Unit
- Sound wave properties: Sound travels as longitudinal pressure waves through a medium. Key properties include frequency (pitch), wavelength, amplitude (loudness), and velocity. In air at room temperature, sound travels at roughly 343 meters per second.
- Frequency and pitch: Frequency measured in Hertz (Hz) determines pitch. The audible range spans 20 Hz to 20 kHz. Sub-bass frequencies (20-60 Hz) are felt more than heard, while high frequencies (8-20 kHz) add air and detail to a mix.
- Amplitude and loudness: Amplitude corresponds to the wave's peak pressure variation, perceived as loudness. Measured in decibels (dB), every 10 dB increase represents a tenfold increase in sound intensity. Sustained exposure above 85 dB can cause hearing damage.
- Inverse Square Law: Sound intensity decreases proportionally to the square of distance from the source. Doubling distance reduces sound level by 6 dB. This law governs speaker placement and listener coverage in concert venues.
- Reflection and absorption: Sound waves interact with surfaces, reflecting off hard materials like concrete and absorbing into porous ones like acoustic foam. Reflections create echoes and comb filtering, while absorption reduces overall reverberation time.
- Diffraction and refraction: Diffraction bends sound waves around obstacles, allowing low frequencies to wrap around corners. Refraction occurs when sound passes through mediums of different temperatures, bending its path. Operators must account for these in outdoor concerts.
- Standing waves and room modes: When reflected waves interfere constructively, standing waves create peaks and nulls at specific frequencies. Room dimensions determine modal frequencies, requiring strategic speaker placement and acoustic treatment to flatten response.
- Phase and interference: When two identical sound waves combine, constructive interference boosts amplitude if in phase, while destructive interference cancels sound if out of phase. Phase alignment is critical for subwoofer arrays and multi-speaker systems.
- Harmonic content and timbre: Complex sounds consist of a fundamental frequency plus integer multiples called harmonics. Harmonics give each instrument its unique timbre. Understanding the harmonic series helps engineers apply equalization effectively, as overlapping harmonics can create masking. Skillful frequency carving makes each element audible in a dense mix.
- Dynamic range and headroom: Dynamic range is the ratio between the loudest and quietest sound levels, measured in decibels. Live sound systems must handle wide dynamic ranges without distortion. Headroom is the safety margin between nominal operating level and clipping. Proper gain staging across all gear preserves dynamic range and prevents unwanted artifacts.
- Transient response: Transients are short, high energy peaks at the onset of a sound, such as a drum hit or a plucked string. Sound system components like speakers and amplifiers must reproduce transients accurately to deliver punch and clarity. Poor transient response results in muffled attacks and loss of detail, especially with percussive instruments.
Design Principle
Principle: Acoustic Space Mapping
Definition: Analyzing a venue's acoustic characteristics (reverberation time, reflective surfaces, and modal frequencies) before system deployment to anticipate sound behavior and optimize speaker coverage.
Application: Before a concert, measure the venue's RT60 reverberation time using a impulse response. For a hall with high RT60 (over 2 seconds), reduce low-frequency content to avoid muddiness and close-mic sources to minimize room coloration. Use directional speakers to reduce reflections.
Quick Revision
- Sound waves are longitudinal pressure waves with properties of frequency, amplitude, and velocity.
- Frequency determines pitch; audible range is 20 Hz to 20 kHz.
- Amplitude determines loudness, measured in dB; Inverse Square Law reduces intensity by 6 dB per doubling of distance.
- Reflection, absorption, diffraction, and refraction affect sound behavior in venues.
- Standing waves and phase interference require careful system alignment to avoid cancellations.