To understand the best ways to mitigate sound issues and properly identify them, it is critical to have a basic understanding of acoustics. By understanding how we measure and define core acoustical properties, you can diagnose issues and right-size solutions. Below we will cover the simplified Basic Theory of sound:
Core Acoustical Properties
Core acoustical properties include:
- Frequency (Hz): measured in Hertz, frequency is the pitch of a sound. There are low-frequency sounds such as fog horns, designed to travel far distances (since low frequency waves travel much further). High frequency noises such as whistles, however, are much more easily blocked.
- Amplitude (dB): measures how “loud” a sound is. Most complex sounds have differing amplitudes (or volumes) at various frequencies. The average volume at all of the frequencies is called dBA.
- Duration/Fluctuation (sec): some sounds can be much more annoying or distracting than others. This is often a product of its fluctuation. Just like how a fan can help you sleep at night, our ears adjust to noise levels over time. Quickly changing noise levels catch our attention. The more impulsive a noise is, the more problematic it often becomes.
How Sound Travels – Why Can’t Sound Travel in Space?
Unlike other waves, sound waves travel directly through the material (or medium) that they’re passing through. For instance, light waves travel on photons that shoot through a space.
Sound, on the other hand, actually moves the air particles (or wood, water, metal). Because the particles are more closely spaced in wood, sound travels much faster through it. In general, sound travels more quickly through wood and metal, than it does through air.
This explains why sound cannot travel in a vacuum or in space. Without air particles to transmit the sound, the sound wave has no medium to move the energy.
Additive Properties of Sound
Decibels are measured on a logarithmic scale. This means that every increase of 10 dBA results in a doubling of loudness. Similar to the Richter scale for an earthquake, a level 7 earthquake may be moderate, while a level 8 may be destructive and level 9 catastrophic. Without context, Level 9 versus Level 7 does not appear to be a major change, but the intensity doubles with each increase.
Similarly, 60 dBA sound appears to be twice as loud as a 50 dBA sound, which is twice as loud as a 40 dBA sound.
dBAs cannot be added together. For instance, if a hand drill operates at 80 dBA, two hand drills will not operate at 160 dBA. In fact, through modeling and field testing, it has been determined that it takes 4 hand drills to reach 90 dBA, and it would take 76 hand drills to reach 100 dBA.
- 1 Hand Drill -> 80 dBA
- 4 Hand Drills -> 90 dBA
- 76 Hand Drills -> 100 dBA
When dB Values differ, they may be added together with the following rules of thumb:
- Differ by 0-1 dB, Add 3 dB to the Higher value
- Differ by 2-3 dB, Add 2 dB to the Higher value
- Differ by 4-8 dB, Add 1 dB to the Higher value
- Differ by 9+ dB, There is no change to the Higher Value
White noise is a mixture of high frequencies and low frequencies. Pink Noise, on the other hand, has the same amount of low frequency, but reduced high frequency noises. This is because many people consider high frequencies to sound “hissy” and cacophonous. Over tens of thousands of years, our ears became more sensitive to higher pitches.
Most sounds are made up of a combination of different frequencies (unless it’s a tuning fork, or a note on your piano, which is designed to resonate at only a single frequency).
For instance, a train whistle may be 90 dB at 500 Hz and 20 dB at 50 Hz. To create a single number to simulate how much sound an object makes, we use a rating system known as dBA. This is an “A-weighted” single-number that combines and “averages” the dB level at each frequency. Therefore, you can then determine that the train noise is only 70 dBA.
How is dBA Calculated for a Noise Source with Multiple Components?
A-weighting is the most common average sound level used in the acoustics industry. Because our ears are more sensitive to high-frequency noise, the A-weighting system reduces the weight, or importance, of the lower-frequency bands.
For instance, see the motorcycle sound levels below to determine its dBA level.
Properties of Sound Simplified
Although there is much more to Basic Theory of sound, you don’t need to know all of it to understand the basics of soundproofing. So you’re not faced with too much information all at once, only a few of of the topics are covered above:
- How Sound Travels
- Additive Properties of Sound
- How to Calculate dBA
More information on the basic theory of sound can be found in the Fundamentals of Architectural Acoustics, the above article is an excerpt from the publication.
