Adding high notes is what mostly discussed in articles that talks about how to increase vocal range and ignoring altos and basses. Lower voices, this one's for you! We'll explore some ways to add low notes by using your chest voice.
Everyone uses the chest voice for normal speaking. Your speaking voice can teach you a lot about your singing voice as a matter of fact. The way you use your speaking voice can either help or hinder your singing voice.
Let us explore your speaking voice for starter. Laugh, cry, yawn, sign etc are your various non-speech sounds. Try voicing it out. Find the nearest pitch to the sounds you made if you have a piano or pitch pipe available. Now speak a few monosyllables: uh-huh, mm-hmm, aha. Once again, using the piano or pitch pipe, speak a few monosyllables and match the pitch you produce.
Now speak a few simple sentences, such as "my name is_____" or "I love to sing". Like the past exercise, find the matching pitch. Many people try to speak at a lower pitch than is natural for their voice when ideally, the pitch should be the same for the speaking as it is for monosyllables or non-speech sounds. This is not a healthy thing to do.
Speak monosyllables at various pitch levels on a piano and continue exploring your voice. Find the lowest pitch you can speak without sounding gravelly. "Vocal Fry" is the term used for the gravelly sound and sustaining this is not healthy. Your ideal speaking pitch should be about four to five steps above your vocal fry level.
After that, try reading a paragraph or speak some sentences. To find out how high you can go, experiment with higher speaking pitches. Along the way, note where your voice is most comfortable and where you start to hear and feel strain.
You will feel vibration or resonance in your chest when you use your 'chest voice'. This is when you produce tones in that pitch range. With you thumb and fingers resting on your collarbones, put your hand gently on your upper chest. Do a yawn-slide (exhale on the syllable "hee" or "hoo" while sliding from the top of your range to the bottom). Your hand should feel vibration as you slide down into your chest voice.
Although it feels like the resonance is occurring in your chest, it's actually happening in your throat and mouth. The air moving from your lungs and across your vocal folds is vibration that you feel.
The fifth slide is the simple low-range singing exercise. Use the buzz (that's the puckered lips vibrating as air is expelled) or a syllable such as "vaw", while in the comfortable middle part of your range and sing the starting pitch and slide down five steps. In the key of C major it would be G-C, so-do. The slide should be smooth, not bumpy or creaky. Start each repetition a half-step below the previous one.
If you feel bumpy or creaky sensations as you descend the scale, you're probably holding some tension. You stop for a while and do some relaxation exercises for your face and neck. Gently massage your face and throat, then try again. Close your mouth slightly from its starting position as you descend the scale.
Next, using again the buzz or "vax", sing an octave scale up and back down. As you go up the scale, allow your jaw to drop and your mouth to open a bit wider, then reverse that as you come back down. It may be helpful to imagine your tone on a path leading away from yourself, with low notes nearest and high notes farthest away. You can move one hand back to your side as you descend and move it away from your body as you ascend the scan. Well, that's one thing to try out.
The arpeggio is another helpful exercise. Sing do-mi-so-do-so-mi-do on a vowel sound, such as "oo", "ee", or "ah". Start each new arpeggio a half-step lower than the last.
Adding a lower range will take time and effort too, just like any other singing technique. If you are patient and persistent, you will see positive results.