Basic concepts of sound engineering. Terms in sound engineering
In a sense, sound engineering is the science of distortion, with which there is a continuous struggle on all sections of the sound path. All distortions can be divided into linear and nonlinear. The former only affect the level and shape of the signal, changing them, while the latter add foreign products to the signal that were not in the original input signal.
Nonlinear distortion includes THD (Total Harmonic Distortion) – harmonic (and subharmonic) distortion, IMD (Intermodulation distortion) – intermodulation distortion (combination, difference and total frequency), the most noticeable by ear, dynamic (clipping, “step”, overmodulation) .
To linear – amplitude-frequency (distortion of the frequency response, that is, unevenness), phase-frequency (phase response), temporal, spatial (for example, related to directivity), transient (distortion of transients).
Frequency response – amplitude-frequency characteristic – graphical relationship between a constant level of the input signal and the level of the output on the working frequency band. A very interesting (and tricky) characteristic, which is usually not bad at the declared nominal levels of the input signal, although in the event of a change (decrease) in this level, it often travels around the seams. That is, at a level of -20 dB (10 times less), the formerly relatively linear characteristic of the amplifier (owners of Soviet amplifiers are usually very proud of these characteristics), for example 20-20.000 Hz in the average octave level of -0.2 dB with an input signal of 1v for UMZCH or 0.25v for a full / pre-amplifier, becomes completely non-linear and can be stored in this frequency band even with a deviation of -5 dB. dB is the decibel, in this case the generally accepted unit of the relative logarithmic scale when comparing signal levels. Relatively honestly, this characteristic is used by manufacturers of high-quality magnetic tapes, which measure the frequency response at saturation levels of the magnetic tape (0 dB) and at the level of -20 dB, which is more informative and most statistically probable on a real musical signal.
Frequency unevenness in the frequency range 100-8000 Hz in speakers of the Hi-Fi category (High Fidelity – high fidelity, pretty worn term) should be no more than 4 dB per octave. And in studio speakers (monitors) no more than 1dB.
An effectively reproducible frequency range is a range within which the sound pressure level decreases by a predetermined amount relative to the level averaged over a certain frequency band. In the international standard IEC 581-7, which defines the minimum requirements for Hi-Fi speakers, it is 50-12500 Hz at a level of -8 dB with respect to the sound pressure level averaged in the band 100-8000 Hz.
According to the standard ST SEV 1356-78, the range of reproducible frequencies, at the lower and upper boundaries of which there is a decrease in the sound pressure level, should also not be 50-12500 Hz. The deviation of the frequency characteristics of speakers shared in stereo equipment should not exceed 3 dB in the frequency band 250-8000 Hz with averaging characteristics in each octave.
In the case of speakers, there is still such a characteristic as directivity. Each speaker has its own directional characteristic. Usually, acoustic power decreases nonlinearly when deviating from the central axis of the speaker (this is not entirely true, often there is a petal structure of the characteristic, where the side petals are, of course, weaker than the main one), and at different frequencies in different ways. At low frequencies, the decrease in acoustic power when deviating from the axis of the speaker is less, and in the case of subwoofers, the directivity is generally neglected. The listener is far from always located exactly opposite the speaker system, and when the listener is shifted to the side, the level of various frequencies changes. First of all, losses are noticeable in the field of higher sound frequencies. In multi-band speakers, the use of narrowly directed tweeters (tweeters) can dramatically degrade the sound picture when the listener is shifted a small distance away from the axis. Acoustic systems that have good frequency response along the axis, but a narrow directivity characteristic, especially with a pronounced dependence of its width on the signal frequency, sound tiring, harsh, the stereo image is unstable and shifts depending on the frequency distribution in the signal. A very sharp directivity characteristic is inherent in planar acoustic systems based on electrostatics and isodynamic systems, which makes them very sensitive to the location relative to the listener.
According to existing standards, the deviation of the frequency response from the average value at angles + – 20-30 ° horizontally or + – 5-10 ° vertically should not exceed + – 4dB.