Gustavo Pires (Vicoustic Technical Director)

 

 

Does room acoustics still have a strong role in designing a Home Cinema in the age of powerful technologies like Dolby Atmos? The answer is yes. Actually, if you are thinking in upgrading your system to the new and more powerful solutions which require more and more speakers distributed throughout the room, acoustic treatment becomes even more relevant to control sound reflections. 



 

The advent of Dolby Atmos technology allows up to 128 audio tracks and associated spatial audio description metadata. In result, sound moves around you since each ‘sound source’ can be precisely placed and moved in a three-dimensional space. Most systems use a soundbar or loudspeakers redirected to the ceiling and from there sound is spread to multiple places in the audience. This adds a new step forward to the necessity of acoustic treatment within a cinema room, because reflections in the walls and ceilings may affect the spatial audio distribution. To completely enjoy the innovation developed by Dolby Laboratories that is becoming common in home cinemas, it is vital you treat reflections in the room.

It’s true that some of the new powerful sound reproduction and signal processing tools already include acoustic correction features. Nevertheless, even considering the most efficient audio system, if the acoustics of the room is not well treated and considered since project day one, it is likely that the listener will not be able to totally enjoy and take advantage of the full power of the audio system. Ultimately, this will raise doubts regarding the investment made.

It is common nowadays to encounter Cinema Rooms and Home Theatres with moderately small dimensions. Such rooms are prone to have sound field anomalies related to room modes. These modes can cause audible effects in the room’s sound field at low frequencies, by originating areas with minimum pressure levels and areas with maximum pressure levels that can vary as much as 15 dB. This will naturally affect the listener’s correct perception of sound at low frequencies.

Furthermore, modern home theatres are increasingly considering the use of sub-woofers in order to reproduce low frequency components of the film audio tracks. The placement of the sub-woofers and the listening positions will determine how the room modes are excited and heard by the listener, affecting the perception of the material being reproduced. It is therefore crucial to also include some acoustic treatment to control the low frequencies within the Cinema Room.


The Goal

The goal in a Cinema Room or a Home Theatre acoustic design should be to provide the audience a neutral acoustic environment, in order to assure them a clear and complete film audio experience without introducing any acoustic distortions that could compromise its perception.

In other words, the home cinema listener should be able to clearly hear what has been recorded with minimum influence from external sources such as: i) room’s acoustics; ii) noise from mechanical sources (e.g. cinema’s HVAC systems); and iii) noise from sources located in theatre’s adjacent spaces. Furthermore, the noise generated within the cinema room should not be a source of nuisance to potential noise sensitive receptors located nearby.

To achieve this, there are mainly three areas where the acoustic design of a Home Cinema should act on: i) Improving sound insulation between home cinema and adjacent spaces; ii) Limiting internal background noise levels by controlling noise from mechanical sources such as HVAC; iii) Designing internal acoustic treatment in order to control Cinema’s reverberation time and avoid acoustic defects such as echo, flutter echoes, room resonances, etc. 


 

Sound Insulation

Generally, home cinema walls, ceiling and floor sound insulation performance should be designed taking into account: i) the background noise criteria selected for the home cinema; ii) the anticipated noise generated in theatre’s adjacent spaces; iii) the degree of noise sensitivity of the theatre’s adjacent spaces.

Based on these three subjects the sound insulation performance should be defined in terms of STC or Rw. Standard in situR’w values for Home Cinemas are typically between 50 dB to 65 dB, depending on the three subjects mentioned above.

Flanking issues should be well controlled in order to guaranty that the in situ sound insulation performance is not compromised. Attention should also be addressed for home cinema entrances. Usually, it is recommended the use of a vestibule in order to maximise the sound insulation between the home cinema and the adjacent spaces. 


 

Background noise

High levels of internal background noise may also compromise signal to noise ratios. Therefore, controlling noise from mechanical sources such as HVAC is considered essential.

Here are some noise control guidelines that should be taken into account: i) Locate all mechanical sources outside the cinema room (if possible); ii) Properly control both noise and vibration generated by the equipment: iii) Properly seal all penetrations over acoustically rating elements such as cinema walls, floors and ceiling. 

Standard acoustic background noise criteria for Home Cinemas are typically between NR25 to NR35.

 

Acoustic Treatment

Finally, in order to achieve a neutral acoustic environment within the home cinema there are three main areas where internal acoustic treatment should act on: i)Reverberation Time Optimization; ii) Early Reflections control; iii) Sound field anomalies control (room modes, flutter echoes, etc.). Usually the design of a home cinema will require a significant amount of acoustic absorbing panels.

There are several guidance providing RT criteria for cinema rooms. Typically criterion calls for very short reverberation times. Depending on the room’s volume, basic mid-frequency RT values (500 Hz) are recommended to be within 0,2 s to 0,6 s. There are also recommendations for RT changes with frequency, typically lengthening RT at low frequencies and shortening RT at high frequencies. 

A reasonable effort should be made to achieve these recommended values, since high RT values may result in detriment of the sound information being transmitted and consequently reducing speech intelligibility whenever dialog is present in the material being reproduced and, as we know, dialogues are of major importance in most films.

Early reflections should also be well treated. As mentioned, the goal is for the listener to be able to feel the ambiance and reverb contained in the film audio track. In addition, the listener should be able to clearly distinguish sound sources and locate them in the sound field. Therefore, the sound should reach the listener’s ears with very few reflections and remain “uncoloured” by the room itself. It is recommended to treat first reflections with sound absorbing panels, this will take energy from these early reflections and improve sound clarity, source location, etc. within the room. 

It should be noted that late reflections do not present the same issues mentioned for early reflections. They even might be helpful in order to avoid the room becoming too dead, as long as they are well controlled and without too much energy. Moreover, if the delay time of these reflections is within the human’s ear fusion zone, their energy will be perceived as coming naturally from the sound sources. Nevertheless, specular reflections should be avoided, and therefore it is recommended to treat these late reflections with sound diffusing panels, which will spread its energy by the room and will create a sense of spaciousness inside.

As a rule of thumb we should treat reflection in the following way: i) surfaces near the loudspeakers should be deadened, i.e. should be controlled using sound absorbing panels; ii) other surfaces should provide good diffusion; iii) specular reflections should be avoided.

 

The Vicoustic Solution 

Vicoustic has several innovative acoustic solutions both to address acoustic treatment and sound insulation issues. In addition, our project team has a group of acoustic engineers and audio professionals with a decade long experience. 

We can help you getting the best performance of your home cinema, with a detailed project that tells you what and where to install every acoustic panels, side by side with our own professional acousticians. 

 

For more information please visit our website