a triangle wave only the odd harmonics are present.

With a sawtooth wave all harmonics are present.

As an ambulance speeds towards you, sirens blazing, the sound you hear is rather high in pitch. This is because the sound waves in front of the vehicle are being squashed together by the moving ambulance. This causes more vibrations to reach your ear per second. As you know, more vibrations per second results in a higher pitched sound. When the ambulance passes you, the sound becomes lower in pitch. Behind the ambulance there are fewer vibrations per second, and a lower sound is heard. This change in pitch is known as the Doppler Effect. When a vehicle travels faster than the speed of sound, about 330 meters per second, a sonic boom can be heard. As the vehicle overtakes its own sound, the sound waves spread out behind in a shockwave, or sonic boom.

Doppler Applet