| Print & Noise Printers > Methods > Acoustic Noise |  | Navigation Icons Guide
|
Acoustic Noise
Acoustic noise is unwanted sound. Almost all sound produced by IT equipment counts as noise. People would prefer equipment to be completely silent unless it has been specifically told to make a sound.The important figure is sound power declared according to ISO 9296:1988. This gives a measure of all the sound produced by the machine where less is better.
What ISO 9296 of 1988 actually does is to set out how noise figures are to be given "Declared noise emissions". To be in accordance with ISO 9296 measurements are made in accordance with ISO 7779 and then expressed in product specifications according to ISO 9296. Typically we get three figures:
Active Standby Sleep
LWAd x.x B y.y B z.z B
The figure is sound power in Bels, weighted for human perception and statitstically corrected for the size of the sample measured. The aim is to have the lowest figures for x.x , y.y and z.z.
--
Noisy Machines
IT equipment rarely is completely silent.
Most of the noise from PCs and the standby noise from printers is created by fans. Recent equipment often needs fans to give some measure of cooling to the innards, particularly the processor, graphics chip and power supply in a PC. Printers often have fans as well. Laser printers use at least one fan to take excess heat from the fuser and to cool the power supply. Some fast colour printers have three or more fans. Large inkjet printers often have fans to help dry the page or direct air flow containing fine droplets towards a filter.
Printers can be very noisy because the print process is innevitably so. With dot matrix printers a series of needles strike the ribbon and the paper and they often move several hundred to several thousand times per second making an intense noise.
Laser printer processes are not inherently noisy. The core process involves a drum turning quite slowly and a series of rollers. The paper pickup rollers make some noise as they engage and push sheets off the input stack. Other noises are the polygon mirror spinning up which may be heard as a whining noise and the crinkle of paper as it moves out of the delivery rollers. The main noise is usually a fan spinning up to keep other parts of the machine cool whilst the fuser is driving heat into the paper.
Inkjets make little or no noise from the printheads themselves however most printers scan the prithead across the page and this makes a steady thumping noise as the carriage reverses. Most of the noise comes from vibration of the desk the printer is on, rather than from the printer itself.
A few years ago noise from IT equipment wasn't considered very important - there wasn't much of it. however IT equipment is now ubiquitous so the noise can be quite noticeable. Quieteness has also become important because a lot of IT equipment is used for audio-visual purposes and it isn't much use a PC being used for audio if the machine is spoiling the signal.
Manufacturers have only quite recently begun reporting the acoustic noise from IT equipment. Standards had to be developed for the figures to be measured and reported in a meaningful way.
Sound can fairly easily be measured with a microphone and a meter and professional equipment to do this is quite readily available. There are problems however because sound measurements vary greatly with distance from equipment, are often much more intense in one position than in another and sounds need not be loud to be irritating.
Given freedom to measure and eport sound in different ways manufacturers would naturally tend to pick ways favouring themselves. The resulting figures would probably be of little value as a way for buyers to compare the sound produced by equipment.
Measurement
From the manufacturers point of view there are three standards of interest.
ISO 7779 - 1999
"Acoustics -- Measurement of airborne noise emitted by computers and business equipment", is the international accepted one used for measuring noise emissions from personal computer system units, hard disks, printers and projectors.
ISO 9295
The ISO 9295 standard, "Acoustics -- Measurement of high-frequency noise emitted by computer and business equipment", is used as a complement to ISO 7779 for measuring high frequency noise (in the 16 kHz octave band). This kind of noise is beyond direct hearing but is perceived. It was a particular problem with CRT based TVs and monitors because the horizontal scan coils produced this kind of noise, perceived by children as a shrieking noise and by some adults as unpleasant.
ISO 10302
The ISO 10302 standard, "Acoustics -- Method for the measurement of noise emitted by small air-moving devices", is the international accepted one used for noise measurement of fans.
Declaration
From the buyers point of view the only standard that should be needed is ISO 9296 which requires measurements in line with ISO 7779 and then states how they should be reported.
ISO 9296:1988
Acoustics -- Declared noise emission values of computer and business equipment
Acoustic noise level are usually given in accord with ISO 9296 : 1988 "Acoustics - Declared noise emission values of computers and business equipment." (The standard is available from www.iso.org). This specifies information to be given in technical documents supplied to users by manufacturers.
ISO 9296 specifies use of measurements taken according to ISO 7779 and reports statistical maximum values of the:
- sound power, A-weighted in decibels or Bels. Abreviated LWA
- sound pressure levels at the operator or bystander positions, again A-weighted in decibels. Abreviated LpA
ISO 7779 requires both figures be given.
Specialists seem to consider the sound power figure to be reproduceable and reliable. Some manufacturers have argued for sound pressure on the grounds that it is a traditional and more readily understood figure. Less prominently their arguments may also be because sound pressure is always a lower figure and because it is possible to argue about the figures allowing them to pass off unrepresentatively good figures as accurate.
Sound power is the total noise radiated by the machine in all directions. The sound power is a measure of the machine and is (nearly) independent of the measuring environment. The measuring apparatus is usually an array of mirophones forming a hemisphere surrounding the sample in an anechoic chamber.
Sound pressure is a measurement of the pressure fluctuations in the air. It is easily measured by a calibrated microphone and meter but the measurement depends on how far the microphone is from the machine and the properties of the environment. Given the distance from a machine a resonable figure for sound pressure can be calculated from sound power.
The a weighting adjusts the figures to some extent to human perception. A weighting is crude in psychoacoustic terms but widely accepted.
both figures may be followed by a "d" such as LWAd
The significance of the d is that the ISO 9296 standard corrects for uncertainty in both different machines of the same type and uncertainties in measurement. A mathematical adjustment is applied depending on sample size and a larger sample size will give a lower figure.
So ideally a potential just looks for the lowest LWAd figure and that should be the least irritating machine to have around - at least if power consumption, consumable prices and other capabilities all match up.
-
ISO 9296 is derived from ECMA 109. A fairly common procedure has developed in the IT industry that standards are often developed by ECMA because it has a quick process. ECMA standards are then submitted for approval by ISO which takes longer. Finally the standard is submitted to ANSI for adoption as a concensus international standard. At the end of the process the three standards are equivalent.