The nasal, trachea and lung cavities also add resonances that change the timbre of a person’s voice. Some people are gifted with internal air cavities that lead to formants which emphasize frequencies that we find pleasant to listen to; for example a fundamental and several overtones which are exact harmonics. If they are further gifted with (or can train) an ear-brain system that can distinguish frequencies well, they can become a good singer. Training of the voice can improve the range and ability of a singer. Opera sopranos, for example, undergo a long period of training to be able to sing higher notes and to sing loud enough to be heard over the orchestra.
Being heard over the sound of an orchestra seems like an impossible task but opera singers are helped by the fact that the formants involved emphasize many frequencies between 2000 Hz and 3000 Hz. Trained singers create this formant by lowering the larynx and expanding the throat region above the larynx. Orchestral instruments are louder but in a lower frequency range. That plus the fact that humans hear better in the range of frequencies that the voice uses makes it possible for an opera singer to sound as loud as the orchestra without using a microphone (although many performances today do use artificial amplification).
Although the formants used by an opera singer makes the sound more easily heard, they also have an effect on the way certain phonemes sound. The result is that in opera singing, particularly in the higher register of singing, it is harder to distinguish the words being sung. Here are examples of an opera soprano singing scales using a given phoneme. Notice that in the higher pitches it becomes difficult to distinguish different phonemes.
Because there are two or three predominant vocal formants, some singers can emphasize a note and a higher partial at the same time so that they can be heard as distinct tones. This is often called overtone or throat singing. Singers in Mongolia and some other regions use this technique as demonstrated in this video of Mongolian throat singing. About 20 seconds into the video you will hear a whistling sound singing a melody along with a lower drone sound at the same time. Both sounds are coming from the singer’s vocal system. This is something that can be learned by almost anyone, as demonstrated by the dozens of YouTube videos on “Tuva Throat Singing”.
We know that the speed of sound changes if the density of the medium in which it is traveling changes. We also know that since the speed of sound is related to wavelength and frequency by v = λ f, changing the speed of a wave will change its frequency (the wavelength remains fixed). You may have heard the voice of someone change when they inhale helium. This occurs because the speed of sound changes and therefore so does the frequency. The effect is as if the formants have shifted upward. The following video is of a voice with helium and with sulfur hexafluoride Sound in sulfur hexafluoride is slower so the frequencies of the formant will be lower. NOTE: DO NOT TRY THIS AT HOME. Helium and sulfur hexafluoride do not have oxygen in them and it is possible to suffocate. In particular sulfur hexafluoride is heavier than air and is difficult to get out of the lungs.
A little science behind “beatboxing”.
Many animals use sounds either attract mates or warn away competitors. A few animals use sounds to navigate (discussed in Chapter 16: Acoustics). In general, smaller animals make higher frequencies and larger animals make lower frequencies but there are plenty of exceptions. Elephants and whales use infrasound (below 20 Hz) to communicate over long distances but both mammals can also make higher frequency sounds.
Most mammals have vocal cords much like humans. Some marine mammals such as dolphins have phonic lips in their nasal cavities instead of vocal cords in their throats. These phonic lips act the same way vocal cords do, vibrating when the dolphin exhales and are controlled by muscles in the nasal passage. Whales do not have vocal cords but can pass air between two large internal cavities in their bodies and the passage way vibrates, allowing the whale to make sounds while submerged without exhaling.
Most insects and some crustacean make sounds by rubbing body parts together. This form of making sound is called stridulation. Insects also use resonating body parts such as wings and hollow body cavities to amplify the sound. Cicadas have an air filled abdominal cavity ribbed with cartilage and muscles and it is the contraction of these muscles which makes the sound we hear. Cicadas much louder than other insects which generate sound by stridulation because of their unique method of making sound. A few insects such as crickets use leaf structures or tunnels in the ground to amplify their calls. The treehopper, an insect, communicates to others of its species by sending vibrations through the branches of the plant it is on.
Fish generally make sounds by contracting the muscles around their swim bladder if they have one. This typically produces a broad spectrum of low frequency sounds (under 1000 Hz) and is called drumming. At least one type of herring emits gas through its anus to produce sounds.
Most birds have two sets of syrinx, one on each of their two bronchial tubes. These small areas of the bronchus can vibrate and because there are usually two regions, some birds can make two sounds at the same time. This is part of the reason birdsong can be very complex. This arrangement also allows birds to produces short sounds with durations of 1/200th of a second, 10 times faster than humans. This is probably why birds also have absolute pitch (their ear/brain system hears exact frequencies) whereas humans have relative pitch. Some birds and some frogs have resonating sacs that amplify the sounds being produced by vocal cords or syrinx.
Some interesting information about unusual animal noises:
Recordings of soundscapes (natural ambient sound – there is more there than you think!).