Reason #1: The Melody Is In The Middle
The melody of a song is the part that people hum or whistle when they have a song stuck in their heads. If there's a sing-a-long, most people involved will sing the melody, since that's what they know best. When people sing Happy Birthday To You, it's the melody that's heard most prominently.
Now, with most styles of singing, the melody is usually the highest part. In church hymnals and most choral music, for example, the sopranos have the melody and all the harmony parts are sung on lower notes. The altos are right below the melody, then the tenors, and finally, singing those very low notes, the basses.
In barbershop, the melody is usually not the highest part. The tenor sings above the melody, then comes the lead (he sings the melody) and baritone, who are constantly swapping places, and finally the bass who, once again, sings those very low notes.
Barbershop is not the only style that utilizes this structure (gospel quartets also use it), but it's the first element that starts to separate it from other styles.
Reason #2: The Dominant Seventh Chord
A musical chord is when more than one note is sung or played at the same time. In barbershop, there are four singing parts, so most of the time we hear four-part chords, and one of those four-part chords is called the Dominant Seventh.
Down through the years, chords have been given names to help musicians communicate more efficiently. Since I'm keeping things simple here, I'm not going to explain why one of the chords was named the Dominant Seventh, but suffice it to say that this particular chord is so prevalent in barbershop music that it has come to be known as the Barbershop Seventh Chord to many musicians.
The dominant seventh chord is found in virtually every musical style of the western hemisphere, but in barbershop, this chord is used as often as possible. In a typical, traditional song, 35 to 60% of the chords are dominant sevenths.
Here's an example of a song that's chock full of dominant sevenths: Story Of The Rose (written in 1899, arranged and sung by me) (© 2008 Jon Nicholas)
Reason #3: Ear Singing
Though many early barbershop quartets (the late 1800s to the early 1900s) used musical instruments to accompany their singing, most modern quartets sing in the a cappella style, which means without instrumental accompaniment.
The reason they prefer singing a cappella is because they can "bend" notes in order to make the chords sound better.
On many musical instruments, notes are separated in equal distances from each other. I won't go into the details about why, since I'm keeping it simple.
Chords sound better, though, when some of their notes are altered a bit. The reason why is because sound waves are involved, and sound waves don't follow a simple mathematical system. The human brain can detect when a chord "locks" into place, and most singers, with experience, can automatically adjust their notes slightly up or down to make a chord sound better than when played on an instrument like a piano. A locked chord is called so because once achieved, the singer feels vocally locked into place. It's a strange sensation, actually, because it becomes difficult to move up or down once the chord has locked.
Although bending notes like this is technically known as Just Intonation, in barbershop circles it's often called ear singing. Each of the four singers must listen to the other parts while singing their own. By using their ears (listening), they are able to determine when to raise or lower a note in order to achieve a locked chord.
Here's an ear-tuned dominant seventh chord: Tenor (7th note of the scale) and Lead (3rd note of the scale) are singing slightly lower in pitch than a typical instrument would play. (© 2013 Jon Nicholas)
Here's one that has all four parts singing on pitches matching a well-tuned instrument: No bending of notes here. (© 2013 Jon Nicholas)
Finally, here's a series of chords where the first one is ear-tuned, the second is evenly tuned, the third is ear-tuned, and so on back and forth to allow you to compare: Good, Not so good, Good, Not so good, Good, Not so good. (© 2013 Jon Nicholas) The difference is quite subtle, but the ear-tuned chord locks into place better. If you're having trouble hearing the difference, don't worry, most people struggle with it. Here's a tip: Try listening to the highest note in the chord. Which high note sounds more pleasant?
Reason #4: Overtones
If you listen to a really good barbershop quartet, you may find yourself wondering who is singing those amazingly high notes. You can pick out the note the tenor is singing, but there are notes even higher coming out of the group. Sometimes the notes are so high, you can't believe a grown man could sing them. Well, that's because they aren't singing them. You're hearing overtones.
An overtone is a natural phenomenon that occurs when sound waves rub against each other and produce a higher note. Overtones are occurring all around us every day, but most of the time we don't notice them because they are fleeting and usually soft in volume. In a well-tuned barbershop quartet, however, overtones are enhanced, even to the point where the loudest note is not being sung by any of the four singers.
Since each voice can produce its own overtones, it's quite possible to have a five, six, or even an eight-part chord being sung by four people. Any time that overtones can be heard during a song, barbershoppers, if you will, call this expanded sound.
Expanded sound is a wonderful experience for both the listener and the singer. Listeners are treated to a sound that cannot be duplicated on a typical musical instrument. One of the most common effects of listening to this sound (especially when it involves the above-mentioned barbershop seventh chord) is the rise of goose bumps on the skin. I've heard countless people tell me this after a performance. The singers, on the other hand, are feeling something that is hard to describe. It simply must be experienced to be understood. By the way, another benefit of this phenomenon is that the singer can relax his volume a bit because the over-all volume has now increased with the addition of the overtones.
Unlike most singers in other musical styles, barbershoppers actively work towards producing overtones.
Of Course, There's More...
I've covered four of the biggest reasons why barbershop singing sounds so unique, but there are more reasons than that.
For one, there's a little thing called the Circle of Fifths, and a classic barbershop song moves around that circle quite a bit. You may want to do your own research on the topic, but to keep it simple, most modern songs do not travel around the circle very far. Conversely, songs from the early days of American Popular Music, including those from the Tin Pan Alley era, do and this is why so many barbershop songs are from days gone by.
There are many modern songs that are sung in the barbershop style, though they usually don't have a high percentage of dominant seventh chords (partially due to the fact that they don't travel around the above-mentioned circle), but they still sound barbershoppy because they utilize several other elements I've mentioned in this article. Some people call this BarberPop, and it's a wonderful new subset under the broader umbrella of barbershop music.
You want more reasons? Okay then, but you'll have to research them yourself because I'm keeping it simple here.
Some other elements that set barbershop apart:
Echos and Patter Songs
Unexpected Chord Progressions
Want to know more about the genre?
I've read this book and it's very informative about the history of the barbershop quartet. If you're serious about learning about the genre, I recommend it.