Room Acoustics and Reverberation Time - Essay Example

? Room Acoustics (Insert Room Acoustics Introduction Room Acoustics refers to how sound behaves in an enclosed space; more specifically, in a room. People have the general perception that the speakers in our stereo or home theatre systems are the final sound link in the whole sound chain. In other words, people believe that the speakers in the music system make the biggest difference to the sound that reaches our ears. But from a sound engineering point of view, the placement of speakers and the sound that proceeds from them in a stereo and home theatre setup is but partly responsible for what we hear. What many people do not know is that the room in which the sound has been set up determines to a certain and large extent the final sound that one hears from the sound system. Room acoustics looks at how the room as a component of sound can be structured in order to improve the dispersion of sound in a room (Rossing, p.303). Sound in any room is transmitted or reaches the listener in two distinct ways. The first way in which sound is transmitted is directly from the speakers. This is referred to as direct sound. The second way in which sound reaches the listener is through reflection of sound. The reflected sound is referred to as indirect sound. In a studio setup, the indirect sound is as a result of the direct sound being reflected by various surfaces in the studio or in the room. For example, sound can be reflected by walls, floors and the ceiling. From this description, it can be derived that the sound one hears in any room is a combination of direct sound, directly from the speaker(s); and indirect sound - the sound from the speakers that is reflected. Indirect or reflected sound can be or is both good and bad. It is good because it amplifies sound to sound much fuller and louder. A simple experiment which can be used to prove this phenomenon is by placing speakers in an open field where there are no walls or barriers that can reflect the sound. In this experiment, the speakers will sound less powerful simply because the sound is directional. The bass of the speakers seems to be dull. However, when the very same speakers are placed in a room, the reflection adds some spaciousness to the sound that is projected from them. Reflected sound on the other hand is bad because it can lead to sound distortion. If the room is structured poorly, then there is a possibility that it will amplify certain sound notes which will mute others. Assuming that there is video footage that is accompanying the sound, the sound may be distorted and there may be a discord between the image and the sound reaching the ear because the sounds reach the ear at different times. Room Acoustics are more important when building a recording studio as compared to the basic stereo or home theatre setup. The main reason why it is important is because any reflected sound that is distorted can compromise the quality of the sound, or music that is being recorded. It is therefore imperative for producers to have an understanding of how they can perfect the recording studios and structure them in a such a way that sound is propagated in the right amount. In other words, the surfaces of the studio should be built in a way that diffuses sound as opposed to completely reflecting it or absorbing it. The surfaces should therefore be made of a material(s) that is both reflective and absorptive, the result of which is a diffusive surface. Reverberation Time Reverberation can be described as the collection of reflected sound in an enclosed room, or auditorium. It can also be described as the persistence or sustaining of sound in a room after the sound has been produced. Reverberation time (RT60 or T60) on the other hand refers to the time taken for the intensity of produced sound to reduce by 60 dB from its original level. Reverberation time is a very important parameter in describing the acoustics or acoustic behavior of a room (Goldstein, 2007, p. 305) and it is affected or dependent on two factors. One of the factors is the volume of the space and the other is the sound absorption characteristics of the space. Sound absorption is measured in metric Sabins. Different surfaces which include walls, floors and the ceiling will absorb sound at different rates. The rate at which the different surfaces absorb the sound is entirely dependent on the material that has been used to build that surface. Since there are different material which can be used in building the surfaces of a room, one has to consider that they have different absorption rates. This rate of absorption is referred to as absorption coefficient. Absorption coefficient is a value between 0 and 1. The absorption power of materials is therefore rated on the basis of the absorption coefficient. When the absorption coefficient of a material is 1, then that material is regarded as completely absorptive. When the absorption coefficient value is 0, then it can be said that that material has no absorptive capabilities. For example, the typical absorption coefficient of a carpet is 0.35. Therefore if a carpet of 35 square meters is used to cover the floor, then the sound absorption of the floor can be computed as: 35 X 0.35 = 12.25 metric sabins. The formula for computing the reverberation time is as follows: V is the volume of the room provided in cubic meters while A is the total absorption of the surfaces of the room, that is, the walls, ceiling and the roof. The reverberation time differs depending on the room that is being used. Large audio rooms such as auditoriums and concert halls require to have a reverberation time of between 1.5 seconds and 2 seconds. For recording studios and conference rooms, the reverberation time should be below 1 second. It is therefore evident that the larger the room, the higher the reverberation time. The value 0.161 is a constant in the formula (Pierce, 1991). When computing reverberation time, it is important to