Acoustic Elements. Acoustics is the science that studies the production, transmission and perception of sound both in the range of human hearing and in ultrasonic and infrasonic frequencies. Given the variety of situations where sound is of great importance, there are many areas of interest for its study: voice, music , sound recording and reproduction, telephony, acoustic reinforcement, audiology, architectural acoustics, noise control , underwater acoustics, medical applications, etc…
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- 1 Sound characteristics
- 2 ultrasonic transducers
- 3 sound receivers
- 1 Hearing
We have previously used the concept of sound as an elastic wave of small intensity several times. However, in a narrower sense, sound is only understood as audible sound, that is, elastic waves perceptible by the human ear .
Experience shows that the ear perceives oscillations with frequencies between the limits of 20 Hz and 20 kHz as sound. Elastic waves with frequencies below 20 Hz are called infrasound, and those with frequencies above 20 kHz ultrasound . Elastic waves with frequencies of 10 10 Hz and higher, corresponding to thermal Debye waves in liquids and solids, are sometimes called hypersound.
Depending on the structure of the spectrum, a distinction is made between noises and musical sounds . The noises are aperiodic oscillations. Its spectrum is continuous, that is, it is formed by a set of frequencies that continuously fill a certain interval. Musical sounds have spectrum lines with multiple frequencies and, therefore, are periodic oscillations.
Each sinusoidal sound wave is a single tone. The height of a tone depends on the frequency of the oscillations. The frequency scale currently adopted in music is composed as follows. Each octave is divided into twelve intervals; on the piano, seven white and five black keys correspond to these intervals; the latter are indicated in musical writing with the diesi sign.
To complex musical sounds there corresponds a fundamental tone (first harmonic ), and a set of overtones (higher harmonics). If two musical sounds have the same fundamental tone but different overtones (that is, different spectrum), they are said to differ by their timbre Precisely by the timbre we distinguish the sounds emitted by different musical instruments and human voices.
In addition to pitch and timbre, sounds are distinguished by their intensity. In the general case, the intensity of the sound depends on the intensity of the sound wave, but since the sensitivity of the ear is not the same for all sounds of different frequencies, this dependence turns out to be very complex. The maximum sensitivity of the ear corresponds to sounds from 700 to 6000 Hz in frequency . In this frequency range, the ear can perceive sounds with an intensity of approximately 10— -11 _10 -12 W/m².
The minimum intensity of an acoustic wave that the ear is still capable of perceiving is called the threshold of hearing. The threshold of normal hearing is taken equal to I 0 =10 -12 W/m² for the frequency v 0 =1kHz
The maximum intensity of an acoustic wave that we can still perceive as sound, and not as a painful sensation, is called the pain threshold. For different frequencies, the pain threshold is different, varying from 0.1 W/m² for 6000 Hz to 10 W/m² for low and high frequencies.
As can be seen, the sensitivity of our ear is very large, the range of intensities from the threshold of hearing to the threshold of pain is about 10 12 _ 10 13 W/m². With this huge range, it is convenient to use a logarithmic scale. For this purpose, the magnitude called intensity level L = 10 lg (I/I 0 ), in which 1 is the intensity of the sound under investigation and I0, the normal hearing threshold, is introduced. The unit of sound intensity level is the decibel: L = 1 dB if I = 1.26I 0 (in this case lg (I/I 0 ) = lg 1.26 = 0.1).So that they can be compared, in the table. the intensities and intensity levels of some sounds compared to the normal hearing threshold are indicated. The distance from the sound source to the ear is given in meters.
|Falling drops of water
|conversation in low voice
|Car driving on paved road
Fig. 2. Effect of Electrostriction
In some crystals (such as quartz) the effect of electrostriction is observed. This effect consists in that if an electric field directed along the x axis is created, the crystal contracts or lengthens in this direction, and along the y axis, on the contrary, it lengthens or contracts. In the direction of the z axis the dimensions of the [crystal] do not vary.
Fig. 3. Ultrasonic Translator
The figure shows the schematic of the ultrasonic translator. In it, a quartz lamina 1 serves as a radiating element, as shown in the figure below, perpendicular to the crystallographic axis, that is, according to the so-called x-cut; the steel sheets serve as armor; The alternative voltage of an ultrasound generator is connected to them , by means of the cable. Above the sheet is the air, in which the ultrasonic wave is almost completely reflected. All the radiation is directed towards the water.
In order to obtain high-power radiation with directed character, the diameter of the radiator should be made as large as possible. In this case, a single crystal is not used, but rather a mosaic of several sheets of strictly equal thickness and type of cut.
