RU2201044C2 - Acoustic system - Google Patents

Abstract

FIELD: electro-acoustics. SUBSTANCE: proposed acoustic system has loudspeaker whose front part functions as primary acoustic signal radiator and acoustic horn in the form of three- dimensional gradually widening round-section symmetrical figure such as truncated cone that passes acoustic signal of loudspeaker rear part. Acoustic horn is made of thin flexible material capable of retaining original desired shape ; tightly mounted in smaller hole of horn is loudspeaker whose rear part faces acoustic horn interior; horn aperture is joined with double-layer coating whose inner layer is made of sound- absorbing material and outer one, of hard material that functions as base for device fixation or installation. EFFECT: improved gain-frequency response with respect to sound pressure, enhanced sound capacity, simplified design, reduced size and weight of acoustic system. 1 dwg

Description

 The invention relates to the field of electroacoustics. As a broadband speaker system and as a subwoofer, the device can be used in household and professional sound reproducing equipment, in particular for high-quality reproduction of musical works.

 One of the central tasks of modern electroacoustics is the creation of relatively small-sized and light-weight acoustic systems that provide high-quality sound reproduction, including in the low-frequency region. Downsizing and improving the performance of the acoustic system at low frequencies are two mutually exclusive trends.

 The decisive factor in improving the performance of small-sized acoustic systems at low frequencies is the choice of the optimal type of acoustic design based on the use of one of the following components or their combinations: phase inverter; additional passive emitter; acoustic labyrinth; acoustic low-pass filter (see, for example, Broadcasting and acoustics, edited by Yu.A. Kovalgin. - Radio and communications, 1998).

 Currently, there are several dozen designs of bass-reflex speaker systems (see, for example, US patent US 05696357: "Bass-reflex loudspeaker" - "Loudspeaker with bass reflection"). The known device contains an acoustic box in which two tubular resonators are installed, which act as phase inverters at two different frequencies. Sound waves emitted from the rear side of the woofer cone are out of phase with respect to waves emitted from the front side of the cone. The phase inverter is turned on in such a way that sound waves emitted from the rear side of the diffuser pass through it. The purpose of the phase inverter, which is formed by the internal volume of the speaker system body and two resonators, is to make sound waves emitted from the rear and front parts of the speaker cone approximately in phase at low frequencies of sound reproduction in the spatial sector of listening to the speaker system. This leads to increased sound pressure at low frequencies.

 The main disadvantages of the known device are the relatively large volume of the case, the effect of “mumbling” at low frequencies, the rapid decline in the amplitude-frequency characteristics of sound pressure at low frequencies outside the operating range of the speaker system, the “organ” stylization of bass sound at the frequencies of the bass reflex tuners, the acoustic noise of airborne flows in pipes of phase inverters.

Closest to the claimed device is an acoustic system according to US patent US 04930596 "Loudspeaker system". The known device contains a loudspeaker installed in a pre-hull enclosure of the closed type. The front part of the loudspeaker cone serves as the emitter of the primary acoustic signal. In the rear part of the pre-corpus casing there is an opening to which an acoustic waveguide made in the form of a thin tube is tightly connected. To ensure a single-mode mode of operation of the waveguide, the diameter of the pipe is chosen to be much smaller than its length. The other end of the acoustic waveguide is connected to the input of the acoustic horn, made in the form of a three-dimensional smoothly expanding figure with a circular cross section, for example, in the form of a truncated cone or with an exponential longitudinal profile. The length of the pipe L of the acoustic waveguide is chosen approximately equal to half the wavelength corresponding to the lower boundary frequency f _{L of the} frequency response of the speaker system. In this regard, the acoustic signal emitted from the rear of the loudspeaker, the frequency f _{L of} which is close to the lower cut-off frequency of the device, experiences a phase delay of approximately 180 ^° when passing through the acoustic waveguide and the acoustic horn. Therefore, at a frequency f _{L, the} sound waves emitted by the front of the loudspeaker are in phase with the sound waves emitted by the speaker. All this provides an improvement in the frequency response of the speaker system at low frequencies.

 Let us dwell on the main disadvantages of the known device.

 The first disadvantage of the known acoustic system can be attributed to the relatively large length L of the waveguide tube, reaching several meters. In this regard, to reduce the linear dimensions of the known device, the waveguide has to be made in the form of a curved pipe (labyrinth). The presence of bends leads to a complication of the design and manufacturing technology of the device, to the appearance of additional noise of the air flow and reflected acoustic waves.

