BUILDING ACOUSTICS

acoustics

“Acoustics is a science of sound including its production, transmission and effects. In present usage the term sound implies not only the phenomena in air responsible for the sensation of hearing but also whatever is governed by analogous physical principles. Thus, disturbances with frequencies too low (infrasound) or too high (ultrasound) to be heard by normal person are also regarded as sound. One may speak of underwater sounds, sound in solids, or structure-bourne sound.”

Allan D. Pierce, ACOUSTICS, an introduction to its Physical Principles and Applications.

Sound and noise

What is the difference?

Sound: changing air pressure

Noise: unwanted sound!

Eg: DRUM –

when mallet hits drum the surface (head) vibrates and pushes / pulls air – it changes air pressure and ripples (waves) in air moves in all directions

They strike our eardrums (which also moves) and that’s how we perceive sound.

Humans generally hear sounds that vibrate the air as slow as 20 times per second (or 20 Hertz, or 20Hz). This is often referred to as pitch, frequency, cycles per second or Hertz.

humans are considered to have hearing that spans from 20Hz to 20kHz

The bass drum has a very low pitched sound. A flute may have a high-pitched sound.

LOUDNESS (volume): Where pitch is how fast the air pressure changes, loudness is determined by how much the air pressure changes.

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Here is a 20Hz sound shown for

one second of time. Notice that

during the one second of time we

can count 20 complete cycles. The

waveform shown is a simple

sinusoidal wave, or sine wave for

short.

In real life, there are few things that generate single frequencies at a time. Even musical instruments have a fundamental tone, as well as “overtones”.

These other tones are frequencies that are generated in addition to the main frequency that is generated, and help us differentiate a violin from a flute, even if they are playing the same note.

other common objects, such as motors or engines generate a host of frequencies all at once…with each moving part generating its own sound and adding it to the other sounds. The resulting sound is called a complex sine wave, and these make up virtually all of the sounds we hear.

Complex sine waves are the movement of air back and forth at many frequencies and many volume levels all at once. After these hit our eardrum, our brain is able to recognize and even dissect most of the sounds into things we have heard in the past (such as a violin, a jet engine, a blender, or a child crying).

IMPORTANT POINTS:

A) Sound is generated by

changing air pressure

B) The pitch is how fast the air

pressure changes

C) The volume is how much

the air pressure changes

D) Most sounds we hear are

complex sine waves

DECIBELS: The volumeof a sound is measured in decibels (or dB).

The general range of human hearing is from ~0dB to 120dB. As a quiet library is about 30dB, we are rarely in environments that are quieter than that, so we don’t often hear sounds below that level as they would be masked by the other sounds (or one might say noise) in the environment around us.

On the other end of the scale at 120dB, this is considered the threshold of pain, where the ears begin to feel pain from the volume and above which, permanent damage to human hearing will occur. Rock concerts often run at 120dB and sometimes over 130dB if you are near the speakers.

Noise Propagation

Soundwaves move through the air with some natural loss

sound waves reflect off of other surfaces, some more, and some less depending on the material, so the sound coming from one source can easily fill every corner of a room by propagating out in all directions and by reflecting off of the surfaces in the room.

changes in air pressure not only move our eardrums back and forth (that is how we hear), but also move other objects back and forth.

The diagrams below demonstrate a variety of sound propagation through common materials and the amount of sound on 1 side versus the other:

Surace, Kevin (Dec, 2003), Quiet Solution Ver 2.5 – Sound & Noise, generation, propagation and reduction

Building Acoustics

• The field of building acoustics consists of two parts; – Room Acoustics which deals with sound propagation in a

room

and

– Building Acoustics, which deals with sound propagation between rooms.

• Typically, room acoustics is related to the quality of sound, e.g. concert hall acoustics, while building acoustics is more related to unwanted sound, i.e. when you want to hear as little as possible of what is going on in adjacent rooms.

http://www.norsonic.com/index.php?sideID=2494&ledd1=248

Sound in a room

• In normal conditions for a room: rate of sound energy input to the room is exactly compensated by the rate of energy absorbed in the room.

