Physics of Sound

Explanations by Malia L.

Sound is created by vibrations which displace air molecules, these vibrations and sound itself is depicted and understood as a sine wave. There are three main descriptors used to characterize and explain sound waves- phase, frequency, and amplitude.

Sine Wave Phase:

Phase has to do with the timing of a wave and is used to specify a certain position in the wave’s cycle. Phase is typically used when talking about multiple sound waves, which is typically what we are hearing. A pure tone is a sound wave with a single frequency and is the easiest type of sound wave to understand. However, our world is made up of many complex sounds that are made up of many frequencies combining together into complex tones and phases.

Phase is used to understand the difference between waves. When sound waves are out of phase or reaching the ears at different points in their cycle, it helps us localize and interpret where sound is coming from. When the same sound waves enter our ears at the same time and are therefore in sync, it intensifies the sound entering our ears and makes it louder.

If two of the same sound waves are reaching our ears perfectly out of sync so that the waves are entering the ear at opposite phases (i.e the peak of one wave is entering the ear while the trough of the other wave enters at the same time), the two waves cancel each other out and no sound can be heard.  Noise canceling headphones work in this way.

Sine Wave Frequency:

Frequency is another important characteristic in understanding and describing sound waves. We often understand frequency as the pitch of a sound, but pitch is subjective and personally understood while frequency can be measured. High-frequency sounds result in short, tight wavelengths while low-frequency tones create wider wavelengths. Frequency is an important factor to understand when it comes to hearing as the anatomy of our ears allows us to hear certain frequencies better than others. Our ears are equipped to best understand mid-range frequencies that are in the range of speech, so very high and very low frequencies need to be heard at a much greater intensity for us to even hear them at all.

Sine Wave Amplitude:

The last characteristic of a sound wave that is often used is amplitude. Amplitude is the measurable intensity of a sound and is related to how loud we perceive a sound to be. On a sound wave the amplitude is represented by the height of the wave. The taller the wave, the higher the amplitude, the more intense or loud the sound is perceived.

Sound Pressure Level in relation to Amplitude:

Another important aspect of understanding sound and sound waves is how it is measured. We often use Sound Pressure Level (SPL) to assign sound and its loudness a numerical value. SPL is measured by taking the difference in atmospheric pressure as sound passes through. Just like sound waves cause our eardrum to vibrate, they also cause vibrations in the air and that is what we measure. SPL changes based on how close the measurement is taken to the sound source so that the intensity of a sound decreases as you move away from the sound source (which is important when thinking about protecting your hearing, say at a loud concert).

Sound Pressure Level is an important factor to understand when fitting a hearing aid:

Whether the amplification source of a hearing aid is sitting in the ear canal or behind the ear needs to be considered as this will affect the SPL. This also relates to Boyle’s Law which states that as the physical volume of a gas decreases the pressure of the gas increases. Think about blowing up a balloon, as you blow into it the pressure increases to fill the space of the balloon, causing the volume within the balloon to decrease. This is also the case when fitting hearing aids. Because hearing aids sit in the ear canal and are closer to the eardrum the volume of the ear canal is decreased, which increases the Sound Pressure Level. This gives the hearing aid an extra loudness gain because of that decrease in volume of the ear canal and is called insertion gain.

Another important measurement to understand is Hearing Level (HL):

HL is used when doing a hearing test and is referenced to audiometric zero (0 dB HL), which is a soft sound heard by someone with normal hearing. The amount of SPL needed differs depending on the frequency of the sound being heard. High frequency and low frequency tones will need more SPL to be heard than mid-range tones. We use Hearing Level so that it is easier to understand and visualize an audiogram and what “normal hearing” looks like as it gives us a uniform reference point to go off of.

Another measurement used in hearing healthcare is Sensation Level (SL).

This is unique for each person and is based on an individual’s hearing thresholds. So that a 10dB SL would be 10dB louder than the softest sound an individual can hear at a specified frequency of a measured HL. This measurement is not used as frequently but is important to understand and especially useful when testing someone’s speech reception threshold.

If you are interested in learning more, contact us at Hear to U Audiology or Hears Hearing & Hearables!

Thank you for reading the physics of sound and how it begins to relate to testing one’s hearing.