Room Acoustics

Room Acoustics - Room Acoustics Room Bouncing Around...

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Unformatted text preview: Room Acoustics Room Bouncing Around October 26, 2007 Music and Other Sounds Come from a source. The source is not isolated, it is in an The environment. environment. The environment can affect what the The listener will hear: listener 07/24/11 07/24/11 Ambient noise level Properties of the wall, ceiling, etc. Other sources producing sound at the same Other time. time. Room Acoustics Consider a Pulse of Sound WALL Reflection Sound Changes Different Travel Distances Many reflections occur at the same “time” 07/24/11 07/24/11 Room Acoustics Consider the following: 270 ft 100 ft ear source 250 ft d = vt t = d / v 500d / 1100 ft/s = ft. t1 = 500 / 1100 = 0.45 sec t 2 = 540 / 1100 = 0.49 07/24/11 07/24/11 ∆ = 0.04 s = 40ms Room Acoustics 200 ft Note Our ear will hear two sounds 40 ms apart Our as a but “rough” and, perhaps, as a fast echo. A room should be carefully designed to maintain a “pleasant” aural experience. maintain This will be our concern today. 07/24/11 07/24/11 Room Acoustics A wall Sends a “delayed” reflection of the sound Sends to the ear. to A matter of distance. The reflection may be synchronized with The the source so that they may “interfere” the The reflection may, be hindered by the The absorption of the sound energy by the wall. wall. There may be an echo. There 07/24/11 07/24/11 Room Acoustics Example - Interference “Wall Wall 07/24/11 07/24/11 Room Acoustics Also consider Wavelengths in music Note different wavelengths and compare with the size of a room. Wavelength will be an important variable in a room. 07/24/11 07/24/11 Room Acoustics Surfaces (Walls, floors, etc.) Rough or Smooth Hard or soft Location with respect to listener Characteristics depend on the sound Characteristics being detected. being 07/24/11 07/24/11 Room Acoustics Two surfaces 07/24/11 07/24/11 Room Acoustics IS THIS A ROUGH SURFACE??? 1 nm = 10-12 meters =0.000000000001 m 07/24/11 07/24/11 Room Acoustics Again, Consider a Wall How smooth is it? Smooth is in the feel of the feeler! Smooth or Rough are Relative terms. We define: 07/24/11 07/24/11 SMOOTH – Variations occur on a scale much SMOOTH smaller than a wavelength of the sound we are considering. are ROUGH – The variations in the surface are ROUGH comparable to the size of the wavelength. comparable Room Acoustics SMOOTH ROUGH 07/24/11 07/24/11 Room Acoustics SPECULAR DIFFUSE SOFT Walls A soft wall (like rubber or cork) will yield when soft you push on it. you Sound (music) pressure pushes on the wall. IF the wall deforms, than a force (pA) times a IF distance (the deformation), means that the wave does WORK. does The sound therefore loses some energy when it The hits such a wall. hits The reflection isn’t as strong as one from an “unyielding” wall. 07/24/11 07/24/11 Room Acoustics Consider an outdoor concert Musicians on stage People in the audience No Walls or Ceilings Only reflections possible are from Only structures in back of the musicians. structures 07/24/11 07/24/11 And possibly the ground Room Acoustics Useful aspects of reflection Think about the reverse! 07/24/11 07/24/11 Room Acoustics The old Greek Amphitheater 07/24/11 07/24/11 Room Acoustics 07/24/11 07/24/11 Room Acoustics Closer Audience “Band Shell” 07/24/11 07/24/11 Room Acoustics 07/24/11 07/24/11 Room Acoustics 07/24/11 07/24/11 Room Acoustics Care in a band-shell The focus can’t be too good because then The all of the performers need to be at the same place. same Since they can’t be, a vertical wall might Since be better. be Real Band shells look right but really do Real NOT properly focus. ON PURPOSE! NOT 07/24/11 07/24/11 Room Acoustics What does “focus” mean Sound waves hit a surface which can be called a Sound mirror. mirror. The mirror surface can be curved so that rays of The sound from different directions can be made to come together at the same place. come Like a lens In a concert hall, too much focusing can also In mean that there is only ONE good seat in the house! house! 07/24/11 07/24/11 Room Acoustics EXAMPLE: The Ellipse A & B = foci 07/24/11 07/24/11 Room Acoustics Whispering Gallery Note – This Wren design was actually a spherical surface that doesn’t really focus that well. It probably comes close to a portion of an ellipse. 07/24/11 07/24/11 Room Acoustics APPROXIMATION ?? 07/24/11 07/24/11 Room Acoustics 07/24/11 07/24/11 Room Acoustics Parabolic Reflector 07/24/11 07/24/11 Room Acoustics Parabolic Receiver 07/24/11 07/24/11 Room Acoustics What about REAL Rooms??? Rooms??? In a Real Room What about What the walls? the Smooth How Smooth? WALL Rough How Rough? How Transmission properties? properties? 