The lamellae work at the resonance frequency, which makes it possible to obtain the maximum amplitude of the oscillations. The frequency of a sheet is determined by its thickness and by the speed of sound propagation in it. Indeed, a number of half-waves fit into the thickness of the sheet, so their natural frequencies can be calculated
In electrostriction transducers, in addition to quartz, polycrystalline ceramics of barium titanate (BaTiO 3 ) are used. For this purpose, tiny crystals are grown, about a millimeter in size; these crystals are mixed with a small amount of cementing material (barium salt) and, heating the mixture to 1300—1400° C, fired. This makes it possible to obtain samples of any dimensions and shapes. The ceramic is polarized in an electric field of about 10 6 V/m intensity. After removing the polarizing field, in ferroelectrics (to which barium titanate belongs) the remanent polarization is conserved, in a way analogous to permanent magnetization in ferromagnetics.
It turns out that if an alternating electric field is applied to a previously launched polycrystalline sample in the direction of the polarizing field, longitudinal oscillations are produced in this direction analogous to those of the x-cut of quartz.
In practice, magnetostrictive transducers made of nickel or permandur (alloy of 49% Fe, 49% Ni and 2% V) are widely used; Manganese-zinc (ferroxcub A) and nickel-zinc (ferroxcub B) ferrites are also used. The transducers are made up of sheets, so that the Foucault currents are smaller; High-frequency current from the generator passes through the winding.
Any sound or ultrasound radiator can also serve as a receiver. Indeed, the wave, hitting the oscillating component of the radiator, causes it to move. In this case the elastic oscillations are transformed into electrical ones.
The electrodynamic microphone , widely used in practice, consists of the same parts as the dynamic loudspeaker, only instead of the diaphragm, a light membrane is used in the microphone. The wave, upon reaching the membrane, makes it vibrate; together with the membrane oscillates the mobile coil, located in the air gap of a strong permanent magnet. In the coil , when oscillating in the magnetic field, an induced current is produced that goes to the amplifier.
In the condenser microphone, the membrane and the fixed plate form the condenser, whose electrical capacity varies by separating more or less from one another. The sound wave causes the membrane to vibrate and with this the capacity of the capacitor varies, which produces an alternating voltage in the load resistance. The frequency of this voltage is equal to the frequency of the wave and its amplitude is proportional to the amplitude of the wave.
In the carbon microphone, the membrane periodically compresses the carbon powder, thus changing its resistance and the current in the circuit. Increasing pressure causes resistance to decrease and therefore current to increase, and decreasing pressure causes current to decrease. As in the types of microphones mentioned above, the weak oscillations of the current (or voltage) are intensified by means of thermionic amplifiers.
It should be noted that, in all types of microphones, the natural frequency of the mobile system must differ greatly from the frequency of the oscillations that it perceives, in order to avoid that, due to resonance, one of the frequencies of the total perceived spectrum stands out. Ultrasound receivers, on the other hand, work with the resonance frequency. The most frequent, in this case, is that the same transducer serves alternately as radiator and receiver.
The hearing organ of mammals , and of man among them, has a very complex structure. The external ear is made up of the pinna and the ear canal. The tympanic membrane 1′ separates the external ear from the middle ear, a small chamber 2 that contains three small bones: the malleus, the incus and the stirrup. The malleus is in contact with the tympanic membrane; the stapes, with the oval window 3, which serves as the entrance to the inner ear. The middle ear communicates with the nosopharynx via the Eustachian tube.
The inner ear is made up of a series of communicating channels that make up the labyrinth. Of this labyrinth, only cochlea 4, attached to the auditory nerve, is related to the ear. Three semicircular canals form the organ of balance.
Inside the snail there are channels filled with fluid 1 (lymph). In the middle channel is the auditory receptor or organ of Corti, which consists of five rows of cells from which cilia or eyelashes protrude; the hair cells extend the entire length of the spiral of the snail. They constitute a total of about 4800 fibers that contain five cells each. These cells form the basilar membrane, in which the fibers have different lengths: at the base of the cochlea they are shorter and at the apex, longer.
Sound perception is effected as follows. The sound wave passes through the external auditory canal, reaches the tympanic membrane 1 and causes forced oscillations in it. These oscillations are transmitted through the ossicles of the middle ear, which act as an amplifier, and reach the oval window. The oval window produces oscillations in the lymph and, by means of it, oscillations of the cochlea fibers. The strongest excitation is experienced by fibers whose natural frequency coincides with the sound frequency. Precisely because of this we can distinguish the tones and perceive the difference in timbre. In fact, the organ of Corti performs the spectral analysis of the sound waves that reach the ear and transmits the corresponding information to the brain.
The fact that we have two ears gives us the possibility to determine the direction in which the source of a sound is located (binaural effect). If the source is directly in front of the observer, the sound reaches both ears simultaneously, but if it is to the side, the sound reaches one ear before the other and we perceive this delay as a phase difference.