8.1 Physics of Sound, Microphones, and Audio Capture

8.1 Physics of Sound, Microphones, and Audio Capture

Audio engineering in digital multimedia education requires an understanding of acoustic physics, transducer mechanics, and signal transmission. Capturing pristine audio in the field or studio demands an understanding of how physical sound waves behave, how different microphone designs convert kinetic acoustic energy into electrical voltage, and how cabling architectures preserve signal integrity.

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The Physics of Acoustic Sound

Sound is mechanical energy transmitted through an elastic physical medium (such as air, water, or dense solids). Unlike electromagnetic radiation, sound cannot propagate through a vacuum. It travels as a longitudinal pressure wave, where medium molecules oscillate parallel to the direction of wave travel, creating alternating regions of high and low pressure:

• Compressions (Condensations): Regions where air molecules are forced tightly together, producing localized increases in physical air density and acoustic pressure above ambient barometric levels.
• Rarefactions: Regions where air molecules rebound and spread apart, producing localized decreases in density and pressure below ambient levels.

One complete sequence of a single compression and an adjacent rarefaction constitutes an acoustic cycle.

Atmospheric Pressure (Equilibrium) --------------------------------------
                              .-.                 .-.
Compression (High Pressure)  /   '               /   '
----------------------------+-----+-------------+-----+------------------
Rarefaction (Low Pressure)         '.-.                '.-.
                            <--- One Cycle --->

Frequency and Pitch

Frequency ($f$) is the number of complete compression-rarefaction cycles that occur within one second. It is measured in Hertz (Hz), where 1 Hz = 1 cycle per second (and 1 kHz = 1,000 Hz):

• Audible Spectrum: The nominal range of human hearing spans 20 Hz to 20,000 Hz (20 kHz). Frequencies below 20 Hz are termed infrasonic (often felt as physical vibrations rather than heard), while frequencies exceeding 20 kHz are termed ultrasonic.
• Pitch Perception: Frequency governs the psychological perception of pitch. This relationship is logarithmic rather than linear; an octave increase represents a doubling of the physical frequency (220 Hz -> 440 Hz -> 880 Hz). Human speech fundamentals typically inhabit 85 Hz to 255 Hz, while consonant clarity and sibilance reside between 2 kHz and 8 kHz.

Amplitude and Perceived Loudness

Amplitude corresponds to the magnitude of acoustic pressure deviation from ambient equilibrium. In physical acoustics, it represents the degree of displacement experienced by air particles:

• Sound Pressure Level (dB SPL): In the analog physical domain, acoustic sound pressure is quantified logarithmically in decibels of Sound Pressure Level (dB SPL). The baseline reference point (0 dB SPL) represents the nominal threshold of human hearing at 1 kHz (defined as 20 μPa or 2 × 10^-5 N/m²). Normal conversational dialogue measures roughly 60 dB SPL; sustained exposure exceeding 85 dB SPL induces permanent sensorineural hearing damage; the physical threshold of acute pain sits at 120 to 130 dB SPL.
• Decibels Relative to Full Scale (dBFS): In digital audio systems, amplitude is measured on a negative logarithmic scale in dBFS, where 0 dBFS represents the absolute digital maximum ceiling before clipping distortion occurs. In digital tracking, engineers record nominal averages around -18 to -12 dBFS to preserve overhead margin.

Timbre and Harmonic Complexity

Timbre (tone color) is the acoustic characteristic that enables a listener to distinguish between two distinct sound sources—such as a cello, an acoustic guitar, and a human voice—producing the identical fundamental pitch ($f_0$) and amplitude. Timbre is determined by:

1. The Fundamental Frequency ($f_0$): The lowest physical frequency component of the vibrating body, which defines the perceived musical pitch.
2. Harmonics and Overtones: Secondary higher frequencies produced simultaneously by complex vibrating materials. In musical and vocal sounds, these overtones exist as integer multiples of the fundamental ($2f_0, 3f_0, 4f_0, ...$), known as the harmonic series.
3. Spectral Envelope (ADSR): The temporal evolution of sound energy through four distinct phases: Attack (initial rise time to peak amplitude), Decay (drop to sustain level), Sustain (duration at steady state), and Release (decay to silence).

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Transducer Types: Dynamic, Condenser, and Ribbon Microphones

A transducer is any physical device that converts one form of energy into another. A microphone is an electro-acoustic transducer that converts mechanical sound wave energy into an alternating electrical voltage signal.

[ Acoustic Wave ] ---> [ Diaphragm Vibration ] ---> [ Transducer Mechanism ] ---> [ Electrical Audio Signal ]

1. Dynamic Microphones (Moving-Coil)

Dynamic microphones operate via electromagnetic induction, grounded in Faraday's Law of Induction: when an electrical conductor moves through a magnetic field, an electrical voltage is induced across the conductor.

