Акустика помещений
Online Acoustic Calculators [English version]Contents:
Loudspeakers position in rectangular room
Setting up speakers in a rectangular room Locating of first reflections points Helmholtz slot Resonator Acoustic lowfrequency conversion absorber Acceptable Room dimensions according to EBU/ITU Schroeder diffuser calculator Room dimensions in compliance with international standards Axial room modes calculator Simplified analysis of axial room modes Reverberation time calculator It’s necessary to optimize the room’s acoustic characteristics according to definite terms in order to achieve high quality sound reproduction. This is achieved by forming “acoustically correct” geometry of the area, and also by using special acoustic treatment for inner surface of the walls and ceiling. It’s a common thing to deal with a nonchangeable form room. In these cases, natural resonance of the room can extremely negatively affect the equipment’s sounding. Correct mutual position of acoustic systems, enclosing constructions and listening zone can be an important tool to help reduce room resonance and achieve realistic sound stage. In order to correctly place acoustic systems in a rectangular symmetrical area, we recommend using acoustic calculators for each of the three typical cases:
Using the calculation results in practice will help to reduce room resonance influence, improve tonal balance and make sound stage more realistic. Calculations methodology is based on stereophonic principle of loudspeakers placing, recommendations on implementing “Golden ratio” principle by George Cardas, and general theory of psychoacoustics. Find out more about this methodology from article "Placing loudspeakers in a rectangular room".
Manipulating the intensity of early reflections allows to improve sound stage quality and to make AC sounding more detailed and vivid. The most important are those early reflections off the areas, situated on sidewalls and the ceiling between the listening zone and AS. Furthermore, reflections off rear wall can affect sound quality if the listening zone is situated too close to the wall.
It’s recommended to place soundabsorbing materials or sounddispersing constructions (acoustic diffusers) at places with areas of first reflections. Acoustic treating must be suitable for frequency range, where acoustic distortions (plicate filtration effect) are observed the most often.
Line sizes of applied acoustic coating should be 500600 mm less than areas of first reflections. It’s recommended to agree the parameters of necessary acoustic treating with acoustic engineer in each definite case.
Acoustic slot absorber (Helmholtz Resonator) Slot Helmholtz Resonator is a vibrating singleDOF system, so it has the ability to respond to one definite frequency, corresponding to its own frequency.
Helmholtz Resonator’s distinctive feature is its ability to perform own lowfrequency vibration, where wavelength is much bigger than the resonator’s size.
This Helmholtz Resonator’s ability is used in architectural acoustics while creating so called slot resonator absorbers (Slot Resonator).
In general, absorber’s construction is a timber framing erected on a wall or a ceiling surface. A set of slats with slots between them is put on the framing. Inner framing space is filled with soundabsorbing material. Resonant frequency of absorbing depends on slats cut, framing depth and insulation absorbing effectiveness.
It’s possible to calculate resonant frequency of such construction with the help of evaluation formula:
fo = (c/(2*PI))*sqrt(r/((d*1.2*D)*(r+w))), where
w  slat width,
r  slot width,
d  slat thickness,
D  frame depth,
с  speed of sound in the air.
If you use different width slats and adjust them to different slots, or use a frame with variable depth, it’s possible to build an effective absorber to work in wide range of frequencies.
Helmholtz Resonator construction is simple enough and may be made from nonexpensive and available materials in a music room or a studio while performing building works.
Acoustic lowfrequency conversion absorber (LFCA) Panel conversion absorber is quite popular means of acoustic treating for critical listening rooms due to its simple construction and rather wide range of absorbing diapason in the area of low frequencies. Panel absorber is represented by hard resonatorframing with enclosed air volume, which is hermetically covered with flexible and massive panel (membrane). Plywood or MDF with glued inner layer of damping material are used as membrane material. Effective soundabsorbing material is put into the frame inner space. Sound vibrations motivate the membrane (panel) and the connected air. In this case kinetic energy o the membrane is converted into heat energy due to inner loss in membrane material, while kinetic energy of the air molecules is converted into heat energy due to viscous friction in soundabsorber. That is why we call this type of absorber conversional.
