29-January-2012

Acoustics - important part of your hi-fi system




The sound quality in the listening room does not only depend on the class of audio equipment. Significant influence has correct speaker position, the location of the listening position, size of the room, decorating materials and finally psychoacoustic ability to perceive and distinguish the sounds by the listener.

Unfortunately, the properties of the listening room are often omitted and most attention is placed on the highest class of equipment. In case of budget configuration, this approach usually works - further investments noticeably improve sound quality. With time, however, we reach the point where spending increasing amounts of money, accomplish less and less improvement in sound.

Buying expensive amplifiers, record players and speakers with more linear band, sophisticated speaker cables, interconnects and sophisticated power sources can bring only minimal improvement, if not dealt with the arrangement of the listening room.

All the problems associated with the acoustics are caused by reflections of sound waves from the walls, floor and ceiling. In rooms such as auditoriums or sports halls large distances between the walls and between floor and ceiling cause a big echo effect.

Reflective surfaces are extensive, so that a sound wave can be more completely reflect it expires. After simple test such as hands tap in this room can take up to several seconds before the mute the sound completely. It is not difficult to guess that listening to music in this room is not a pleasure.

In a typical living room or a dedicated listening room limiting the distance between the walls are too small to create a long echo. Most such areas are dominated by the reverberation in the region of medium and high frequencies. This can be discovered by walking around such room and clapping hands.

Unfortunately, much more serious problems occur at low frequencies, and these are difficult to determine.

In a medium sized furnished room critical distance is typically two meters. This is the distance measured from the line of speakers to the listening position where 50% of the sound reaching the listener comes from reflections.

These reflections cause the phase change, non-linearity characteristics of the frequency, aggressive highs, weak creation of virtual sources, and finally a low buzzing bass, which is lacking in the band and is poorly controlled. The further you sit from the speakers, the greater part of reflected sound causes even more serious distortion of the sound.


What are the main aspects of acoustics and how they influence sound quality

Echo is a set of repeated reflections between two parallel walls. It prolongs the disappearance of the sound and causes blurring of detail. It is determined by a parameter called the reverberation time (RT60).

Reverberation time is expressed in seconds as a time it is needed to hear weakened sound by 60dB. Reverberation time 60 in a room without acoustic adaptation ranges from 0.5 to 2 seconds, depending on sound frequency. The values recommended for a good listening room is 0.3-0.5 seconds.

A small value of direct to reverberant ration causes the degradation of the details. Particularly a combination of a small ratio with a long reverberation time is resulting in a significant degradation in shaping of the music scene (only direct sound provides information about the position of the virtual sources).

Early reflections are those that reach the listener delayed no more than about 10 ms compared to the direct sound. Early reflections, merging with the direct sound are causing a phase shift. This results in a number of holes in the frequency range. This phenomenon is called a comb filter.

Room resonances (standing waves) are repeated reflections of sound waves forming between two parallel walls. Resonances are responsible for modal ringing which is a clear boost of low frequencies: between any two surfaces there is a series of low frequencies, which are louder and longer than the low frequency band disappear. This results in poorly controlled bass. The bass is present but can not distinguish between different tones.

Another even bigger problem is a distortion of low-frequencies. Speakers - no matter how expensive - placed in an ordinary room without proper adaptation will reveal a series of increases and decreases in the frequency range.





Measurement of room in frequency range from 20 to 1500 Hz.

Low frequency waves reflecting from walls, floor and ceiling linked with direct sound will create huge distortions. When waves of opposite phase overlap they result in silencing frequency. Highest increases can reach of up to + 6 dB, biggest reductions can reach up to about -30 dB and this appears a middle and upper bass band.



How to improve acoustics

After identifying the main sources of our problems we have to resolve them.
The easiest way to improve the sound is to experiment with speaker positioning.
It is worth bearing in mind the two aspects:
The first is to place the columns in front of a solid wall, which is reflecting the kinetic energy and stop the bass inside the room. Without this solid foundation we will lose the bass.
The second is symmetry.
This applies both to the room and correct speaker placing.
In the room where on the right hand side we have a solid wall and on the left side we have passage into another room, it is impossible to obtain the correct music scene with a suitably located sources. Correct placement of the speakers can partially compensate tonal characteristics. Also, change in the listening position will affect the above-mentioned aspects.
Sharp high frequencies.

