What do Adam Levine, One Republic, and Usher have in common? Well, each lead singer can comfortably sing higher than most altos.
There's nothing more embarrassing than singing to the radio, only to jump to head voice when the boys are still going strong.
Among those of us with lower ranges, there is a category that my group prefers to refer to as "lady bass" which is exactly what is sounds like: the deepest voices of an all-female group.
The Queen of Lady Basses is none other Johanna Vinson of Delilah and Musae. Jo described herself as a bass more than a singer, and even mentioned that she finds it strange to use her "girl voice," which can be interpreted as anything above middle C.
Dmm, and Other Sounds Basses Make
In order to maximize the bass sound from a female voice, it is essential that you stick to sounds that involves getting the most resonance possible. Words are the most difficult to produce at the lowest notes, so it's best to stick to sounds hums with an open throat. Dmms, bums, thms and other sounds with almost no vowel are going to resonate the most.
If you're lucky enough to have access to an octavizer, that will only enhance the sound you already have. It's a bit controversial however, and is colloquially referred to as "cheating" in the a cappella world. A Cappella 101 discusses the concept of cheating in a cappella in their poll here.