Home Theater Audio 101: Essential Basic Terms
1. Speakers
A speaker is a device that converts an electrical signal back into an acoustic signal; how faithfully it reproduces the original is an important standard for judging speaker performance.
2. Power
There is no direct relationship between sound quality and power. Power determines only the maximum sound intensity a speaker can produce — in practical terms, how much punch the sound has. Under international standards there are two ways to rate power: rated power (RMS: root mean square of a sine wave) and instantaneous peak power (PMPO).
3. Frequency Range and Frequency Response
The former is the span between the lowest and highest effectively reproducible frequencies of an audio system; the latter describes what happens when an audio signal at constant voltage is fed to the system: the sound pressure the speaker produces rises or falls with frequency, and the phase shifts with frequency. This frequency-linked relationship between sound pressure and phase (the amount of variation) is called frequency response, measured in decibels (Db).
4. Loudness
The strength of a sound is called intensity, and it is determined by the amplitude of rapid air-pressure changes (sound pressure). But the ear's subjective sense of intensity does not match the objective physical intensity; this subjective sense of strength is called loudness, also measured in decibels (Db). It is derived from the ratio of the sound pressures of a 1000Hz tone at different intensities, taking one tenth of the common logarithm of that ratio.
5. Distortion
Distortion falls into harmonic distortion, intermodulation distortion and transient distortion. Harmonic distortion is the distortion caused by high-order harmonic components being added during playback that were not in the original signal; intermodulation distortion mainly affects the pitch of the sound; transient distortion arises because a speaker has a certain inertial mass, so this metric is closely tied to speaker quality. It is usually expressed as a percentage — the smaller the figure, the lower the distortion. For ordinary multimedia speakers a distortion below 0.5% is desirable, while subwoofers generally have higher distortion, where below 5% is acceptable.
6. Speaker Sensitivity (unit: Db)
For every 3 dB difference in speaker sensitivity, the sound pressure output doubles. Around 87 Db is medium sensitivity, below 84 Db low sensitivity, and above 90 Db high sensitivity.
7. Impedance
This is the ratio of voltage to current in the signal fed to a speaker. Speaker input impedance is generally classed as high or low: above 16Ω is high impedance, below 8Ω is low impedance, and the standard speaker impedance is 8Ω.
8. Signal-to-Noise Ratio
This is the ratio of the normal sound signal a speaker reproduces to the noise signal (power) when there is no signal. It is also expressed in Db.
9. Speaker Materials
Cheap plastic speakers have thin cabinets that cannot suppress resonance, so there is no real sound quality to speak of; wooden speakers reduce the coloration caused by cabinet resonance and generally sound better than plastic ones. Multimedia speakers are usually two-way, two-driver designs: a smaller driver handles the mid and high frequencies, while a larger one handles the mid and low frequencies.
10. Speaker Construction and Characteristics
By construction, speakers divide into bookshelf and floor-standing types. The former are compact, with clear layering and precise imaging, but their power is limited and their low-frequency extension and weight are insufficient; they suit listeners who mainly enjoy high-fidelity music and are the first choice of multimedia enthusiasts like us. The latter are larger and handle more power, with stronger low-frequency weight and springiness, good at conveying sweeping momentum and powerful impact — though poorly executed examples fall a little short in layering and imaging.
11. Expandability
This refers to whether a speaker supports simultaneous multi-channel input, whether it has output jacks for passive surround speakers, whether it has a USB input, and so on. How many surround speakers a subwoofer can drive externally is also one measure of expandability.
12. Sound Effect Technologies
Among hardware 3D sound technologies, the more common ones today are SRS, APX, Spatializer 3D, Q-SOUND, Virtual Dolby and Ymersion. Each achieves the effect differently, but all give the listener a clear three-dimensional sound field; the first three are the most common.
13. Pitch
A signal with a particular and usually stable pitch — in plain terms, how high or low a sound seems. It depends mainly on frequency and is also related to sound intensity.
14. Timbre
The felt quality of a sound, and the characteristic that distinguishes one sound from another. Different instruments sounding the same pitch can have utterly different timbres, because although their fundamental frequencies match, their harmonic content differs greatly. Timbre therefore depends not only on the fundamental frequency but also closely on harmonics at whole-number multiples of it, which is why every instrument and every person has a distinct timbre.
