I. Firearms and Ammunition Basics

Understanding the source before the sound

The Cartridge

• Casing — brass cylinder, holds propellant
• Primer — impact-sensitive ignition cap
• Propellant — gunpowder, becomes hot gas
• Bullet — the projectile itself

Shotgun Ammunition

• Plastic shell with metal base
• Contains primer and propellant
• Wadding seals gas behind the load
• Fires pellets (shot) or a single slug

Firearm Types

• Pistol — Glock semiautomatic pistol; closed chamber, single barrel -
• Revolver — Ruger .357 Magnum revolver; rotating cylinder, multiple chambers -
• Rifle — AK-47 pattern rifle; long barrel with rifling grooves -
• Shotgun — Pump-action shotgun; smooth bore, large diameter -

What Is Caliber?

• Bullet diameter in inches or millimeters
• Same diameter, different cartridges
• More powder = more acoustic energy
• Helpful reference: Ammo Caliber Size Chart

Mechanical Actions

Percussion lock - hammer moves from half-cock to full-cock
Hear example at 1:53

Break-action - hinged barrel opens and locks closed
Hear shotgun breech at 5:07

Pump-action - sliding fore-end
Hear example at 10:35

Lever-action - lever near trigger
Hear example at 12:02

Bolt-action - manual bolt handle
Hear example at 14:57

Semiautomatic - gas/recoil cycles the action
Hear pistol slide at 33:07

Acoustic Clues from Mechanics

• Firing pin strike (example video)
• Slide or bolt cycling - (example video)
• Spent casing ejection - (example video)

The Revolver Cylinder Gap

• Small gap between cylinder and barrel
• Hot gas leaks from this gap
• Creates a distinct pre-blast impulse
• Visible in off-axis recordings

Discussion

• Why might caliber alone be misleading?
• How could mechanical sounds help an examiner?
• What makes the revolver acoustically unique?

II. The Muzzle Blast

The primary “bang”

What Causes the Blast?

• Propellant ignites inside the casing
• Hot gas builds extreme pressure
• Gas exits the muzzle behind the bullet
• Rapid expansion creates a pressure wave

How Loud Is It?

• Peak levels exceed 150 dB SPL
• For reference: jet engine at 30m is ~140 dB
• Instant hearing damage without protection
• Overwhelms most consumer microphones

Duration: Surprisingly Brief

• True muzzle blast lasts only 1-3 ms
• That is 0.001 to 0.003 seconds
• Hollywood gunshots are heavily processed
• Echoes make it seem much longer

Directivity: Not a Point Source

The 20 dB Difference

• Most energy goes forward (barrel direction)
• Rear levels are ~20 dB lower
• Measured for a .308 rifle
• Critical for shooter location estimates

Discussion

• Why does the blast last only 1-3 ms?
• How does directivity affect witness accounts?
• What happens to a microphone at 150+ dB?

III. Ballistic Shock Waves

When bullets break the sound barrier

Supersonic vs. Subsonic

• Supersonic: bullet faster than sound (~343 m/s)
• Subsonic: bullet slower than sound
• Supersonic rounds produce a shock wave
• Subsonic rounds produce only muzzle blast

The Mach Number

• M = V / c (velocity / speed of sound)
• M > 1 means supersonic
• Higher M = narrower shock cone
• M = 1.8 → cone angle of 34 degrees

The Shock Wave Cone

• Cone trails behind the bullet
• Angle: theta = arcsin(1/M)
• M = 1.8 → 34 degree cone
• M = 3.5 → 16.6 degree cone
• Interactive Example:
  • https://physics-simulations.org/simulation/sonic-boom-mach-cone

The “N” Wave

• Distinctive pressure signature
• Abrupt onset, then abrupt offset
• Looks like the letter “N” in waveform
• Caused by nonlinear air propagation

What a Microphone Hears

Downrange mic: shock wave arrives first

Then the muzzle blast arrives later

Behind the shooter: only muzzle blast

Speed of Sound and Temperature

$$c = 331\sqrt{1 + \frac{T}{273}} \text{ m/s}$$

• T = temperature in Celsius
• At 20C: c is approximately 343 m/s
• Must account for this in forensic timing

Discussion

• Why does a downrange mic hear two events?
• How could temperature affect a forensic analysis?
• Why might subsonic ammo be forensically harder?

