In audio system design and subjective listening evaluation, frequency response and frequency range are two fundamental parameters that determine a system’s sound reproduction capability. Together, they define the amplitude-frequency behavior of audio equipment, directly influencing sound fidelity, tonal balance, and suitability for various applications. This article explains their acoustic principles, from definition and measurement to perceptual impact and practical application, and discusses their relationship with dynamic range.

Ⅰ. Frequency Response: The Objective Characterization of Amplitude-Frequency Behavior

1. Definition and Measurement

Frequency response describes the variation in output sound pressure level as a function of frequency under steady-state conditions, typically represented as an amplitude-frequency curve. In specifications, it is commonly expressed as “20 Hz – 20 kHz ±3 dB,” meaning the output level stays within ±3 dB of a reference value across that range. A flatter response indicates more accurate sound reproduction.

2. Perceptual Impact by Frequency Band

• High-frequency attenuation (2 kHz – 20 kHz): Loss of harmonic detail and airiness, resulting in a “dull” or “muffled” sound.
• Low-frequency deficiency (20 Hz – 250 Hz): Reduced impact and physical sensation, making dynamic content sound weak or loose.
• Mid-frequency dip (200 Hz – 2 kHz): Vocals and instruments lose fullness and body, sounding thin and lacking texture.

3. Common Response Curves and Their Acoustic Characteristics

Note: Roll-off Slope
The roll-off slope (e.g., 12 dB/octave) indicates how quickly output declines outside the passband. Gentle slopes preserve phase coherence and detail; steep slopes (>24 dB/octave) may cause phase distortion and impair sound localization.

Ⅱ. Frequency Range: The Reproducible Spectrum

The human ear can theoretically perceive 20 Hz – 20 kHz, though adults often hear only up to 15 kHz. A high-quality sound system should cover a broad range to faithfully reproduce diverse program material:

III. Application-Specific Tuning Guidelines

1. Conference Sound Reinforcement

• Key range: 200 Hz – 10 kHz
• Goal: Maximize speech intelligibility, reduce low-frequency noise.

2. Stadium Broadcasting

Full-range balance:

• Bass (60 – 250 Hz) enhances crowd atmosphere.
• Mid-highs (200 Hz – 10 kHz) ensure clarity of announcements and whistle sounds.

3. Outdoor Music Festival

• Bass requirement: >110 dB SPL in 20 – 250 Hz range to support electronic music.
• Mid-high accuracy: Faithful reproduction of distorted guitars and vocal articulation up to 20 kHz.

Ⅳ. The Interplay of Frequency Response, Range, and Dynamic Range

1. Response and Bandwidth Coordination

Wide frequency range alone does not guarantee fidelity. A pronounced boost in the low end (e.g., +6 dB) can muddy kick drums and weaken definition. Standards such as THX recommend delegating frequencies below 80 Hz to a dedicated subwoofer, with main speakers handling 80 Hz – 20 kHz for optimal phase alignment and clarity.

2. The Role of Dynamic Range

Dynamic range (DR) is the ratio between the maximum undistorted output and the noise floor: DR = 20log10(Pmax/Pnoise) (Unit: dB)

• CD audio: ~96 dB DR, capable of rendering whispers and orchestral climaxes without compromise.
• Perceptual analogy: Similar to HDR imaging, preserving detail in both quiet and loud passages.

Ⅴ. Recommended Listening Tests

• Bass extension and control: Sister Drum by Zhu Zheqin (tests 20 – 40 Hz depth and clarity).
• High-frequency resolution and air: Hotel California (live version) (reveals ambient detail above 12 kHz).

Frequency response and frequency range form the physical basis of sound system performance, directly shaping realism and emotional impact. In system design and tuning, these parameters should be evaluated through both objective measurement and subjective listening, tailored to the content and environment, to achieve a harmonious balance between technical precision and auditory experience.