| How does one draw a sound? What elements of sound are there that we can show graphically in order to help us better understand and recognize important patterns in sound? What do you think a high tone looks like, or rolling thunder, or a chirping cricket? A spectrogram, or sonogram, is a visual representation of sounds that decomposes the sounds into the different frequencies that make up the sound. Scientists use spectrograms in a variety of fields, including linguistics and bioacoustics. Having a graph of sounds helps scientists determine their structure and recognize similarities and differences between sounds. Understanding the basic components of sound is necessary in order to comprehend the elements of a typical spectrogram. Sound travels by waves through some medium, such as air or water. This makes sound waves different than light waves, which can travel through a vacuum such as space. A sound wave can be pictured as a ripple in a still pool of water when a stone is dropped in the pool. The wave travels through the medium, but only displaces it in an up-and-down or back-and-forth motion. Sound waves are longitudinal waves, which means that the medium moves back-and-forth in the same direction the sound wave is moving. 
Fathom | Simple representations of sound waves as pure tones. The frequency of a sound wave, or interval between wave peaks or troughs, represents a tone's pitch and is measured in Hertz (Hz) which is how many times the wave moves the water back-and-forth per second. The higher the frequency, the higher the pitch to the human ear. The amplitude, or height, of the wave represents volume, or loudness. Humans can detect frequencies between 16 Hz and 20,000 Hz, although this varies widely with individuals and with age. An optimum range for human hearing is 1,000-2,000 Hz. The middle figure shows a pure tone of a violin being played softly at high pitch, which we would hear as high notes. The bottom figure shows a low-pitched instrument such as a double bass or tuba being played loudly. Note these are simple representations and do not show the complexities of harmonics, which all sounds have. | Harmonics is an important concept to understand with sounds in the real world, as almost all natural sounds have harmonics. To use the example of a vibrating string, the main pitch we hear comes from the vibration of the whole string, but other frequencies are also generated from parts of the string vibrating in relation to the whole string in precise ways that obey mathematical laws. The first harmonic is called the fundamental, and is the lowest frequency in the sound. The second harmonic is twice the frequency of the fundamental, the third is three times, etc. In a spectrogram, frequency is represented in kilohertz (kHz), or 1000 Hz, on the vertical line and volume is represented by varying colors, with red being the loudest and light blue being a faint sound. Time intervals are represented along the horizontal access. In the following spectrogram and in the spectrograms in subsequent sessions, each of the three bars is 10 seconds of sound. See if you can tell where the harmonics are in the spectrogram, which shows up as frequency "bands." |