Absorber is represented by a system massspring, due to which it has resonant frequency, which makes it work the most effective. Absorber can be tuned to desirable diapason often by changing its form, volume and the membrane’s parameters. Precise calculation of panel absorber resonant frequency is a complex mathematical problem, where the result depends on big quantity of initial parameters: fixation way of the membrane, framing construction, absorber’s characteristics, etc. Nonetheless, using some admissions and simplifications allows achieving acceptable practical result. Let’s consider that a membrane with surface density m is fixed on a framing with depth d and they vibrate as an integral unit with no surface deformation. In such case, resonant frequency fo can be described as follows: fo=600/sqrt(m*d), where
m – membrane surface density kg/sq.m
d – framing depth cm
This formula is fair in case, where absorber’s inner space is filled with air. If you put porous soundabsorbing material, processes in the system quit being adiabatic and the formula transforms into the following:
fo=500/sqrt(m*d)
Filling inner volume of the construction with soundabsorbing material reduces Qvalue (Q) of the absorber, which leads to its working diapason extension. It’s possible to achieve bigger inner loss and enhanced absorbing effectiveness by putting thin, but massive layer of tough and flexible material. Theoretical working diapason of panel absorber is in +/ one octave relatively to calculated resonant frequency.
It’s necessary to note that in most cases (simple homecinema or studios treatment) the mentioned simplified approach is quite enough. But sometimes the solution of highlevel acoustic problems requires more precise definition of panel absorber resonant characteristics, taking into consideration complex mechanism of the membrane flexural deformations. This requires holding/using more precise and rather lengthy acoustic calculations.
Acceptable room dimensions according to EBU/ITU, 1998 recommendations This calculator helps to work out an acceptable proportion between studio line sizes, control rooms and musicrooms from point of view of reducing the impact of lowfrequency resonances.
It’s based on a methodology, created by Robert Walker in 1993, after a series of researches, held in Research Department Engineering Division of ВВС. As a result, a formula has been introduced to regulate the balance of room’s line sizes in quite wide limitations. In 1998 this formula was accepted as a standard by European Broadcasting Union (European Broadcasting Union, Technical Recommendation R221998) and International Telecommunication Union (International Telecommunication Union Recommendation ITUR BS.11161, 1998), also, it was recommended for use while building studios and musicrooms. The proportion is the following: 1.1w/h <= l/h <= 4.5w/h  4, l/h < 3, w/h < 3
where l – length, w – width, и h – height of the room.
Besides, it should exclude integral balance of the room’s length and width with its height in bounds of +/ 5%.
All sizes should correspond to the distances between the main enclosing constructions of the room.
Schroeder acoustic diffuser Schroeder diffusers, which construction is based on numbers theory, are an excellent tool for receiving diffuse sound field in different purpose rooms. Actually, Schroeder diffuser is a diffraction grid, which diffuses sound energy in wide range of frequencies, which falls to it, even at big size of falling angle.
Schroeder diffuser consists of series of cells with different depth but equal width, made in a timber, MDF or other plate framing. Typical diffuser construction cut (p=7) is shown in the picture on the left.
Diffuser’s construction is based on mathematical sequence of quadratic deductions from the numbers theory, which is evaluated as follows:
s_{n} = n^{2 }*mod(p), where
s_{n }– sequence of meanings of diffuser cells apparent depth,
n – nonnegative number {0, 1, 2, 3 ...}, determining the number of corresponding cell,
p – prime number {2, 3, 5, 7, 11, 13, 17...}. (Prime number is the one which is different from 0 and 1 and is divisible by 1 or itself)
The cell’s actual depth dn in the diffuser’s construction depends on its project frequency meaning for:
d_{n} = s_{n} * с /(f_{o} * 2 * p), where d_{n} –the depth of cell n,
f_{o} – diffuser’s project frequency,
с – speed of sound in the air,
p –prime number (diffuser order), corresponding to the quantity of cells.
Schroeder diffuser has the highest effectiveness of diffusing sound energy at the project frequency. Working frequency diapason depends on line sizes of the diffuser cells. The bottom of diffuser’s working diapason (flow) depends on the deepest cell size. The high bound of working diapason (fhigh) depends on cell width meaning.
The diagram of sound energy dispersion at Schroeder onedimensional diffuser is semicylinder. Room dimensions in compliance with international standards The influence of main resonances in a smallsize room often leads to increase of reverberation time and frequency characteristics irregularities, which in its turn often leads to sound coloration. Problems may occur at low frequencies due to comparatively low mode density.
