ECHOES IN MAN-MADE STRUCTURES

Transcription

1 ECHOES IN MAN-MADE STRUCTURES COMPILED BY AKINWUMI O.I ARC/09/7359 AND AKINTAYO J.A. ARC/09/7357 (GROUP 9) SUBMITTED TO THE DEPARTMENT OF ARCHITECTURE IN PARTIAL FULFILMENT OF THE COURSE ENVIROMNMENTAL CONTROL 111 (ACOUSTIC AND NOISE CONTROL) (ARC 507) LECTURER: PROF. OGUNSOTE DATE: JULY Page 0

2 TABLE OF CONTENT Abstract 1 Introduction..3 Key terms..4 Echo..5 Cause of echo 6 Echo in rooms/auditoriums..6 Controlling echo... 7 Case studies.. 9 References...14 Echoes in man-made structures Page 1

3 INTRODUCTION Hearing is one of the most crucial means of survival, and speech is one of the most distinctive characteristics of human development and culture. Accordingly, the science of acoustics spreads across many facets of human society music, medicine, architecture, industrial production, warfare and more. The word "acoustic" is derived from the Greek word ἀκουστικός (akoustikos), meaning "of or for hearing, ready to hear" and that from ἀκουστός (akoustos), "heard, audible", which in turn derives from the verb ἀκούω (akouo), "I hear". The Latin synonym is "sonic", after which the term sonics used to be a synonym for acoustics and later a branch of acoustics. Frequenciesabove and below the audible range are called "ultrasonic" and "infrasonic", respectively. The term Acoustics of a building describes the production and transmission of sound inside the building. The following are to be considered for the acoustic design of a building, auditorium, cinema, or theatres etc. The sound heard by the audience should be sufficiently loud in any part of the hall. The quality of the speech and music should not be changed anywhere inside the hall. There should not be focusing of sound in any part of the hall. There should not be any vibrations due to resonance. There should not be any other noise from other sources in the hall. Hence the need to study Echoes in manmade structures Echoes in man-made structures Page 2

4 KEY TERMS Acoustics: Acoustics is defined as the science that deals with the production, control, transmission, reception, and effects of sound. The term is sometimes used for the science of sound in general. It is more commonly used for the special branch of that science, architectural acoustics that deals with the construction of enclosed areas so as to enhance the hearing of speech or music Reflection: It is seen as the phenomenon of wave motion, in which a wave such as sound wave is returned after impinging on a surface. Diffusion: In Rough surfaces, sound is reflected in many directions, and such reflection is called diffusion. Absorption: Sound waves are gradually absorbed by the medium through which they are traveling. The amount of absorption is dependent on the frequency of the sound wave, with higher frequencies being absorbed faster than lower sounds. The type, humidity and temperature of the material also plays an important part. Diffraction: It is the property of wave motion in which waves (such as sound) spread and bend as they pass through small openings or around barriers. Diffraction is more pronounced when the opening or the barrier is similar in size to or smaller than the wavelength of the incoming wave. Reverberation: In terms of acoustics, it is the interpretation of the persistence of sound after a sound is produced. A reverberation, or reverb, is created when a sound or signal is reflected causing a large number of reflections to build up and then decay as the sound is absorbed by the surfaces of objects in the space which could include furniture and people, and air. This is most noticeable when the sound source stops but the reflections continue, decreasing in amplitude, until they reach zero amplitude. If so many reflections arrive at a listener that they are unable to distinguish between them, the proper term is reverberation. Reverberation is frequency dependent. The length of the decay, or reverberation time, receives special consideration in the architectural design of spaces which need to have specific reverberation times to achieve optimum performance for their intended activity. In comparison to a distinct echo that is a minimum of 50 to 100 ms after the initial sound, reverberation is reflections that arrive in less than approximately 50ms. As time passes, the amplitude of the reflections is reduced until it is reduced to zero. Reverberation is not limited to indoor spaces as it exists in forests and other outdoor environments where reflection exists. Reverberation Time: The time it takes for a signal to drop by 60dB is the reverberation time. RT60 is the time required for reflections of a direct sound to decay 60 db. Reverberation time is frequently stated as a single value, if measured as a wide band signal (20 Hz to 20kHz), Echoes in man-made structures Page 3