consider several aspects of sound. These aspects or factors that need to be considered are reflection, absorption and diffusion. Reflection Reflection of sound is an aspect that helps us to locate or determine the origin of a certain sound. The reason why reflection is easily detected is because the human auditory system, which is also inclusive of the brain, is extremely sensitive. We do not just locate sound by some miraculous way. The brain is the main component of the auditory system that helps in locating the source of sound. It uses the timing difference between the original sound and the reflected sound to locate the source. This is the reason why it is even possible to locate the source of sound in the dark or with your eyes closed. Reflection of sound highlights two major factors that determine the eventuality of sound. The two factors are loudness and length of delay. If the reflected sound is too loud, or if the length of delay between the original sound and the reflected sound is too long, then the eventuality of the original sound is echoes. This explains why it is difficult in detecting the source or identifying the source of sound in a room that is highly reflective or in an open field, where the ground is the only reflective surface. Shifting focus to a recording studio, echoes are highly unpleasant. Room acoustics is important in designing a modern recording studio simply because it helps the designers to establish the right amount or rate of reflection of sound in the room. If the recording studio reflects more sound and leads to the formation of echoes, then it alters the whole recording process especially in the booth since the echoes will be received by the recorder. Sound reflections in any room can interfere with the sound either destructively or constructively based on the size, shape, or dimensions of the room. This section will focus on the adverse or destructive interferences that can be borne as a result of reflection in highlighting the importance of room acoustics in designing a recording studio. The first negative impact that can be realized as a result of sound reflection is standing waves. In any given structural design, there is always a pair or two of parallel surfaces. When sound is reflected back and forth between the parallel surfaces, standing waves are created. Sound waves are created along the three dimensions of a room. That is, the height, the width and the length. When the sound waves are produced, what happens is that there are regions of constructive and destructive interference of the sound waves owing to collision. This makes sound at different regions in the room to differ in terms of pressure or loudness. In the areas of constructive interference, the sound is louder and the sound is very low in areas of destructive interference. It is important to conduct a sound test during the design process of a recording studio. During this test, one can realize that the bass sounds are stronger near the walls and at the corners of the room. This is because the standing waves tend to recollect at these points in the recording studio. These standing waves are referred to as room resonance modes. There are different types of modes. The first type is the axial modes. These modes are as a result of reflection of sound between two opposite and parallel surfaces. The second type of room modes is tangential modes. These modes are formed as a result of sound being reflected from or by four surfaces. The final type of modes is oblique modes. These modes are formed when sound is reflected from six surfaces; that is, four walls and the floor and ceiling (Reese, 2008, p. 6). Determining the frequencies of the axial modes is important since it helps one to understand how the sound system and the room interact, being more keen on the bass notes that are 300 Hz and below. The formula that is used to calculate the axial room resonance modes is as follows: Where F is the frequency of the axial room resonance, C is the speed of sound and D is the distance between two opposite surfaces (Pierce, 1991). If for example the axial mode frequency of a room is 30 Hz, then the room will resonate strongly at that frequency. If the sound frequencies go below that, then it becomes problematic especially in a recording studio control room simply because the resonance will overwhelm the sound being produced. Though it will be possible to hear sounds that are below 30 Hz, the room will not be able to offer reinforcements or withhold sounds that are below a frequency of 30 Hz. From this, it is rather evident that room acoustics helps in the determination of the axial mode frequencies of a room. This way, a producer is able to set the sound in the room to frequencies that are higher than the axial mode frequency to avoid a scenario where the resonance of the room interferes with a recording session. The room resonance modes are determined by the dimensions of the room. Therefore for a recording studio to have the desired frequencies, then it is necessary that the designers use the formula to determine the dimensions of the room that will help in achieving the required frequencies. The smaller the room, the more its resonance modes will color bass response. The second negative impact to sound of sound reflection is flutter echo. Flutter echo is the ringing reverberation that remains after the direct sound has ceased. Flutter echo is caused when the surfaces of a room have a hard finishing, and they end up reflecting the direct sound. Flutter echo adds an unpleasant harshness to the midrange and treble sounds and consequently blurring the transients of the music. If a recording studio is therefore finished with hard surfaces that reflect sound, the outcome is flutter echo which affects the final sound of recording since the transients will be overpowered by the midrange and trebles. It is therefore very important for a designer to install sound absorbing or sound diffusing materials to or in walls when