Identifying Frequencies

Terms: LoudnessPitchTimbre

Loudness Sound loudness is a subjective term describing the strength of the ear's perception of a sound. It is intimately related to sound intensity but can by no means be considered identical to intensity. The sound intensity must be factored by the ear's sensitivity to the particular frequencies contained in the sound. This is the kind of information contained in equal loudness curves for the human ear. It must also be considered that the ear's response to increasing sound intensity is a "power of ten" or logarithmic relationship. This is one of the motivations for using the decibel scale to measure sound intensity. A general "rule of thumb" for loudness is that the power must be increased by about a factor of ten to sound twice as loud.

A widely used "rule of thumb" for the loudness of a particular sound is that the sound must be increased in intensity by a factor of ten for the sound to be perceived as twice as loud. A common way of stating it is that it takes 10 violins to sound twice as loud as one violin. Another way to state the rule is to say that the loudness doubles for every 10 phon increase in the sound loudness level. Although this rule is widely used, it must be emphasized that it is an approximate general statement based upon a great deal of investigation of average human hearing but it is not to be taken as a hard and fast rule.

Two different 60 decibel sounds will not in general have the same loudness Saying that two sounds have equal intensity is not the same thing as saying that they have equal loudness. Since the human hearing sensitivity varies with frequency, it is useful to plot equal loudness curves which show that variation for the average human ear. If 1000 Hz is chosen as a standard frequency, then each equal loudness curve can be referenced to the decibel level at 1000 Hz. This is the basis for the measurement of loudness in phons. If a given sound is perceived to be as loud as a 60 dB sound at 1000 Hz, then it is said to have a loudness of 60 phons.

Fletcher-Munson Curve

Estimating Changes in Loudness

There are variations in individual perception of the strength of sound and studies have shown that in general the sound is perceived twice as loud if the sound level increases by 10 dB. Similarly, a 20 dB increase in the sound level is perceived as four times as loud by the normal human ear.

Changing 1 dB in sound pressure level - Insignificant change in apparent loudness Changing 3 dB in sound pressure level - Just perceptible change in apparent loudness Changing 5 dB in sound pressure level - Clearly noticeable change in apparent loudness Changing 10 dB in sound pressure level - Twice or half as loud Changing 15 dB in sound pressure level - Significant change in apparent loudness Changing 20 dB in sound pressure level - Much louder or quieter, four times as loud

Pitch Pitch = frequency of sound For example, middle C in equal temperament = Hz Sounds may be generally characterized by pitch, loudness, and quality. The perceived pitch of a sound is just the ear's response to frequency, i.e., for most practical purposes the pitch is just the frequency.

Timbre Color, or timbre (pronounced "TAM-ber"). Timbre describes all of the aspects of a musical sound that do not have anything to do with the sound's pitch, loudness, or length. In other words, if a flute plays a note, and then an oboe plays the same note, for the same length of time, at the same loudness, you can still easily distinguish between the two sounds, because a flute sounds different from an oboe. This difference is in the timbre of the sounds. Timbre is caused by the fact that each note from a musical instrument is a complex wave containing more than one frequency. For instruments that produce notes with a clear and specific pitch, the frequencies involved are part of a harmonic series. For other instruments (such as drums), the sound wave may have an even greater variety of frequencies. We hear each mixture of frequencies not as separate sounds, but as the color of the sound. Small differences in the balance of the frequencies - how many you can hear, their relationship to the fundamental pitch, and how loud they are compared to each other - create the many different musical colors. The harmonics at the beginning of each note - the attack - are especially important for timbre, so it is actually easier to identify instruments that are playing short notes with strong articulations than it is to identify instruments playing long, smooth notes.

The human ear and brain are capable of hearing and appreciating very small variations in timbre. A listener can hear not only the difference between an oboe and a flute, but also the difference between two different oboes. The general sound that one would expect of a type of instrument - a trombone for example - is usually called its timbre or color. Variations in timbre between specific instruments - two different trombones, for example, or two different trombone players, or the same trombone player using different types of sound in different pieces - may be called differences in timbre or color, or may be called differences in tone or in tone quality. Tone quality may refer specifically to "quality", as when a young trombonist is encouraged to have a "fuller" or "more focussed" tone quality, or it can refer neutrally to differences in sound, as when an orchestral trombonist is asked to play with a "brassy" tone quality in one passage and a "mellow" tone quality in another.

Many words are used to describe timbre. Some are somewhat interchangeable, and some may have slightly different meanings for different musicians, so no attempt will be made to provide definitions. Here are a few words commonly used to describe either timbre or tone quality. Reedy Reedy Brassy Brassy Clear Clear Focused or unfocussed Focused or unfocussed Breathy (pronounced "BRETH-ee") Breathy (pronounced "BRETH-ee") Rounded Rounded Piercing Piercing Strident Strident Harsh Harsh Warm Warm Mellow Mellow Resonant Resonant Dark or Bright Dark or Bright Heavy or Light Heavy or Light Flat Flat Having much, little, or no vibrato (a controlled wavering in the sound); or narrow or wide, or slow or fast, vibrato Having much, little, or no vibrato (a controlled wavering in the sound); or narrow or wide, or slow or fast, vibrato