Explains basics about sound and what classroom issues are present due to sound effects which causes problem for students to hear teacher properly. Explains concept of reverberation and other issues and suggests about its solution for better classroom sound efficiency

1. Acoustics
Reverberation
and
Classroom Issue
Ashvary Jain
0201ME101017
BE III sem, Mechanical
Jabalpur Engineering College

2.  Introduction
Acoustics and Classroom
 Problems in a classroom
• Room Noise
• Reverberation
• Speaker Listener Distance
 Reverberation – In Detail
 Solutions
Summary

3. Introduction
Sound .
• It is a mechanical wave that undergo oscillation.
• Transmits through solid liquid or gas.
• Composed of frequencies within range of hearing or
create sensation of vibration.

4. Acoustics
• Study of all mechanical waves in gases, liquids, and
solids.
• Including vibration, sound, ultrasound and infrasound.
• Everything related to hearing and speaking comes in its
scope.

5. What happens to sound when
it hits a barrier?
• Transmitted (pass through)
• Absorbed
• Reflected
• Diffracted

7. Acoustics and
Classroom
Importance of hearing/listening
• Approximately 45% of school time spent listening
• Approximately 30% speaking
• Approximately 16% reading
• Approximately 9% writing

8. • Elements of Listening
• Detection – Identifying the sound
• Discrimination – Differentiating between various words
• Recognition – Catching the exact word
• Comprehension – Noting correct thing

9. Problems in
Classroom
Often students are not able to get what teacher is
speaking, sometimes even misinterpreted. The main reasons
behind this are
• Room noise
• Speaker-listener distance
• Room reverberation
Lets discuss these problems individually….

10. Room
Noise
• Acoustical noise is any unwanted
sound
• Could be external or internal
• Includes whispers, shut
noises, external disturbances.
• Noise is around 55-60 dB in
classroom

11. Speaker Listener
Distance
• Audibility decreases as the distance from the speaker
increases
• About a 6dB drop for every doubling of distance.
• At certain distance, background noise can mask speaker’s
voice.
• A typical teacher’s voice measures 60 to 65dB at a distance of
1m/3ft.

12. • Noise is generally at 55-60 dB in a classroom.
• First row student gets decent sound, about 65 dB
• Forth row student get 60 dB of voice signal, same as noise
• For back-row students, teacher is at 53dB, receive 55% of the
speech signal, and achieve just 60% word recognition

13. Reverberation

14. Reverberation
• It is the persistence of sound in a particular space
after the original sound is removed.
• Large number of echoes build up and then slowly
decay as the sound is absorbed
• Causes prolongation of sound.
• Longer the reverberation the more “smear” of speech
sounds.

15. Reverberation Time
•Time it takes a sound to stop reflecting
•Is the time in seconds for a sound to reduce 60 dB in
intensity once the generation of sound has stopped
•Hard room -> long RT time
•RT of room alters the effect of the speaker-listener
distance upon sound intensity

16. Sound Source

17. Classroom study of
Reverberation

18. Classroom Calculations
Time Difference Calculations
• Assuming the dimensions of a class room to be – 15 x 10x
10 metres.
• Speed of light = 330 m/s
T1 = Time taken by sound to travel via 10m straight path
T2 = Time Take by sound to travel via 12+12m reflected path
Time = Distance /Speed

19. T1 = 10/330 = 0.0303 sec
T2 = 28/330 = 0.0727 sec
T2 – T1 = .0424 sec
10m 12m
Time difference of .0424
sec between two sound
waves cause the confusion
while detecting the
speech
10m

20. Reverberation Time Calculations
Sabine equation
RT60 - Reverberation Time in sec
V - Volume of Room in cubic metres
S - Total Surface Area in square metres
c – Speed of Light in metres
a – Average Absorption Coefficient

21. By using a pre build
Reverb Time
calculator and
putting data close
to the reality, the
reverberation time
was found to be
around 1.8 seconds

22. In general, for a class room, the reverberation
time must be between .4 to .6 sec

23. Solutions for Better
Acoustics
Importance of Better Acoustics
• Improvement in speech recognition
• Improvement in speech recognition
• Reduced vocal strain and voice fatigue for teachers
• Increased teacher mobility
Some of the solutions for better Acoustics are -

24. 1. False Ceiling
• Decreases the volume of room.
• Time variation between two rays also reduces
• Acoustical ceiling tiles in the room to decrease reverb due to
absorption
• Available in panels, which are installed using a grid attached to the
existing ceiling.
• Wood planks provide a similar benefit as acoustical ceiling tiles.
• Thermocol sheet also help for the purpose

25. Acoustic Tiles used as False Ceiling

26. 2. Carpeting Floor
•Helps in absorption of reverb sound.
•Replace hardwood, laminate, tile or cement floors
with carpet
•cover the floors with area rugs.
•More open area on floor = more reverberation

27. Room with good False Ceiling and Carpeting

28. 3. Heavy Fabric on Windows
•Cover windows with heavy drapes.
•Drapes made from soundproofing material are
available but are costly
•Instead choose a heavy fabric that covers the entire
window
•Also cover walls with heavy fabric or velvet
•Appears similar to Movie Theatre fabrication

29. Drapes in an Auditorium
Movie Theatre Fabrication

30. 4. Mic and Speaker System
• This is specially for classrooms.
• Speakers are installed in various part of the room
• Teacher is supposed to speak on mic
•Appears as teacher being near to most of the
students
•Reduces the Speaker – Listener Issue as if teacher is
just near to him.
•Background noise is suppressed.

31. An Auditorium
with perfect
use of Acoustic
Fundamentals

32. Summary
• Acoustics make big impact on any vocal oriented operation
• Classrooms often suffer with poor acoustics
• Classroom related issues include – Room Noise, Speaker-Listener
distance, Reverberation
• Various measures could be taken to enhance acoustics
• For Classrooms, this include False Ceiling, Carpeting, Drapes and
Speaker-Mic system

33. Thank You