building a recording studio in order to avoid the occurrence of flutter echo. To reduce the flutter echo which is produced by the floor, the designer can install a permanent carpet. The ceiling is a surface that is parallel to the floor hence it has the capability of producing flutter echo. To avoid this, the designer should use acoustic ceiling tiles which are a perfect diffusing material of sound. Absorption & Diffusion Sound is a form of energy. During a recording process, there will obviously be some unwanted sound that is produced as a result of reflection. However, considering that this unwanted sound is a form of energy, it cannot be destroyed. Therefore the only way to eliminate it is by converting it in to a different form of energy. Most recording studios will use fiberglass to absorb this unwanted sound and convert it to energy. The fiberglass is a dense semi-permeable material. When the excess acoustical sound hits it, it converts it in to heat energy. However, some materials, as pointed out earlier, may have an absorption coefficient of 1. What this means is that all the acoustical sound incident to the absorptive surface will be converted to a different form of energy thus all the sound ends up being lost; both the necessary and unnecessary sound. This is where diffusion comes in to the picture. Diffusion eliminates the conversion of the excess acoustical sound because rather than absorbing it, it scatters it in the room. Therefore diffusion creates a rich surround sound. Diffusion is not necessarily reliant on the material that is used to cover the surfaces of the recording studio. It depends on the irregularity of a surface. However, the advancement of technology has led to the development of materials that act as both absorbents and diffusers. Mercury wedges are a good example of such material. The mercury wedges are placed at the rear wall of a recording studio to absorb the sound that is being projected from the speakers and also to scatter it across the room in a proportionate manner (universal-acoustics.com). Conclusion The above sections have highlighted the basics of room acoustics and how they apply to building a modern recording studio. This section will be summarizing the essay and assessing the importance of acoustic treatment of a recording studio in more depth. Acoustic treatment simply refers to the application of the theories discussed above in any room that has been built specifically for sound or in a room that needs to be modified to produce the required sound. Just like in any other scenario, the sound that arrives to the ears of the producer in a recording studio or the sound that is finally recorded is a mixture of direct sound from the speakers and reflected sound from the surfaces of the recording studio. This reflection of sound is very much necessary in the recording process but only if it is controlled. If uncontrolled, the eventuality would be bass notes sounding muddy; some being louder than others and others sounding as if they are of key; a lack of focus in the highs and mids; and vague stereo imaging. All of this translates to distorted sound. In many cases, designers or enthusiastic producers will muffle the surfaces of a recording studio with carpets. Carpets are an ideal absorbent and in addition to that, they are cheap to install. However, this is an unprofessional way of designing the studio. When one gets an understanding of room acoustics, s/he is able to understand that there is more to controlling reflected sound. When a recording studio is muffled with a carpet, what it does is it dries up the high sounds. As for the mids, the carpet only absorbs very little and absorbs no bass sounds whatsoever. Therefore when recording, what happens is that the final sound will be dominated by mid and low frequencies, both of which will overpower the highs which are easily absorbed by the carpet. At the end of it all, the recording is of poor quality. To ensure that the final sound is well balanced and controlled, a designer has to ensure that thickness of the absorbent is adequate enough to absorb the bass frequencies which will be produced in the studio. When this is achieved, then it can be said that the acoustic treatment has been fully implemented in the design. Another important aspect of room acoustics in the designing of a recording studio is the fact that it helps the designer establish the dimensions of the room to match the desired frequencies of the room resonance modes. When designing the rooms, a designer should ensure that the measurements accommodate a fair and even distribution of room modes so that reasonable and consistent bass responses are maintained. An ideal recording studio should not have two sets of spaces between walls that are identical or are a multiple of each other. This way, the room can support more room modes. In this respect, it is also important to point out that larger rooms are more ideal than smaller ones because they can accommodate more room modes. In conclusion, room acoustics are important in the building of a modern recording studio since they help the designer to fully implement the acoustic treatment when structuring the room and they also help in enhancing the product quality of the recordings since the sound produced and the sound reflected and diffused, are at an equilibrium. Reference List Goldstein, E. B. 2010. Sensation and perception. Belmont, Calif: Thomson Wadsworth. Pierce, A. D. 1991. Acoustics an introduction to its physical principles and applications. Woodbury, NY: Acoustical Soc. of America. Reese, P., & CASE, A. U. 2008.Constructing a small critical listening room for a regular classroom using readily obtainable materials and construction techniques. Thesis (M.S.)- : University of Massachusetts at Lowell, 2008. Rossing, T. D. 2007. Springer handbook of acoustics.New York, N.Y: Springer. Universal Acoustics. 2012. Acoustic Room Treatment Guide. [Online].Available at: [Accessed on December 13, 2012]. Read More