Studio and musicroom designers try to solve the problem by using rooms of relevant sizes, and also by situating listeners and loudspeakers in definite places. There have been introduced a lot of approaches to find optimal proportion in room sizes during recent decades. Mostly, these methods are aiming to avoid the cases where repeated modes are situated in narrow band. The major assumption states that if there’s music playing in a room, lack or boost of definite tone elements will reduce the sounding quality and affect tone structure. Usually, the starting point for these methods is defining modes in an enclosed rectangular room with solid walls. Richard Bolt (Richard H. Bolt, 1946) introduced a design chart, which allows defining preferred proportion of the room walls. Bolt was studying sound behavior in a camera, where enclosures in size were comparable with wave length. He introduced such group of proportions between height to length, and length to width, where modal frequencies were distributed sufficiently. Louden (M. M. Louden, 1971) calculated modes distribution for lots of proportions of room sizes and published list of preferable sizes, based on one control chart. Due to this method the wellknown proportion 1:1,4:1,9 was received. Louden made researches over 125 combinations of room sizes balance with step 1,0. Robert Walker (Robert Walker, 1996) developed the criteria of room quality, based on calculating rms distance between mode frequencies. This method allows receiving range of practical and nearly optimal room sizes. In 1998 Walker’s formula was taken as a standard by European Broadcasting Union (EBU) and International Telecommunication Union (ITU). Trevor Cox recommendations (Trevor Cox, 2004) are based on working with a room with the most even mode frequency characteristics. Computer algorithm is used for multiple seeking for the best room sizes. Similar seeking is used while defining the best placement for sound source and sound detector. All the given methods, this way or another, have their own limitations. However, applying them in practice helps to identify the diapason of acceptable room proportions, avoid the worst cases and minimize the influence of room modes on tone balance and tone quality. Axial room modes
Each room has its own acoustic resonances or, as they are also called, room modes. Room proportions, e.g. length, width and height proportions set the position of room modes in frequency spectrum, and also the density of their distribution. Sizes as they are define the frequencies, where resonances take place, i.e. whether important frequencies will be enhanced or suppressed. When a perfectly rectangular room has perfectly smooth reflecting surfaces (walls, ceiling and floor), these resonances can be easily calculated according to the following wellknown formula:
where n_{x}, n_{y} and n_{z} – are whole numbers, and L_{x}, L_{y} and L_{z} – are relatively length, width and height of the room.
Для вычисления всех мод необходимо перебрать все возможные комбинации из трех целых чисел Nx, Ny, Nz. На практике же достаточно вычислить только низкочастотные моды, т.е. ограничиться максимальным N=4.
It’s necessary to try all the possible combinations of three whole numbers Nx, Ny, Nz. in order to calculate all the modes. In practice, it’s enough to calculate only lowfrequency modes, i.e. to confine to the maximal N=4.
f (1,0,0) = c/2/L
these modes are called axle or axial and, as a rule, are the most intense.
Tangential modes emerge when sound wave is repeatedly reflected by the four surfaces, where pairs are parallel to each other. These modes can be calculated by combining two whole numbers and zero. For instance, array (1,1,0) allows to calculate a firstorder mode in xy (lengthwidth) dimension. Such stationary modes emerge between the four walls of the room and are situated in parallel with the ceiling and the floor.
Oblique modes emerge between all the six walls of the room. As mode energy accumulation takes place during each sound circle (a wave here locks itself after six rereflections, losing part of its energy each time) oblique modes affect the sound less than any other stationary wave. It’s possible to calculate frequencies enhanced by these modes if you combine the set of three (1, 1, 1 – oblique mode of first order).
Axial modes are, as a rule, the most intense and you can ignore the influence of tangential and oblique modes while giving rough estimates for room resonances distribution.
Colorations largely determine the quality of sound for a small studio or listening room. The big task then is to determine which, if any, of the hundreds of modal frequencies in a room are likely to create colorations.
Reverberation time calculation Reverberation time (T, s), a concept introduced by W.C. Sabine at the end of 19th century, remains one of the most important acoustic properties of the room. Standard reverberation time is defined as that time required for the soundpressure level in a room to decay 60 dB after a source stops generating sound. It characterizes degree of boominess of the room. Т (sec) = 0,163*V / (−ln(1−α)*S + 4*µ*V)
V – volume of the room, m3 Calculated value of reverberation time is graphically compared to recommended (optimum) values are in compliance with international standards:
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