Understanding Loudness, Pitch, and Quality of Sound
There are many ways to describe a sound, but the perceptual attributes of a sound can typically be divided into three main categories-namely, loudness, pitch, and timbre. These attributes are crucial to how we perceive and interpret the sounds around us.
Loudness
Loudness is a subjective measure of the intensity of sound as perceived by the human ear. The most direct physical correlate of loudness is sound intensity (or sound pressure) measured close to the eardrum. It depends primarily on the amplitude of the sound wave. The intensity ($I$) of a sound wave is the sound power transmitted per unit area, measured in $\mathrm{W/m}^2$. Loudness ($L$) is commonly measured in decibels (dB). Loudness enables distinction between faint and strong sounds even if the frequencies are identical. Sound intensity $I$ is defined as the power transmitted per unit area. Loudness in decibels can be calculated using $L = 10 \log_{10} \left(\dfrac{I}{I_0}\right)$.
However, many other factors also influence the loudness of a sound, including its frequency content, its duration, and the context in which it is presented. Some of the earliest psychophysical studies of auditory perception, going back more than a century, were aimed at examining the relationships between perceived loudness, the physical sound intensity, and the just-noticeable differences in loudness (Fechner, 1860; Stevens, 1957). A great deal of time and effort has been spent refining various measurement methods. These methods involve techniques such as magnitude estimation, where a series of sounds (often sinusoids, or pure tones of single frequency) are presented sequentially at different sound levels, and subjects are asked to assign numbers to each tone, corresponding to the perceived loudness.
Other studies have examined how loudness changes as a function of the frequency of a tone, resulting in the international standard iso-loudness-level contours (ISO, 2003), which are used in many areas of industry to assess noise and annoyance issues.
Factors affecting loudness:
- Distance from the sound source
- Intervening materials
- Sensitivity of the detecting instrument (e.g., ear vs. microphone)
Pitch
Pitch refers to the sensation that allows the ear to distinguish between sharp (high) and flat (low) sounds. It is directly associated with the frequency of the sound wave. Pitch is crucial to our perception and understanding of music and language. Pitch plays a crucial role in acoustic communication. Pitch variations over time provide the basis of melody for most types of music; pitch contours in speech provide us with important prosodic information in non-tone languages, such as English, and help define the meaning of words in tone languages, such as Mandarin Chinese.
Human ears typically respond to frequencies between $20\ \mathrm{Hz}$ and $20,000\ \mathrm{Hz}$, known as the audible range. Frequencies above $20,000\ \mathrm{Hz}$ are called ultrasonic, and those below $20\ \mathrm{Hz}$ are called infrasonic. The pitch of a sound is independent of its loudness and is purely determined by the frequency of vibration.
Pitch is essentially the perceptual correlate of waveform periodicity, or repetition rate: The faster a waveform repeats over time, the higher is its perceived pitch. The most common pitch-evoking sounds are known as harmonic complex tones. They are complex because they consist of more than one frequency, and they are harmonic because the frequencies are all integer multiples of a common fundamental frequency (F0). For instance, a harmonic complex tone with a F0 of 100 Hz would also contain energy at frequencies of 200, 300, 400 Hz, and so on. These higher frequencies are known as harmonics or overtones, and they also play an important role in determining the pitch of a sound.
In fact, even if the energy at the F0 is absent or masked, we generally still perceive the remaining sound to have a pitch corresponding to the F0. This phenomenon is known as the “pitch of the missing fundamental,” and it has played an important role in the formation of theories and models about pitch (de Cheveigné, 2005).
We hear pitch with sufficient accuracy to perceive melodies over a range of F0s from about 30 Hz (Pressnitzer, Patterson, & Krumbholz, 2001) up to about 4-5 kHz (Attneave & Olson, 1971; Oxenham, Micheyl, Keebler, Loper, & Santurette, 2011). This range also corresponds quite well to the range covered by musical instruments; for instance, the modern grand piano has notes that extend from 27.5 Hz to 4,186 Hz.
Timbre (Quality)
The quality or timbre of sound is the characteristic that distinguishes two sounds of the same pitch and loudness produced by different sources. Timbre refers to the quality of sound, and is often described using words such as bright, dull, harsh, and hollow. Technically, timbre includes anything that allows us to distinguish two sounds that have the same loudness, pitch, and duration.
The distinction among loudness, pitch, and quality is fundamental for recognizing sound patterns. Loudness allows one to sense how strong or weak a sound is. Pitch is responsible for the perceived frequency and sharpness of a note.
An important aspect of timbre is the spectral content of a sound. Sounds with more high-frequency energy tend to sound brighter, tinnier, or harsher than sounds with more low-frequency content, which might be described as deep, rich, or dull. Other important aspects of timbre include the temporal envelope (or outline) of the sound, especially how it begins and ends. For instance, a piano has a rapid onset, or attack, produced by the hammer striking the string, whereas the attack of a clarinet note can be much more gradual. Artificially changing the onset of a piano note by, for instance, playing a recording backwards, can dramatically alter its character so that it is no longer recognizable as a piano note.
A pure tone has only one frequency, whereas most natural sounds are complex and contain several frequencies. Timbre enables recognition of musical instruments, voices, or other sound sources, even if they produce sounds of the same pitch and loudness.
In fact, the same instrument can create different tone qualities. If you pluck a guitar in different ways, you can get quite different tones. Try it!
Key Differences: Loudness, Pitch, and Timbre
| Attribute | Definition | Physical Property |
|---|---|---|
| Loudness | Perceived intensity of sound | Amplitude |
| Pitch | Perceived highness or lowness of sound | Frequency |
| Timbre | Quality of sound | Waveform and harmonic content |
Understanding the physical basis of sound characteristics enables accurate characterization and manipulation of sound in various applications, including acoustics, communication, and technology.