While having a beer prior to going onstage may not seem like it would have any effect on a vocal performance, there is a price to pay. Among other effects, beer can dry out vocal cords and produce mucus in the throat---both of which can spell disaster for a singer's instrument.
Beer and other forms of alcohol are diuretics that cause increase urination, which, simply put, removes fluids from the body. The vocal cords perform best when hydrated. Drinking beer prior to singing can dry out and shrivel the vocal cords, making it difficult to hit correct notes. Singing with dry vocal cords can also lead to injury.
Beer is slightly acidic, so it creates mucus in the throat---a detriment to any signer. Mucus interferes with a singer's air stream by fragmenting the air flowing through the throat and vocal cords. Interruptions in the air stream can lead to cracking and loss of notes altogether.
Effects of alcohol
Drinking beer and alcohol lowers inhibitions and relaxes reflexes. This can lead to over-signing and loosing control of the voice---leading in turn to missed and off-pitch notes. Being intoxicated will also of course affect a singer's ability to pronounce words clearly in some cases.
Other drinks to avoid
Beyond beer and alcoholic drinks, vocalists should limit or avoid caffeinated drinks such as sodas and coffee before a performance; both soda and coffee are diuretics. Lemon and other acidic juices and even milk should also be avoided because they trigger the body to create mucus. Instead, vocalists should drink plenty of water and maintain their health to give their best performances.
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