Chapter 6 Book Interaction between Sound and Media Notes

1. What is Attenuation?

• Attenuation means the sound wave gets weaker as it travels through the body.

• It's like how your voice sounds quieter when someone is far away.

• It's measured in decibels (dB), which is a special way of measuring how much sound has been lost.

Key points:

• +3 dB = sound is twice as strong

• –3 dB = sound is half as strong

• +10 dB = 10 times stronger

• –10 dB = 10 times weaker

2. What Causes Attenuation?

There are 3 main reasons why sound gets weaker:

1. Absorption – the sound turns into heat (this happens most often).

2. Scattering – sound bounces off in different directions (like light hitting rough water).

3. Reflection – sound bounces straight back (like an echo).

More distance = more weakening
Higher frequency (pitch) = more weakening

3. What is a Decibel (dB)?

• A decibel is a way to compare two sound levels using math.

• It uses logarithms (just a fancy math tool that helps with really big or small numbers).

Example:

• If the sound gets 10 times stronger, that’s +10 dB.

• If it gets 10 times weaker, that’s –10 dB.

4. Reflection & Transmission (How sound moves at boundaries)

When sound hits a boundary between two tissues (like muscle and fat), three things can happen:

1. Reflection – part of the sound bounces back (this is how we make ultrasound images).

2. Transmission – some sound keeps going into the next tissue.

3. Scattering – sound spreads out in different directions (usually in rough or small objects).

Smooth surfaces = clear echo (specular reflection)
Rough surfaces = scattered echo (diffuse reflection)

5. Acoustic Impedance (Z)

• This is how much a tissue resists the sound wave.

• Each type of tissue (like bone, muscle, or fat) has its own impedance.

• When two tissues have very different impedance, more sound reflects back (like sound hitting bone).

6. Refraction (Bending of Sound)

• If the sound hits a boundary at an angle, and the second tissue has a different speed, the sound bends.

• This is called refraction, like how a straw looks bent in water.

• It only happens when:

  1. The sound hits at an angle (not straight).

  2. The two tissues have different speeds.

7. Attenuation Coefficient

• This tells us how much sound is lost per cm as it travels.

• It depends on frequency:

  • High frequency = more sound lost (less deep penetration)

  • Low frequency = less sound lost (can go deeper)

Example:

• In soft tissue, about 0.5 dB is lost per cm per MHz.

8. Half-Value Layer (HVL)

• This is the distance it takes for the sound to lose half its strength.

• If HVL is thin, the sound gets weak quickly (high frequency).

• If HVL is thicker, the sound goes deeper before getting weak (low frequency).

Simple Summary

• Sound gets weaker as it travels—this is called attenuation.

• It happens because of absorption, scattering, and reflection.

• Decibels help us measure how much is lost.

• Reflections create ultrasound images.

• Sound bends at tissue boundaries if the angle and speed change (refraction).

• High-frequency sound gives better pictures but can’t go deep.