ABBEYnet

Sound

Module of Leaving Certificate Construction Studies

Content

Nature of sound

• Sound Generation
• Behaviour of Sound
• Terms related to Sound

Sound in Buildings

• Sound Proofing materials
• Sound Proofing Detail
  

Aims

• To develop an awareness of sound the environment.
• To develop our knowledge and understanding of acoustics.
• To apply our understanding of the principals of sound to building design.
• To integrate our I.T. skills with our study of Construction.
• Enhance our research, communication and project compilation skills.

Objectives

Students will be able to:

• Demonstrate a knowledge and understanding of the concepts and principals of sound generation and transmission.
• Demonstrate an understanding of the acoustic properties of materials and sound transmission.
• Apply our knowledge of sound to design principals in Building

Definition:

Sound is a sensation produced through the ear resulting from fluctuations in the pressure of the air
These fluctuations usually originate from a vibrating object and are in the form of a longitudinal wave motion.

 

The Structure of the Ear

Sound Generation (1)

There are three types of vibrating sources

• strings and rods
  -violin and harp
• surfaces
  -drum, diaphragm of a telephone receiver
• vibrating air columns
  -vocal organs, organ & wind instruments

 

• Consider the air close to the surface of some object that is vibrating. As the surface moves outwards the air molecules next to the surface are pushed closer together, I.e. the air is compressed.
• The air cannot move back into its original position for the moment because the space is occupied by the advanced surface of the vibration object and therefore a movement of air occurs away from the surface.
• This movement in turn causes the compression of another layer of air. A knock-on affect occurs. This can be easily seen in the example of a spring.

Spring

o The result of the outward displacement of the vibrating surface is thus to produce layers of compressed air, decompressed, compressed and decompressed air… progressing outwards from and parallel to the surface.
o This movement is similar to ever-increasing concentric spheres originating at the source and expanding outwards until the sound dissipates.

o The wave consists of positive pressure and negative pressure. The result is a pattern of peaks and valleys.

o Sound requires a medium to travel through (sound will not travel in a vacuum). Depending on the elasticity and density, some mediums transmit waves faster than others. (Sound travels through glass/metal fourteen times faster than it does through dry air)

Frequency

The frequency of a sound is the number of cycles, or oscillations, a sound wave completes in a given time(per second). Frequency is measured in hertz, or cycles per second.

"Frequency," Microsoft® Encarta® 98 Encyclopedia. © 1993-1997 Microsoft Corporation. All rights reserved.

Wavelength x Frequency = Speed of Sound

The human ear responds to frequencies from about 30 cycles per second (hertz) up to about 20,000hz.

 

Sound Intensity

• This is the amount of energy that is in each wave.
• Sound intensities are measured in decibels (dB). Sound intensities are arranged on a logarithmic scale, which means that an increase of 10 dB corresponds to a 10 times increase in intensity.

"Sound Intensities," Microsoft® Encarta® 98 Encyclopedia. © 1993-1997 Microsoft Corporation. All rights reserved.

For example, the intensity at the threshold of hearing is 0 dB, the intensity of whispering is typically about 10 dB, and the intensity of rustling leaves reaches almost 20 dB. Using the logarithmic scale, which means that an increase of 10 dB corresponds to an increase in intensity by a factor of 10. Thus, rustling leaves are about 10 times louder than whispering.

"Sound Intensities," Microsoft® Encarta® 98 Encyclopedia. © 1993-1997 Microsoft Corporation. All rights reserved.