High tones contain the most detail. Higher harmonics also carry information about the definition and location of the instruments on stage. Speaker positioned towards achieving best bass will not always be the best configuration for medium and high frequencies.
Increase in high frequencies can be caused by lack of their absorption.
This is especially noticeable when floor is covered with terracotta or laminate flooring. In this case, even the location of the carpet in place of the first bounce from the floor can improve a lot. However, if there is still too much of height frequencies it will be necessary to increase absorption with more professional methods. For this purpose, walls can be covered with width range absorbers.





Greatest impact of the absorbers can be heard by placing them in places of early reflections from the walls. It must be remembered that this will also decrease the sound pressure of reflected waves and will result in more pointed-source apparent that not everyone has to like. Using absorbers should be done careful not to soundproof the room to much.
Aim of the acoustic adaptation is to achieve neutral not deaf room. Atmosphere and reverb are already recorded with our favorite music, the room should only ensure correct their restoration.


Scene backed up

Medium frequencies are the most critical and also the hardest to master.
Most of the acoustic instruments and voices are recorded in this area.
Medium frequencies are also responsible for sound energy and attack.
When this range is not clear quick drum hits are illegible, and sounds blend together.
Midrange improvement can be achieved using absorbers tuned to the appropriate frequency.
With it is easier to get flat frequency response of the room.




To flatten the frequency response and improve the creation of music scene, combination of diffusers and absorbers should be applied.
Diffusers will not only weaken the reflected wave energy, but also "detain" wave, which is delayed relative to the direct wave. This results in increased precision of creating music scene, both in depth and width. It also improves focus of virtual sources, shaping instruments better and more natural. Different depths of diffuser slots will cause different phase shifts of sound wave so that reflections are more stretched in time without loosing as much energy as using absorbers. This results in a more lively acoustics.



Boomy bass

Controlled and precise bass is the foundation of musical presentation. If something is wrong with it, the whole system will never play well. If the bass sounds correctly listening becomes fun. The ideal way to correct problems with this range in almost all areas is the use of so-called basstraps. These are the acoustical absorbers of low frequencies. At this point, many readers are probably wondering why I'm writing about bass absorption, since there is usually not enough and needs to be highlighted. Well, decreases in low frequency band are caused by direct waves overlapped by reflected waves. Bass Traps absorb low frequency energy causing better alignment of the bass characteristics.

There are several types of low frequency absorbers:
The first resonator is called Hermholtz-a resonator. It is tuned to a specific frequency. Usually it has the shape of a cylinder with a bass-reflex hole.
It is used as a separate system placed usually in a corner, but it may also have character of structure in the corner.
The principle of its work is simple. Sound waves cause movement of air in the bass reflex hole. It falls to the centre and loses its energy within the body.

Another type of absorber is a membrane resonator. It is a shape with front made of solid plywood, which absorbs certain resonant frequency. Resonance of such membrane can be tuned to a specific range of frequencies within the medium and high-bandwidth bass.




The last of the presented types of bass absorbers is called Tube Trap. This system has a cylindrical shape, but is not tuned to specific frequency. It absorbs bandwidth from 400 Hz down, ensuring of medium and high frequencies distortion.




(Low frequency absorber , which also distorts mid and high-frequency)


Ceiling

The last area that we discuss is ceiling. Often this is a flat, undeveloped space with a large surface area, which adversely affects the acoustics. There are several possibilities to adapt the ceiling. One of them is building a perforated plate. Increasing share of the entire surface of perforation plates will obtained higher absorption which in addition to echo cancellation will also improve the music scene.

If there is no need to increase the absorption, the omnipolare diffusers can be used. They disperse the sound waves in all directions so that the reflected waves have less energy (compared to the diffuser slot).






Summary

Most of the time you will not find listening rooms with expensive hi-fi systems without even a single sound absorbers. Listening to music in the room properly adapted is a wonderful experience. It's amazing to hear a powerful, low and controlled bass, remaining at the same level as the rest of the band as well as be able to experience all the details and extensive details on the music scene. Almost as if the musicians had materialized in the room and play just for us.
I hope that through this article encouraged readers to improve the listening room acoustics. You can start right away - finding the optimal settings for the columns in relation to the listening position.

Original article from HiFi I Muzyka posted by Miroslaw Andrejuk www.hi-fi.com.pl

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