• Internal Construction: A flexible Mylar or plastic diaphragm is mechanically attached to a fine, tightly wound copper voice coil. This voice coil is suspended inside the microscopic gap of a permanent magnet.
• Operational Behavior: Incident acoustic pressure waves strike the diaphragm, driving the attached voice coil back and forth through the magnetic flux lines. This motion generates a small, fluctuating alternating current (AC) voltage proportional to the velocity of the air particles.
• Physical Attributes:
  • Ruggedness: Highly resistant to physical drops, mechanical shock, and environmental moisture or temperature swings.
  • High SPL Handling: Can tolerate extreme Sound Pressure Levels (>140 dB SPL) without internal mechanical damage or signal clipping.
  • Power Independence: Completely passive; requires no external power source or battery.
  • Transient Limitations: The physical mass of the diaphragm-and-coil assembly creates mechanical inertia, yielding a slightly slower transient response and rounded high frequencies compared to condensers.
• Common Applications: Live stage vocal performance (e.g., Shure SM58), close-mic guitar amplifier cabinets and snare drums (e.g., Shure SM57), and broadcast speech in untreated acoustic rooms (e.g., Shure SM7B, Electro-Voice RE20).

2. Condenser Microphones (Electrostatic / Capacitor)

Condenser microphones operate via electrostatic capacitance. In electronics, a capacitor consists of two conductive plates separated by an insulating dielectric material, storing charge according to the relationship $C = (\varepsilon \times A) / d$ (where $C$ is capacitance, $\varepsilon$ is dielectric permittivity, $A$ is plate area, and $d$ is the distance between plates).

• Internal Construction: The capsule houses an ultra-thin, flexible gold-sputtered Mylar diaphragm positioned parallel to a fixed, rigid metal backplate, separated by a microscopic air gap.
• Operational Behavior: A DC polarizing voltage is applied across the plates, charging the capsule capacitor. When acoustic waves displace the flexible diaphragm, the distance ($d$) between the plates fluctuates. This continuously alters the capsule's capacitance, generating an electrical signal: $V = Q / C$
• Phantom Power (+48V DC): Because the raw electrical signal generated by the capacitor capsule has exceptionally high impedance and microscopic current, condenser microphones require active onboard impedance-converting electronics (typically a JFET or vacuum tube preamplifier). Condensers require external electrical power—most commonly +48V Phantom Power—delivered directly from an audio interface or mixing console across conductors in a balanced XLR cable.
• Physical Attributes:
  • Transient Response: The microscopic mass of the gold-sputtered diaphragm accelerates and decelerates near-instantaneously, capturing subtle percussive attacks, acoustic transients, and high-frequency harmonics up to and beyond 20 kHz.
  • Sensitivity: Extremely sensitive to low-amplitude sounds and distant room reflections.
  • Environmental Fragility: Delicate internal tolerances make condensers susceptible to damage from drops, physical shocks, and high humidity (which causes condensation across the capsule, resulting in crackling, sputtering, or signal dropouts).
• Common Applications: Studio vocal tracking, voice-over recording, acoustic string instruments, drum overhead cymbals, orchestral tracking, and broadcast lavaliers.

3. Ribbon Microphones (Velocity Transducers)

Ribbon microphones are a specialized form of electromagnetic transducer, operating on magnetic induction principles similar to moving-coil dynamics.

• Internal Construction: Houses an ultra-thin (often ≈ 2 μm), corrugated strip of pure aluminum foil suspended freely between the magnetic poles of a strong permanent magnet.
• Operational Behavior: The ribbon serves simultaneously as both the acoustic diaphragm and the electrical conductor. Rather than responding to sound pressure at a single point, the ribbon responds to the pressure gradient—the difference in acoustic velocity between the front and rear of the element. As the ribbon moves through the magnetic gap, an audio signal is induced directly across its ends.
• Physical Attributes:
  • Acoustic Character: Delivers an organic, warm vintage timbre characterized by a natural, smooth high-frequency rolloff that tames harsh brass or shrill string instruments.
  • Bidirectional Geometry: Because both sides of the ribbon are completely exposed to the acoustic environment, traditional ribbon capsules naturally exhibit a pure figure-8 polar pattern.
  • Fragility: Extremely delicate. Strong blasts of air (vocal plosives or outdoor wind gusts) can stretch, warp, or tear the aluminum ribbon. Furthermore, applying +48V phantom power through an incorrectly wired or patched cable can permanently destroy vintage passive ribbons (though modern active ribbons incorporate protective buffer circuitry).
• Common Applications: Studio guitar cabinets, brass sections, orchestral room ambience, and vintage vocal styling.