IV. Other Acoustic Components

Beyond the blast and the shock wave

Mechanical Action Sounds

• Trigger mechanism click
• Firing pin striking the primer
• Slide or bolt cycling
• Spent casing hitting the ground

Cylinder Gap: Timing Analysis

• Time between the two peaks varies
• Depends on barrel length and bullet speed
• Longer barrel = greater separation
• Can narrow down revolver model

Surface Vibration

• Gunshot shakes the ground and surfaces
• Vibration travels through solids
• At least 5x faster than airborne sound
• Can help resolve location ambiguities

The Complete Acoustic Signature

1. Cylinder gap gas (revolvers only)

2. Muzzle blast (all firearms)

3. Ballistic shock wave (supersonic only)

4. Mechanical action sounds

5. Surface vibrations

What Determines the Signature?

• Firearm type (pistol, revolver, rifle)
• Caliber and cartridge
• Barrel length
• Supersonic vs. subsonic ammunition
• Recording position relative to shooter

Discussion

• How could surface vibration help in casework?
• Why does position matter so much?
• What combination of clues identifies a revolver?

V. Environmental Acoustics

How the world reshapes the gunshot

Reflections and Echoes

• Sound bounces off buildings, ground, cars
• Each reflection is a delayed copy
• Multiple reflections overlap in time
• Creates a complex, extended waveform

From 3 ms to 700+ ms

• Source blast: 1-3 ms
• Urban recording: 700+ ms
• Overlapping reflections fill the gap
• A “pop” becomes a sustained “boom”

Reverberation

• Dense overlap of many reflections
• Characteristic decay pattern
• Depends on the space geometry
• Indoor vs. outdoor: very different

Absorption

• Surfaces absorb some sound energy
• Soft materials absorb more (grass, fabric)
• Hard materials reflect more (concrete, glass)
• Changes the spectral content over distance

Estimating Reflector Distance

• Time delay between direct and reflected sound
• Distance = (delay x speed of sound) / 2
• Divide by 2 because sound travels there and back
• Can identify nearby surfaces

Forensic Implications

• Environment is part of every recording
• Same gun sounds different in every location
• Reflections can obscure shot timing
• But reflections also carry information

Discussion

• Why is a 3 ms blast recorded as 700 ms?
• How might reflections help, not just hinder?
• Why does the same gun sound different indoors?

VI. Laboratory vs. Forensic Recordings

Pristine data meets messy reality

Laboratory Recordings

• Specialized mics (up to 500 kHz sample rate)
• Carefully leveled to avoid clipping
• Quasi-anechoic (echo-free) environments
• Capture true waveform shapes

Forensic Recordings

• Body cameras, surveillance, phones
• Clipped and distorted waveforms
• Heavy background noise
• Dense reverberation

The Clipping Problem

• Gunshot pressure overloads the microphone
• Waveform peaks are chopped flat
• True amplitude information is lost
• Non-linear distortion adds false frequencies

Bridging the Gap

• Lab data provides reference signatures
• Forensic recordings require interpretation
• Pattern matching, not exact comparison
• Experience and methodology matter

Discussion

• Why can’t we just use better mics at crime scenes?
• What information survives clipping?
• How do lab recordings help forensic work?

VII. Forensic Recording Example 1

Taser, gunshots, and speech on one recording

Discussion

• How do overlaid envelopes help identify a weapon?
• Why might Taser timing matter legally?
• What are the risks of pitch-based speaker ID?

VIII. Forensic Recording Example 2

Synchronizing multiple devices

Discussion

• Why are distant recordings sometimes more useful?
• What makes a good synchronization reference?
• How does multi-device analysis strengthen a case?