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Flying circus of physics

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Cocktail-party effect

In a small party with people standing and talking in pairs, each member of a pair stands at a “socially acceptable” distance from the other and the two can hear each other without any trouble. However, if the density of people in the room increases, why does hearing become more difficult, and what does each member of a pair do in response? Why can a voice still be distinguished? You might notice these same effects in many other environments, such as a noisy restaurant or subway car.

Answer  
As the density of people increases, the background noise of their conversations (sounds coming directly to you from them plus sounds reflecting from the walls and ceiling, as well as from other people) also increases. When the background noise becomes about as loud as the conservation you are having, you and your partner automatically raise your voices, an effect known as the Lombard effect, after Etienne Lombard who studied the phenomenon in 1911. Because all the other pairs of people have the same problem of hearing, they also raise their voices and so, you still have a problem hearing your partner. At some point, to avoid screaming, you and your partner step closer than normal (inside the “personal space” of one another). If someone quiets the party, say for an announcement, and then the conversations resume, the voice levels quickly (exponentially with time) return to their former values. The Lombard effect has been studied in some animals, such as birds that automatically raise the sound level of their calls when confronted with increased background sounds from other birds.

If someone were to record your conversation with your partner, using a single microphone, and then play it back later (in a quiet room), you probably would not be able to distinguish your partner’s words as well as you did when you heard them “live.” The difference is that, in person, you hear the partner with two ears---the slight delay between what the ears hear and the slight difference in sound intensities at the ears help you to distinguish your partner’s voice from the on slaught of other voices. The effect is dubbed the cocktail party effect. Being able to see your partner’s mouth motion and “bodylanguage” can also help you fill in for words or even for entire sentences that you did not hear clearly. None of these clues are available if you listen to a single-microphone recording of the conversation. Then you must use other clues, such as searching for intelligent thoughts or recognizable tones buried in the background noise. Being able to pull a conversation out of such background noise is sometimes too easy, such as when you can easily hear the voice of an audience member who was sitting near the recorder when a bootleg recording of a concert was made. Being able to recognize familiar sounds in a noisy background is used by some animals, such as a chick king penguin hearing its parents in the din produced by thousands of other nearby king penguins.

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