07/24/11 07/24/11 Room Acoustics Another Factor RESONANCE 07/24/11 07/24/11 Room Acoustics Resonance Examples 07/24/11 07/24/11 Room Acoustics Speakers? 07/24/11 07/24/11 Room Acoustics At home with Shostakovich 07/24/11 07/24/11 If you can see it, you can hear it! Wherever you see your speaker reflected in the mirror, that's a point of reflection that should receive absorptive, or in some cases, diffusive acoustic treatment. Room Acoustics 07/24/11 07/24/11 Room Acoustics Diffraction Sound can “bend” around objects. Sound can change its properties Sound depending upon the size of the wavelength compared to objects. wavelength The Diffraction effect can be understood The via one of the early theories of waves. via 07/24/11 07/24/11 Room Acoustics A Bad Photo .. sorry ploop 07/24/11 07/24/11 Room Acoustics Huygen's Principle 1678 Polaroid Photo 07/24/11 07/24/11 Room Acoustics Huygen's Principle Every point on the front Every of a wave (wave front) acts as a source of spherical waves. spherical The next position of the The wave front will be the surface that is tangent to all of the other parts of the surface created in the same way. in The spherical wave The travels at the speed of sound. sound. vt 07/24/11 07/24/11 Room Acoustics Another View 07/24/11 07/24/11 Room Acoustics A Slit (Window) 07/24/11 07/24/11 Room Acoustics Diffraction Through a SMALL Diffraction Opening (comparable to λ ) DIFFRACTION 07/24/11 07/24/11 Room Acoustics 07/24/11 07/24/11 Room Acoustics An Edge 07/24/11 07/24/11 Room Acoustics Sound Travels in straight Lines. Travels in crooked lines. Can be focused. Can be absorbed by a surface Can be diffracted Can interfere “with itself” Is dependent on the properties of the Is room. room. 07/24/11 07/24/11 Room Acoustics What else? Small objects will scatter or diffract sound so it Small can be heard in non-straight lines. can Around edges, etc. Small objects do very little to long wavelength Small sounds (low tones). They are like the Eveready Battery … they keep going and going and going ….. ….. Higher frequency sounds will be deflected or Higher absorbed more than low frequency sounds. absorbed 07/24/11 07/24/11 Room Acoustics We discussed Reflections 07/24/11 07/24/11 Room Acoustics What Do You Think? 07/24/11 07/24/11 Room Acoustics Or a school performance hall 07/24/11 07/24/11 Room Acoustics Professional Concert Hall (mucho Dolleros ) (mucho 07/24/11 07/24/11 Room Acoustics Surfaces 07/24/11 07/24/11 Room Acoustics B le s aff Soft Walls 07/24/11 07/24/11 People??? Room Acoustics Note Modern halls are adjustable for 07/24/11 07/24/11 The piece being played The size of the audience Room Acoustics Create a SUDDEN Sound loudness time Listen & Record with a microphone 07/24/11 07/24/11 Room Acoustics Real Example: Royal Festival Hall 07/24/11 07/24/11 Room Acoustics 07/24/11 07/24/11 Room Acoustics Room Reflections Room full of sound! 07/24/11 07/24/11 Room Acoustics Room Full Of Sound Cut a small Window into the wall EACH SECOND THE SAME FRACTION OF SOUND WILL LEAK FROM THE ROOM LEADING TO WHAT IS CALLED EXPONENTIAL DECAY. 07/24/11 Room Acoustics Listen to the Room! 07/24/11 07/24/11 Room Acoustics Lets start a musical tone and listen to Lets the auditorium with a sound recorder. the 07/24/11 07/24/11 Room Acoustics How about the return to silence? There is a steady musical sound in the There auditorium. auditorium. The symphony is over. The music suddenly stops. It takes a The certain time for the sound level to get to a very small level. very The time it takes for the auditorium sound The to drop to 1/1,000,000th of the steady level to is called the REVERBERATION TIME. REVER 07/24/11 07/24/11 Room Acoustics The Return to Peace Reverberation Time Reverberation time is the time the sound takes to be reduced to one millionth of its initial level. 07/24/11 07/24/11 Room Acoustics More Absorbing Absorbing Materials 07/24/11 07/24/11 Room Acoustics A Formula NOT to be Remembered 0.16 x Volume of the room reverberation time = " effective" area of all absorbing surfaces 07/24/11 07/24/11 Room Acoustics Let’s try a calculation – Living Room @ Let’s 500 Hz (Book states this wrong) 500 3m 4m 07/24/11 07/24/11 Ceiling Area = 4 x 5 = 20 m2 Effective = 0.1 x 20 = 2m2 5m Room Acoustics Another Example 300 x 0.1 same 07/24/11 07/24/11 Room Acoustics Reverberation Times Desired 07/24/11 07/24/11 Room Acoustics For Music Rooms must be carefully designed. The Rooms “engineering” contains a lot of “Kentucky Windage”. Windage”. Different kinds of music require different Different acoustical designs. acoustical In the right room, you hear what the In composer intended you to hear. composer 07/24/11 07/24/11 Room Acoustics http://www.crutchfieldadvisor.com/ISEOrgbtcspd/learningcenter/home/speakers_roomacoustics.html 07/24/11 07/24/11 Room Acoustics ...
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