Building Utilities 3: Acoustics and Lighting Systems PDF

BUILDING UTILITIES 3 ACOUSTICS AND LIGHTING SYSTEMS WEEK 3  1.1 DEFINITION COMPLEX TONE The vibration in pressure caused music, speech or noise. Noise- unwanted sound; amount of ACOUSTICS annoyance produced by noise is...

BUILDING UTILITIES 3 ACOUSTICS AND LIGHTING SYSTEMS WEEK 3  1.1 DEFINITION COMPLEX TONE The vibration in pressure caused music, speech or noise. Noise- unwanted sound; amount of ACOUSTICS annoyance produced by noise is subjective, psychological, and proportional to the loudness of the noise; most annoying sounds are high frequency rather than low frequency as well as intermittent rather than continuous noise; sounds bearing information are harder to ignore than nonsense noise. ACOUSTICS 1.1 DEFINITION COMPLEX TONE Noise Ambient Noise- the noise associated with an environment at a given time; no dominant sound in particular. Background Noise- is the total noise from all sources other than a particular sound. White Noise- undistinguishable background noise. ACOUSTICS 1.2 BEHAVIOR OF SOUND Sound moves in a straight-line path unless it is interrupted, when sound waves strike an object, it can be reflected, absorbed or transmitted. Reflection and absorption of sound is dependent on the wavelength of the sound. Sound transmission is dependent on how much sound is reflected and how much is absorbed.  ACOUSTICS 1.2 BEHAVIOR OF SOUND High frequency sound tends to pass through thin materials and curve more easily around barriers, low frequency sounds are easily reflected by thin materials and do not bend around barriers. Sound level decreases as the distance from sound source increases, if the distance is doubled (increased by factor of 2) the sound energy is decreased by a factor of 4.  ACOUSTICS 1.3 PHYSICAL QUANTITIES OF SOUND FREQUENCY (f) It is the measure of the tone or pitch of sound. The number of cycles that the air particles move back and forth in one second in a sound wave is called the frequency of the wave. Its unit is cycles per second (c/s) which is also termed Hertz (Hz) after the Austrian physicist Heinrich Hertz.  ACOUSTICS 1.3 PHYSICAL QUANTITIES OF SOUND FREQUENCY (f) A normal young adult is capable of hearing sounds ranging from 20Hz to 20 kHz audible to the human ear meaning waves occur between 20 and 20,000 times per second. ACOUSTICS 1.3 PHYSICAL QUANTITIES OF SOUND FREQUENCY (f) Frequencies below 20Hz are called infrasonic frequencies. They are not heard but are perceived by humans as vibrations. Frequencies above 20kHz are referred to as ultrasonic frequencies. These are also not heard by the humans, although some animals can hear them. Eight frequency bands or octaves are considered in room acoustics with the following center frequencies: 63Hz, 125Hz, 250Hz, 500Hz, 1kHz, 2kHz, 4kHz and 8kHz. ACOUSTICS 1.3 PHYSICAL QUANTITIES OF SOUND SPEED (c) The speed of sound in air has been measured as 344 m/sec. This corresponds to 1,240 km/hr (770mi/hr) which is extremely small as compared to the speed of light (300,000km/sec) The speed of sound in air does not vary with the frequency of sound or its loudness. Sounds at all audible frequencies, regardless of their loudness, travel at the same speed. In solids, the speed of sound (that is, the speed of travel of vibrational energy) is considerably greater than in gases or in liquids. ACOUSTICS 1.3 PHYSICAL QUANTITIES OF SOUND WAVELENGTH (‫)ג‬ Distance between wave crests; waves are farther for low frequency sound and waves are closer for high frequency sound. The wavelength and the frequency of sound are related to each other as shown in the equation below: c= f‫ג‬ Where c= speed f= frequency in cycles per time ‫ = ג‬meters  ACOUSTICS 1.3 PHYSICAL QUANTITIES OF SOUND WAVELENGTH (‫)ג‬ The wavelength of sound corresponding to the center frequencies are as shown:  1.3 PHYSICAL QUANTITIES OF SOUND INTENSITY Is defined as the amount of sound power falling on (or passing through, or crossing) a unit area. Since the unit of power is watt, the ACOUSTICS unit of sound intensity is watt per square meter (W/m²). The sound intensity which is just audible, called the threshold of audibility, has been determined to be 10˄-12 W/m². The intensity that corresponds to the sensation of pain or threshold of pain in the human ear is approximately 10 W/m². Decibel – is a measure of the intensity of sound; 10dB = 1B  ACOUSTICS 1.4 CHARACTERICTICS OF SOUND LOUDNESS (INTENSITY) Sound pressure level (loudness) – is the human perception of the strength of sound Is a measure of the intensity of sound and is expressed in decibels. It is a quantity called the sound intensity level (IL). ACOUSTICS 1.4 CHARACTERICTICS OF SOUND PITCH Is the frequency of sound wave perceived by the human ear. A high- pitched sound means that it has a high frequency. The female voice is slightly higher pitched than male voice.