Glossary — H

Haas Effect (Precedence Effect): A psychoacoustic phenomenon where the brain localizes a sound based on the first arriving wavefront, provided the second wavefront (reflection or another speaker) arrives within 10-35 milliseconds. In car audio, the Haas effect is used during time alignment to "center" the stereo image by delaying the closer speakers so their sound reaches the listener at the same time as the further speakers.
Harness Adapter: A wiring interface that plugs into a vehicle's factory radio connector, allowing an aftermarket head unit or DSP to be installed without cutting the original vehicle wiring. High-quality harnesses are "pinned" for all factory functions, including steering wheel controls and data-bus signals.
Harmonic Distortion: The addition of unwanted frequencies to an audio signal that are integer multiples of the original frequency. In automotive amplifiers, "Total Harmonic Distortion" (THD) is a measure of signal purity. While even-order harmonics can sound "musical," odd-order harmonics (common in clipping) sound harsh and fatiguing.
HD Radio: A digital broadcasting standard that allows AM and FM stations to transmit audio and data as a digital signal over the same frequencies as their analog broadcasts. HD Radio offers higher fidelity, multicasting (additional stations on one frequency), and metadata like album art and traffic info.
Headroom: The difference between the normal operating level of an audio system and the maximum level it can handle before clipping (distortion). Ample headroom is essential for reproducing musical transients (like a snare drum hit) cleanly. A system with low headroom will sound "compressed" or "thin" when pushed to higher volumes.
Headset / Headrest Speakers: Small drivers mounted within the headrest of a vehicle seat. In immersive systems (like those from Bose or Burmester), headrest speakers are used to provide personal surround cues and to enable localized Active Noise Cancellation (ANC) near the passenger's ears.
Headtracking: A technology that uses sensors (often cameras in modern vehicles) to monitor the position and orientation of a listener's head. The DSP can then dynamically adjust the HRTF filters and time alignment to maintain a perfect 3D soundstage even as the driver moves or leans.
Head Unit: The primary control component of a vehicle's audio system, located in the dashboard. Modern "Infotainment" head units handle not only music playback (via Bluetooth, USB, or CarPlay) but also navigation, climate control, and vehicle settings. In high-end builds, the head unit is often used only as a digital transport to a standalone DSP.
Hearing Sensitivity: The human ear's varying ability to detect different frequencies. According to the Fletcher-Munson curves, we are most sensitive to frequencies between 2kHz and 5kHz (the vocal range) and much less sensitive to deep bass at low volumes. This is why "Loudness" curves are used to boost bass during quiet listening.
Heat Sink: A passive heat exchanger that transfers the thermal energy generated by an amplifier's transistors to the surrounding air. In automotive applications, heavy aluminum heatsinks are critical for preventing thermal shutdown during long periods of high-power operation in a hot trunk or under a seat.
Helmholtz Resonance: The phenomenon of air resonance in a cavity, such as a vented subwoofer enclosure. The air in the port acts as a mass, bouncing against the "spring" of the air inside the box. The frequency of this resonance (Fb) is determined by the box volume and the port's dimensions, providing a massive boost in efficiency at the tuning point.
Henry (H): The SI unit of inductance. In car audio, millihenries (mH) are the standard unit for measuring the inductors used in passive crossover networks. A larger inductor (more mH) results in a lower low-pass crossover frequency.
Hertz (Hz): The SI unit of frequency, equal to one cycle per second. Named after Heinrich Hertz. In audio, Hz describes the pitch of a sound. Humans typically hear from 20 Hz (deep bass) to 20,000 Hz (high-frequency air). In vehicle tuning, 1/3rd octave intervals (e.g., 63Hz, 80Hz, 100Hz) are common benchmarks.
High-Level Input (Speaker Level): An amplifier input stage designed to accept the high-voltage signal directly from a factory head unit's speaker wires. Unlike low-level RCA inputs, high-level inputs include internal voltage dividers to reduce the signal to a safe level for processing.
High-Pass Filter (HPF): A filter that allows frequencies above a certain cutoff point to pass while attenuating frequencies below it. An HPF is essential for protecting small speakers (tweeters and midranges) from low-frequency energy that would cause over-excursion and physical damage.
Hiss: Broadband high-frequency noise, often caused by poor gain staging or a low-quality preamplifier stage. If the gain on an amplifier is set too high, the "noise floor" of the upstream source is amplified, resulting in an audible hiss even when the music is paused.
Horn-Loaded Compression Driver: A type of high-frequency loudspeaker that uses a small diaphragm coupled to a large flared horn. This design is extremely efficient and offers controlled dispersion, making it popular in high-output competition vehicles, though it requires precise EQ to sound natural in a small cabin.
HRTF (Head-Related Transfer Function): A set of complex filters that characterize how a sound from a specific location is filtered by the listener's head, shoulders, and outer ears (pinnae) before reaching the eardrum. DSPs use HRTF data to "trick" the brain into hearing sounds as coming from specific 3D coordinates in a Dolby Atmos mix.
Hybrid Vehicle Audio: Audio system design for vehicles with both ICE and electric motors. These cars present a challenge for ANC (Active Noise Cancellation) because the noise floor changes dramatically as the car switches between silent electric mode and noisy gas-engine mode.
Hysteresis: A phenomenon in magnetic materials (like speaker magnets and transformers) where the output lags behind the input. In audio, magnetic hysteresis in an inductor or voice coil can cause non-linear distortion, particularly at high power levels.

END OF INDEX — H