Decibel Scale (dB)

This is a log scale which gives the ratio of sound levels relative to a basic level which is humans 'threshold of hearing'. (i.e. 0dB)
The upper end of the scale is the maximum sound that the ear can tolerate (i.e. 130dB)

Decibels (dB)	Situation
0	Threshold of hearing
20-60	Human speech
60-80	Vacuum cleaner
70-90	Motorcycle
95-115	Thunder
120-140	Jet plane taking off
15-190	Rocket launch

Inverse-square law

"The sound intensity from a point source decreases in inverse proportion to the square of the distance from the source."

o If a person stands 2m (A) from a sound of 100dB, the sound level at A will decreased in inverse proportion to the square on the distance from the sound source. So, at point A, the sound level will have dropped by a factor of 22, (i.e.4 times)
o At 4m (B) the sound level will have dropped by 42 (i.e. 16 times)

Reverberation Time

o This basically is the length of time it takes for a sound to die away. When a sound is produced in a room it reflects off the walls etc. and sustains longer than if the sound was produced in the open.
o Hard surfaces in a room will reflect sound, whereas soft furnishings absorb sound and thereby reduce the reverberation time.

Sabine's Formula

oThis formula gives the reverberation time for sound to drop by 60dB from its original value.

T =	0.16 V
	A

T = Reverberation time in seconds
V= Volume of the room in m3
A = Total absorption of the room surfaces

Sound Absorption

o When a sound hits an obstacle (room walls), some of the energy is absorbed, some is reflected and some is transmitted through it.
o The Sound Absorption Coefficient measures absorption.
- No sound absorption is Zero
- Total absorption is One

Resonance
o Resonance is where a vibrating body will cause adjacent objects to vibrate at the same frequency.

Sound in Buildings

Sound in buildings is transmitted in two ways
> Airborne transmission,
> Structure borne transmission

Airborne Sound
o Sound travels from the source to the receiver through the air. Often this sound strikes a wall which in turn causes it to vibrate at the same frequency, this causes the air in the adjacent room to vibrate and thereby the sound can be heard in that room.
o E.g. hearing the radio in the next room.

Structural(Impact) Sound
o The structure e.g. walls, act as the transfer medium for the sound. As the sound source vibrates the adjacent structure vibrates similarly and the sound travels through the structure.
o E.g. The impact sound of foot-steps upstairs is transferred through the joists to the ceiling and the room underneath.

Direct/Indirect Transmission
o Where sound passes in a linear path this is known Direct Transmission.
o Where the sound takes a different path through the structure is known as Flanking or Indirect Transmission

Airborne Sound Insulation
o Airborne Sound is reduced by placing a Dense or Thick obstacle in its path (e.g. increased thickness of the wall).
o An increase in the Mass improves the insulation properties. (The Mass Law)
o There should not be a gap in the division as this would significantly reduce the Airborne Sound Insulation Property.

Impact Sound Insulation
o Preventing direct contact between the sound source and the structure reduces the effect of Impact sound.

o Sound absorbent materials such as carpet and cork absorb the vibrations from the sound source and reduce the impact on the structure.

o Discontinuous Construction also prevents direct transmission of the sound to the structure; this can include a cavity wall or a floating floor.

Sound Insulation Properties
There are two categories of sound insulating materials.
o Heavy, Dense materials (brick & concrete) are good Airborne Sound Insulators. The vibrations die away among the tightly packed molecules of a dense material.
However these materials are poor Impact Sound Insulators.

o Porous Material (fibreglass wool, cork) are good as Impact sound insulators. The sound is reflected and trapped within the voids.

Sound level in receiving room is influenced by:
1. The sound insulation of the separating wall or floor
2. The area of the separating wall or floor
3. The volume of the receiving room
4. The amount of flanking transmission
5. The amount of absorbing material, such as furniture, in the receiving room.

Sound proofing existing walls
1. Fill any joints or cracks in the walls or plaster
2. Increase the mass of the wall by plastering (over brick)
3. Addition of plasterboard on internal surfaces
4. Replace timber partitions with block or brick
5. The joint between wall and roof should be filled (normally with fire stopping material)
6. Walls completed up to the roof
7. Install Rockwool insulation slabs in partitions
8. Stagger timber studs in partition with insulation between
9. Install acoustic panels on walls and ceilings

Sound proofing floors
1. Place a soft covering or resilient material on floor
2. Place floating floor on top of existing floor with resilient material between.
3. Seal gap between floating floor and skirting or wall with a resilient strip