You're at a conference and excited to hear the keynote speaker. The speaker is an expert in the field and you've learned a lot from reading his books. Alas, five minutes into the talk you start to check your phone, and ten minutes in you're drowsy. Finally, you leave the talk early to catch up on some work calls.

Why did this speaker fail to capture your attention and deliver value despite his credentials?

Because he didn't pay attention to his voice.

The one secret that successful presenters use to communicate their feelings about a topic and keep the audience engaged is vocal variety. Great speakers find ways to vary key elements of their delivery to create suspense and surprise.

Research shows that our body language and tone of voice telegraph how we feel about the words we speak. To be more believable and bring your words to life, increase fidelity between your delivery and actions by paying attention to how you use your voice.

Varying your vocal intonations plays another critical role -- that of influencing others. You can strategically change how you say things to emphasize a point, communicate how strongly you feel about it, enlist others, and more clearly communicate a call to action.

The three elements to master to keep your audience awake, engaged, and comprehending are:

• Volume. Loud or soft.

• Pitch. High or low.

• Rate. Quickly or slowly.

You can vary and combine these three elements to achieve multiple vocal combinations giving your speech texture and nuance.

Here are three tips to get you started:

1. Start with one element

Assess if there's one speech element that's an issue for you. Record yourself speaking and listen. For example, women sometimes talk in too high a pitch. Notice the difference in Margaret Thatcher's speech when she lowers her pitch.

You might also be someone who speaks too fast to compensate for your nerves when presenting to the CEO. At first, vary only one problematic element by doing the opposite of what you typically do: Lower your pitch or slow down your rate.

As you gain control, you can add other elements.

2. Don't over think it

At first, just try to change the volume, pitch, or rate for any one word in each sentence. It doesn't matter which word or which element you vary.

Notice how it wakes up your speech and your audience. Unsure how to do this? Take a tip from Hans Rosling's TED talk on global population. Notice that Rosling varies about every third word to increase engagement and comprehension.

3. Turn to poetry

Practice with a nursery rhyme you know well to help you get out of your head and disentangled from the actual content. You can then focus on how the words sound when you vary your voice each time.

Try saying the rhyme out loud six times highlighting one variation of each vocal element. Exaggerate it. Notice how you sound and how each element makes you feel when you emphasize it. Now, tone down the variation and say the rhyme again.

When you want others to pay attention to what you have to say, don't just work on your phrasing. Take the time to practice varying the three vocal elements of volume, pitch, and rate.

Do this well, and your audience will loudly sing your praises.