Psychoacoustic perception of sound

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In order to discern their place in the world around them, people are endowed with five senses, touch, smell, taste, vision and hearing. For the purposes of the use of modern technologies of entertainment, such as a home theater are important vision and especially hearing. Today's technology digitizing video and music content undoubtedly has its advantages and disadvantages, but for knowingly sacrificing quality content in order to adapt the same for transmission through the communication systems. One of the solutions that are used for this purpose are the limits of such a system in terms of DVB transmission of images and music.

Regarding the video content is already quite described through descriptions of computer systems. Is used to compress content, which fortunately is not as drastic in the matter provided for distribution through BR (Blu-ray) media. Standards for BR media are usually encoded with 'MPEG-4 AVC' (Advanced Video Coding) coder for video content and 'Dolby Digital Plus' (DD+) expanded 'Dolby' standard system for 7.1 audio playback. From Figure 7.1.1 it is clear that the transfer of digital content better if it is used multiple levels of sampling in amplitude analog signal if the sampling period as short as possible. Thus, the BR media normally put video content in HD resolution of 1920x1080 pixels, with a sampling rate of 8 bits or 10 bits, image display frequency of 25 Hz (i - interlaced mode) or 50 Hz (p - progressive mode) for distribution in Europe, and 30 Hz or 60 Hz for distribution in America. Audio content on the BR media are usually encoded with 'MPEG-4 AAC' (Advanced Audio Coding) coder and sampled with 16, 20 or 24 bits at a sampling frequency of 48 kHz, 96 kHz or 192 kHz, of course, with the use of 6 or 8 independent channels for 5.1 or 7.1 system playing music. BR media should add content related to subtitles. It must be noted that the above standards for encoding video and audio compression method used by which a portion of the content is lost (lossy compression), but nevertheless sufficiently good quality for human perception of image and sound, and the default for most multimedia devices.

The human ear is the the most magnificent of the senses. Of course, the fact that about the organ that allows seeing what is written or read using information from small cells in the eye is also amazing, but the way in which sound affects the brain is much more important than it is through the eye presenting as brain images. Therefore, this organ of human, in order to understand how it works, should be given a more thorough description which includes the next image.

Figure 1.1 The human ear - a system of human hearing mechanism.

The outer ear, or eye, is of great importance with regard to 'capture' the sound and determine the direction of the sound source (some animals can rotate and ear). Collected sound through auditory channel lined with fine hairs and sweat glands to protect the entry of foreign matter and polluted air leads to the eardrum. The eardrum is a membrane that separates the outer ear from the middle ear. Eardrum in the middle ear is connected to three ossicles, called the hammer, anvil and stirrup, which act as a lever mechanism that pushes the small stirrup across the membrane attached to the cochlea. It is managed by a volume increase of approximately 20 times. Auditory ossicles serve as an amplifier of pressure. Sound vibrations are transmitted through the membrane stirrups into the cochlea in the inner ear that transmits sound through the liquid and which is longitudinally divided into two channels basilar membrane, so that the sound vibrations one channel cochlea spread to her environment and continue their way towards the other channel and outlet with membrane which serves as the 'elastic cap'. In this way it creates pressure in the cochlea are created by running molecular dynamic fluid cochlea and auditory nerve is distributed along the cochlea is in charge of analyzing the sounds of different frequencies.

There are quite a few extras, the labyrinth and vestibular nerve in the supervision of balance when walking (or sitting), and the Eustachian tube that connects the middle ear with the oral cavity regarding equalizing air pressure on both sides of the eardrum. Chewing gum, yawn, or chew candy help if there is such an imbalance caused by the rapid descent of aircraft and cars down the mountain about equalizing the external pressure and the pressure in the middle ear. Thus, the outer and middle ear contains air, while the inner ear (cochlea) is filled with fluid. In general, low-frequency tones are analyzed at the end of the cochlea while the high-frequency tones are analyzed at the beginning of the cochlea. Cochlea nerve system monitors both incoming and outgoing sound wave to the outlet side. Depending on the intensity of sound brain volume difference between channels is interpreted as a volume.

Thirties at Bell Laboratories in the United States have been working tests between sound sources and human perception, and as a result published diagram that contains curves of equal volume - Fletcher Munson curves, which show that the ear does not respond equally to different frequencies of sound at different volume, which is shown in Figure 1.2a.

Figure** 1.2. Hearing features. ( + / - )

Thus, the ear is a kind of probe or sensor. Transducer that converts one form of energy into another, acoustic energy (sound pressure) is converted into kinetic energy (movement of bones of the middle ear), and then into electrical energy (nerve impulses that the brain interprets). Speaking skills are less of a human listening skills, as best illustrated in Figure 1.2b which highlighted specific areas around the ear can distinguish. Ear covers the range of tones is greater than 8 octaves. It is a musical interval of eight tones, each of which begins the next octave frequency twice the initial frequency of the previous octave, which is best illustrated in Figure 1.2c. However, the figure shows only the basic sounds that each musical instrument created without the harmonic components.

