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Facts about sound
Acoustics and the Terms that Define it
Sound—Sound is identified as when something causes the air pressure to fluctuate rapidly above and below the local ambient air pressure our eardrums respond by vibrating similarly.
Noise—Noise is unwanted sound, whether it is a simple short duration annoyance, or an extended duration sound. It is determined noise when it is perceived negatively by the average human, community, etc.
Loudness—One of our subjective reactions to sound is what we sense as loudness. This sensation is related to the amplitude of the pressure fluctuation above and below the existing ambient pressure. For a given frequency, the greater the amplitude of the pressure fluctuation, the louder the sound is perceived to be. Loudness is a function of both the amplitude and frequency of an air pressure fluctuation, both must be considered simultaneously.
Sound Pressure—Sound pressure is the numerical value for a pressure fluctuation in the atmosphere. Often, sound pressure is confused with another acoustical term sound pressure level. These are different terms and the difference between the two must be understood.
The accepted units for sound pressure are metric, Newton per one square meter (N/m2). To gain an understanding of what this unite represents, it is approximately 0.00015 psi. The human ear is extremely responsive to low amplitudes of air pressure fluctuations. The quietest sound an average person is likely to hear is 0.00006 N/m2 with the threshold of discomfort at 6.3 N/m2. Pressure fluctuations above 6.3 N/m2 can result in permanent hearing damage.
Sound Pressure Level (Lp)—Sound Pressure is typically quantified in decibels (dB) not (N/m2). When expressed in decibels, the value of air pressure fluctuation is termed sound pressure level. Due to its logarithmic derivation, the use of decibels in acoustics lends itself well to correlating equal numerical changes in decibels with the perceived human response of equal changes in loudness. The following rules hold true:
- A 10 dB increase in sound pressure level is perceived by the average human as sounding twice as loud, or doubling of sound.
- A 5 dB increase in sound pressure level is clearly noticeable.
- A 3 dB increase in sound pressure level is barley noticeable.
- Sound pressure level differences of less than 3 dB will generally be undetectable even by those with very acute hearing.
Sound Power—Sound power is the actual amount of radiated acoustical energy expressed in terms of watts of power. The goal in all acoustical designs is to provide specific sound pressure levels at specific locations. To achieve these design sound pressure levels, the designer must analyze the surroundings of all receivers, analyze the characteristics of all sound paths to the receivers and must have an accurate analysis of the acoustical characteristics of the sound source. An extremely wide range of radiated sound power is encountered in everyday life. The range extends from (10-11) watts up to (108) watts.
The standard method of characterizing a sound source in singular and reproducible manner is by determining the sound power of that source. Using sound power data, one can analyze the effects of the environments and paths and predict sound pressure levels at various locations. If the predicted sound pressure levels at eh receiver are different than required, sound blocking and sound absorbing solutions can be introduced to achieve the design goals.
Sound Power Level (Lw)—Sound sources are not usually rated in terms of watts, since the numbers become so cumbersome, but instead in decibels (dB). Sound power level data can be broken down into octave band frequency components.
Pitch—Subjectively, we react to the rate of pressure fluctuation by assigning a certain pitch to it. What is sensed as a low pitched sound (foghorn) is composed of relatively slow pressure fluctuations. What is sensed as a high pitched sound (police whistle) is composed of relatively fast pressure fluctuations. Low pitch sound has a long wavelength and high pitch sound has a chart wavelength.
Tone—Often people subjectively classify sounds as a function of their tonal quality. Examples of this are when we characterize sounds as hissing, rumbling, roaring or whistling.
Wavelength—Wavelength is defined as the speed of sound (1120 ft/sec) divided by the frequency (Hz) reported in units of feet. For example, the wavelength of 63 Hz is 18 ft. and the wavelength of 4000 Hz is 0.28 ft.
Frequency—The frequency of a pure tone is defined as the number of cycles per second at which ambient air pressure is fluctuating. The unit of frequency measurement is hertz (Hz). One Hz corresponds to 1 cycle per second. The human ear responds to a wide range of sound frequencies. A young person with excellent hearing may be able to discern sounds between 20 Hz and 20,000 Hz. As we get older, the audible-range width has a tendency to shrink at both the low and high frequency ends of this range.
Broadband Sound Level—Very few of us are ever exposed to pure-tone sounds. Most of the sounds that we are subjected to on a daily basis are a combination of thousands of frequency components. These are all broadband sounds. Although we may be able to characterize them by their pitch or tonal qualities, they are not pure tones.
Components of Broad Band Sound—The various amplitudes of frequency components of a broadband sound level differ significantly among sound sources. It is not enough to say that equipment produces a certain type of noise since different equipment types exhibit differing sound spectra. Paired with the fact that sound reduction properties of various noise control products are a function of frequency, it can be easily seen that the components of a broadband sound level need to be quantified before any noise problem can be effectively solved.
Octave Bands—The way of expressing the amount of sounds in various frequencies is by the use of octave bands. An octave band is a frequency band in which the highest frequency in the band is two times the lowest frequency in the band. This results in a geometric progression where the actual bandwidth increases as frequency increases. Octave bands are typically expressed in one of three ways: (1) as an octave band number, (2) as a center frequency, (3) as a listing of the upper and lower frequencies of the octave band.
Octave Band Number—Octave band numbers represent frequency segments usually evaluated in an acoustical design. Typically, octave band (1) refers to a center frequency of 63 Hz and (2) 125 Hz, (3) 250 Hz, (4) 500 Hz, (5) 1000 Hz, (6) 2000 Hz, (7) 4000 Hz, (8) 8000 Hz.
Center Frequency—Center frequencies are the geometric center frequency of each respective octave band.
A-weighted Sound Level (dBA)—A means of adjusting a linear noise spectrum to closely reflect the response of the human ear; it requires adjustments to be made in all frequencies except 1000 Hz; once adjustments are made decibel addition can be used to yield a single “A-weighted” sound pressure level, expressed as a single number for correlation against a single-number value specification such as a city noise ordinance or OSHA regulation.
Ambient (Background) Noise Level—The sound pressure level measured with sound source in off position expressed in individual octave bands (dB) and one overall sound level reading (dBA)
Source—Noise radiates from a source
Path—Noise is transmitted via sound waves through a space that separates the source from the receiver.
Receiver—The receiver can also be termed critical location. It consists of a person, neighbor or community that is being affected by the sound generated by the source and traveling across the path. The main goal of any acoustical design is to protect the receiver from hearing damage and also create and pleasant acoustical environment.
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