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Multimedia (Home Theater) - Basic characteristics of sound signal

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What is the sound, right?

Sound is a form of energy that manifests as a fluctuation of pressure in the medium it travels through. The most widespread medium that human uses is air, but the sound is spread through water, brick, glass, wood, and everything has a density. The lower the temperature below the speed of sound. The higher the density of air (or any media) provides higher speed of sound. Typically, the sound is associated with propagation through the air and the speed of propagation in normal conditions is about 340 m/s The light is spread far more quickly, approximately 300'000'000 m/s. Therefore, it is easy to calculate how far away a storm is 'burst' if eve flashes because 'thunder always comes after lightning'. The only place where the sound can not exist is in a vacuum or the environment at a temperature of absolute zero (-273°C). So, all of those science fiction movies with photon torpedoes and dazzling explosions (no oxygen) with a tremendous noise, are pure fiction, it can not happen in the universe. The absence of media prevents propagation of sound. The biggest stunt was affected when the spaceship 'fall' :-).

Sound is a series of waves of ever changing compression and rarefaction of molecules media. Compression occurs when the sound source makes the surrounding air molecules together and thus increases the air pressure, and dilution occurs when the molecules of air expands to fill a larger space and cause a drop in air pressure. Four measurable parameters describing sound: amplitude, frequency, wavelength and phase. Amplitude describes the magnitude of change in the air pressure. The amplitude can be measured as shown in Figure 2.1.

 Amplitudes of signal
Figure 2.1: Amplitude of the signal.

By labels in Figure 2.1 apply to the following notion of amplitude:

Under the amplitude of power entails the notion that it was an electrical sinusoidal signal of constant amplitude that an amplifier can play for about 15 minutes without emerging signal degradation and for that amount usually says it is NOMINAL or RATED POWER (RMS power).

With the concept of signal is closely related the term 'frequency'.

Description of sinusoidal signal is covered under Chapter 1.3. Phases within the audio content describes the relationship between two or more sinusoidal waveforms that form the final waveform audio content. In Figure 2.2 shows the possible phase between two sinusoidal signals of the same amplitude. Analogous to the description in Chapter 1.3, figure that follows is described as what are doing two mutually spaced vectors over time.

 Phase shift
Figure 2.2 Phase shift between the two signals.

In the graph at Figure 2.2 red curve is used as a reference, and the blue curve in relation to it lags by 90°. That is to say, the displayed signal vectors rotate at the same speed, but they are mutually spaced by 90°. If using sources that are waveforms of the same amplitude and frequency, and the phase difference is 0°, the result will be the same waveform with twice the amplitude. If sources are the wave of the same amplitude and frequency but 180° phase difference, the result will be a amplitude of size zero, or silence. Any other phase relationship will result in modifying the phase of the waveform that is between these two phases. Almost all sounds, except sine waves are complex waveforms contain more signals of different amplitudes and frequencies.

Most sounds consist of signals that describe the following four groups of frequencies:

The theory deals with sound waves and their way through the air or through a material. In this sense propagation of acoustic waves is considered in several categories:

Due to above, in the untreated room can expect a number of problems concerning its acceptable acoustics. Very different ways and materials to make a room acoustically friendly. Not the same design for the home theater room and the right theater. Besides the theater entirely full of visitors has better acoustic characteristics in relation to the half empty theater. The human is a good acoustic absorber.

There are two ways of understanding the propagation of sound waves:

Sound has features like it has a light or X-rays, and has measurable parameters. According to Figure 2.1 are connected by the following equation (wave equation):

 Wave equation

The above equation allows to calculate the several of critical data relating to the room where the home theater and the perception of the listener.

That music is a set of sound waves of different amplitudes and frequencies, it is clear that their sum total does not care about proper form, as shown in Figure 2.1, but the waveform looked to the example on the next set of pictures.

 Digital music signal
Figure*** 2.3 Signal of music file - Ravel: Bolero, The New York Philharmonic. ( +/- )

From the example shows that the dynamic range of a considerable amplitude, and that not come even close a continuous signal of the same amplitude and frequency, but it's a wealth of colorful signals. The dynamic range is:

The stated values refer to the reference value: 0 dB = FS Square Wave. So it is a measure for the digital recording and the image can be read 2.4d each sampling point, based on the RMS range above it is clear that this is a 24-bit sampling (sample depth), the sampling frequency of 44.1 kHz (sampling rate), the degree of compression mp3 320 kbps (bit rate). The average RMS value indicates that the amplifier does not have to play a huge RMS continuous power, but that must allow as much dynamic range.

So let's assume that the playback amplifier uses declared (nominal) power of 200 W, which can develop on the consumer (resistor) than 4 Ω or 8 Ω. Supply voltage output transistor amplifier is ±62 V, which gives the maximum amplitude of 60 V to the position ' 1 ' in Figure 2.1, the RMS value of the voltage of 42 V (RMS) to position ' 3 ' in Figure 2.1, and the range between the two peaks amplitude of 120 V to the position ' 2 ' in Figure 2.1. This last measure would be happy to show most manufacturers of second-rate equipment, but no notes on where the amplitude does. According to Ohm's law (P=U 2 /R) RMS power of the consumer is a 440 W at resistor of 4 Ω, with above 200 W RMS refers to the power load on the criteria for a period of 15 minutes with a constant signal amplitude and frequency of 1 kHz, with no increase of distortion. Power signal amplitude by these parameters is 900 W. The range between the two amplitudes gives another striking value - 3600 W, a soul given to boasting about selling second-rate equipment.

By construction and characteristics that are primarily related to the signal distortion, power amplifiers can be classified into several categories.

So, when buying the audio system, the default is that the preamplifier working class ' A '. A huge amplification with strong negative feedback connections distortion of power amplifier in class ' AB ' are almost negligible. What we should take into account is that the larger the available dynamic range of the power amplifier. The distortion should be less than 0.05%, and the dynamics of about 90 dB. Regarding the achievement of greater dynamics must take account of its own amplifier noise. For high quality playback noise level should be at least 20 dB lower than the quietest music signals. Taking into account all of the above is satisfactory power amplifiers for home use is about 150 W per channel.



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