5 however, being frequency dependent, it can be more precisely described in terms of frequency bands (one octave, 1/3 octave, 1/6 octave, etc.). Being frequency dependent, the reverb time measured in narrow bands will differ depending on the frequency band being measured. For precision, it is important to know what ranges of frequencies are being described by a reverberation time measurement. Echo: An echo is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. Echolocation: It is the use of sound by some animals to perceive surroundings. By emitting sounds and listening for their echoes, animals are able to find prey, avoid obstacles, and navigate without using vision. Echolocation is used at night or in environments that are perpetually dark, such as inside caves, underground, or in the deep sea. Cetaceans (whales and dolphins), bats, birds, and shrews use make use of echolocation. Echoes in man-made structures Page 4

6 ECHO An echo can be explained as a wave that has been reflected by a discontinuity in the propagation medium, and returns with sufficient magnitude and delay to be perceived. Echoes are reflected off walls or hard surfaces like mountains and privacy fences. When dealing with audible frequencies, the human ear cannot distinguish an echo from the original sound if the delay is less than 1/15 of a second. Thus, since the velocity of sound is approximately 343 m/s at a normal room temperature of about 25 C, the reflecting object must be more than 11.3m from the sound source at this temperature for an echo to be heard by a person at the source. Sound travels approximately 343 metres/s (1100 ft/s). If a sound produces an echo in 2 seconds, the object producing the echo would be precisely that distance away (the sound takes half the time to get to the object and half the time to return). The distance for an object with a 2-second echo return would be 1 sec X 343 metres/s or 343 metres (1100 ft). In most situations with human hearing, echoes are about one-half second or about half this distance, since sounds grow fainter with distance. In nature, canyon walls or rock cliffs facing water are the most common natural settings for hearing echoes. The strength of an echo is frequently measured in db sound pressure level SPL relative to the directly transmitted wave. Echoes may be desirable (as in sonar) or undesirable (as in telephone systems). CAUSES OF ECHO A space with a tall ceiling is visually appealing and can feel very comfortable and spacious. However, high ceilings can also result in severe echoes in the space. The presence of this significant empty space promotes the bouncing around of sound waves that can make echoing a real problem. Secondly sound waves not absorbed will bounce around cause hearing distortion, henece the need to use soft absorbent materials and finishes. ECHO IN ROOMS/AUDITORIUMS The adverse effect of echo can be felt in auditoriums, theatres, big lecture halls, large concert halls etc in which this effect of echo has not been adequately tackled at the initial design stage. Sound produced in an ordinary room is somewhat modified by reverberations due to reflections from walls and furniture; for this reason, a broadcasting studio should have a normal degree of reverberation to ensure natural reproduction of sound. For best acoustic qualities, rooms are designed to produce sufficient reflections for naturalness, without introducing excessive reverberation at any frequency and without echoing certain frequencies unnaturally. In most cases, the acoustics of a room will be satisfactory if a proper balance between soundabsorbing and sound-reflecting materials is created. Troublesome echoes may frequently Echoes in man-made structures Page 5

7 occur in a room that otherwise has a proper overall reverberation time if the ceiling or a wall is concave in shape and is highly reflecting; in such cases, sound may be focused at a particular point, making the acoustics bad at that point in the room. Similarly, a narrow corridor between parallel reflecting walls may trap sound by repeated reflection and cause troublesome echoes, even though the overall absorption is sufficient. Attention must also be given to the elimination of interference. Such interference arises from the difference in the distances traversed by the direct and the reflected sound and produces so-called dead spots, in which certain ranges of frequency are canceled out. Reproduction of sound picked up by microphones also requires the elimination of echoes and interference. CONTROLLING ECHO Ceiling finishes: Installing acoustic tiles on a tall ceiling vastly improves the sound quality in a space that echoes due to the presence of high ceilings. It works by absorbing some of the sound waves thereby reducing reflected sound waves. Wall finishes: Floor finishes; Lay carpet or rugs on the floor. Just like hard, bare ceilings and walls, floors also bounce the sound waves around and promote echoes when they too are hard and empty. Replacing a concrete or terrazzo floor with carpet can drastically reduce echo in a space with tall ceilings. Since hard flooring surfaces are visually appealing, this may not be desirable. Still, an area rug or two can help reduce echo. Objects or furniture: Fill space with more objects. The more things you have throughout the space, the less the sound waves will be able to bounce around. Add furniture with absorbing soft materials or rearrange so that they are away from the walls and more toward the centre of the space to reduce echo. Objects on the walls: Much like acoustic ceiling tiles absorb the sound waves that cause echoes, objects hanging on walls have the same effect. Large paintings, tapestries, photographs and other objects add beauty and personality to a space in many cases, and they also help stop echoes from happening. The more you have hanging on the walls, the fewer echoes will be produced regardless of ceiling height or floor finishes Openings: Glass is among the worst surfaces for adding to echoes in a space. Glass windows should be covered with drapes this helps reduces the problem echo. Materials: Materials that help reduce echo include, Wall hangings or drapes Carpet or rugs Furniture Floor acoustic tiles Acoustic ceilings Echoes in man-made structures Page 6

