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PHYS 105 Auditorium Acoustics Dr. James van Howe Lecture 20 Symphony Hall, Boston Who is the Artist? Modest Mouse Flaming Lips Radio Head Shins True or False Reverberation times over 5 seconds muddies the sound too much in a concert hall (not very useful for most kinds of music)

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slide1

PHYS 105

Auditorium Acoustics

Dr. James van Howe

Lecture 20

Symphony Hall, Boston

slide2

Who is the Artist?

  • Modest Mouse
  • Flaming Lips
  • Radio Head
  • Shins
true or false
True or False

Reverberation times over 5 seconds muddies the sound too much in a concert hall (not very useful for most kinds of music)

sample problem
Sample Problem

What is for the concert hall below (shoebox design) at 500 Hz?

Walls: plaster on lath; a=0.06

Ceiling: Acoustic Tile; a=0.83

Floor: Carpet on pad; a=0.57

10 m

15 m

25 m

sample problem cont
Sample Problem cont.

I just designed a very “dry” hall; a good lecture hall but bad for music

slide8

What is for the modified concert hall below?

Walls: plaster on lath; a=0.06

Ceiling: plaster on lath; a=0.06

Floor: Carpet on pad; a=0.57

10 m

15 m

25 m

If I changed my ceiling to plaster rather than acoustical tile, just part 3 of drain changes:

Much better hall, good for music and speaking!

what makes a good concert hall
What makes a good concert hall?

Study of 22 European concert halls

  • Long reverberation up to 2.0 s; however > 2.0 s, in general is less pleasing
  • Narrow halls

-early reflection from side walls very important (concert halls good acoustically often shoebox design)

-drawback is you can’t see performance as well; often sacrifice sound for ability to see performance (fan-out design)

  • High binaural dissimilarity (one ear hears something slightly different than other)
  • Less definition: sound energy in the first 50 ms lower in good halls
things to avoid in auditorium design
Things to avoid in auditorium design
  • Echoes

-usually culprit is the rear wall

  • Flutter Echoes

-rapid series of echoes from two hard parallel surfaces

  • Sound focusing

-curved walls (concave) focus sound causing shadows in some spots and loud spots in other locations

  • Sound shadows

-often occur under balconies because reflections are blocked

  • Background noise

-Heating and cooling systems usually culprit

shower modes
Shower Modes

Resonant modes of a box

H

Shower strengthens the specific frequencies given by fl,m,n and kills the others

W

L

Example: Tahiti Sand Shower Stall from Home Depot

why are shower modes bad

Frequency Response Concert Hall

Frequency Response Shower

discrete spikes

continuous

Amplitude

Amplitude

Frequency

Frequency

Holes

Why are Shower Modes Bad?

-The room acts as a filter that takes away frequencies you might want!

-Not so good for your home theatre or recording studio

Why doesn’t this happen in a Concert Hall?

-It does happen in rectangular halls, but since the volume of the hall is so big it really isn’t noticeable

ways to avoid strong room resonances
Ways to avoid strong room resonances
  • For rectangular room, make or choose one whose ratio of dimensions is non-integer
  • Make the room irregular shape (avoid the rectangle)

-crucial for recording studio

  • Electronically compensate
  • Spread sound with sound diffusers

-crucial for recording studios

non integer ratios
Non-integer ratios

A room whose dimensions (L x W x H) give a ratio of 5:3:2 has a more continuous frequency response than 1:2:3

This is because for integer ratios, many fl,m,nare the same number (degenerate), making these peaks stronger in the response curve

A cube has a ratio of 1:1:1 is the worst possible choice

If you want your home theatre to sound the best, put it in a room whose dimensions are way off from integer ratios

irregularly shaped rooms walls surfaces
Irregularly shaped rooms, walls, surfaces

-Lack of parallel surfaces reduces constructive and destructive interference that leads to box resonances

-Irregular surfaces help smooth out spectral response

-Also eliminates flutter echoes

Recording Studio Schematic Utusan Malaysia

Irregularly shaped reflecting surfaces

electronically compensate
Electronically compensate

Intelligent sound field tuning system for home theater systems

IEEE Transactions on Consumer Electronics 51, pg 635 – 639, 2005.

System responds to acoustics of room by adding/subtracting the appropriate sound field, like very fancy noise canceling headphones

Can make the room sound less like a box without changing the structure of the room!

sound diffusion
Sound diffusion

-Same reasoning behind irregular shape of room

-Want sound to spread in many directions to avoid resonance

-Additional effect is to allow sound to “reverberate” and mix

Sound diffusers are angular shapes or convex; often rough surfaces

slide19

One of the better concert halls in the U.S

Why? What do you notice?

Symphony Hall, Boston

true or false20
True or False

Reverberation times over 5 seconds muddies the sound too much in a concert hall (not very useful for most kinds of music)