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Microphone Polar Pickup Patterns

A polar pickup pattern illustrates a microphone capsule's three-dimensional sensitivity to acoustic sound waves arriving from various directional angles (0° on-axis front, 90° / 270° sides, and 180° rear).

             0° (Front On-Axis)
                     ▲
               .---' | '---.
            .-'      |      '-.
  90° <--- [         ●         ] ---> 270°
  (Left)    '-.             .-'      (Right)
               '---. | .---'
                     ▼
             180° (Rear Null)

1. Omnidirectional

• Acoustic Sensitivity: Uniform sensitivity across all 360° in horizontal and vertical planes. Captures sounds arriving equally from the front, sides, and rear.
• Acoustic Properties: Operates as a pure pressure sensor. Omnidirectional capsules are immune to the proximity effect (no artificial bass boost occurs when working close to the capsule). They exhibit superior resistance to mechanical wind turbulence and vocal plosives, and capture realistic ambient room tone.
• Limitations: In untreated acoustic environments, omnidirectional mics capture unwanted reverberation and room reflections. In live sound environments, they are highly vulnerable to acoustic feedback if placed near stage monitor speakers.

2. Cardioid (Unidirectional)

• Acoustic Sensitivity: Heart-shaped directional pickup pattern. Maximum sensitivity resides directly on-axis (0° front); sensitivity drops by 6 dB at the sides (90° and 270°), with near-total rejection (null point) directly behind the capsule at 180°.
• Acoustic Properties: Utilizes rear acoustic delay ports that allow sound from the rear to reach both sides of the diaphragm simultaneously, canceling itself out via destructive phase interference. This makes cardioid microphones the primary tool for isolating a specific sound source while rejecting room reverberation or live stage monitors positioned at 180°.
• The Proximity Effect: Directional pressure-gradient microphones exhibit a physical bass buildup when the sound source moves into extreme close proximity (within a few inches) of the capsule. The low frequencies (<200 Hz) receive up to +10 dB or more of unnatural boost. While radio announcers exploit this for vocal presence, unmanaged proximity effect generates muffled, muddy, and boomy student dialogue.

3. Supercardioid and Hypercardioid

• Acoustic Sensitivity: Progressive variations of the cardioid pattern offering narrower front pickup angles (115° for supercardioid; 105° for hypercardioid) and enhanced side rejection.
• Rear Sensitivity Lobe: Achieving a narrower front lobe introduces a small pickup lobe directly at the rear (180°). The null points of maximum rejection shift to the rear-quarters (126° for supercardioid; 110° for hypercardioid).
• Practical Deployment: In live sound reinforcement, stage monitor wedges must be angled at 120° to 135° off-axis rather than placed directly behind the microphone stand to prevent feedback.

4. Shotgun / Interference Tube

• Acoustic Sensitivity: Narrow, highly directional line-gradient pickup pattern engineered to isolate distant sound sources.
• Interference Tube Mechanics: A hypercardioid capsule is mounted at the rear base of a long, acoustically slotted barrel (interference tube). Sound arriving directly on-axis (0°) travels straight down the tube unhindered. Off-axis sounds entering the side slots at different distances along the tube arrive at the diaphragm out of phase with each other, canceling out via destructive acoustic phase interference.
• Practical Deployment: The standard tool for film, television, and sports broadcasting. The microphone does not "zoom" into sound; rather, it cancels out off-axis ambient noise, allowing clean dialogue capture from outside the camera frame.

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Form Factors, Field Placement, and Wind Mitigation

Selecting the appropriate microphone form factor and physical placement determines dialogue clarity in multimedia productions.

Form Factor Characteristics and Uses

• Lavalier (Lapel) Microphones: Miniature electret condenser capsules designed to clip unobtrusively to a speaker's clothing (typically mounted along the center sternum, approximately 6 to 8 inches below the chin). They provide consistent speech levels regardless of head movements during instructional lectures, broadcast news interviews, and panel discussions. Best practices require dressing cables underneath clothing to eliminate rustling noise.
• Handheld Vocal Microphones: Ergonomic, cylindrical enclosures designed for on-stage vocalists, field reporters, and student interviewers. They incorporate internal mechanical shock mounts and heavy wire mesh baskets to minimize handling noise.
• Boundary / Pressure Zone Microphones (PZM): A small condenser capsule mounted face-down millimeters above a flat, reflective boundary plate. Placing the capsule very close to a large boundary minimizes the short time difference between direct sound and the first boundary reflection, reducing the comb-filtering problems that occur when a conventional microphone is positioned farther above that surface. Other room reflections can still cause interference. Ideal for placement on conference tables, theatrical stage floors, and courtroom benches.
• Boom-Mounted Shotgun Microphones: Suspended on a carbon-fiber boom pole using an elastic suspension shock mount (lyre cradle) to isolate mechanical handling vibrations. Standard location sound technique positions the shotgun slightly in front of and above the actor's head, angled down at roughly 45° pointing directly at the mouth and sternum, hovering inches outside the top edge of the camera frame.

