Room Acoustics: Preparing a Home Cinema Room
It is well known that the sound of audio equipment is influenced by many factors and one of the most important components of sound quality is room acoustics. Considering the numerous requests of our readers and the active discussion of the issue on GT, we decided to devote several publications to this topic.
In this article we will look at the acoustic preparation of the premises for the home theater . It should also be noted that the principles described in this article will be very useful in creating rooms for listening to music, home music studios and rehearsal bases.
Today, many acquire quite expensive audio equipment for home theater, and it often happens that due to the problems of the room where it is installed, the sound leaves much to be desired. Room acoustics plays one of the key roles in shaping the stage, the nature of the sound, the transfer of various timbral and frequency features. In many ways, thanks to the acoustics of the room, it is possible to achieve a so-called. presence effect when watching movies and using surround sound systems.
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The journey of sound waves through the room begins at the exit from the radiators, after which they are repeatedly reflected from the walls, floor and ceiling. It is these obstacles in their path that create the greatest number of problems. Everything affects the sound, from furniture, flooring, ceiling type, to the material from which the building and interior partitions are made. In some rooms, not only the reflection, but also the resonance of the finishing materials or furniture becomes a significant problem.
The pursuit of high-quality sound begins with reducing the number of reflections, eliminating resonant surfaces, and obtaining the maximum possible sound dispersion in the room. When preparing the premises, it is necessary that the acoustic properties of the surfaces have a minimal effect on the frequency parameters of the signal, do not introduce significant distortions, do not generate negatively affecting reflections and echoes.
To this end, when preparing such a room, it is strongly recommended to minimize the number of flat surfaces, wooden and glass interior items. Quite an ideal option is to leave in the room only upholstered furniture, as well as furnishings with a raised surface. In addition, it is important to understand that the larger the room, the more significant changes will have to be made to the decoration.
The overwhelming number of acoustic problems is caused by the so-called. "Standing waves" and the reflection of sound waves. Using a number of improvements in the decoration, you can minimize the influence of these factors on the sound of a home theater. The most simple and affordable ways to change the acoustic properties of a room are the use of sound-absorbing and sound-scattering materials. For clarity, the diagram below reflects the effect of absorbing and scattering surfaces on sound waves:
Sound-absorbing panels are widely used to decorate the walls of home theaters today. It should be understood that the different frequencies are absorbed by the materials unequally. Most modern panels, created on the basis of fiberglass, with a thickness of 2.5 cm are able to effectively absorb sound waves with a frequency up to the lower boundary of 850-1000 Hz.
At the same time to achieve effective absorption of lower frequencies can be using an increase in the thickness of the layer. It is natural that with such an approach, you will inevitably have to sacrifice the volume of the room. Considering the linearity of the dependence, when using fiberglass and similar panels, for effective absorption of sound waves of the entire audible spectrum, it is necessary to use a layer about 15 cm thick. With more effective sound absorption of the material, the thickness of this layer may be less.
A special coefficient is used to evaluate sound absorption.
At the same time, it should be remembered that when calculating the coefficient for materials, not the entire audible frequency spectrum is estimated, but only the range from 125 to 4000 Hz.
Below is a comparative table with the coefficients of some materials:
The most important role of the absorbing panels is the elimination of the so-called “first reflection.” As you know, the sound from the speaker reaches our ears both directly and due to reflections from several surfaces, and, naturally, the reflected wave does this within a few milliseconds after the straight line. The negative impact of the first reflection inevitably distorts the scene (the location of apparent sound sources), can contribute to the formation of undesirable spatial sound illusions when using surround sound systems (4.0, 5.1, 7.1 and others).
Accordingly, first of all, it is necessary to equip surfaces with sound-absorbing panels on which projections of the “first reflection” points will be located. The number of such points will depend on the number of columns (cabinets) in the used speaker system.
Below are simplified diagrams giving an idea of ​​the points of the first reflection:
The approximate location of the first reflection points is easy to determine with a mirror. The essence of the method: one person is at the viewing site, the other moves along the walls with a mirror. As soon as the first one sees the reflection of the dynamics in the mirror, the point of the first reflection is determined. Most experts believe that for effective sound absorption there is no need to cover all surfaces with materials; it is enough to close the reflection points with panels with a width of 70-90 X 200-250 cm (or more) depending on the height of the walls. Also, in order to reduce the negative impact of low medium, it makes sense to install panels on the ceiling.
After proper installation of the panels, there is only one absorption problem - waves with a frequency below 140 Hz. Ensuring optimal bass sound requires a special approach - the use of special low-frequency absorbers. There are several designs of such sinks, and I do not dare to assert which of them is better (many praise pipes). It is important to understand that their number and absorption coefficient will depend on the sound pressure level of the subwoofer or subwoofer speakers. Also, as an alternative to sinks, you can use upholstered furniture, which is not always effective, but in some cases can help to cope with low.
