Acoustics: An Analysis

ACOUSTICSACOUSTICSAn analysis

James Rodney P. Santiago

STUDY OF ACOUSTICS


Why is there a need to study the behavior of Why is there a need to study the behavior of sounds?sounds?

What are the properties of sound?What are the properties of sound?

What are the methods used to measure the What are the methods used to measure the properties of sound waves?properties of sound waves?

STUDY OF ACOUSTICS


Why is there a need to study the behavior of Why is there a need to study the behavior of sounds?sounds?

What are the properties of sound?

What are the methods used to measure the properties of sound waves?

NATURE OF SOUNDWAVES


Sound waves is similar with ______________ and ___________, only that, sound waves need _______________ for it to be propagated.

PROPERTY OF SOUND WAVES


SOUND WAVESOUND WAVE

MOLECULES MOLECULES

STUDY OF ACOUSTICS


Why is there a need to study the behavior of sounds?

What are the properties of sound?What are the properties of sound?

What are the methods used to measure the properties of sound waves?

SOUND WAVES


FREE FIELD CONDITION


A condition wherein a sound wave if free to propagate infinitely away from the source not being reflected back.

In enclosed space design, free field conditions only exists in theory, though, it is possible to come close to that by approximating the conditions similar to free field. A simulated free field enclosure is called an Anechoic room.

COMPONENTS OF SOUND IN AN ENCLOSED SPACE


- Direct Sound- Reflected Sound

STUDY OF ACOUSTICS


Why is there a need to study the behavior of sounds?

What are the properties of sound?

What are the methods used to measure the What are the methods used to measure the properties of sound waves?properties of sound waves?

SPEED OF SOUND


Speed of Sound = 1,087 ft./sec. @ 32 deg. FSpeed of Sound = 1,087 ft./sec. @ 32 deg. F

For other temperatures :For other temperatures :

Speed of Sound = 1,087 + 1.1 (T Speed of Sound = 1,087 + 1.1 (T 32) ft./sec.32) ft./sec.Where T = temperature in degrees Where T = temperature in degrees farenheitfarenheitnote that ambient pressure is not significant in the measurementnote that ambient pressure is not significant in the measurement of of the speed of sound.the speed of sound.

SPEED OF SOUND IN VARIOUS MEDIUMS


QUESTIONS


1. How much in sound pressure attenuation does a sound wave suffer after going through 60dB?

2. To simulate free field conditions, what should be the property of all the materials inside an enclosed space?

Elements of Acoustics Modes of Sound

Reverberation Time

Absorption

Diffusion

Modes of Sound Deal with resonant frequencies of room Constructive interference A mode consists of the fundamental

frequency and all associated harmonics. Also called eigentones, room

resonances, or natural frequencies.

Modes of Sound

Axial Modes involve reflections between two parallel surfaces.

Smallest energy loss

Modes of Sound

Tangential Modes involve reflections between four surfaces

Less powerful than Axial Modes but still significant

Modes of Sound

Oblique Modes involve reflections between sixsurfaces

These are the weakest modes

Modes of Sound Acoustical quality depends partially on how

well the modes are spaced

Reverberation TimeThe Reverberation time of a concert hall refers to the time it takes a sound to decay

60 dB

Reverberation times might vary depending on the frequency of the sound and on

position in the hall.

Reverberation Time There is no absolute optimum reverberation

time for concert halls. However, there is a window of generally acceptable reverberation times.

Absorption Sound waves make inelastic collisions

with surfaces

Some of the energy dissipates as heat

Different materials absorb certain frequencies more efficiently than others

Absorption Here is an equation that determines the

rate of energy dissipation:

a = log( 1 )

a = Sabine absorption coefficient = energy absorption coefficient

Absorption This is an equation for Reverberation time after

calculating absorption:

RT60 = 0.049V / Sa (English System)

RT60 = 0.161V / Sa (Metric System)

V = total volumeS = surface areaa = absorption coefficient

Diffusion How well an environment disperses

sound. Some criterion for good diffusion:

a. Minimal frequency and spatial effects

b. Decays are perfectly exponentialc. Consistent reverberation timed. Negligible decay beats (fluctuations

due to nearby modes)

Diffusion Splaying provides some help Acoustic installations:

Schoeder diffusors (reflection phase-grating)

Flat-panel with distributed absorption

WAVE PROPAGATIONJames Rodney P. Santiago

Wave Propagation

Speed of sound is approx. = 344 m/s or 1130 ft/s.

Combining sine waves

Direct and Indirect Sound

Combining Delayed signals

Diffraction

Effect of Humidity

Loudness Contour

Weighted Sound Level

Enclosure Directivity

Reverberation time calculation

Wave Equation-

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Loudness- sound is the change/fluctuations in air pressure above and below atmospheric pressure. This variation of pressure is perceived by the human ear as the apparent loudness, the greater the pressure change, the louder the sound.- At ideal conditions, a human ear can sense a pressure change of 0.0002 microbars which is equivalent to 1/1,000,000 atmospheric pressure. This amplitude range is commonly measured in dBSPL relative to 0.0002uBars (0 dBSPL) which is the threshold of hearing while 120dBSPL is the threshold of pain.- 1dB is about the smallest change in SPL that can be heard. A 3dB change is slightly noticeable while 6dB is very noticeable.

Direct vs. Ambient Sound- A direct sound becomes weaker as it travels away from the sound source. The amount of change is inversely proportional to the square of the distance change.

- Ambient sound in a room is at nearly the same level throughout the room due to sound being reflected many times within the room until it becomes non-directional. (Reverberation).Note :Ambient sound of the room becomes increasingly apparent as a microphone is placed farther away from the direct sound source. In every room, there is a distance (measured from the sound source) where the direct and reflected sound becomes equal in intensity. In acoustics, this is called Critical Distance.

Critical Distance- How do you maintain/estimate the critical distance?

A unidirectional microphone should be positioned not greater than 50% of the Critical distance, highly reverberant rooms may require even lesser distance.

Phase Relationship between Sound and Interference

- The phase of a single freq. Sound wave is described relative to the starting point of the wave (0 ). The peak of high pressure zone is at 90, the peak of the low pressure zone is at 270 .

Boundary Condition- When a microphone element is placed very close to a reflective surface, both the reflected and direct sound are in phase to the microphone resulting in the doubling of its intensity.- This occurs for reflected frequencies whose wavelength is greater than the distance from the microphone to the surface.

MICROPHONE PLACEMENT


Chassis Ground vs. Signal Ground-

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Acoustics: An Analysis

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