An Introduction to Sound, Frequency, and the Acoustic Spectrum.

Before learning about ultrasound, it’s important to first have an introduction to sound itself. Most of us hear a multitude of different sounds every minute, but few can explain how it all works together.

Simply put, sound is the vibration of air molecules (That’s why there is no sound in space!). When an object vibrates, it causes the air molecules around it to move. These molecules bump into the molecules next to them, and so on. This creates a wave of vibrations that travels through the air - and this is what we perceive as sound. Air itself does not fully travel with the sound wave. Rather, each air molecule moves away from a rest point, but eventually returns to it. When sound is created, the process can be broken into 3 stages:

1. The source vibration

2. Wave propagation (transfer of energy through a medium, i.e., air molecules).

3. Biological detection (eardrum)
   

When we hear something, our ears are sensing the pressure waves that are moving through the air. The number of pressure oscillations per second is known as frequency, which is measured in Hertz (1 Hz = 1 oscillation per second). These pressure waves enter the outer ear and cause the eardrum to vibrate. The soundwaves collected by the outer ear are passed farther into the inner ear where electrical signals are created. These signals are carried to the brain which translates them to sound that we can recognize and hear.

The Acoustic Spectrum

The audible range of humans - the frequency range of sounds that we can audibly hear - is 20 Hz to 20 kHz (20,000 Hz). Frequency is directly tied to pitch, so a 20 Hz sound would have an extremely low pitch (e.g., a deep bass). While the other end of the audible range is technically 20 kHz, most adults have trouble hearing anything near this. Even a shrill whistle has a frequency of just 10 kHz. Decibels (dB) are another primary measurement of sound. However, while frequency and pitch relate to the vibrations per second, decibels are an independent measurement related to intensity/loudness, or amplitude of the pressure wave. As we’ve just reviewed, the audible range contains soundwaves that we can hear. However, there are many sounds out in the world that we can’t hear. The acoustic spectrum refers to the full range of sound wave frequencies that can propagate through a medium as mechanical vibrations and waves. The acoustic spectrum is commonly divided into four major bands of frequency:

• Infrasound: Less than 20 Hz. Too low for humans to hear but can be physically felt.

  • E.g., The rumble of a distant volcano or earthquake.

• Audible Sound: 20 Hz to 20 kHz. Considered within the standard range of human hearing.

  • E.g., Voices, music, cars, etc.

• Ultrasound: 20 kHz to at least 100 kHz. This is well above the range of human hearing.

  • E.g., Gas, electrical, and mechanical issues, animal echolocation.

• Hypersound: 10^13 Hz (10,000,000,000,000 Hz).

  • Classical acoustic wave theory begins to break down. This is far outside our range of interest.

Frequency vs Amplitude

We may think that sounds outside the audible range don’t produce decibels because we don’t hear them. However, remember that frequency is the primary factor that determines if we can hear sound or not. Sounds throughout the entire acoustic spectrum can still produce decibels. It's common knowledge that loud sounds can damage your hearing. Concerts, explosions, and other sounds that are higher than 90 dB generally fall in the danger zone for long-term exposure. However, we’ve just established that sounds in other acoustic ranges (like ultrasound) also produce decibels. Although they won’t damage your hearing, these sounds can still cause adverse, non-audible effects. Sounds in the infrasound range can cause feelings of anxiety or nausea, and many ultrasonic sounds can cause headaches, dizziness, and pressure in the ears. In fact, extremely high-amplitude (~180 dB) ultrasound can cause severe internal organ damage, and you wouldn’t hear a thing. Remember that frequency, and the respective Hz, does not objectively categorize what can be heard. Bats can hear sounds from ~1 kHz to over 200 kHz. Their audible range simply includes the ultrasound range, whereas ours does not. Humans must visualize techniques like echolocation, whereas bats and dolphins audibly hear the associated ultrasonic frequencies.

Congratulations - You've completed your introduction to sound, frequency, and the acoustic spectrum!