The Subjective Annoyance Value Of Birdsong

Pink noise reduces the “subjective annoyance value” of air conditioning

One of the great* things about scientists is their tendency to quantify everything – including, it now appears, how annoyed we get by the noises coming out of our central heating and air-conditioning systems. The latest issue of the Journal of Sound and Vibration showcases research by Guo-qing Di and colleagues, based at Zheijang University in China, in which the subjective annoyance value (SAV) of the noise made by an office air conditioning unit was calculated.

Heating and air conditioning, whilst undoubtedly making our bodies more comfortable, can have some undesirable consequences for our peace
of mind. The whirring of extractor fans, for example, can generate infrasound
that is, noise with a frequency of less than 20 Hz, which is felt rather than heard. (Human ears are, in the main, only capable of hearing sounds of between 20 and 20, 000 Hz.) Infrasound can induce deep feelings of unease, and has convinced many people that their homes are haunted.

The air conditioner featured in of Guo-qing Di’s study wasn’t actually spooking anyone – instead, it produced low frequency sound at around 100 Hz. Those frequencies aren’t going to induce the sensations of dread that infrasound creates, but a team of volunteers recruited by Guo-qing Di and colleagues
consistently rated the sound as highly annoying. But what can you do, short of
ripping out the aircon and going back to the days of stifling office buildings?

The team decided to try adding different sounds to the drone of their aircon, in the hope of masking or altering the low-frequency noise and, consequently,
reducing the calculated annoyance factor. The sounds they chose included natural sounds like birdsong and flowing water. They also tried frequency modulated pure tones, and something called pink noise. Volunteers were asked to listen first to the plain aircon noise, and then to the aircon noise mixed with the added sound. Finally, the volunteers rated on a numerical scale how much more or less annoying the new, modified aircon sound was when compared to the original. This numerical rating was called the Subjective Annoyance Value (SAV) of the modified sound.

So, how successful were these attempts at making aircon less annoying? Well, the results were a little bit surprising. Trying to mask aircon noise with sounds from nature, whilst in principle a good idea, wasn’t very effective at all in reality. The SAVs of combined aircon and nature sounds were, generally, higher than the aircon alone. Birdsong – which most people would consider a very pleasant sound – and wind noises were generally rated as more annoying than the sound of a ticking clock, or flowing water, when combined with the aircon drone.

The best way to reduce aircon annoyance, it turned out, was with a peculiar acoustic phenomenon called pink noise. Everyone’s heard of white
noise, of course, and can imagine what it sounds like. White noise contains all
the frequencies of sound that human ears can detect, and all the frequencies sound equally loud to our ears. Pink noise also contains that full range of frequencies, but, unlike white noise, the loudness of the frequencies is not equal across the range. In pink noise, the higher the frequency, the lower the loudness – which, in effect, makes for a more bass-heavy noise. You can see that more clearly by looking at the pictures, below:

The spectral density of white noise. Frequency lies on the horizontal axis, and intensity (loudness) on the vertical

The spectral density of pink noise

And you can hear the difference here:

What Guo-qing Di and colleagues found was that adding pink
noise to the aircon drone did actually reduce the subjective annoyance of the
noise – all the volunteers in the panel agreed on this. But you had to be a bit
careful with your pink noise. You need to add it at a decibel range of between
15 and 25, and with a frequency range of just 250 – 1000 Hz. Pink noise with a
larger range of frequencies – between 250 and 20,000 Hz – just made the aircon drone even more annoying.

Is this going to revolutionise our offices? Will manufacturers
start adding pink noise generators to their aircon units? It’s too early to
say. But we do know, now, that it is possible to make the sound of air conditioning a little bit more pleasant – although you may need the services of a skilled sound engineer to do it. And it’s good to know that within the sometimes lofty world of science are everyday heroes like Guo-qing Di and his colleagues – on a mission to make our daily lives, in this one respect, just a little bit less

*Well, to me it’s a great thing. You’d be entirely justified in replacing “great”, here, with “worst”.


Guo-qing Di et al. In press. Adjustment on subjective annoyance of low frequency noise by adding  additional sound. Journal of Sound and Vibration (2011)  doi:10.1016/j.jsv.2011.06.014

Tandy, V (1998). The ghost in the machine. Journal of the Society for Psychical Research 62 (851) pp 360 – 364

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