Если π²V/S>L, то в зависимости от величины V/S длина трубы может оказаться в несколько раз меньшей, чем половина длины волны, соответствующей резонансной частоте.The second drawback is the difficulty of correctly calculating the length of the waveguide tube. Indeed, under real conditions with a certain degree of approximation, the air in the pipe can be considered as an oscillating system with both concentrated and distributed parameters. In the first case, the air in the pipe having a cross section S is considered as an incompressible medium, the oscillations of which occur due to elastic forces from the side of the air volume V concentrated inside the speaker housing. Resonance is achieved for sound waves whose wavelength

If π ² V / S> L, then, depending on the value of V / S, the length of the pipe can be several times smaller than half the wavelength corresponding to the resonant frequency.

In the case of considering the waveguide as an oscillatory system with distributed parameters, air in the tube is considered as an elastic medium that ensures the propagation of sound waves. In the ideal case, if the waveguide is matched in input and output and the reflected waves are absent, then the traveling wave mode is realized. To invert the phase of the oscillations, the length L is chosen equal to half the wavelength corresponding to the frequency f _L adjusts the phase inverter. In real conditions, there are always, to one degree or another, reflected waves, which lead to the appearance of standing acoustic waves. In this case, when calculating the resonant frequencies, it is necessary to take into account the reflection of waves (and the phase relations of the reflected waves) in all places of the inhomogeneities of the acoustic path (connection of the pipe to the pre-chamber chamber, connection of the pipe to the speaker, speaker output, etc.). In practice, the second disadvantage of the known device means the impossibility of accurate preliminary calculation of all components of the system and the need for a long and time-consuming experimental fine-tuning of the product to achieve the required technical characteristics.

As a third disadvantage of the known device, we point out that the correct phase relations of the acoustic waveguide, which contribute to the amplification of sound in the listening sector (phase delay from 90 to 270 ^o ) are approximately reached within one octave. When you try to increase the sound pressure at low frequencies by enhancing the effect of the horn system, a peak appears on the amplitude-frequency characteristic of the speaker system, the height of which directly depends on the ratio between the sound pressures generated by the horn and the front side of the speaker cone. The higher the peak height in the amplitude-frequency characteristic, the subjectively higher is the sound efficiency at low frequencies. Along with this, with a pronounced peak in the amplitude-frequency characteristic, the known acoustic system emits most effectively stylized sounds of approximately the same tone, reminiscent of the sound of an organ for attack and after-sounding. In this situation, a compromise must be sought between the low-frequency reproduction efficiency and the uniformity of the amplitude-frequency characteristic.

 The technical result of this invention is to improve the amplitude-frequency characteristics of sound pressure, giving surround sound, simplifying the design, reducing the size and weight of the speaker system.

 The technical result is achieved by the fact that in the known device comprising a loudspeaker and an acoustic horn of circular cross section, the following changes are introduced. The loudspeaker is mounted in the smaller hole of the speaker in such a way that its rear part faces the inside of the acoustic speaker. Opening the horn is closed with a two-layer coating, the inner layer of which is made of sound-absorbing material, and the outer layer is made of solid material, such as wood. The outer side of the two-layer coating serves as the basis for fastening or installing the device. The acoustic horn is made of thin elastic material with the ability to preserve the original shape. To obtain lower values of the lower cutoff frequency of the device when choosing the material from which the horn is made, preference should be given to samples having low values of the coefficient of elasticity (E <5 MPa) and high values of the attenuation coefficient of surface acoustic waves.

 The drawing shows a drawing of the inventive device. The device comprises a single-band or multi-band dynamic loudspeaker 1, the front part of which diffuser serves as the emitter of the primary signal. The loudspeaker with the back side is tightly installed in the hole of the tapering part of the casing of the acoustic speaker 2 having the shape of a truncated cone. The walls 3 of the speaker 2 are made of a thin elastic material capable of preserving the initially given shape. With its opening, the horn is closely docked with a two-layer coating, the inner layer of which is formed by a sound-absorbing panel 4, which completely covers the wide opening of the horn. The device is installed or attached to a rigid base 5, made, for example, of wood. As the base 5 of the speaker system can be used a fragment of the floor, ceiling, walls or furniture of the room. For this, the device is tightly attached to the corresponding part of the interior of the room.

