Lecture 04 - Acoustics PDF

Acoustic in indoor space Lecture-04 Topic - Acoustics PRECEDENCE EFFECT When a sound is reflected off a wall or other solid surface, the returning sound waves reinforce the original sound and the phenomena is called the precedence effect or Haas effect (1951) i.e., when a sound is...

Acoustic in indoor space Lecture-04 Topic - Acoustics PRECEDENCE EFFECT When a sound is reflected off a wall or other solid surface, the returning sound waves reinforce the original sound and the phenomena is called the precedence effect or Haas effect (1951) i.e., when a sound is followed by another sound separated by a sufficiently short time delay. First established by scientist, Joseph Henry in 1940. The precedence effect or the Haas effect is of considerable importance in architectural acoustics both for the natural reinforcement of live sounds and sound from reflecting surfaces If a reinforcing sound (even from loudspeaker) if is within the below mentioned time of the initial sound, then a clear understanding can be expected For speech, a delay of 25 msec For music, a delay of 35 msec For romantic music, delays as high as 50 msec. SIGNAL-TO-NOISE RATIO The degree to which noise inhibits intelligibility is dependent on the signal-to-noise ratio, which is simply the signal level minus the noise level in dB. i.e., a measure used in science and engineering that compares the level of a desired signal to the level of background noise Aingerand Strutt (1935) coined the ratio impression (Q metric) Q = Ed + Ee/ El + En Desired Q>1 where Ed = direct field energy (N m) Ee= early part of the reflected energy (N m) The higher the ratio, the better the signal El = late portion of the reflected energy (N m) quality En = constant noise energy (N m) Early-reflected sounds with the direct sound increases the apparent strength of the whole When Direct and early sound = Noise and reverberant sound (Q= 1) Aingerand Strutthad set the dividing line between early and late reflections at 1/16 second (62.5 ms) and set a lower limit of 1 for a satisfactory value of Q. ROOM FOR SPEECH Fundamental requirements in designing rooms for speech (Doelle, 1972) There must be adequate loudness -high direct field level. The sound level must be relatively uniform. The reverberation characteristics of the room must be appropriate. There must be a high signal-to-noise ratio. Background noise levels must be low enough to not interfere with the listening environment. The room must be free from acoustical defects such as long delayed reflections, flutter echoes, focusing, and resonance. Beyond 30 to 40 feet it is difficult to understand unreinforced speech Optimum volume per seat should be 110 cu ft (3.1 cu m) (80 –150 cuft)Less volume increases loudness and decreases RT for same area of absorption ACOUSTIC REQUIREMENT FOR CLASSROOM DESIGN For children specially primary section 100% intelligibility is desired. Children should also be heard to the teacher. Low noise level or low reverberation or both is desired Additional aid for students with hearing or language disability Avoid sources of noise –HVAC ducts, waste water lines Inverse square law: pressure reduces with increased distance Sound pressure decreases by 6dB for every doubling of distance ACOUSTIC REQUIREMENT FOR CLASSROOM DESIGN Critical distance is the distance at which the source sound pressure equals the reflected sound pressure Critical distance and then source sound decays. Source sound level drop in a classroom Reflected sound support from the walls / ceiling provides sufficient loudness The orientation of a reflective element is determined by the required coverage ACOUSTIC REQUIREMENT FOR CLASSROOM DESIGN For a Flat floor: Grazing attenuation of direct sound is observed ACOUSTIC REQUIREMENT FOR CLASSROOM DESIGN Absorbing panels for For a Flat floor: Grazing low frequency sound attenuation of direct sound is observed Critical distance is the distance at which the source sound pressure equals the reflected sound pressure and then source sound decays. Critical distance depends on Volume of room Reverberation time Beneficial reflections, preferably from overhead, should be designed Frequency of human voice is 600Hz to 4000Hz Human voice so absorbers should be porous in nature. Absorbing panels Classroom Plan at back wall ACOUSTIC REQUIREMENT FOR CLASSROOM DESIGN In a large flat-floored classroom, a platform of 1 ft (0.3 m) height can improve the sight lines significantly. RT = Height of ceiling (in feet) / 20α (α= absorption coefficient) (mid-frequency reverberation time can be considered) for RT = 0.6 sec and ceiling height is 10 feet, then desired α= 0.83 Reverberation time or RTx, is a metric which describes the length of time taken for a sound to decay by x dB from its original level CONCLUSION FOR CLASSROOM DESIGN Typical classrooms size 8 m wide by 10 m deep accommodates 30 to 40 students Ceilings in classrooms are low enough to add to early reflections Low frequency sound is of no importance in speech and hence appropriate absorbers may be added Reverberation times preferred between 0.4 to 0.8 seconds Intelligibility should be maximum (at least 80%) Reflective surfaces i.e. surfaces near source should not be covered. Make the side walls reflective surfaces in order to increase the signal intensity. OPEN OFFICE DESIGN Privacy in an open-office work environment can be achieved when careful arrangement of the furniture orientation of both talkers and listeners Partial height barriers