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SOUND PRESSURE LEVEL (SPL) vs. SOUND POWER
Thus far, all the references to decibels have been in terms of sound pressure level. To obtain a SPL reading is relatively simple: position the meter at the specified point and measure in dBA. As long as the background noise is held at least ~6 dBA below the sound being measured, and the meter is accurate and calibrated, the result is clear. Because it is so simple to conduct, SPL measurement in decibels at 1-meter distance has become a de facto sound measurement standard, especially where a purpose-specific standard does not exist. However, this measurement is best likened to a single snapshot photograph of the object from one particular point of view. It cannot show the whole acoustic picture. The measured SPL for a device varies with angle, position and acoustical environment.
Sound power, while also a sound measurement expressed in decibels, is a more complete measurement that expresses the total amount of acoustic energy emitted by a sound source.
Think of it like a light bulb, which radiates light in every direction. If you could measure all the energy radiated by a bulb, then this would be the equivalent of sound power.
Noise generally emanates from most sources in some omnidirectional fashion, like light from a light bulb, although the intensity in different directions may vary considerably, unlike a light bulb. Measure all the acoustic energy radiated in every direction by a sound source: This is sound power.
Sound power is akin to a 3-D image compiled from many photographs. It involves multiple microphone measurements from many positions around the sound source, and calculations to convert these measurements into a single value. Unlike SPL measurements, it is not dependent on environmental factors. Sound power is a more accurate predictor of noise under a wide range of environments than SPL readings, and correlates better with human perception, especially for comparative purposes.
To distinguish sound power from SPL, sound power is commonly expressed in bel (a decibel is 1/10th of a bel). The A weighting scale is also generally applied to sound power measurements. For the purposes of this article I will use the A-weighted bel scale for sound power where possible, and refer to SPL in dBA @1M where applicable.
Used with permission from Tomas Risberg, creator of the pioneering web site, The Silent PC. The bel table is located on the ISO 9296 page, along with a detailed explanation of the items on it.
OTHER ASPECTS OF SOUND AND NOISE
Thus far we have focused on perceived and measured loudness, and touched on frequency. There are many other aspects of noise that affect human perception and reaction.
Variability: In other words, how a sound changes over time. One plain fact comes up over and over in dealing with noise: A variable noise that changes relatively quickly or dramatically over time is more noticeable and annoying than one that stays constant. Hence, a thermally-controlled fan that responds quickly to changes in temperature can be more intrusive than one that has a louder default noise level but stays constant. This is the reason why Seagate states in one of their white papers that minimizing the difference in noise level between idle and seek for hard drives is psychoacoustically more significant than achieving the lowest idle loudness.
Pure Tones Vs. Broadband: The plucking of a guitar string was cited as an example of a pure tone. It is one that decays, unlike a held key on an electronic organ, which is also a pure tone. The undecaying steady tone is much more common in computers, with rotating devices like fans and hard drives. The sound of a waterfall or of the surf heard from a distance is broadband; it is composed of random noise in the entire audible frequency spectrum. Such broadband noise that occurs in nature can generally be described as pink noise - its loudness decreases with increasing frequency. Most machine-generated broadband noise has a flatter frequency balance, with equal loudness at high and low frequencies. This is called white noise, usually perceived as more intrusive than pink. Most human beings are more comfortable with broadband noise than with pure tones, if there is little loudness difference between them. So the "wooosh" of air blown by a fan is less intrusive than the high frequency "whine" that can emanate simulutaneously from that same fan if it is small and spinning at high speed.
Directionality: Low frequency sounds emanate from any source in an omnidirectional pattern. As we move up in frequency, the sound becomes more and more directional. In other words, it cannot bend around corners, al though it can reflect off hard adjacent surfaces to reach around corners. High frequency directionality explains, for example, why the high pitch whine that comes from some CRT monitors is not always audible from every angle. If you own such an afflicted monitor, you will have had the experience of moving your head a few inches or turning your head and finding that the noise has disappeared - only to reappear when you put your head back to another position. Directionality can be exploited with baffles in PC cases to contain higher frequency noise.
Decrease with Distance: Distance makes the sound grow fainter. Sound reduces in intensity at the rate of 6 decibels for each doubling of distance when there are no reflective surfaces, such as when a loudspeaker is suspended 50 feet up in mid-air in the middle of a field. In an enclosed space like a room, the sound that radiates away from you, which would not be heard outdoors, reflects off walls to reinforce the direct sound. This explains why indoors, sound generally reduces in intensity by the slower rate of 3 dB for each doubling of distance.
Mechanical Vibrations: All sound is caused by modulation of the air by a moving, vibrating object. Most noise measurement techniques isolate the noise source so that its mechanical vibration does not interfere with the airborne noise that is being measured. But even if those vibrations do not translate directly into noise, when the object is coupled to - bolted, screwed, glued or even set down upon - another object. A fan or hard drive in free air sounds different compared to when it is firmly screwed into chassis of a PC case. Generally, panels in the PC case resonate easily in response to the low frequency vibrations in the fan or hard drive -- caused usually by imperfectly balanced bearings or uneven weight distribution of the moving mass.
Room Acoustics: The particular acoustic qualities of a room can have an additive or subtractive effect on noise. Obviously, if your office is an anechoic chamber, you'll probably hear less noise from your computer. But things such as heavy carpeting or bare wooden floors, overstuffed furniture or glass and metal furniture, drapes and curtains or bare glass windows and walls, and the asymetry or symmetry of your room -- these can all affect dramatically the noise that a listener perceives.
Despite the efforts of scientists and engineers to advance the metrology of acoustics, there is still no single objective or numeric summation that tells about the quality of a sound. By the term quality, I do not mean how good it sounds, but rather, the nature of a sound. This is best illustrated with an example:
The sound of a gas lawn mower a few houses away is measured at the point of reception as 50 dBA average SPL. The sound of the distant surf is measured at the point of reception as 50 dBA average SPL. The measurements are correct; the SPL values are the same. Do they have the same value, meaning or effect on human beings? No. Most of us perceive these sounds as fundamentally different. A sound level meter does not. It takes sophisticated frequency spectrum analysis plotted over time, and someone trained enough to interpret the data in order to identify roughly what you and I can hear and characterize in seconds.
In my work, I repeatedly encounter compelling evidence that while measurements are important, they only tell half of the acoustic story. There is simply no substitute as yet for a careful, trained listener who can describe accurately what is heard and correlate that description to an analysis of its source.
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