The Physics of Sound

The physics of sound is the study of the physical attributes of sound waves. There are many ways that sound can be described and measured. The following audio program discusses some of the ways these properties can be understood and applied in the recording studio.

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Sound Travels In Waves

When a disturbance of air particles happens a series of waves spread out from the source in all directions. Those waveforms are composed of cycles of compressed air molecules and decompressed air molecules. These waves will eventually lose their energy and settle back to a resting state unless disturbed again.

The ability of air to return back to a resting state is the elastic property that also allows sound to travel so efficiently and effectively. The more reflective surfaces in the space the longer it will take for the room to return to a stable resting state. The more absorbent the materials are in the room the quicker the room will lose its acoustic energy.


Propagation Of Sound

One of the most basic principles of the physics of sound is that sound completely fills a space. When a disturbance or a sound, occurs in a space such as a room it will fill that space entirely. In fact, there is no place in that room you could go where you would not be able to perceive the sound created within that space. The sound may be colored in terms of its frequency response or acoustic energy but it will completely fill every space of that room.

In an acoustic environment the energy of the initial sound wave will reflect off of surfaces and the energy of that waveform will have a certain amount of sustain. This is called reverb. The amount of reverb can be accentuated within certain frequencies areas that the room will naturally resonate at. These are called resonant frequencies, room modes or standing waves.

Frequencies

Even though sound waves propagate in all directions of a space they do not do so evenly. A sound wave will project more efficiently in a given direction if the sound is focused that way, as with a speaker. Sound waves also project differently with respect to frequency.

Higher frequencies have a tendency to be more directional and the energy dissipates more rapidly. Lower frequencies tend to be omni directional and our ability to localize them is more difficult than with higher frequencies. Our hearing is optimized to localize and interpret frequencies that are in the range of the human voice.

Transmission Of Sound

Another basic of the physics of sound is that low-frequency sound waves transmit through services more readily than do higher frequencies. Low frequency sound waves maintain so much energy that they can literally vibrate the walls, floor and ceiling of any room. This allows those sound waves to propagate or transmit into adjacent spaces.


The Propagation Of Sound In The Recording Studio

The whole purpose of the study of the physics of sound for the recording studio is to learn how to best control it and optimize it for the purpose of recording music. The design principles used to create recording spaces is based on the basic principles of the propagation of sound in a space.

The live room, or recording space, is typically designed to create a more or less even sound field throughout the room. Although this is technically impossible from a purely physical point of view, the goal is always to achieve this result to whatever degree possible.

The design of a control room, conversely, is to create a reflection free zone around the listening position of the engineer. The purpose here is to minimize the effect of the room environment on the sound coming from the speakers. The total elimination of reflections is not possible and would sound completely unnatural. The goal here is to limit early reflections within 20 ms that would tonally color the direct sound coming from the speakers.


The Physical Components Of Sound

The physics of sound in terms of sound waves can be broken down into a series of basic physical characteristics. These characteristics can then he measured, equated to the music or material that would be recorded in a space, and then worked with to achieve the optimal effect for capturing a recording accurately.

Frequency and pitch

In the physics of sound, frequency is defined as the number of cycles, or complete waveforms, that occur within the period of 1 second. A complete cycle is defined as one compression and rarefaction cycle. The number of cycles within a second can also be given a musical value that is called a pitch. Pitch is the relative highness or lowness of a tone or frequency.

There is a musical relationship between these different pitches that can be measured. The doubling of any given pitch will be perceived as the same note but at a higher frequency called an octave. The additive of any given pitch will create what is called a harmonic. Harmonics are musically related but are not necessarily perceived as the same note. It is the harmonics that help to define a sound as emanating from a particular instrument. It helps ups to distinguish a flute from a piano that are playing the same note.

The Range Of Human Hearing

The range of perceptible frequencies of our hearing mechanism is generally defined as being between 20 Hz and 20,000 Hz. The study of the physics of sound shows a wide range of frequencies that appear as sound waves. Although most people cannot hear this full range of frequencies, it is generally considered the standard measurement of our hearing ability.

Hearing this full range of frequencies is not necessary for the average person. The most important range of perceptible frequencies are the ones that are defined by the frequency range of the human voice. Hearing frequencies above or below this range adds detail to the sound but is not necessary for its perception.

It is only through regular study and use that we can maintain these extended frequency ranges in our hearing. The more you listen and put your attention to the detail of a sound the more acute your hearing will become to those details. It is a practice like any other physical activity that would get better when repeated over and over again.


Physics Of Sound And Speaker Design

Speaker systems are designed to most efficiently and effectively project sounds over the course of our range of hearing. Very few speakers can effectively reproduce frequencies from 20 cycles up to 20 kHz. Because our hearing range is compromised in those lower and higher frequencies already, our ability to perceive them even if accurately reproduced is going to be limited.

Understanding this principle is an important part of the study of engineering and the physics of sound. The use of subwoofers, for example, takes the workload off of the low-frequency drivers of the left and right speakers. If that low-frequency driver were required to project those low frequencies, it would negatively affect the ability of the speaker to project the frequencies the speaker is most efficient at reproducing.


Working With Different Frequency Ranges

The range of human hearing covers approximately 10 octaves of frequencies. Understanding these different frequency ranges and their importance in our perception is key to engineering any audio program. The following physics of sound information will outline those different ranges and their affect.

Low Frequencies

Low frequencies generally encompass the 1st 2 octaves of the range of our hearing. This would start at 20 cycles and end around 80 cycles. The general perception of low frequencies extends a bit higher than 80 cycles, generally up to hundred or 125 Hz. It is this frequency range that gives us the feeling sensation of sound in the form of vibration.

Low Mid Frequencies

The low mid frequencies pick up where the low frequencies leave off and extend up to about 400 cycles or so. These frequencies encompass the fundamental frequencies of a majority of the musical instruments. Therefore, this frequency range is critically important to capture accurately. It will also help to define the low frequency range.

Midrange Frequencies

The midrange frequencies generally start from around 400 cycles and extend up to approximately 2.5 kHz. This range of frequencies adds definition and intensity to a sound. It is this frequency range that gives us most of our localization cues. Balancing these frequencies between the instruments is critical to creating separation and definition within each of the individual instruments.

Hi Mid Frequencies

Hi mid frequencies generally start around 2.5 kHz and extend up to approximately 5 or 6 kHz. This frequency range provides the presence of individual sounds. Our hearing is optimized in this range because it is the area where we define the hard consonants of different words. This frequency range is all about detail and our ability to clearly understand what it is we are hearing.

High Frequencies

The high-frequency range starts around 5 or 6 kHz and extends up to the limit of our hearing at approximately 20 kHz. This frequency range is important to create the sense of size and space in a recording. Because high frequencies travel most efficiently in the upper area of a room, they give a sense of height that is necessary for the proper imaging and accurate representation of any recording.


Conclusion:

Understanding the physical properties of sound and how they transmitted into space and through speakers is an important part of recording in mixing any kind of audio. The use of this basic information should help guide you in terms of your work within any acoustic space for recording and mixing.

Understanding how sound propagates in a space and the interrelationship of the different frequency areas will help you to work with them more intelligently and effectively. With a minimum of effort and time, you can learn to work more efficiently with all your recording and mixing work.


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Anatomy of the Ear

Protecting Your Hearing

Temporary Threshold Shift

Critical Listening

Audio Ear Training

Physics of Sound

The Decibel

Fletcher and Munson

Selective Hearing

Speed of Sound and Wavelength

Acoustical Phase

The Sound Envelope

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