of the correct type and correct height Highly absorbent ceiling and wall panels Masking sound should have the proper spectral content and level No reverberant field exists, particularly because of intervening barriers and highly absorptive ceilings. RESTAURANT DESIGN In restaurants or private homes, the noise may be generated by conversations other than those of interest. Objective is to talk comfortably across a table (1.2 -2m), but conversations not to be overheard by someone at a neighboring table (say 3-4m away) A normal conversational level = 70dB At Hard-surfaced restaurants, it is very difficult for hearing. Addition of absorbing materials can control reverberant noise. An area approximately equal to the restaurant ceiling area. PERCEPTION OF DIRECTION The perception of direction is controlled by two factors: o the interaural (situated between or connecting the ears the interaural plane)delay time between the ears o the level difference created by the interaction between the head and the ears The first ear receives sound a millisecond before the other ear Shielding provided by the head helps brain differentiate the loudness and hence the direction is perceived When two sounds arrive at a listener simultaneously, the louder sound determines the direction. RECAPITULATION Clarity of speech is our objective Three basic types of acoustical issues o Isolation from outside sound o Source sound and reflection o Internal absorption o Background noise Signal to noise ratio -Single source, Multiple source Direct sound can reach the audience further if the reverberation time is lower and the volume is lower CONFERENCE AND BOARD ROOM DESIGN Comes under medium live room, reverberation time (0.8-0.9 sec) Capacity determines the volume, architectural components of these rooms- size, shape is known. Seating arrangement determines the acoustical plan Usually oval / rectangular seating with chairs around is desired, it allows discussion with each other. Unreinforced speech is heard over distances up to about 35 ft or 12 m reflective ceiling at 3m from floor Can cause flutter add absorbers 3.5m Typical layout plan of a conference room CONFERENCE AND BOARD ROOM DESIGN Volume of room = 288cum 3m x 8m x 12m Area of ceiling absorber 8x12 -4x8 =64 sqm Area of wall absorber 2x3m x (8m+12m) = 120sqm Area of exposed carpet= 64sqm Total absorption = 0.52x64+0.17x120+0.14x64 =62.64 sabine RT = 0.16X288/62.64 = 0.735 sec CONFERENCE AND BOARD ROOM DESIGN SUGGESTED ACOUSTICAL MEASURES Reflective ceiling preferably at 3m height or lower for the central part Absorptive ceiling preferably at the edges of the ceiling to reduce reverberation time Sound absorptive treatment in side walls at least upto50% of wall surface area Cloth-wrapped panels, preferably with dense fiberglass or cork infill –porous absorber The chairs, curtains and human beings –act as absorbers Avoiding mechanical noise from fans, ducts that can mask useful sound Carpet as floor finish to stop foot sound SMALL CONFERENCE ROOM SMALL CONFERENCE ROOM LARGE CONFERENCE ROOM LECTURE HALLS- LARGE SPACE DESIGN Capacity: 50 -350 Caution: echo, flutters Area per person = 0.45 to 0.5 sqm Volume –80 –150cum / person Choice of room shape: Rectilinear -provides better frontal view drawback –flutter echo Fan shape -brings audience closer, drawback –sound concentration Appropriate use of absorbers Fan shape configuration brings the audience close to the platform seating layout should be contained within a 125◦ Rooms being large: the direct field should be augmented by strong early reflections from hard surfaces (within 35ms). Advantage of overhead reflections –source direction perceived same as actual Disadvantage: can mask source sound RAISING THE FLOOR Direct sound energy reaches the audience in different extent without any acoustical measure. Increases the critical distance i.e. direct sound reaches further Raising floor (Case 3) also gives a better view to the audience Case 2 and Case 3 helps o useful reflections o reduces grazing attenuation. Slope of 180–220 Suggested for bigger lecture halls A stepped or sloped floor Halls beyond 25m: hard to hear UTILISING THE CEILING UTILISING THE CEILING SIDE AND BACK WALLS Beyond critical distance direct sound becomes weaker and reflected sound reinforces it. Stage enclosure area must have reflective surface. Lateral reflections smear the perceived source direction hence absorbers to be added. Thus side walls should have absorbers to Absorbers on stop lateral reflection side walls Absorbers on back wall prevent delayed reflection Absorbers also control reverberation time Absorbers on back wall SMALL LECTURE HALLS SMALL LECTURE HALLS SMALL LECTURE HALLS LARGE LECTURE HALLS The ceiling can be a series of flat-stepped elements, which provide beneficial early reflections. Flat ceiling elements are both more practical to build and better for intra-class discussions LARGE LECTURE HALLS LARGE LECTURE HALLS Introduction to Auditorium Design SHAPES OF AUDITORIUM Strong side reflections are generated by wall surfaces of narrow rectangular rooms Narrow halls also yield low delay times for early reflected sound. Intimacy has the prime weightage of 40% (Beranek, 1962) Fan shaped surfaces bring audience closer, Accommodates more people in lesser distance Curved back wall may lead to sound focusing SHAPES OF AUDITORIUM For the same capacity of audience, a fan shaped auditorium will allow lesser distance between the source and the listener than a rectangular shape (d1