[Musical Acoustics Donald Hall Ebook 14
Rancul Ratha
1998: The last note of the Bach Cello Suites reverberates throughouta state-of-the-art concert hall, and the audience stands in applauseas Yo-Yo Ma takes a final bow with his 1712 Davidoff Stradivariuscello.
These four moments in history demonstrate music's power totransform, excite, and entertain people. They illustrate a technicaland artistic mastery of sound production and music-making by highlyskilled artists. And they show the dedication of audiences around theworld who travel great distances and ignore numerous technicalchallenges to hear their music.
Musicians spend years practicing the manipulation of sound. And theyare always on the lookout for just the right instrument. Fans spendthousands of dollars on music collections, hi-fi stereo equipment,and concert tickets, all in pursuit of a good listening experience.So what does all this boil down to? The scientific phenomena ofacoustics and sound wave production.
The difference between the sound an artist produces in a concert halland the sound produced by the beginning student with a MIDIsynthesizer player at home has everything to do with how the quality,tone, richness, and substance of the sound waves reaches our ears.This chapter won't teach you how to become the next Hendrix,but it does examine the scientific phenomena of sound to help youdesign and produce better web audio.
Sound is often measured and described using two distinct reference points: the psychophysical perception of sound in the human ear and brain, and the scientific quantification of sound with acoustic measuring devices. Psychophysical terms such as loudness and pitch describe the human perception of the same acoustical phenomena as amplitude and frequency. For example, amplitude is the measurement of the range of air pressure quantified in decibels or dB. Loudness is the human perception of the range of air pressure in a waveform. Frequency is the measurement of the rate of repetition of a waveform. Pitch is the perception of how high or low a waveform sounds. Many audio equipment manuals and professionals use these terms interchangeably, which can create confusion for newcomers.
Sound is thevibration of airmolecules or variation in air pressure that can be sensed by the ear.The pattern and rate of audible vibrations give sound its uniquequality. The range of auditory perception is approximately 20 cycles(or fluctuations) of air pressure per second to 20,000 cycles persecond. Air pressure fluctuations outside of this range are notaudible to the human ear and are called subsonic (less than 20 cycles per second)and ultrasonic (more than 20,000 cycles persecond) vibrations.
Sound is generatedbya friction-producing force such as a drum stick hitting a cymbal, abow moving across a cello string, or a vibrating speaker cone thatsets the surrounding air molecules into motion. From the point ofimpact or disturbance, sound waves or patterns of vibrating airmolecules radiate outwards through the atmosphere to the ear like theripples of water on a pond. Figure 2-1 shows asound produced by a cello as it emanates from a speaker cone.
As sound waves produced from a cello move rapidly outward through theatmosphere, they reflect off the various surfaces in the room anddivide and multiply into thousands of reflections. Once thesereflections reach the ear, they are converted into electrical nerveimpulses and sent to the brain where they are stored and interpretedas a beautiful "cello" sound. Similarly, these airpressure fluctuations or reflections can be converted into electricalwaves or signals with a microphone and sent to a recording devicethat stores a "waveform" pattern.
The unique pattern of air pressure variations or sound reflectionsproduced by an instrument or a speaker cone are knownaswaveforms. Figure 2-2illustrates three different sounds and their unique waveforms. Inaudio recording and web broadcasting, it is the ability to accuratelycapture and reproduce these waveforms that results in good soundquality. Improper recording and mastering techniques, poor equipment,and other mistakes may result in a blurred or distorted replicationof the original waveform, resulting in poor sound quality.
Most musical sounds vibrate in orderly repetitive patterns (with theexception of percussive instruments such as rattles and shakers). Incontrast,noiseproduces random, chaotic wave patterns. Figure 2-3 shows these two types of waveforms.
Each type of disturbance in the atmosphere produces a distinctpattern of vibrations. These patterns of vibrating air molecules givea sound its unique quality. The sound is composed of threeelements: loudness,pitch, andtimbre.
These elements are the three fundamental qualities of sound thatscientists, audio professionals, and equipment manufacturers use tounderstand, measure, and control the audio production process. Aclear understanding of how these terms are used will help you masterproper recording, editing, and encoding techniques for web audiobroadcasting.
