Acoustic room treatment

by Bruno Brozovic

Today I will give you some basic information about acoustic room treatment so you can enjoy better in your music listening.

Acoustic Room Treatment – Intro

Sound is something that surrounds us. Most people are not even aware of the acoustic properties of the space around us, offices, workshops, shops, stations, and even underground garages have acoustic properties, some of which are extremely acoustic: cinemas, theaters, etc. while in some like the train station sound loses definition to speech recognition. Large empty corridors and stairs that mostly have an echo and workshops that are so polluted with noise that they can even permanently damage your hearing. In order to convey the concept of acoustics, sound, and finally the acoustic treatment of the auditorium and the placement of the speakers, we must return the whole story to the very beginning.

Acoustic Room Treatment

Acoustic Room Treatment – What Is Sound?

What is sound? Sound is the energy that is created when a particle moves due to a certain excitation and transfers its energy to the next particle by its inertia, which creates a sound wave by the domino effect by the excitation of the next particle. This means that in order to hear something, a source must expel a certain amount of energy, which is then usually transmitted to our ear by air as a medium (it can be water or some other substance).

Acoustic Room Treatment – Something About Hearing

Hearing is our most sensitive sense. Man can hear frequencies of 11 octaves with a dynamic range of 140 dB. In cooperation with the brain, we have the most accurate, most sensitive microphone and frequency analyzer, but with all its perfection, it is very easy to confuse and damage. The ear is not equally sensitive at all frequencies, it is most sensitive in the range of 350 – 3500Hz (human voice area) where it is able to notice a difference of 10 * 10-6 pa (pascal) while the maximum sound pressure it can withstand without damage is 100 pa. Depending on the age, we hear sound from 16 Hz to 20000Hz, but in an as yet unexplained way we “feel” higher and lower frequencies.

What is hearingNoise is an unwanted sound that disables communication, disrupts concentration, and impairs hearing. Noise as such is undesirable in auditoriums and an effort is made to minimize the impact of ambient noise and the room in which the music is listened to.

On the other hand, the human ear can judge sound as simple or complex. Simple sounds are composed of only one frequency and do not exist in nature. Complex sounds are those that, in addition to a larger number of fundamental frequencies, also have multiples of the fundamental frequencies or higher harmonics that give the tone color and which can differ even when the fundamental frequency is the same. If the number of frequencies and higher harmonics is very large and densely distributed, our ear hears the noise. Noise does not have a specific frequency and is divided into white and pink. White noise is the noise that has a uniformly distributed acoustic energy, while in pink noise the spectrum decreases by 3 dB per octave. Pink noise is used to play components and to test speaker resonance (especially in the treble range).

Sound Physics

To understand what happens when listening to a musical composition, we must also know that sound propagates through space like a wave and is thus limited by pure physical guidelines on the propagation of waves through space. At low frequencies, the speaker diaphragm has a large shift so that the low-frequency waves have large amplitude and are often longer than the listening room itself. A 20Hz frequency wave extends over 18m, a 200Hz wave is slightly shorter than 2m while a 2000Hz wave is shorter than 20cm. Thus, it is determined that lower frequency waves propagate in a circle throughout the room and the orientation of the source is lost (so it is difficult to locate the sound source in subwoofers), while with increasing frequency waves become narrower and more directional and easier to locate. In an acoustic deaf chamber (a room designed to absorb and scatter all sound waves so as to minimize the impact of space on the propagation of sound waves) there are no problems with sound waves but the problem is that no one has a deaf chamber at home.

Most Common Problems With Room Acoustics

Most problems in home listening rooms are caused by reflected waves, which collide with other waves of the same frequency and the same phase, causing standing waves. Standing waves can be detected when listening as unnaturally elevated and blurred bass and as a loss of individual frequencies. It is impossible to completely suppress such phenomena, but it is possible to reduce them to satisfactory frames by choosing a room whose size (volume) and ratios between wall, floor, and ceiling surfaces favor a more correct response of certain frequencies in the music section. The calculations themselves are dealt with by many experts in the field of acoustics, and some theories have proven to be quite effective in combating negative phenomena. Most of these theories are implemented in studies around the world and are very effective, but when the whole problem is transferred to the home auditorium, it is concluded that no theory is fully consistent and difficult to apply in normal circumstances, more precisely, each of the theories there is some parameters that suit the home auditorium while others do not. The golden middle is a mix of several recognized theories from which it takes the parameters that best suit real home headphones.