[ Boom Operator Position ] ---> [ Carbon-Fiber Pole ]
                                      │
                                [ Lyre Shock Mount ]
                                      │
                                [ Shotgun Capsule ]
                                      │ (Angled 45° downward)
                                      ▼
                              (Actor Mouth / Sternum)

Wind and Plosive Mitigation

Air turbulence striking an exposed microphone diaphragm produces high-amplitude, low-frequency electrical overloads ("wind rumble" and "plosives") that clip preamplifiers:

• Pop Filters: Circular nylon or perforated metal mesh screens placed 2 to 4 inches in front of studio vocal condenser microphones. They diffuse high-velocity air bursts produced by explosive consonant phonemes ("p", "b", "t") before the moving air packet can strike the capsule.
• Foam Windscreens: Open-cell polyurethane foam sleeves slipped over capsules. Designed for indoor use to disperse gentle air currents from heating, ventilation, and air conditioning (HVAC) ducts or slow actor movement.
• Blimps (Zeppelins): Large, rigid aerodynamic mesh cylinders that enclose a shotgun microphone entirely, decoupling the capsule from turbulent airflow and creating a pocket of dead air around the transducer.
• Furry Windjammers ("Deadcats"): High-density faux-fur covers slipped over blimps for high-velocity outdoor wind. The fine, flexible synthetic hair filaments dissipate the kinetic energy of turbulent wind gusts before the air can impact the solid boundaries of the inner enclosure.

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Audio Cabling: Balanced vs. Unbalanced Architectures

Preserving audio fidelity across physical transmission pathways requires selecting between balanced and unbalanced electrical interconnects.

UNBALANCED CABLE (2-Conductor / Vulnerable to Interference)
+--------------------------------------------------------------+
|  [Signal Wire]   -------------------- (Audio + Induced Noise)|
|  [Ground Shield] -------------------- (Ground Return)        |
+--------------------------------------------------------------+

BALANCED CABLE (3-Conductor / Common-Mode Rejection)
+--------------------------------------------------------------+
|  [Hot Wire (+)]  -------------------- (+Audio + Induced Noise)|
|  [Cold Wire (-)] -------------------- (-Audio + Induced Noise)|
|  [Ground Shield] -------------------- (Chassis Ground)       |
+--------------------------------------------------------------+

Common-Mode Rejection (CMR) in Balanced Lines

A balanced connection has two signal conductors with equal impedance to ground plus a shield. Many outputs drive equal-and-opposite audio voltages on the two conductors, while impedance-balanced outputs may drive audio on only one conductor. In either design, interference tends to couple similarly onto both conductors.

The receiving differential input responds to the voltage difference between the two conductors and rejects voltage common to both. Its common-mode rejection ratio (CMRR) describes how effectively it suppresses that shared interference. Rejection is substantial rather than mathematically perfect; it depends on impedance matching, cable construction, frequency, and the receiving circuit. XLR and TRS connectors can carry balanced signals when the connected equipment is wired for balanced operation, but the connector shape alone does not guarantee it.

• Balanced Connectors: 3-pin XLR is common for professional microphones, and 1/4-inch TRS is common for balanced line-level interconnects. TRS is also used for unbalanced stereo headphones, so always check the circuit specification.

Unbalanced Audio Lines

Unbalanced Audio Lines

Unbalanced cables employ only two electrical conductors:

1. Center Conductor: Carries the audio signal ($+S$).
2. Ground Shield: Carries the electrical ground reference and serves as the negative return path.

Because there is no inverted out-of-phase reference signal, any electromagnetic interference induced onto the center conductor enters the audio signal path directly. If an unbalanced cable run exceeds 15 to 20 feet (4.5 to 6 meters), it acts as an antenna, picking up 60 Hz electrical mains hum, dimmer buzz, and high-frequency RF interference. Unbalanced lines are restricted to short instrument and patch connections.

• Unbalanced Connectors: 1/4" TS (Tip-Sleeve; electric guitars, keyboards, amplifiers), 3.5mm (1/8") mini-jacks (consumer headphones, auxiliary audio), and RCA phono jacks (consumer AV receivers, turntables).

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Microphone Transducer and Polar Pattern Matrix