Dear Colleagues
Please forgive me dear (engineering) reader (and perhaps, gardener) but surely there is nothing more irritating than the strident noise of lawnmowers or mulching machines with their staccato crackling sound on a quiet Saturday afternoon ? In my book, silence is often more desirable than privacy. This short note is on noise and why and how we deal with it.
The decibel
The decibel was created in the 1920’s by the Bell Telephone Labs and is widely used in audio measurement. A write up on audio (and public address systems is contained at the end of this note). The decibel is ten times the logarithm of one power quantity relative to a reference quantity. The difference in decibels between two powers, P1 and P2 (the louder one) is 10 log (P2/P1) dB where the log is to base 10. So it can easily be used to measure the level of everything. The most common usage is of loudness (of noise) - or relative sound pressure. If you halve the power (relative to the reference source), you reduce the power and sound level by 3dB (10log(1/2)).
A complex animal
Sound and loudness is quite a complex animal. On the transmitting side, it depends on the frequencies and amplitudes; while on the receiving side (the ear); we have a non-linear device with an enormous dynamic range. Non-linear means that the ear is not equally sensitive to all frequencies but works best from 1kHz to 5kHz (presumably the noises emitted by our predators in prehistoric times). A weighting curve (or filter) is thus used to make sounds measured by “non-ear” devices such as microphones and other acoustic devices more representative of what people actually hear. The one most widely used today is the A weighting scheme and thus we refer to dBA when talking about noise.
(Interestingly enough, the human eye is similar – twice the amount of Lumens or light does not look twice as bright – you need to increase the light intensity by ten times to achieve this).
Crazy examples of dBA
Typical numbers for dBA include:
• Just audible is 10 dBA
• Soft whisper at 15 feet is 30 dBA
• A quiet office is about 40 dBA
• Air conditioner, normal speech, 60 dBA
• Noisy restaurant, freeway traffic, noisy office, 70 dBA
• Hearing protection recommended at 80 dBA
• Heavy truck in traffic measures 90 dBA
• Rock concert is 110 dBA
• Thunderclap is 130 dBA
• Jet air ops on a US Navy carrier deck is 140 dBA
Sound becomes irritating when averaged over 24 hours, it exceeds 65dBA.
Reductions in noise – really ?
There has been a significant reduction in noise creating devices. As a growing example, electric cars have become dangerously quiet (and can be a danger to pedestrians). The biggest noise on the road these days is the ‘screech and whine’ of rubber on the tarmac. Airports have become more subdued with the insistence on use of high-bypass jet engines (with slower turning compressor fans). And welded track and electric locomotives have made noise from the railways more acceptable.
However machinery makers have been a bit more reluctant to keep their noises down. Because it costs them. Most countries set 85dBA as the maximum level of noise (up to 8 hours at a time). There has been the suggestion, that domestic appliances are deliberately made louder to make them sound more powerful than they actually are (e.g. a blender) Although OSHA in the USA allowed unsilenced machines to be increased from 90dBA to 100dBA (thus allowing a doubling in loudness). This has made it increasingly difficult for Americans to sell their products outside the USA (e.g. into fussy but shrewd Europe)
Noise cancellation
Noise cancellation can work either at the source or at the listener end. Sometimes. And not always perfectly. The closer your noise is to a point source, the easier it is to cancel. But bear in mind that a large shape radiating noise from all surfaces in all directions would be almost impossible to apply noise cancellation technology to (thus best to engineer the noisy machinery to be quiet). And low frequencies (large wavelengths) are easier to dampen, since the distance between the noise source and noise dampener needs to be significantly less than the wavelength of the noise to work properly.
So what can you do about this ?
• Consider noise reduction as a key element when engaging on your next project
• Be aware that everyone is increasingly demanding quieter machinery
• To make your product more saleable and your workforce happier – go for quieter equipment
• Look at opportunities to dampen noise in all your designs
I wondered about the rather irate (and perhaps patronising ?) note from the noted philosopher, Arthur Schopenhauer:
The amount of noise which anyone can bear undisturbed stands in inverse proportion to his mental capacity.
Thanks to The Economist and Joe Wolfe of the University of NSW for a great web site on audio.
Yours in engineering learning
Steve
