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3-D Sound and Spatial Audio. MUS_TECH 348. Environmental Acoustics and Computational Simulation. Indirect Sound. Indirect sound exists in all environments though we may not attend to it consciously. Large-Space Acoustics.

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indirect sound

Indirect Sound

Indirect sound exists in all environments though we may not attend to it consciously.

large space acoustics

Large-Space Acoustics

We are most able to attend to indirect sound in large-spaces like concert halls or cathedrals.

Reverberation time is the duration in which the indirect sound decreases to -60 dB of the direct sound.

large space acoustics1

Large-Space Acoustics

Typical reverberation times:

large space acoustics3
Large-space Acoustics

Indirect sound that reflects off of one wall is called a first-order reflection, off two walls a second-order reflection, etc. As the density of reflections increases, the higher-order reflections merge into the continuous sound with at least 1000 reflections/sec we call reverberation or the late field.

reverberation simulation
Reverberation Simulation

By 1963 Manfred Schroeder has perfected the first digital reverberator. His purpose was to simulate concert hall acoustics including a multiple delay line to simulate the initial first- and second-order reflections.

slide9

Schroeder Reverberator

decay

density

Schroeder’s basic building blocks were the delay line (a non-recursive comb filter), the recursive comb filter and the all-pass comb filter.

delay line non recursive comb filter

-1

f

f = SR/m

Delay line: Non-recursive Comb Filter

a

K

z-m

amplitude response:

(explains the name comb filer)

impulse response

K

Ka

t

m samples

recursive comb filter
Recursive Comb Filter

K

z-m

b < 1

impulse response:

amplitude response:

-1

K

f/2

Kb

Kb2

Kb3

Kb4

t

f

m samples

f = SR/m

all pass comb filter

-b

z-m

b

All-pass Comb Filter

amplitude response is constant: (recursive and non recursive parts cancel each other)

impulse response

b

b2

b3

b4

t

f

-b

slide13

Schroeder Reverberator

(density)

Allpass

Block diagram of a single channel (monophonic) reverberator

recursive comb

(decay)

schroeder reverberator
Schroeder Reverberator

29.7 msec

c

37.1

c

5.0

1.7

A

A

41.1

c

density

Two all-pass filters in series insure that there are 1000 reflections/sec.

43.7

c

decay

Four recursive comb filters in parallel determine the reverb time

early reflections
Early Reflections

input

delay buffer

reverb

output

input

early reflections

a

reverb

t

slide16

f

+

Summation of recursive comb filters should approximate a flat response

f

+

f

=

f

problems
Problems

a

1)

Response is not flat

f

2)

t

Impulse response has periodicities that produce pitch percepts

periodic

air and wall absorption

Air and wall absorption combine to create a reverberation time that is frequency dependent. For concert halls the low-frequency reverberation time is longer:

Air and Wall Absorption

air and wall absorption1
Air and Wall Absorption

Air absorption and the absorption of walls, seats and people help to shape the reverberation.

air and wall absorption2
Air and Wall Absorption

In everyday environments the building materials have a strong impact on reverberation of the acoustic environment.

air and wall absorption3
Air and Wall Absorption

K

z-m

LP

b < 1

Moorer suggests putting low-pass filter in the feedback loop of the recursive comb filter in order to simulate air and wall absorption. This causes the high frequencies in the reverberation to die out before the low frequencies.

slide22

Air & wall absorption filter

Usually the low-pass filter inserted into the recursive comb filter is a first-order filter. Here is a second-order filter that is more accurate.

Low-frequency loss

(not present in concert halls but present in other kinds of construction)

Power in dB

Frequency in Hz (linear)

slide23

simulated

Comparison of simulated and actual rooms

real

other methods for creating reverberation

Other methods for creating reverberation

David Griesinger Lexicon

modulating delays in reverb

lexicon style reverb
Lexicon-Style Reverb

Predelay

lowpass

filters

All-pass Combs

Diffusion

Modulated

All-pass

low-pass filter

Delay

All-pass

Delay

feedback matrix

Waveguide Reverberation

Feedback Matrix

Julius Smith

A

Strautner & Puckette 1982

Jot & Chaigne 1991