Absorption should be optimal, excessive sound absorption will make the sound less realistic, the illusion of volume will disappear. Sound scattering is used to balance the reflection and absorption. As we saw on the table in the second part of the article, scattering allows to increase the number of vectors of each reflection. Due to surface irregularities, additional sound fields are produced, which are formed as a result of sound diffraction. Thus, the sound "fills" the room more evenly.
There are a variety of diffusing panels that can be created from different materials, for example, hard woods, stone, synthetic materials. In some cases, diffusing panels are covered with cloth or felt. A characteristic feature of the scattering panels is the uneven surface texture, often used conical shape of the elements. It is known that in our near Soviet past, with an acute shortage of acoustic finishing materials, some musicians and music lovers used egg trays as scattering panels.
For a more accurate determination of the parameters of low-frequency sound absorption, the calculation of room modes (room resonances) will be useful. In any room, there are so-called room fashion or room resonances. Their formation is affected by the ratio of the length, width and height of the room. These parameters set the location of room resonances in the frequency spectrum and the density of their distribution.
Thus, using the known dimensions of the room, it is possible to calculate in advance the frequencies at which resonances will occur, and, therefore, to know whether these or other frequencies will be amplified or suppressed.
For an ideal rectangular room with smooth surfaces of the walls of the floor and ceiling, resonances are calculated by the formula:
To calculate all the resonances, you must alternately select all combinations of three integers Nx, Ny, Nz. Experts note that in practice it is important to calculate only low-frequency modes, i.e. maximum N = 4.
Individual resonances are described by different combinations of integers Nx, Ny, Nz. So, for example, the combination (1, 0, 0) will describe a first-order mode along the side, taken as “x”, and (0, 2, 0) describes a second-order mode along the side, taken as “y,” and so on. P…
When 2 of the 3 integers are zero, the calculation is greatly simplified and allows you to easily calculate the frequency of "standing waves" that occur between a pair of opposite walls along one of the dimensions of the room. f (1,0,0) = c / 2 / L
These modes are called axial or axial and are the most intense of all. It is their calculation that represents the most significant importance.
Optimal use of scattering and absorbing materials will allow you to get rid of the negative effects in the acoustics of the room, and adherence to simple principles, the use of technically sophisticated techniques can significantly improve your feelings from watching movies and listening to music. In the following material on this topic, our company's specialists will share the experience they gained in creating our demonstration rooms.
In this article we will look at the acoustic preparation of the premises for the home theater . It should also be noted that the principles described in this article will be very useful in creating rooms for listening to music, home music studios and rehearsal bases.
Today, many acquire quite expensive audio equipment for home theater, and it often happens that due to the problems of the room where it is installed, the sound leaves much to be desired. Room acoustics plays one of the key roles in shaping the stage, the nature of the sound, the transfer of various timbral and frequency features. In many ways, thanks to the acoustics of the room, it is possible to achieve a so-called. presence effect when watching movies and using surround sound systems.
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General principles
The journey of sound waves through the room begins at the exit from the radiators, after which they are repeatedly reflected from the walls, floor and ceiling. It is these obstacles in their path that create the greatest number of problems. Everything affects the sound, from furniture, flooring, ceiling type, to the material from which the building and interior partitions are made. In some rooms, not only the reflection, but also the resonance of the finishing materials or furniture becomes a significant problem.
The pursuit of high-quality sound begins with reducing the number of reflections, eliminating resonant surfaces, and obtaining the maximum possible sound dispersion in the room. When preparing the premises, it is necessary that the acoustic properties of the surfaces have a minimal effect on the frequency parameters of the signal, do not introduce significant distortions, do not generate negatively affecting reflections and echoes.
To this end, when preparing such a room, it is strongly recommended to minimize the number of flat surfaces, wooden and glass interior items. Quite an ideal option is to leave in the room only upholstered furniture, as well as furnishings with a raised surface. In addition, it is important to understand that the larger the room, the more significant changes will have to be made to the decoration.
The overwhelming number of acoustic problems is caused by the so-called. "Standing waves" and the reflection of sound waves. Using a number of improvements in the decoration, you can minimize the influence of these factors on the sound of a home theater. The most simple and affordable ways to change the acoustic properties of a room are the use of sound-absorbing and sound-scattering materials. For clarity, the diagram below reflects the effect of absorbing and scattering surfaces on sound waves:
Absorption
Sound-absorbing panels are widely used to decorate the walls of home theaters today. It should be understood that the different frequencies are absorbed by the materials unequally. Most modern panels, created on the basis of fiberglass, with a thickness of 2.5 cm are able to effectively absorb sound waves with a frequency up to the lower boundary of 850-1000 Hz.
At the same time to achieve effective absorption of lower frequencies can be using an increase in the thickness of the layer. It is natural that with such an approach, you will inevitably have to sacrifice the volume of the room. Considering the linearity of the dependence, when using fiberglass and similar panels, for effective absorption of sound waves of the entire audible spectrum, it is necessary to use a layer about 15 cm thick. With more effective sound absorption of the material, the thickness of this layer may be less.