Different frequencies and amplitudes do not hear the sound below a certain sound pressure levels, and if the sound pressure exceeds a certain measure causes a feeling of discomfort and pain and can lead to physical damage. Generally, no consequences can follow the loudness of 85 dB for about 12 hours (listening to music), then it takes the same number of long pauses. And every time you add 3 dB above the level of the volume while listening must be reduced by half. So, if you are listening to music volume 95 dB, one can listen to about an hour and a half but then must take a break of an hour and a half. Church organ, for example, can get the volume up to 100 dB. Limit of pain exceeds the noise generated by a jet engine. All described the anatomical features of the human ear, a human has two. Based on this, enables still a few features that the human brain recognizes:

• The brain can detect the difference between the amplitude of the sound our ears. Using this distinction brain will determine the direction of the sound source.
• If the distance from the sound source is too large, except the ears, the brain will compare the arrival time of sound between two ears and conclude where is the sound source.
• When sound comes from one side of the head, must 'swerve' around the head before can get to the other ear. High frequency sound is not capable tour the head for short wavelengths, so that the ear on the opposite side from the source to read much less volume.
• Masking, the ability to lower the volume of the audio source other conceals a much stronger sound source. High levels of sound will mask the sounds low, especially if both are two sources of the same frequency. Masking is possible and if the time of arrival of the sound source low bit earlier or later than the time of arrival of the sound from the stronger sources. Specified and enabled the 'mp3' compressing of audio content.
• Hass effect, the concept is based compared to the time of arrival of sound in the brain. Given the shape of the head and the distance between the ears, the time difference is usually 35 ms. Thus, the brain will favor sound coming for example within 15 ms compared to the sound that would come after 35 ms even if it is 10 dB louder than the first came. This peculiarity allows full spatial (stereo) image with only two speakers.
• The effect of the crowd, the ability to isolate one sound or group of frequencies in one group of people over another, just as in conversations at cocktail parties.
• Octave illusion, the ability of our brain to reconstruct if it finds that the missing fundamental frequency based on the recognition of frequency of received sound waves.

Small curiosity, there are several frequencies that have enormous negative effects on the human body:

• 3-7 Hz - The resonant frequency of fluid in the labyrinth of the inner ear. Impossible to stay on balance and movement through space. These are also the frequency used by a majority of the cardiovascular system. It can cause a lack of blood flow to the extremities, breathing problems, dizziness, hyperventilation and theoretical death.
• 12 Hz - The resonant frequency of the common human in the chest cavity. Too long exposure at this frequency causes disturbances in the gastrointestinal tract, dizziness, nausea and feeling sick.
• 14 Hz - Causes some of the similar effects as 12 Hz, but also stronger so that the present inability to concentrate, feeling of fullness in the inner ear, irritability, and prolonged exposure leads to a feeling of exhaustion.
• 1250 Hz - With a sound wave power more than 3'000 dB human head will explode. Yet acoustic impedance of air on Earth prevents the spread of sounds strength above 194 dB. However, exposure to sound pressure of 194 dB means roughly that the former music listener is not interesting anymore :-).

Human hears / submits sound pressures ranging from 20 µPa - 20 Pa (1 Pa = 1 N/m ² ). The minimum value specified is called the THRESHOLD of AUDIBILITY, and the maximum value specified THRESHOLD of PAIN and defines the signal frequency 1 kHz. This huge numeric range is impractical, and therefore prefer to use a logarithmic scale as shown in Figure 1.2. Threshold of audibility is defined as the value of 0 dB, and the threshold of pain as the value of 120 dB. ' dB ' (deci Bell), the tenth part of the ' B ' (Bell), is a unit for measuring sound pressure and acoustic power, and it is taken as a measure of well-established as the ' B ' as a measure too high.

What is a ' dB '? Although sometimes considered to be ' dB ' absolute value type 'number without dimension', not exactly. Zero ' dB ' is not the same as zero in mathematics but a minimum volume of sound that the human ear can register listening acoustic wave (signal) frequency of 1 kHz. Several standards established to define the reference value in accordance with sound and electrical sizes.

• 0 dB SPL = 20 µPascals (Sound Pressure Level) - the lowest sound pressure that the human ear can register. This refers to the actual value of the sound pressure in the acoustic space or media.
• 0 dBm = 1 mW power dissipation at 600 Ω impedance of the voltage source of 0.775 V (775 mV), referent sizes based on telephony systems.
• 0 dBu = 775 mV, similar as dBm but does not define to which the impedance relations, but just observed voltage value.
• 0 dBV = 1 V RMS (Root Mean Square), but does not define to which the impedance relations.
• dBFS = dynamically varies in relation to the used digital scale, which is the reference value when all bits sampling ' 1 '. Other values are therefore negative. For 16-bit sampling of what the range of -96 dBFS, 20 bit sampling of what the range of -120 dBFS, and 24 bit sampling correspondingly range from -144 dBFS. Thus, a measure of digital systems. Each ' 1 ' converted to ' 0 ' represents a difference of 6 dB.