• To be accurate, the absorbed energy consists of two parts; the energy actually absorbed by the room boundaries (the walls, ceiling and floor) and the energy transmitted through the boundaries into other rooms or out in the open.

• Consequently, the sound level in the room will be reduced if more absorbing materials are brought into the room or more energy is transmitted through the boundaries.

• We can increase the amount of absorption by bringing in carpets, soft furniture, curtains etc.

• A very effective way of increasing the amount of transmission through the boundaries is simply to open a window!

If the room is equipped with different absorbers absorbing different frequencies to different extents you may "tune" the room to a certain sound balance by adjusting the amount of the different type of absorbers.

Typically, rooms can in this way be more or less optimized to their uses such as for romantic orchestra music, baroque music, pop-music or speech.

The Reverberation Time Must Match the Use of the RoomRooms with highly reflecting surfaces such as bathrooms will have a relatively long reverberation time while rooms with lots of absorbing materials will have short reverberation times.

Sound Distribution

Reverberation time is used in both building acoustics and room acoustics. Room acoustics uses it for "tuning" a room into the intended use (speech, music), while building acoustics uses it when measuring sound transmission through a wall.

In building acoustics where we concentrate on the noise level due to what goes on in adjacent rooms (or outside) an important parameter is the sound insulation.

Values in building acoustics should be normalized and objective, i.e. independent of the current conditions (amount of absorption) of the room. Primarily we want to characterize a wall's ability to insulate rather than the current sound level in the receiving room. This is very much because most countries have regulations on how good the insulation at least should be.

We are here talking about the sending room - which is where the noise originates - and the receiving room - where we measure the amount of noise coming through the wall.

Noise Reduction

Why is noise an important workplace hazard?

Noise is one of the most common occupational health hazards. In heavy industrial and manufacturing environments, as well as in farms, cafeterias, permanent hearing loss is the main health concern. Annoyance, stress and interference with speech communication is the main concern in noisy offices, schools and computer rooms.

To prevent adverse outcomes of noise exposure, noise levels should be reduced to acceptable levels. The best method of noise reduction is to use engineering modifications to the noise source itself, or to the workplace environment.

Surace, Kevin (Dec, 2003), Quiet Solution Ver 2.5 – Sound & Noise, generation, propagation and reduction

basics of how sound behave

When sound strikes a surface, some of it is absorbed, some of it is reflected and some of it is transmitted through the surface. Dense surfaces, for the most part, will isolate sound well, but reflect sound back into the room. Porous surfaces, for the most part, will absorb sound well, but will not isolate.

The best way to stop sound transmission through a building structure is to isolate the sound source from the structure before the structure has a chance to vibrate.

http://www.acoustics101.com/

Figure 1. Principles of isolation of a noisy machine

CBD-236. Introduction to Building AcousticsOriginally published February 1985.

A.C.C. Warnock

False ceilings are absorbent

Use of fabrics is a pleasant way of sound and noise management

Proper use of plants and landscape elements can help create

barriers, reduce and control noise

Rules of thumb

Do not be tempted by the wow factor

The most common ways of increasing the sound

insulation is to:

1. Use materials with a high mass.

2. Use of a cavity between the internal and external

leaves.

3. Avoiding air paths by filling all joints properly.

Write your own Rules of thumb

Good sound environment?

Noise should be dampened to such an extent that it no longer

interferes with the activity you were set out to do.

Just 30 dB(A) is disturbing to sleep.

Noise with sound levels of 35 dB(A) or more interferes with the

intelligibility of speech in smaller rooms.

Even lower background levels are needed for adequate speech

intelligibility for vulnerable groups - such as the hearing

impaired, the elderly, children in the process of language and

reading development, and individuals who are not familiar with

the spoken language.

Avoid many hard surfaces to eliminate disturbing ‘echo’. A

reverberation time below 0.6 seconds is desirable, even in a quiet

environment.

A 10 dB difference is perceived by the human ear as a doubling (or

halving) of the audible sound.

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