Basic Guidelines Derived From The Presented Theories Of Room Acoustics

  • It is useful to enter a small angle (about 5 °) between opposite parallel surfaces, which eliminates the flute echo of the room (not very effective in smaller rooms).
  • Louden calculated 216 modes for 125 different room ratios and came to the conclusion that the ideal room ratio is 1: 1.4: 1.9 regardless of room volume.
  • In some recent studies, Bonell has concluded that an ideal ratio cannot be obtained without knowing the volume of the room that affects the frequency range and coloration, its ratio is 1: 1.25: 1.60 for rooms of 60m ^ 3, and for larger spaces ratio of 1: 1.45: 3.27.
  • The most ideal is a combination of these modes and the use of attenuation which reduces the excitation of the modes and their resonant peaks.
    Since we are usually not able to influence the dimensions of the rooms, we only have to supply the existing auditorium with the necessary acoustically treated materials by measuring frequency responses, which will absorb, reflect or scatter critical frequencies in space and reduce unwanted colorations to acceptable limits. Acceptable frames are a very subjective concept, the most important parameter in the acoustic treatment of a room is a purely subjective “sense of space” that defines the overall absorption of the room itself. High absorption will cause a lack of reflections of elemental noises which will eventually lead to discomfort when listening. In principle, values ​​of less than 0.3 seconds are avoided, unless reflections of surfaces near the listener are not realized. IEC and DIN standards recommend a typical response time of 0.4 +/- 0.05 seconds at medium frequencies from 250Hz to 4000Hz with a maximum value of 0.8 seconds at low frequencies in a room of approximately 80m ^ 3. These standards also serve as speaker constructors as a standard for testing structures in real conditions. These data do not apply to sound studios that try to numb the room as much as possible, and therefore require special equipment that will be able to fully realize the musical spectrum, which is much slower and requires much more power. In such spaces, speaker monitors must realize high power and sound pressure, which often goes from 120-130dB to 30Hz or lower. This same reflection, which studios try to suppress at home, often favors the formation of false “depth” images that occur when early reflections of the back and sides of walls fuse with the direct sound and cause a feeling of deepening of the sound image. In most cases, the listener is not even aware of how the false stereo image was created but attributes the “improvement” to the connecting lines or other factors.
  • The optimal layout of absorbent and reflective surfaces is such that each surface should have a similar absorption coefficient, and in places of maximum pressure (at the edges of the room) the most optimal position is the placement of acoustic panels.

How To Realize This In Practice?

In the most of the listening rooms are mostly plaster walls, painted or with wallpaper, parquet floor, or carpet. In order to prevent strong reflections at mid and high frequencies from the wall behind the listener, it would be good to place on it a heavy curtain or shelf full of books that are unevenly arranged. It would also be good to place a heavy curtain on the side walls, but if this is not practical, a different wall finish can be applied (decking, Bavarian plaster, façade brick, wall cladding with a lower degree of reflection than ordinary wall) or even simpler with side walls. walls set furniture and paintings with thick frames.

Absorption treatment behind the speaker depends on the taste of the listener, if you like a fake spatial and depth image leave that space without any treatment, but if you like a precise and accurate stereo image greatly dampens the walls and floor behind the speaker. Exceptions are electrostatic or panel speakers, which must be slightly more moderate in absorption, which should be a factor of 0.25 at mid frequencies.

Before any acoustic treatment of the room, it is necessary to determine the position of the speakers and listeners. Speakers in the very corner of the room should be avoided, the listener should be as close as possible to the wall opposite the speaker, and never in the middle of the room (as is the case in home theater systems). Wherever possible it would be good to place the speakers at equal distances from the left and right wall. If the room is longer than wider, the speakers should be placed against a longer wall and spaced approximately as wide as the room. For an accurate stereo image, it is necessary to keep the acoustic bisector along the center axis and the speakers should be directed and positioned so that there are no physical barriers between them and the listener that could affect sound propagation and cause possible reflections due to speaker proximity and position.

Another thing that is very important for quality reproduction is the music itself and its dynamics. Musical dynamics is the ratio of volume between the quietest and loudest part of a work. The greatest dynamics are achieved by large orchestras with dynamics exceeding 70dB. When a listening space noise of about 30dB is added to a musical work in a quiet room, it is clear that for a quality reproduction of such a work, the music system should deliver approximately 100dB.

A Word Or Two About Speakers

Here we come to one of the most important links in sound reproduction, speakers. Each speaker is limited by a certain sensitivity due to its construction and quality of workmanship. The sensitivity of the speaker is displayed in dB/m/W, which means that a speaker sensitivity of, say, 90dB/m/W stimulated by an electrical signal of 1W at a distance of 1m will produce a volume of 90dB. With the addition of another speaker, its dynamics will increase by 6dB, and with each doubling of power the volume increases by 3dB, so with 2W it will be 93dB, with 4W 96dB, etc. With each meter of distance, the power level drops by 3dB.

Optimal Speaker Placement


It follows that in order to be able to reproduce a musical work of dynamics of 70dB in a quiet room with a speaker sensitivity of 90dB/m/W at a distance of say 4m (average listener distance) we must supply it with 256W of electricity to deliver 102dB of music at that distance forces. It is important to note that most speakers do not have such a high sensitivity, usually, it is from 85dB to 88dB, so you need a stronger amplifier for quality reproduction of music.

Acoustic Room Treatment – Conclusion

In addition to all of the above, the conclusion is that in addition to the speakers, amplifier, electronics, and cables, the room in which the music is listened to is very important. By improving the acoustic treatment of the room, we can significantly improve the quality of the performance of a musical work, much more than with a much more expensive system in a low-quality or even no acoustically treated room.

Whether I want to admit it or not, the room is one of the most important components on the way to quality and faithful reproduction of music, and very often its impact is minimized and unjustifiably neglected !!!

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