Unit 1 - Room Acoustics Flashcards
What 3 factors influence room acoustics?
- distance
- noise
- reverberation
Describe the rule for determining the effect of distance on sound amplitude when no reflective surfaces are present
For every doubling of the distance, the signal loses 6 dB
Name 3 things that contribute to classroom noise levels (unoccupied and occupied)
- external sources (e.g traffic)
- building equipment (e.g HVAC)
- room equipment (e.g machine, computer, projector)
- activity (e.g staff, pupils)
What is reverberation time?
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)
Describe which spaces have long vs short reverberation times
Closed spaces with reflective surfaces have long reverberation times.
Spaces with absorptive materials (e.g. carpet and drapes) have short reverb times.
What are the 3 types of sound (regarding reverberation)?
- 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
How do we measure room acoustics?
Reverberation Time (RT60) Critical Distance
What is “reverberation time”?
The RT60 is the time it takes for the sound level to drop by 60 dB after the source is turned off
What is the Sabine Equation?
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)
What is a Sabin?
Sa is the total surface absorption of a room
- the sum of all surface areas in the room multiplied by their respective absorption coefficients
In the equation:
RT60=0.161*(V/A)
what does A represent?
A=total surface absorption (Sa) which is equal to: absorption coefficient (alpha) * absorbing surface area in meters squared (S)
What 3 things does the decay rate depend on?
- the amount of sound absorption in a room
- the room geometry
- the frequency of the sound
The Critical Distance involves an interaction between _____ and _____ in a space
distance and reverberation
Define Critical Distance
The point at which the reverberant sound and direct signal levels are equal for a given frequency
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
direct sound
reverberant sound
Beyond the critical distance, ____ sound dominates
Reflected
Within the critical distance, the _____ rule applies
6 dB
For very long reverb times (e.g. 5 seconds), the critical distance is very ____ (long/short)
Short
The SNR will be _____ (positive/negative/”0”) for a student sitting outside of the critical distance
Negative
As the volume of a room decreases, the critical distance ______ (increases/decreases/stays the same). Is this a good thing?
Decreases
No, not a good thing
Dc = 0.20*((VQ/nRT)^1/2)
If the reverberation time (RT) decreases, the critical distance _____ (increases/decreases/stays the same)
Increases
Dc = 0.20*((VQ/nRT)^1/2)
An SNR of ___ is needed for 100% audibility
15 dB
What is the SAI?
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)
What are the target specifications for unoccupied classrooms, with regard to maximum noise and RT60?
Max 35 dBA noise
Max 0.65s RT60
