Pad - An electrical circuit used to attenuate or reduce the amplitude of an audio signal by a fixed amount. For example, a -12dB pad reduces the signal by a fixed 12 decibels.

Parallel Wiring - A circuit in which two or more devices are connected to the same source of voltage, sharing a common positive and negative point, so that each device receives the full applied voltage. For examples, see Speaker Wiring Diagrams.

Parametric EQ - This type of equalizer adjusts sound by controlling three variable parameters - amplitude, center frequency and bandwidth. Conventional graphic EQs control frequency and amplitude, but have limit you to a pre-selected frequency and the bandwidth is fixed.

Passive - (Opposite of active) Refers to a circuit or device that does not require a power source to operate.

Passive Crossover - A crossover network that does not have to be powered to operate. For example: the crossover inside a full range home audio speaker.

Passive Radiator - Similar to a speaker but lack an active motor assembly. They consist of a frame, surround and diaphragm. These passive devices respond to internal pressure within a woofer enclosure and react to it to produce additional bass frequencies (sometimes lower than the woofer itself produces).

PCM - Pulse Code Modulation. A means of digital encoding.

Pe - Driver's rated RMS power handling capability.

Peak - The maximum amplitude of a voltage or current. 

Peak Power - The largest amplitude of signal an amp is capable of producing. A misleading rating because it measured under unstandardized (and sometimes very unrealistic) conditions.

Peak Power Handling (MAX) - Refers to the amount of power a speaker is estimated to handle only during a brief high-intensity musical burst, not a continuous music signal.

Phase - The relative timing of a sound wave that is measured in degrees from 0 to 360. May also refer to relative position of two sound waves with respect to each other. When two waves are 180 degrees out of phase, cancellation occurs.

Piezo (Tweeter) - A tweeter that creates sound when signal current flows through a crystal of Piezo material. It's a high impedance device so it does not need a Crossover in line with the source. Very efficient drivers and relatively inexpensive.

Pink Noise - A signal with all frequencies perceptible to the human ear reduced to an equal energy level. It is the preferred sound source for many acoustical test measurements in respect to the way humans hear. Commonly used (with a calibrated mic) to measure the frequency response curve of a room or area.

Pinswitch - A spring-loaded mechanical switch used in many vehicle applications to turn on interior lights or trigger alarms when doors, hoods and trunk/hatches are opened.

Pitch - The subjective sensation our ears perceive when exposed to various frequencies. Also may refer to a particular musical note and octave.

Polarity - The electrical quality of having two opposite poles, one positive and one negative. Polarity determines the direction in which a current tends to flow.

Polar response - A plot of the of a speaker's output at a given frequencies relative to an off-axis angle. This measures how a speaker sounds when being heard directly in front, compared to how it sounds while standing off to one side.

Pole Piece - A structure in a speaker that is cylindrical with a flat plate on the bottom. It may be solid or have a hollow center (vented). It is encircled by the magnet with a slight gap in between the two (see Parts of a Speaker). The voice coil fits in the gap between the pole piece and the magnet (without making contact) and slides up and down the cylinder like a piston, moving the cone of the speaker.

Polycarbonate - A polymer based plastic material offering superior resistance against resonance.

Polypropylene - A plastic based material used often for speaker cones due to its damping, rigidity and resistance against harsh environmental conditions that exist in vehicles.

Port - Also called a Vent. An opening (commonly a tube) in an enclosure that allows the back wave of a speaker to interact with the front wave. These ports are tuned to increase output of the speaker system and extend the bass frequencies lower. Changing the length or area of a port changes the tuning frequency of the enclosure.

Ported Enclosure - (Vented Enclosure) Any enclosure design with ports. Ported enclosures can offer higher efficiency and increased woofer control (at the tuned frequency). For details and more enclosure types see Woofer Enclosure Designs.

Potentiometer - A three-terminal variable resistor used as a attenuation switch. Two terminals connect to the ends of a flat resistor, while the third terminal is attached to a moveable contact that interfaces with the resistive element. Can be of either rotary or slider design and are available in assorted tapers (the degree of resistive change relative to the travel of the control) .

Power - The amount of work done in a specific amount of time. The electrical unit of power is Watt.

Pre-Amp - The control stage before the main power amp in a receiver or integrated amp. Pre-amps take a small input signal and amplifies it sufficiently before going to the power amplifier for further amplification. A pre-amp regulates all of the control functions such as volume, tone, channel balance and source selection.

Preamp Output - Also called Pre-Outs. Typically found on headunits, a preamp output lets you bypass the power amp stage. It provides a high quality low level audio signal (for each audio channel) out of the device allowing you to connect to larger external amplifiers.

Precedence Effect - Also known as Haas effect, the Precedence Effect describes how we identify the direction of a sound source heard in both ears but arriving at different times. Direct sound from any source first enters the ear closest to the source, then the ear farthest away. This is how we localize a sound source, as long as the sound arrival times to the ears are between 25-35 milliseconds apart. If the later arrivals are longer than this, then two distinct sounds are heard.

Presence - A quality of realism and aliveness.

Presence Range - The lower-treble part of the audio spectrum, approximately 1-3kHz, which contributes to presence in reproduced sound.

Preset - A collection of system settings stored in a memory that is instantly recallable, typically at the touch of a button.

Psychoacoustics - The scientific study of the relationship between human hearing and its stimulus, essentially how the brain perceives sound.

Push-Pull Configuration - A woofer configuration used in isobaric enclosures where one driver is mounted normally, the second is mounted so that it faces into the enclosure. Both share the same internal volume and wired out of phase with one another. The drivers are acoustically in phase since they move in the same direction.