Sound is energy whose source is a series of vibrations that create pressure variations. Air molecules compress and expand or oscillate in a manner best expressed and measured by waves. We hear sound when these pressure waves hit our eardrums, and we express these waves with measurements of time and distance.
The pitch of a sound is a measurement of time. An air molecule that oscillates back and forth once is known as a cycle. The number of cycles per second is a hertz (Hz). It answers the question, how frequently does the molecule oscillate? We call this the frequency and it is measured in Hz. The faster the molecules oscillate, the higher the frequency. If a molecule oscillates 2,000 times in a second, the sound wave has a frequency of 2,000 Hz.
The intensity or loudness of a sound is a measurement of distance. How far each molecule moves or is displaced in a given oscillation is the wave’s amplitude. The further the molecule moves, the greater the energy and the louder the sound. The range of this intensity is so large, that we need to use a ratio or logarithmic scale to measure it called a decibels (dBs). A decibel compares a subject sound relative to the softest sound detectable by humans. This scale is 0 dB (threshold of hearing) to 120 dB (threshold of pain). Since this is a logarithmic scale, moving a few dBs in either direction can be quite dramatic.
So, when we describe sound, we talk about how high or low the pitch is using frequency measured by hertz, and how loud the sound is using decibels (dBs).
Humans hear frequencies between 20 Hz and 20,000 Hz (a dog can hear up to 40,000 Hz). On the intensity scale, 10 dB is a whisper and 120 dB is a jet engine. It is important to point out that higher frequencies are perceived with greater intensity up to about 8,000 Hz. A 70 dB sound at 125 Hz is much less noticeable than a 70 dB sound at 3,000 Hz. This is important to remember as we test solutions and read data. As a scientist, we can measure a wave and know it exists, but as a consumer, if we don’t hear it we don’t know it’s there.
We all remember the philosophical discussion: if a tree falls in the forest and no one is there to hear it does it make a sound? The pressure waves are created, but an ear drum is not available to receive the vibration. In the commercial world, perception is reality.
When these waves are at regular frequencies, between 10 and 60 dB, and are at mathematically regular intervals, sound can be pleasing – music or the voice of a loved one. When these waves come in irregular frequencies, over 70 dB or at odd intervals, sound becomes noise. Given that it’s a wave of energy, it can reverberate and be difficult to control. There are many ways to control it; however, it typically takes a customized solution in context of the source.
Cycle: The path a molecule takes during compression and rarefaction back to compression.
Period: The amount of time it takes for the molecule to complete one cycle (T).
Frequency: How many cycles per second (1/T). This measurement is called a Hertz (Hz).
Amplitude: How far an air molecule moves (compression and rarefaction) from equilibrium or average pressure.
Intensity: Amplitude over time over an area. It is measured in watts per square meter (w/m2).
Decibel (dB): A logarithmic measurement (a ratio) that compares the intensity of a sound to the threshold of human hearing or the least powerful sound that a human can still hear: (10-12w/m2).