AcousticsReverberation

and Classroom Issue

Ashvary Jain0201ME101017BE III sem, MechanicalJabalpur Engineering College

Contents Introduction

Acoustics and Classroom

Problems in a classroom• Room Noise• Reverberation• Speaker Listener Distance

Reverberation – In Detail

Solutions

Summary

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.

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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.

What happens to sound when it hits a barrier?

• Transmitted (pass through)

• Absorbed

• Reflected

• Diffracted

Acoustics and Classroom

Importance of hearing/listening• Approximately 45% of school time spent listening• Approximately 30% speaking• Approximately 16% reading• Approximately 9% writing

• Elements of Listening

• Detection – Identifying the sound

• Discrimination – Differentiating between various words

• Recognition – Catching the exact word

• Comprehension – Noting correct thing

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….

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

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.

• 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

Reverberation

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.

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

Phenomenon of Reverberation

Sound Source

Classroom study of Reverberation

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

T1 = 10/330 = 0.0303 sec

T2 = 28/330 = 0.0727 sec

T2 – T1 = .0424 sec

10m

12m10mTime difference of .0424 sec between two sound waves cause the confusion while detecting the speech

Sabine equation

Reverberation Time Calculations

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

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

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

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 -

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

Acoustic Tiles used as False Ceiling

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

Room with good False Ceiling and Carpeting

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

Drapes in an Auditorium

Movie Theatre Fabrication

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.

An Auditorium with perfect use of Acoustic Fundamentals

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

Thank You