15. Dynamic Range
The ratio of the strongest to the weakest sound, expressed in Db. Speech signals typically span only about 20–45 dB, while some symphonic works reach a dynamic range of 30–130 dB or more.
16. Total Harmonic Distortion (THD)
The extra harmonic components in the output signal, beyond those in the input signal, that arise from non-linear elements as an audio source passes through a power amplifier. Harmonic distortion is caused by the system not being perfectly linear; we express it as the percentage ratio of the RMS of the newly added total harmonic content to the effective value of the original signal.
17. Stereo Separation
The ability of two channels not to interfere with each other's signals — the degree of isolation — usually expressed as the difference between the signal level in one channel and the level that leaks into the other. If stereo separation is poor, the sense of stereo is weakened.
18. Damping Factor
The ratio of an amplifier's rated load (speaker) impedance to the power amplifier's actual impedance. A large damping factor means the power amplifier's output resistance is low; the damping factor is the amplifier's ability to control the motion of the speaker cone after the signal stops. In general, a low output impedance and a large damping factor are desirable.
19. Equal-Loudness Control
Its job is to boost the high and low frequencies at low volume. Equal-loudness control does exactly that, usually by 8 dB or 10 dB.
Range: the span between the highest and lowest notes an instrument or voice can reach.
Timbre: also called tone color, one of the basic attributes of sound — an erhu and a pipa, for instance, have different timbres.
Coloration: the opposite of natural, neutral music — the sound takes on characteristics not present in the program material; the sound you get speaking into a jar is a classic example of coloration. Coloration means the reproduced signal has gained (or lost) certain components, which is plainly a form of distortion.
Distortion: a device's output cannot exactly reproduce its input, producing waveform deformation or gains and losses in signal components.
Dynamic: the ratio of the largest to the smallest information that can be recorded.
Transient response: a component's ability to follow sudden signals in music. A component with good transient response should respond the instant the signal arrives and stop dead the moment it ends, never dragging on. (Classic instrument: piano)
Signal-to-noise ratio: also called the S/N ratio, the contrast between the useful part of a signal and the noise, often expressed in decibels. The higher a device's signal-to-noise ratio, the less noise it produces.
Airiness: an acoustic term for the openness of the treble, or for the sense of space between instruments in the sound stage. Here the high-frequency response can extend to 15kHz–20kHz. Its opposites are "dull" and "thick".
Low-frequency extension: the lowest frequency an audio component can reproduce. It is the yardstick for how far down an audio system or speaker can reach when reproducing bass. For example, a small subwoofer's low-frequency extension may reach 40Hz, while a large subwoofer digs down to 16Hz.
Transient: the ability of a sound to rise or fall suddenly; the less time it takes to rise or fall to a given sound pressure, the better the transient — in practice, the ability of a sound to stop cleanly without smearing.
Dynamic: the ratio of sound pressure, power or voltage at the quietest versus the loudest moments; quantified, this is the dynamic range.
Full-bodied: the reproduced sound has a well-proportioned balance of high, mid and low frequencies — moderate treble, ample midrange and a certain springiness. Layered: a layered sound faithfully conveys the sense of an ensemble as a whole.
Clarity: high intelligibility of speech and clearly separated musical lines.
Balance: the proportions among the parts of the music are well coordinated, and the left and right channels match well.
Fullness: the sound has ample midrange, moderate treble and suitable loudness, sounding warm, comfortable and springy.
Attack: the sound is solid and forceful, with a sense of presence about to burst forth, while also conveying the dynamic range of the source.
Roundness: the sound is beautiful and pleasing, with sheen but without harshness.
Softness: the sound is relaxed rather than taut, the treble is not piercing, and it is pleasant and comfortable to hear.
Cohesion: the sound blends into a single whole, with a good sense of unity and ensemble.
Fidelity: the sound retains the characteristics of the original.
Presence: playback makes the listener feel as if they are there.
Imaging: the sound has a sense of space, the sonic images are positioned essentially accurately, and there is a sense of width and depth.
Overall impression: the general sense one takes from the sound.
Compiled and edited by Snail Power Audio