1 Acoustics

Question 1

Sound

Answer 1

Is a pressure wave consisting of oscillation of particles that make up the medium.

Question 2

Propagation

Answer 2

Is the process of transmitting energy through space.

Question 3

Longitudinal wave

Answer 3

Sound is a longitudinal wave, a disturbance of the medium in the same direction (and opposite) to the direction of propagation of the sound wave. There are areas of compression and rarefaction as the sand travels.

Question 4

Atmospheric pressure and sound?

Answer 4

Equals 1x10^5 Pa caused by the motion of molecules in the air. Pressure at the surface also varies somewhat in response to changing weather systems. When a sound wave travels through air, it causes very small fluctuations in the pressure of air above und below Patm.

Question 5

Sound can be characterised by

Answer 5

Frequency and wavelength.

Question 6

The speed of sound.

Answer 6

The speed of sound through air is dependent on the temperature and to a lesser extent, the humidity. The higher the temperature, the higher the speed of sound. V=331.45+ 0.6T, note that the speed of sound does not depend on the frequency of the sound, so all frequencies of a complex sound arrive at a destination at the same time.

Question 7

Refraction

Answer 7

When a sound enters a new medium, its speed changes.it frequency remains the same, and speed charges, wavelength must also change. Therefore, the direction of propagation changes (rarefraction). Note: different propagation if the thermal profile is right.

Question 8

Sound power P

Answer 8

When a sound is created, energy (or power) is transferred to the medium through which the sound travels and is spread over area.

Question 9

Sound intensity assumptions

Answer 9

Sound propagates equally in all directions. No absorption energy losses associated with sound as it travels.

Question 10

Density of air near surface of earth.

Answer 10

1.3 kg/m^3

Question 11

Sound absorption

Answer 11

Due to internal absorption a certain amount of energy will be lost as sound waves travel due to friction between the molecules, converting some of the sound energy to heat. This loss is most significant in large open areas when the sound travels large distances.

Question 12

Internal absorption

Answer 12

Large open spaces. Within the atmosphere. M is the absorption coefficient of the medium of interest. The extent of absolution of energy of a medium is characterised by its internal absorption coefficient.

Question 13

Internal absorption and humidity.

Answer 13

There is most absorption for high frequencies and in dry conditions. Low humidity, high absorption.

Question 14

Boundary absorption coefficients - small indoor spaces

Answer 14

Indoor, the distance sound travels tends to be short so sound absorption is much more about the surfaces in the room and the extent to which they absorb sound. In this case, external absorption is important.

Question 15

External absorption

Answer 15

The absorption of energy due to space boundaries. (Absorbtion by the walls that make up a room).

Question 16

Alpha

Answer 16

The coefficient of absorption of the surface e.g. If 90% is absorbed by the wall, alpha = 0.9.

Question 17

Beta

Answer 17

The boundary coefficient of reflection (how much is reflected). e.g. If 90% is absorbed then beta = 0.1 note that alpha + beta =1

Question 18

What does the extent of absorption depend on?

Answer 18

Frequency of the sound and the material involved. Soft furnishing are better at absorbing sounds than hard surfaces.

Question 19

Describe reverberation time

Answer 19

When a sound is reflected off a surface, there is a delay in the time it takes for the sound to reach a listener compared to the sound that reaches the ear directly because of the larger distance travelled.in many enclosed spaces there are many sounds from the same source that reach the ear with different amounts of delays. This resulting effect is called reverberation.

Question 20

Small/ large spaces reverberation

Answer 20

Large spaces that lack absorbing surfaces produce alot of reverberation. Small spaces with alot of absorbing materials produce little reverberation.

Question 21

What does k constant depend on and its magnitude

Answer 21

Depends on the volume of the space, the shape of the space and the properties of the boundaries of the space. K = 0.161 s/m at T=22°c

Question 22

Classrooms and RT value

Answer 22

A high level of reverberation means that the space is affected by background noise and this impacts on communication. A good classroom has an RT valve of about 0.6s for a small classroom and 0.7s for a large classroom. 1.2-1.3 can be hard for kids to process sound.

Question 23

3 impacts of noise and its effects

Answer 23

Acoustics
- speech/communication interference
- privacy issues
- performance/ concentration
Occupational Noise Exposure
- hearing loss
Community Noise
- annoyance
- sleep disturbance
- physiological functions
- mental illness