Unit 1 - Room Acoustics

Question 1

What 3 factors influence room acoustics?

Answer 1

• distance
• noise
• reverberation

Question 2

Describe the rule for determining the effect of distance on sound amplitude when no reflective surfaces are present

Answer 2

For every doubling of the distance, the signal loses 6 dB

Question 3

Name 3 things that contribute to classroom noise levels (unoccupied and occupied)

Answer 3

• external sources (e.g traffic)
• building equipment (e.g HVAC)
• room equipment (e.g machine, computer, projector)
• activity (e.g staff, pupils)

Question 4

What is reverberation time?

Answer 4

The amount of time required for the sound field in a space to decay 60 dB (amount of time for sound energy to bounce around before being absorbed by materials and air)

Question 5

Describe which spaces have long vs short reverberation times

Answer 5

Closed spaces with reflective surfaces have long reverberation times.
Spaces with absorptive materials (e.g. carpet and drapes) have short reverb times.

Question 6

What are the 3 types of sound (regarding reverberation)?

Answer 6

• Direct sound (source to listener s/ reflections)
• Early reflections (reflected sounds from nearby surfaces that reach listener within 50 ms)
• Late reflections (reflected sounds that reach listener after 50 ms from the direct sound

Question 7

How do we measure room acoustics?

Answer 7

Reverberation Time (RT60)
Critical Distance

Question 8

What is “reverberation time”?

Answer 8

The RT60 is the time it takes for the sound level to drop by 60 dB after the source is turned off

Question 9

What is the Sabine Equation?

Answer 9

A way to calculate the RT60

Meters:
RT60=0.161*(V/A)

Feet:
RT60=0.049*(V/A)

where V=room volume
and A=total surface absorption (Sa)

Question 10

What is a Sabin?

Answer 10

Sa is the total surface absorption of a room

- the sum of all surface areas in the room multiplied by their respective absorption coefficients

Question 11

In the equation:
RT60=0.161*(V/A)
what does A represent?

Answer 11

A=total surface absorption (Sa)
which is equal to:
absorption coefficient (alpha) * absorbing surface area in meters squared (S)

Question 12

What 3 things does the decay rate depend on?

Answer 12

• the amount of sound absorption in a room
• the room geometry
• the frequency of the sound

Question 13

The Critical Distance involves an interaction between _____ and _____ in a space

Answer 13

distance and reverberation

Question 14

Define Critical Distance

Answer 14

The point at which the reverberant sound and direct signal levels are equal for a given frequency

Question 15

The _____ ______ field decays by 6 dB for every doubling of the distance, while the ___ _____ level should remain reasonably uniform in all parts of the room

Answer 15

direct sound

reverberant sound

Question 16

Beyond the critical distance, ____ sound dominates

Answer 16

Reflected

Question 17

Within the critical distance, the _____ rule applies

Answer 17

6 dB

Question 18

For very long reverb times (e.g. 5 seconds), the critical distance is very ____ (long/short)

Answer 18

Short

Question 19

The SNR will be _____ (positive/negative/”0”) for a student sitting outside of the critical distance

Answer 19

Negative

Question 20

As the volume of a room decreases, the critical distance ______ (increases/decreases/stays the same). Is this a good thing?

Answer 20

Decreases
No, not a good thing

Dc = 0.20*((VQ/nRT)^1/2)

Question 21

If the reverberation time (RT) decreases, the critical distance _____ (increases/decreases/stays the same)

Answer 21

Increases

Dc = 0.20*((VQ/nRT)^1/2)

Question 22

An SNR of ___ is needed for 100% audibility

Answer 22

15 dB

Question 23

What is the SAI?

Answer 23

Speech Audibility Index - the proportion of the useful speech signal (direct speech + early reverberation) that is above the level of the effective noise (actual noise + late reverberation)

Question 24

What are the target specifications for unoccupied classrooms, with regard to maximum noise and RT60?

Answer 24

Max 35 dBA noise

Max 0.65s RT60