Observation
Office Noise Causes and Improvements Stephen Whitlow and Alan Wyman
Chapter 11 Hearing, Sound, Noise, and Vibration
Sound • Human ear transduces sound waves into nerve impulses • Brain interprets impulses leading to perception/recognition of sounds
Decibels • Sound measured in decibels (dB) • relative scale based on infant hearing threshold, express in log scale • 10 dB increase represents doubling of loudness • Human hearing sensitivity varies across frequency: • maximum between 1000 to 4000 Hz (human speech)
Noise • Noise-induced damage to Ear • Exposure to loud noise causes threshold shift at 4000 Hz • Pain is experienced at 130 dB, damage at 140 dB (jet engine at 30m) • Tinnitus (ringing in ears) occurs in nearly ¼ of noise exposed workers (construction, shipbuilding)
Noise • Exposure to continuous and transient sounds regulated: • OSHA has defined 90 dB(A) as maximum exposure to continuous noise over 8 hour shift • OSHA does not permit exposure to continuous noise > 115 dB(A) • EU advises hearing protection at 80 dB(A) • Age-related shift at 4000 Hz: • 12 dB at age 45, 35 dB at age 65 • Sound at 4000 Hz need to be > 4x louder to be detected by 65 year compared to 45 year old
Noise • While sound can be objectively measured, Noise is a subjective perception • Noise defined as sound at an amplitude that causes annoyance or disruption to communication • Noise survey can be useful in assessing noise distribution and impact in working areas
Noise • Management of industrial noise • Short term: • earplugs/muffs • Medium term: • Moving and/or soundproofing noisy machines • Warning signs • Worker rotation between noisy and quiet jobs • Long term: • Comprehensive program • Replacing noisy machine • Acoustic refuges • Audiometric testing
Noise • Hearing protection • Up to 40 dB reduction in SPL at eardrum • Simple ear protection inadequate at noise levels > 140 dB • Correct training is required to maximize reduction • Design of Acoustic Environment: managing ambient and transient noise • Ambient • 55 dB upper limit for mental work and complex supervisory tasks • 45 dB upper limit for creative work • Transient • 75 dB maximum for office spaces
Reverberation • Noise level determined by noise source intensity and reverb of the room • More reflection = more reverb time • Excessive reverb can blur speech signals • Room shape has little effect on reverb time • Reverb field intensity depends on: • Source SL • Volume of space • Amount of sound-absorbing material in room
Reverberation • Reverb can be minimized by increasing sound-absorbing material in room
Speech Intelligibility and Speech Transmission Index (STI) • STI--intelligibility of speech in environment • Designers should try to minimize STI in office environments • Minimizing direct and reflected speech at each workstation • Research indicated STI < .2 needed in open plan office
Speech Intelligibility and Speech Transmission Index (STI) • Sound-absorbent ceiling tiles reduce speech from 55 dB(A) to 40 dB(A) • But hard to reduce speech > 10 dB(A) within 2.5M of source (most open offices have desks closer than this)
Noise Reduction Coefficient (NRC) • 0 = no ambient sound absorbed, 1= all sound absorbed • Researchers recommend .85 for upholstered screens in office • Thick carpets with padding NRC as high as .7
Effects • Effect of Noise on Task Performance • Disruption of verbal communications due to noise known as masking • Continuous noise can impact inner speech many people use to support short term memory • Meta analysis concluded that high levels of irrelevant noise increase error rate in continuous attention task but not work output • Exposure to other people’s speech • Increased error rate in cognitively demanding tasks, but not RT on more perceptual tasks
Noise reduction • Industrial Noise Reduction • Fans: higher noises at higher speed-- use larger fan running at lower speed • Muffing: pneumatic tool produce noise due to compressed air release-- pipe away from operator or use muffler on air output • Positive effect of noise reduction • 23% reduction in absenteeism in punch card workers after noise reduction • 90% reduction in breakage and machine shutdowns after acoustical treatment
Noise on Stress • Researchers identified physiological response to increased noise: • heart rate and stress hormone levels • Correlated with subjective reports of stress and annoyance
Selected research
Introduction • Background noise most frequent complaint about open-office environment • Conflicting requirements must co-exist • Good speech communication • Good speech privacy
Lit Review • 200 workers • 67% disturbed by telephones ringing • 55% by people talking • ~ 50% by air conditioning and office machinery • Boyce 1974 • 2000 workers • 54% bothered by noise – especially talking and telephones ringing • Sundstromet al. 1994
Lit Review • Cognitive psychology literature shows the bad effects of background speech • Supported by observational studies • Banburyet al. 2001 • Memory for math and reading impaired by office noise • Banbury and Berry 1997, 1998
Validation Study • Questionnaires and sound level measurements • 2 locations in U.K. • Open plan offices • One with 5-foot panels • Other office furniture • Both about 140 employees
Office 1 Banking organization Management had windows – clerical in middle of building Measured sound level at 55 dB(A)
Office 2 IT organization Management intermixed with clerical Measured sound level at 60 dB(A)
Demographics 88 employees participated (75% response rate) 65% male, 35% female Mean of 9 months working there Ages 20 – 59 with a mean of 33 years 22% managerial, 22% supervisory, 56% clerical
Disturbance by noise Asked how their concentration was affected Not leading questions using “bothered” or “disturbed”
Disturbance by noise 1 Major improvement 5 Major deterioration to concentration
Correlations • Only one significant • Long they have worked, the more keyboard annoyance • No habituation found (correlation = zero)
Noted results • Telephones left ringing highly disruptive • Possibly due to perception that co-workers should have been at their desk • Simply due to longer disruption?
Noise abatement • Acoustic panels and ceilings • Respondents felt it would help • Babble • Need multiple voices • Naturally occurring in larger offices • White noise • People have been concerned about the ‘hiss’ and think it is HVAC and report being cold
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