BUILDING SERVICE STUDIO – III ESE Jury on ACOUSTICS

Along the direction of propagation or perpendicular to it?
Sound moves as a longitudinal wave, where the particles of the medium oscillate along the direction of propagation of the wave.

Example: When you speak, air molecules vibrate back and forth in the direction of your voice, carrying the sound to your listener. This explains why sound cannot travel in a vacuum, such as outer space.

Sound is not a particle. It is a mechanical wave that propagates through the vibration of particles in a medium.

Design Insight: Always consider the medium of sound propagation in your designs. For example, materials like air, water, or even building structures can influence sound travel.

If I shout and whisper, which parameter of sound will increase: wavelength, amplitude, or frequency?
Amplitude will increase when you shout, as it is directly related to the loudness of sound. Wavelength and frequency remain unchanged.

Application: Loudness impacts space design. For instance, banquet halls require materials that can handle high amplitudes without causing excessive reverberation.

What is decibel, and what does it measure: wavelength, amplitude, or frequency?
A decibel (dB) is a unit that measures the intensity of sound, which is related to the amplitude of the sound wave.

The decibel is a unit used to measure the intensity or loudness of sound, which relates to its amplitude.

Example:

• A whisper is about 30 dB.
• Normal conversation is 60 dB.
• A rock concert can exceed 120 dB.

Architectural acoustics aim to control sound intensity for user comfort. For instance, lecture halls should avoid excessive loudness to maintain speech clarity.

How is reverberation time calculated for a room?
Reverberation time (RT60) is calculated using Sabine’s formula: RT60 =0.161⋅V / A where:
V= Volume of the room (in cubic meters)
A = Total sound absorption (in square meters)

Example: A large hall with reflective surfaces will have a longer reverberation time than a carpeted room. Proper design ensures optimal reverberation time for the room’s purpose.

Unit of sound absorption coefficient?
The sound absorption coefficient is dimensionless, as it is a ratio. However, the unit of sound absorption is sabins.

• Banquet halls: 1.0 to 1.5 seconds to balance music and conversation.
• Lecture halls: 0.6 to 1.1 seconds for speech clarity.

Pro Tip: Incorporate materials like acoustic panels, curtains, or carpets to achieve desired reverberation times.

Difference between reverberation time and echo?

Reverberation Time is the time taken for sound to decay by 60 dB after the source stops.

Echo is a distinct, delayed repetition of sound that occurs when the reflected sound reaches the listener after a minimum of 0.1 seconds.

Design Challenge: Avoid echoes in spaces meant for communication, like classrooms, by using sound-absorbing materials.

Reverberation time of an empty hall will increase or decrease if we add 300 plastic chairs in the hall?
Adding 300 plastic chairs will decrease the reverberation time because they absorb sound energy.

Example: An empty hall may have a reverberation time of 2.5 seconds. Adding 300 chairs reduces it to 1.5 seconds, improving speech intelligibility.

Difference between sound reflection, refraction, and absorption?

1. Reflection: Sound bounces back from a surface.
2. Refraction: Sound bends as it passes from one medium to another.
3. Absorption: Sound energy is absorbed by a material, reducing the reflected sound.

Design Tip: Combine reflective and absorptive materials for balanced acoustics.

If yes, does it absorb, reflect, or refract sound?
Yes, glass is an acoustical material. It primarily reflects sound but can also refract it depending on the angle of incidence.

Example: Double-glazed glass windows in urban homes reduce external noise, enhancing indoor acoustic comfort.

What do we fill in double glass?
Double glass is better for acoustics as it provides superior sound insulation by creating an air gap that dampens sound transmission.

The air gap between panes dampens sound transmission. Filling this gap with inert gases like argon improves both thermal and acoustic performance.

The minimum time required to distinguish between two different sounds?
The minimum time is approximately 0.1 seconds, which is required to perceive a distinct echo.

The minimum distance between the source of sound and the reflecting surface to hear a distinct echo?
To hear a distinct echo, the reflecting surface must be at least 17 meters away from the sound source. This is based on the speed of sound (343 m/s) and the time delay (0.1 seconds).

Example: A canyon is a perfect natural example of echo due to its large reflective surfaces.