 The case of the experimental sample of the acoustic system is made in the form of a horn having the shape of a truncated cone. The horn is made of an elastic material with an elastic coefficient of E≈3 MPa. As a material for the manufacture of the horn walls, for example, some types of rubber or polymeric materials can be used. The wall thickness of the speaker is 2 mm. The distance from the speaker mount to the speaker to the sound-absorbing coating is 110 mm. The internal volume of the speaker is approximately 4.5 liters. The sound-absorbing panel 4 is a fleecy synthetic carpet facing the pile inside the horn. The base 5 is made of boards tightly fitted to each other (wooden floor of the room). Tests conducted with a 10GDSH-2 dynamic loudspeaker showed that the range of effectively reproduced frequencies is 40 - 20 kHz.

The device operates as follows. The sound waves emitted from the rear of the loudspeaker diffuser 1 go to the input of the acoustic horn 2. The total power P ₀ (t) of these waves is divided into three components
P ₀ (t) = P _f (t) + P _b (t) + P _osc (t),
where P _f (t), P _b (t), P _osc (t) are, respectively, the power of the direct wave transmitted through the horn; backward (reflected) waves returning to the input of the horn, and the power of oscillation of the walls 3 of the acoustic horn. It is easy to see that due to the absorption by the panel 4 and directional reflection from the side surfaces of the horn towards its opening, the power P _b (t) of the reflected back acoustic waves is significantly smaller in comparison with the total power at the input of the acoustic horn, i.e.

P _f (t) >> P _b (t).

 This property of the horn is used in the inventive device to reduce the elastic resistance of the closed air column to the vibrations of the speaker cone. In turn, a decrease in the elasticity of air with respect to the diffuser can be used to compensate for the increase in the elasticity of the air, which occurs in connection with a decrease in the volume of the acoustic system. Thus, it is clear that the use of a horn as a speaker enclosure reduces the equivalent volume of the speaker system without compromising the electromechanical characteristics of the emitter at low frequencies.

To maintain high performance of the acoustic system in terms of sound pressure comparable with the prototype, it is necessary to use at least part of the energy emitted from the rear of the diffuser as an additional component for sound waves emitted from the front of the diffuser. In the inventive device, this contributes to the fact that part of the energy P _osc (t) is used to excite vibrations of the side walls 3 of the horn. Note that this approach runs counter to the generally accepted concepts of creating high-quality acoustic systems (see, for example, the book Household electro-acoustic equipment: Reference book / I. S. Aldoshina, V. B. Brevko, G. V. Veselov, etc. - M. : Radio and communications, 1992, p. 7). In order to avoid the appearance of additional nonlinear distortions and unpleasant overtones, the design of the bodies of known acoustic systems is rigid enough to avoid noticeable vibrations of their walls.

 In the course of a theoretical analysis of the calculation results and the experiments conducted, it was possible to find such an optimal combination of material and design of the speaker housing, the joint use of which allows a significant revision of the requirements for the mechanical properties of the speaker housing material. In the inventive device, the walls of the horn of circular cross-section, made, for example, in the form of a truncated cone, should be made of thin highly elastic material capable of preserving the initially given shape of the material. When these conditions are met, the absence of noticeable harmonic components in the vibrations of the housing walls is achieved in a fairly wide range of speaker power. This allows you to use the horn housing as an additional passive emitter of the speaker system.

 According to physical principles, the operation of the claimed device partially resembles the operation of an acoustic system with a passive loudspeaker (see, for example, Household electro-acoustic equipment: Reference Book / I.S. Aldoshina, V. B. Brevko, G.V. Veselov, etc. - M .: Radio and Communications, 1992, p. 6). The difference lies in the fact that in the known device the emitter of sound waves is a passive loudspeaker diffuser, and in the inventive device, the side walls of the acoustic horn.

 The creation of an accurate theory of oscillations of the horn walls is an independent and rather complicated theoretical task of calculating inhomogeneous hollow elastic acoustic waveguides. But the greatly simplified theoretical approximation, which is given below, makes it possible to clarify the essence of the operation of the device.

где P(t) - звуковое давление внутри рупора, которое на достаточно низких частотах приблизительно синфазно для всех рассматриваемых слоев;

- угловая частота собственных колебаний слоя.Let the circular horn and loudspeaker have a common axis of symmetry. Fluctuations in the walls of a speaker are best seen in a cylindrical coordinate system. We mentally divide the horn into a series of thin layers parallel to each other and perpendicular to the axis of symmetry of height Δh. Due to the chosen shape of the horn, the radius r (t, h) of the thin layer of the horn will depend only on its height h relative to the base of the horn (mouth of the horn) and the current time t (due to sound vibrations). Consider a differential equation describing the oscillations of a thin circular layer of a horn wall with a height Δh. Due to axial symmetry of the horn and neglecting surface waves, we can consider the one-dimensional case of oscillations. Calculations show that at sufficiently low frequencies, the differential equation describing a change in radius r (t) will have the form

where P (t) is the sound pressure inside the horn, which at sufficiently low frequencies is approximately in phase for all layers under consideration;

- the angular frequency of the natural vibrations of the layer.