' dB ' is a relationship that describes the following equation:

( + / - )

The first one refers to calculating the power of the observed, and the second equation is applicable to calculating the monitored voltage, wherein the size of the numerator is greater than the size of the denominator of the result if it wants a positive result. Inverse relationship will give a negative result with the same numeric amount. If you consider the power of 10 V to 1 V, adequate power to the consumer size 1 Ω then the 100 W and 1 W, which according to the above equations gives the same amount in ' dB '. Twice the voltage or power given amount increased by 3 dB. The dynamics of hearing is very large, the smallest and the largest sound pressure that the ear can register with 1:10 ⁶  ratio, and that the intensity of sound depends on the square of the sound pressure, minimum and maximum sound intensity in relationship 1:10¹². The intensity of sound can be calculated using the known values sound pressure and known properties of the medium (air), and the reference to the amount of sound intensity was approximately 1•10^-12 W/m ²  for the sound pressure of 20 µPa (micro Pascals). For the range of 120 db, if the lower bound takes the aforementioned reference intensity, it is necessary to increase the intensity of 1 W/m ² . Speaker as a converter of electric signals into acoustic has a very poor efficiency, below 1%. Only high-quality speaker power of 2 kW can be achieved SPL of about 90 dB. Firm 'McIntosh' produces high quality speakers and amplifiers up to 2 kW, which is almost enough to be in a home environment and 'act' a symphony orchestra. But it costs a lot. This really goes a saying, 'How much money, so much music'.

As the sound dissipates during propagation, sound intensity decreases with the square of the distance from the sound source and the sound pressure is achieved at some distance from the source is inversely proportional to the distance. Basic characteristics of a sound wave are:

• Sound pressure ( P ) - Force acting on a surface, it is alternating pressure when spreading sound wave superimposed atmospheric pressure.
  Unit: bar or N/m ²  (1 bar = 10 ⁵  N/m ² ). It is very small compared to the atmospheric pressure.
  The lowest pressure that the ear can register is 2•10^-5 N/m ²  (20•10^-6 N/m ² ). Pressure that causes ear pain is 20 N/m ² .
  Sound pressure decreases proportionally to the distance, and if the double distance falls twice.
• Propagation velocity of sound ( C ) - In the air is about 340 m/s, depends on temperature ( t ), pressure and humidity.
  Calculates with the approximate expression:
  C = 331.4 + 0.6•t [m/s]
  In the gas depends of pressure and density of gas, in liquids depends of density and compressibility ( q ). In water sound propagation velocity is about 1500 m/s.
• Sound intensity ( I ) - Amount of sound energy per unit time through a flow-off surface area of 1 m ²  set perpendicular to the direction of propagation of sound.
  Calculates with expression:
  I = P ² /(q•C) [W/m ² ]
• Sound or acoustic power ( Pa ) – Acoustic energy which flow-off in 1 second through surface area of the S set perpendicular to the direction of propagation of sound.
  Calculates with expression:
  Pa = S•I = S•P•v = S•(P ² /(q•C)) [W]
• Acoustic impedance ( Z ) - Match to the electrical resistance. He is negligibly small for the tube radius larger than 0.1 cm. For the same reason acoustic resistance is very large for a very small diameter tubes. Therefore, substances with narrow pores, such as textile materials are good sound absorbers.
  Calculates with expression:
  Z = q•C
• Density of sound energy ( E ) - Average of sound energy at the time per unit volume.
  Calculates with expression:
  E = I/C [1 Ws/m ³  = N/m ² ]
• Velocity of particle oscillating ( v ) - Speed at which particles passing through a certain position during oscillation.

The curves in Figure 1.2a are called isophone curves. Isophone curve called THRESHOLD of AUDIBILITY corresponds to the subjective intensity of 0 FON. The next curve is the curve of audibility of 10 FON. FON is a unit for loudness, but the methodology is not within the SI system of units, and will not continue to use it.

SUMMARY:

According to these measurements and equations reference sound intensity is approximately 1•10^-12 W/m ² . Include it in the equation for the intensity of sound pressure maximum size that a man can endure will be the sound intensity of 1 W/m ² . Acoustic power of 1 W, assuming that the sound from the source of further expanding spherical (spherical source), at a distance of 5 m from the source can be achieved by the power source of approximately 300 W. The dynamics of auditory fields, i.e. the strongest and the weakest human whom sound can hear, is 120 dB. Under normal circumstances, the most dynamic of music of 100 dB (church organ, for example), for a distance of 5 m from the source, the source should have a force of approximately 3 W, and how the speaker has very poor efficiency, less than 1%, it would necessary to use powerful amplifiers.

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