A special coefficient is used to evaluate sound absorption.
The sound absorption coefficient is the ratio of the magnitude not reflected from the surface of the sound energy to the magnitude of the incident energy. It is calculated by the formula:
A (s) = E (pogl) / E (pad)
where A (S) is the sound absorption coefficient; E (gobl) - absorbed sound wave; E (pad) - incident sound wave; E (otr) - reflected sound wave; E (races) - the sound wave scattered in the material; E (past) - a sound wave passing through the material.
At the same time, it should be remembered that when calculating the coefficient for materials, not the entire audible frequency spectrum is estimated, but only the range from 125 to 4000 Hz.
Below is a comparative table with the coefficients of some materials:
The most important role of the absorbing panels is the elimination of the so-called “first reflection.” As you know, the sound from the speaker reaches our ears both directly and due to reflections from several surfaces, and, naturally, the reflected wave does this within a few milliseconds after the straight line. The negative impact of the first reflection inevitably distorts the scene (the location of apparent sound sources), can contribute to the formation of undesirable spatial sound illusions when using surround sound systems (4.0, 5.1, 7.1 and others).
Accordingly, first of all, it is necessary to equip surfaces with sound-absorbing panels on which projections of the “first reflection” points will be located. The number of such points will depend on the number of columns (cabinets) in the used speaker system.
Below are simplified diagrams giving an idea of ​​the points of the first reflection:
The approximate location of the first reflection points is easy to determine with a mirror. The essence of the method: one person is at the viewing site, the other moves along the walls with a mirror. As soon as the first one sees the reflection of the dynamics in the mirror, the point of the first reflection is determined. Most experts believe that for effective sound absorption there is no need to cover all surfaces with materials; it is enough to close the reflection points with panels with a width of 70-90 X 200-250 cm (or more) depending on the height of the walls. Also, in order to reduce the negative impact of low medium, it makes sense to install panels on the ceiling.
After proper installation of the panels, there is only one absorption problem - waves with a frequency below 140 Hz. Ensuring optimal bass sound requires a special approach - the use of special low-frequency absorbers. There are several designs of such sinks, and I do not dare to assert which of them is better (many praise pipes). It is important to understand that their number and absorption coefficient will depend on the sound pressure level of the subwoofer or subwoofer speakers. Also, as an alternative to sinks, you can use upholstered furniture, which is not always effective, but in some cases can help to cope with low.
Scattering
Absorption should be optimal, excessive sound absorption will make the sound less realistic, the illusion of volume will disappear. Sound scattering is used to balance the reflection and absorption. As we saw on the table in the second part of the article, scattering allows to increase the number of vectors of each reflection. Due to surface irregularities, additional sound fields are produced, which are formed as a result of sound diffraction. Thus, the sound "fills" the room more evenly.
There are a variety of diffusing panels that can be created from different materials, for example, hard woods, stone, synthetic materials. In some cases, diffusing panels are covered with cloth or felt. A characteristic feature of the scattering panels is the uneven surface texture, often used conical shape of the elements. It is known that in our near Soviet past, with an acute shortage of acoustic finishing materials, some musicians and music lovers used egg trays as scattering panels.
Calculation of room resonances
For a more accurate determination of the parameters of low-frequency sound absorption, the calculation of room modes (room resonances) will be useful. In any room, there are so-called room fashion or room resonances. Their formation is affected by the ratio of the length, width and height of the room. These parameters set the location of room resonances in the frequency spectrum and the density of their distribution.
Thus, using the known dimensions of the room, it is possible to calculate in advance the frequencies at which resonances will occur, and, therefore, to know whether these or other frequencies will be amplified or suppressed.
For an ideal rectangular room with smooth surfaces of the walls of the floor and ceiling, resonances are calculated by the formula:
where nx, ny and nz are integers, and Lx, Ly and Lz are the length, width and height of the room, respectively.
To calculate all the resonances, you must alternately select all combinations of three integers Nx, Ny, Nz. Experts note that in practice it is important to calculate only low-frequency modes, i.e. maximum N = 4.
Individual resonances are described by different combinations of integers Nx, Ny, Nz. So, for example, the combination (1, 0, 0) will describe a first-order mode along the side, taken as “x”, and (0, 2, 0) describes a second-order mode along the side, taken as “y,” and so on. P…
When 2 of the 3 integers are zero, the calculation is greatly simplified and allows you to easily calculate the frequency of "standing waves" that occur between a pair of opposite walls along one of the dimensions of the room. f (1,0,0) = c / 2 / L
These modes are called axial or axial and are the most intense of all. It is their calculation that represents the most significant importance.
Total
Optimal use of scattering and absorbing materials will allow you to get rid of the negative effects in the acoustics of the room, and adherence to simple principles, the use of technically sophisticated techniques can significantly improve your feelings from watching movies and listening to music. In the following material on this topic, our company's specialists will share the experience they gained in creating our demonstration rooms.
Source: https://habr.com/ru/post/372983/
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