- показатель затухания собственных колебаний слоя, где b₁ - удельный фактор потерь на единицу площади слоя стенки рупора;
r₀=r₀(h) - радиус рассматриваемого слоя рупора в состоянии равновесия;
ρ - плотность вещества, d - толщина стенок рупора;
Е - модуль упругости материала рупора на растяжение и сжатие.

- the attenuation index of the natural oscillations of the layer, where b ₁ is the specific loss factor per unit area of the horn wall layer;
r ₀ = r ₀ (h) is the radius of the considered layer of the horn in equilibrium;
ρ is the density of the substance, d is the wall thickness of the horn;
E is the elastic modulus of the horn material in tension and compression.

где

- комплексная амплитуда колебаний, модуль которой равен

По отношению к внешнему возбуждению колебания слоя стенки рупора будут отставать по фазе от возбуждающего воздействия на величину φ(r₀), равную

Анализ последней формулы показывает, что вынужденные колебания слоев стенок рупора происходят с задержкой по фазе на угол φ, который меняется в пределах от 0 до 180^o. На частотах, лежащих выше собственной частоты ω₀(r₀), задержка по фазе удовлетворяет условию 90^o≤φ<180^o. В этом случае колебания стенок содержат квадратурную и противофазную компоненты колебаний. Это означает, что на частотах, выше ω₀, стенки рупора могут выполнять функцию фазоинвертера.The solution of equation (1) shows that under a harmonic external action with an amplitude P _{0, the} steady-state oscillations of the horn wall layer will be harmonic oscillations with the frequency of the external influence ω

Where

- complex amplitude of oscillations, the modulus of which is equal to

With respect to external excitation, the oscillations of the horn wall layer will lag in phase from the excitation by a value of φ (r ₀ ) equal to

Analysis of the last formula shows that the forced oscillations of the layers of the walls of the horn occur with a phase delay by an angle φ, which varies from 0 to 180 ^o . At frequencies lying above the natural frequency ω ₀ (r ₀ ), the phase delay satisfies the condition 90 ^o ≤φ <180 ^o . In this case, the wall vibrations contain quadrature and antiphase vibration components. This means that at frequencies above ω ₀ , the horn walls can function as a bass reflex.

If we take into account that the horn consists of many layers with different r ₀ and if we take into account the mechanical connection between the layers, the resonance peak will be substantially blurred in frequency in a certain range, and the dependence of the delay angle on the frequency will be smoother than in the above expression . This circumstance is favorable from the point of view of obtaining a positive effect on the claimed device. Indeed, firstly, the amplitude-frequency characteristic of the speaker system will be more uniform than in known devices with tubular phase inverters-resonators. Secondly, to obtain the necessary phase-frequency characteristics, accurate dimensioning and high accuracy in the manufacture of a horn are not required. Thirdly, there is no need to configure any structural elements of the speaker system.

Given the above calculation results, it is easy to notice that the phases of the oscillations of the front of the diffuser and the walls of the horn will differ from each other by no more than 90 ^o . In this regard, in the working frequency band, sound waves emitted by the front part of the diffuser and the side parts of the horn will be in-phase and (or) quadrature components. Both those and other fluctuations in the aggregate reinforce each other. The presence of quadrature components of the oscillations of the horn walls with respect to the primary signal gives surround sound to the speaker system.

 It should be noted that the implementation of phase relations between the sound waves emitted by the front side of the diffuser and the side walls of the horn will be performed in a wide frequency band. Therefore, in comparison with the prototype, the amplitude-frequency characteristic of the speaker system in terms of sound pressure will be substantially more uniform.

Claims (1)

 An acoustic system comprising a loudspeaker, the front part of which serves as the emitter of the primary acoustic signal, and an acoustic horn in the form of a symmetrical volumetric continuously expanding figure with a circular cross section, having, for example, a truncated cone shape through which the acoustic signal of the rear part of the loudspeaker passes, characterized in that that the acoustic horn is made of thin, elastic, capable of preserving the initially given shape of the material, while in the smaller hole of the horn it is tight A loudspeaker is installed, the rear part of which is facing the inside of the acoustic horn, and the opening of the horn is joined with a two-layer coating, the inner layer of which is made of sound-absorbing material, and the outer layer is made of solid material, which serves as the basis for fastening or installing the device.

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