Loudness, orvolume,is the perception of the strength or weakness of a sound waveresulting from the amount of pressure produced. Sound waves with moreintensity or larger variations in air pressure produce louder sounds.Sound waves with smaller fluctuations in air pressure produce quietersounds, as shown in Figure 2-4. Through theduration of the start and decay of most sounds and passages of music,these sound waves fluctuate at various intensities. This is known asthe dynamic range of a sound or a passage of music. Thedynamic range in an audio file refers to the difference or range inloudness to softness throughout the piece.
Loudness is not to be confused with amplitude. Loudness refers to the human perception of sound, while amplitude refers to quantifiable measurements of air pressure variations. Amplitude is the change in air pressure over time and is universally measured in decibels (dB).
Waveform intensity affects the blend of sounds in a mix, as shown inFigure 2-5. The louder a sound is, the more it willmask or dominate other sounds in a mix.Masking -- thephenomena of louder sounds overpowering quieter sounds -- isadvantageous for an interview with someone at a noisy tradeshow butnot for a voice-over with background music.
Pitch is thepsychoacousticterm for how high or low a sound is perceived by the human ear. Pitchis determined by a sound's frequency, orrateofrepetition. Figure 2-6 shows a one-second durationwaveform and the frequency rate for two different instruments. MiddleC on the piano, for example, vibrates at 261 cycles per second.Frequency is measured inhertzor Hz (also known as cycles per second). The higher the frequency,the higher the pitch. The lower the frequency, the lower the pitch.On home stereo equipment, high-frequency sounds are referred to astreble, and lowfrequency sounds asbass. Figure 2-7 shows waveforms of relatively lower- andhigher-frequency sounds.
Notice that it requires more cycles in the same period of time toreproduce a high-pitch sound than it does to reproduce a low-pitchsound, as shown in Figure 2-7. Thus, high-pitchsounds such as a woman's voice or a buzzing fly require moredigital information to accurately reproduce than do lower pitchedsounds such as a man's voice or a bass guitar. This is whylow-pitched sounds are less degraded by the process of converting asound (encoding) to a low-quality format.
A given note is comprised of a series of "pitches" that vibrate in harmony with its fundamental frequency or pitch. Musical tones contain many such pitches, known as harmonics. You can experience this phenomena both aurally and visually by listening to and watching a guitar string being plucked. The string will vibrate at a root, or fundamental, frequency, as well as at higher multiples of this frequency. These additional frequencies are the harmonics. For example, a cello note playing the pitch of middle C will predominantly resonate at 261 cycles per second, but it will also contain frequencies vibrating at 1,000, 2,000, and 4,000 cycles per second.
It is also important to note that most sounds are a mixture of wavesat various frequencies. A cello note, for example, is composed ofmany frequencies across the frequency spectrum.Thefrequencyspectrum is the complete range of frequencies we can hear, just asthe color spectrum is the range of colors we can see. The term alsois used relative to a particular sound, meaning the frequencyspectrum is the range of frequencies present in that sound. Amid-range cello note, for instance, has a range of 500 Hz to 12,000Hz.
The various frequencies that comprise a sound can be amplified orreduced with equalization (EQ) to change thesound's overall tone and character. Equalization for the Web isdiscussed in detail in Chapter 4, "Optimizing Your Sound Files".
Unlike loudness or amplitude, measured in dB, and pitch or frequency,measured in Hz, timbre is difficult to quantify.Timbre is loosely defined as the tone, color, ortexture that enables the brain to distinguish one type of instrumentsound from another. The term generally encompasses all the qualitiesof a sound besides loudness and pitch, such as "smooth,""rough," "hollow," "peaceful,""shrill," "warm," and so on. In simple terms,timbre is the sonicdifference between a violin and a trumpet playing the same note atthe same loudness or amplitude level.
For further study, read Donald E. Hall's Musical Acoustics (Wadsworth Publishing Co., 1980). This book gives one of the best in-depth explanations of the science of sound and human hearing. You can also check out more information about sound and recording at the University of California, Santa Cruz Electronic Music Studios' web site,
Much of a sound's unique timbre is a result of its particulartransientqualities. Transients are the attack and decay,or beginning and ending characteristics, of a sound, as shown inFigure 2-8. Forexample, the quiver of a violin bow as it strikes the strings or thebrief squawk of a saxophone as the air begins to vibrate the reed aretransients. Different instruments have unique transients that effectthe way we hear a series of notes being played together.
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