8 Sound system design: By paying careful attention to the sound system design, we can improve intelligibility quite significantly. The important factors include: 1) Install accurate speakers which provide controlled dispersion angles and even coverage. The speakers must be selected and installed to provide accurate sound and project it evenly throughout the seating area of the space, and not into the ceiling where more echo is produced. 2) Use the right microphones for each application. By using microphones which are carefully selected for accurate sound reproduction and proper pick-up and proximity characteristics we can greatly improve intelligibility. There is nothing worse that muddy sounding microphones or speakers to destroy intelligibility. 3) Use a speaker delay system in large spaces. In larger spaces with front main speakers and fillin speakers part way back, or under a balcony Echoes in man-made structures Page 7

9 CASE STUDY CCE LECTURE HALL (A WING) The C.C.E lecture room (wing A) in FUTA is built for the major purpose of lecturing students. however, studies carried out, indicate both good and bad acoustics practices.general characteristics include. Floor finish Wall finish Ceiling finish FINISHES Terrazzo floor finish White emulsion paint Asbestos board Echoes in man-made structures Page 8

10 DIMENSIONS OF THE C.C.E HALL (wing A) Highest ceiling point 5.4 meters Lowest ceiling point 4.1 meters Width 16.8 meters Length 16.8 meters Good acoustics materials present in the hall Asbestos ceiling Soft upholstery Poor acoustics materials present in the hall Terrazzo floor Emulsion wall paint finish Wooden chairs and stool Glass walls Glass windows Echoes in man-made structures Page 9

11 Figure 5; showing ceiling, floor, wall and objects in the hall Figure 6; showing furniture, windows and sound system Echoes in man-made structures Page 10

12 Figure 7; showing speakers at the back, with furniture s Figure 8; showing furniture with hard wood Figure 7; showing terrazzo floor finish Figure 8; showing furniture covered with upholstery As indicated by the tables, pictures and drawings above, the high ceilings coupled with hard finishes and furniture s, it can be visually deduced that the Hall A in the C.C.E hall will be potentially unpleasant for lectures to correct this, the following has been proposed. 1. Cover exposed windows and curtain glass wall with draperies 2. Terrazzo floor can be covered with rugs or carpets. The use of acoustic floor tiles is highly advised. 3. Main speakers should be placed in front while support speakers should be placed at the sides. 4. Use a speaker delay system using digital delay system to "time align" the speakers is also recommended 5. Microphones which are carefully selected for accurate sound reproduction and proper pick-up and proximity should be used. 6. Speakers should be properly installed and avoid directing waves into the ceilings. Echoes in man-made structures Page 11

13 REFERENCES Arc 507 course Handouts on acoustics by Prof.O.O Ogunsote, Federal University of technology Akure, Ondo. Alectro Systems Inc: Space Acoustics A Major Problem Crutchfield: Space Acoustics Wall, Judy, Military Use Of Mind Control Weapons, USA, Nexus October / November 1998 Simmons, J., Vernon, J. (1971). Echolocation: discrimination of targets by the bat Hecht, Jeff, Not A Sound Idea, New Scientist, UK, 20 th March 1999 Yost, William A., Fundamentals Of Hearing, Academic Press, Inc., USA, 1994 Kientzle, Tim, A Programmer s guide To Sound, Addison-Wesley, USA, 1998 Madsen, Virginia, Notes Towards Sound Ecology In The Garden Of Listening, Essays In Sound 2, Technophobia, Australia, September 95 Echoes in man-made structures Page 12