Bouncing Around October 26, 2007 Room Acoustics Bouncing Around October 26, 2007

Music and Other Sounds Come from a source. The source is not isolated, it is in an environment. The environment can affect what the listener will hear: Ambient noise level Properties of the wall, ceiling, etc. Other sources producing sound at the same time. 11/27/2018 Room Acoustics

Consider a Pulse of Sound WALL Reflection Sound Changes Different Travel Distances Many reflections occur at the same “time” 11/27/2018 Room Acoustics

Consider the following: 270 ft 100 ft ear source 200 ft 250 ft 500 ft. 11/27/2018 Room Acoustics

Note Our ear will hear two sounds 40 ms apart as a but “rough” and, perhaps, as a fast echo. A room should be carefully designed to maintain a “pleasant” aural experience. This will be our concern today. 11/27/2018 Room Acoustics

A wall Sends a “delayed” reflection of the sound to the ear. A matter of distance. The reflection may be synchronized with the source so that they may “interfere” The reflection may, be hindered by the absorption of the sound energy by the wall. There may be an echo. 11/27/2018 Room Acoustics

Example - Interference “Wall Wall 11/27/2018 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. 11/27/2018 Room Acoustics

Surfaces (Walls, floors, etc.) Rough or Smooth Hard or soft Location with respect to listener Characteristics depend on the sound being detected. 11/27/2018 Room Acoustics

Two surfaces 11/27/2018 Room Acoustics

IS THIS A ROUGH SURFACE??? 1 nm = 10-12 meters =0.000000000001 m 11/27/2018 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: SMOOTH – Variations occur on a scale much smaller than a wavelength of the sound we are considering. ROUGH – The variations in the surface are comparable to the size of the wavelength. 11/27/2018 Room Acoustics

Reflection SMOOTH ROUGH SPECULAR DIFFUSE 11/27/2018 Room Acoustics

SOFT Walls A soft wall (like rubber or cork) will yield when you push on it. Sound (music) pressure pushes on the wall. IF the wall deforms, than a force (pA) times a distance (the deformation), means that the wave does WORK. The sound therefore loses some energy when it hits such a wall. The reflection isn’t as strong as one from an “un-yielding” wall. 11/27/2018 Room Acoustics

Consider an outdoor concert Musicians on stage People in the audience No Walls or Ceilings Only reflections possible are from structures in back of the musicians. And possibly the ground 11/27/2018 Room Acoustics

Useful aspects of reflection Think about the reverse! 11/27/2018 Room Acoustics

The old Greek Amphitheater 11/27/2018 Room Acoustics

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Closer Audience “Band Shell” 11/27/2018 Room Acoustics

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Care in a band-shell The focus can’t be too good because then all of the performers need to be at the same place. Since they can’t be, a vertical wall might be better. Real Band shells look right but really do NOT properly focus. ON PURPOSE! 11/27/2018 Room Acoustics

What does “focus” mean Sound waves hit a surface which can be called a mirror. The mirror surface can be curved so that rays of sound from different directions can be made to come together at the same place. Like a lens In a concert hall, too much focusing can also mean that there is only ONE good seat in the house! 11/27/2018 Room Acoustics

EXAMPLE: The Ellipse A & B = foci 11/27/2018 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. 11/27/2018 Room Acoustics

APPROXIMATION ?? 11/27/2018 Room Acoustics

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Parabolic Reflector 11/27/2018 Room Acoustics

Parabolic Receiver 11/27/2018 Room Acoustics

What about REAL Rooms???

In a Real Room What about the walls? Smooth Rough How Smooth? Rough How Rough? Transmission properties? WALL 11/27/2018 Room Acoustics

Another Factor RESONANCE 11/27/2018 Room Acoustics

Resonance Examples 11/27/2018 Room Acoustics

Speakers? 11/27/2018 Room Acoustics

At home with Shostakovich 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. 11/27/2018 Room Acoustics

A different phenomonon DIFFRACTION 11/27/2018 Room Acoustics

Diffraction Sound can “bend” around objects. Sound can change its properties depending upon the size of the wavelength compared to objects. The Diffraction effect can be understood via one of the early theories of waves. 11/27/2018 Room Acoustics

A Bad Photo .. sorry ploop 11/27/2018 Room Acoustics

Huygen's Principle 1678 Polaroid Photo 11/27/2018 Room Acoustics

Huygen's Principle Every point on the front of a wave (wave front) acts as a source of spherical waves. The next position of the wave front will be the surface that is tangent to all of the other parts of the surface created in the same way. The spherical wave travels at the speed of sound. vt 11/27/2018 Room Acoustics

Another View 11/27/2018 Room Acoustics

A Slit (Window) 11/27/2018 Room Acoustics

Diffraction Through a SMALL Opening (comparable to l) 11/27/2018 Room Acoustics

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An Edge 11/27/2018 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 room. 11/27/2018 Room Acoustics

What else? Small objects will scatter or diffract sound so it can be heard in non-straight lines. Around edges, etc. Small objects do very little to long wavelength sounds (low tones). They are like the Eveready Battery … they keep going and going and going ….. Higher frequency sounds will be deflected or absorbed more than low frequency sounds. 11/27/2018 Room Acoustics

We discussed Reflections 11/27/2018 Room Acoustics

What Do You Think? 11/27/2018 Room Acoustics

Or a school performance hall 11/27/2018 Room Acoustics

Professional Concert Hall (mucho Dolleros ) 11/27/2018 Room Acoustics

Surfaces 11/27/2018 Room Acoustics

$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ Baffles $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ Soft Walls People??? 11/27/2018 Room Acoustics

Note Modern halls are adjustable for The piece being played The size of the audience 11/27/2018 Room Acoustics

Create a SUDDEN Sound Listen & Record with a microphone loudness time 11/27/2018 Room Acoustics

Real Example: Royal Festival Hall 11/27/2018 Room Acoustics

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Room Reflections Room full of sound! 11/27/2018 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. 11/27/2018 Room Acoustics

Listen to the Room! 11/27/2018 Room Acoustics

Lets start a musical tone and listen to the auditorium with a sound recorder. 11/27/2018 Room Acoustics

How about the return to silence? There is a steady musical sound in the auditorium. The symphony is over. The music suddenly stops. It takes a certain time for the sound level to get to a very small level. The time it takes for the auditorium sound to drop to 1/1,000,000th of the steady level is called the REVERBERATION TIME. 11/27/2018 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. 11/27/2018 Room Acoustics

Absorbing Materials More Absorbing 11/27/2018 Room Acoustics

A Formula NOT to be Remembered Wallace Sabine 11/27/2018 Room Acoustics

Let’s try a calculation – Living Room @ 500 Hz (Book states this wrong) Ceiling Area = 4 x 5 = 20 m2 Effective = 0.1 x 20 = 2m2 3m 4m 5m 11/27/2018 Room Acoustics

Another Example 300 x 0.1 same 11/27/2018 Room Acoustics

Reverberation Times Desired 11/27/2018 Room Acoustics

For Music Rooms must be carefully designed. The “engineering” contains a lot of “Kentucky Windage”. Different kinds of music require different acoustical designs. In the right room, you hear what the composer intended you to hear. 11/27/2018 Room Acoustics

http://www. crutchfieldadvisor http://www.crutchfieldadvisor.com/ISEO-rgbtcspd/learningcenter/home/speakers_roomacoustics.html 11/27/2018 Room Acoustics