Shock Waves During Low-Level Supersonic Flight: Why the Sonic Boom Becomes More Intense Near the Ground

When an aircraft exceeds the local speed of sound, it generates a system of shock waves that propagates through the atmosphere.

Contrary to a common misconception, a sonic boom does not occur only at the exact moment an aircraft “breaks the sound barrier.”

As long as the aircraft remains supersonic, it continuously generates shock waves along its flight path.

These pressure disturbances form an approximately cone-shaped region extending behind the aircraft, known as the Mach cone. When this system of shock waves reaches an observer or a structure on the ground, it produces a rapid change in atmospheric pressure, perceived as a powerful sonic boom.

Why Is a Sonic Boom More Intense at Low Altitude?

The lower the aircraft flies, the shorter the distance the shock wave must travel before reaching the ground.

As shock waves propagate through the atmosphere, part of their energy is dispersed and attenuated. Therefore, under comparable conditions, a supersonic aircraft flying at low altitude may produce greater overpressure at ground level than the same aircraft flying at a much higher altitude.

During a low-level supersonic pass, observers are also much closer to the aircraft and its aerodynamic pressure field.

As a result, the sonic boom may be perceived as:

• louder and more intense;

• sharper and more abrupt;

• similar to a nearby explosion or powerful impact;

• accompanied by vibrations in windows, doors and lightweight structures;

• capable of causing damage to fragile components if the overpressure is sufficiently high.

However, altitude is not the only factor affecting sonic-boom intensity.

The pressure signature also depends on:

• aircraft size and weight;

• fuselage geometry;

• wing design;

• flight Mach number;

• aircraft altitude;

• flight attitude;

• acceleration and maneuvering;

• atmospheric temperature;

• wind and atmospheric conditions;

• terrain characteristics.

What Is Sonic-Boom Overpressure?

Before the shock-wave system arrives, atmospheric pressure remains close to its normal local value.

When the shock wave reaches an observer, pressure rises extremely rapidly. Additional pressure variations occur as the aircraft’s complete aerodynamic signature passes over the observation point.

The difference between normal atmospheric pressure and the pressure increase generated by the shock wave is known as overpressure.

In general, greater overpressure produces a more intense sonic boom and increases the possibility of vibration or damage to fragile structures.

Human perception is influenced not only by the maximum pressure increase but also by how rapidly that pressure changes.

A pressure variation that develops gradually may be barely noticeable. A similar variation occurring almost instantaneously may be perceived as a sharp and powerful explosive sound.

The Characteristic N-Wave Pressure Signature

The pressure signature produced by many conventional supersonic aircraft resembles the shape of the letter N.

For this reason, it is commonly called an N-wave.

In simplified terms:

1. pressure rises rapidly when the leading shock reaches the observer;

2. pressure then changes more gradually as the aircraft’s aerodynamic signature passes;

3. a second rapid pressure change occurs near the trailing portion of the signature;

4. atmospheric pressure returns to its normal condition.

This is why observers may hear two closely spaced booms.

The first is primarily associated with shock waves generated near the forward section of the aircraft. The second is associated with pressure disturbances generated farther aft.

Depending on altitude, atmospheric conditions and the aircraft’s geometry, the two pressure events may be perceived as one powerful boom.

A Sonic Boom Is Not Produced at Only One Point

A sonic boom is sometimes described as a single explosion that occurs when an aircraft crosses Mach 1.

That explanation is inaccurate.

A supersonic aircraft continuously generates shock waves while flying faster than the local speed of sound.

As the Mach cone intersects the Earth’s surface, it creates a moving region exposed to the sonic boom. This area is often referred to as a sonic-boom carpet.

People located at different points beneath the flight path may therefore hear the boom at different times as the aircraft continues along its route.

An observer may hear the sonic boom after the aircraft has already passed its apparent closest position because the aircraft is traveling faster than ordinary sound disturbances can propagate through the atmosphere.

Shock-Wave Reflection Near the Ground

At low altitude, interactions between shock waves and the terrain may make the pressure field more complex.

An incident shock wave may reflect from the ground and interact with other pressure disturbances generated by the aircraft.

Depending on altitude, aircraft geometry, terrain and atmospheric conditions, localized regions of increased or reduced pressure may occur.

Mountains, valleys, buildings and other terrain features can also modify how the pressure disturbance propagates.

In urban areas, reflections between buildings may increase the perceived intensity of the sonic boom and produce additional vibration.

Possible Effects on People and Structures

A low-altitude supersonic pass may produce:

• a strong startle response;

• the sensation of an explosion or impact;

• intense noise and temporary auditory discomfort;

• vibration of windows, doors, roofs and loose objects;

• activation of vehicle or building alarms;

• reactions in animals;

• possible damage to windows or fragile structures if overpressure is sufficiently high.

Not every sonic boom causes structural damage.

The consequences depend on the magnitude of the overpressure, the aircraft’s altitude, the strength and condition of nearby structures and local atmospheric conditions.

However, lower altitude generally reduces the propagation distance and may increase the pressure intensity experienced at ground level.

What Does the Shock Wave Do to the Aircraft?

A shock wave should not be understood as an external explosion striking the aircraft.

The shock-wave system is part of the aerodynamic pressure field created by the aircraft during supersonic flight.

For the aircraft itself, the main aerodynamic effects may include:

• increased wave drag;

• changes in pressure distribution;

• movement of the center of pressure;

• changes in stability and control characteristics;

• aerodynamic heating at very high speeds;

• interactions between shock waves and engine air intakes.

These effects must be considered during the aerodynamic, structural and propulsion-system design of a supersonic aircraft.

Low-Level Flight Does Not Necessarily Mean Supersonic Flight

A military aircraft may perform a very fast low-level pass while remaining below Mach 1.

In that case, there is no conventional sonic boom caused by a sustained supersonic shock-wave system reaching the ground.

Even so, the noise may still be extremely intense because of:

• engine exhaust;

• jet noise;

• high-speed airflow;

• aerodynamic pressure disturbances;

• the aircraft’s proximity to observers;

• sound reflections from the terrain.

Therefore, low-level flight and sonic boom are not synonymous.

A true sonic boom requires the aircraft to fly faster than the local speed of sound.

Shock Waves Over the Wing and Sonic Booms Are Related but Different

It is important to distinguish between a local shock wave over an aircraft wing during transonic flight and the sonic boom produced by a fully supersonic aircraft.

During high-subsonic or transonic flight, airflow over parts of the wing may accelerate beyond Mach 1 even though the aircraft itself remains below the speed of sound.

When this local supersonic airflow decelerates back to a subsonic condition, a shock wave may form over the wing.

This can contribute to:

• wave drag;

• shock-induced boundary-layer separation;

• high-speed buffet;

• changes in lift distribution;

• changes in aircraft stability.

However, this does not necessarily produce the same sustained ground-level sonic boom associated with an aircraft flying fully supersonically.

Both phenomena result from air compressibility, but they occur under different aerodynamic conditions.

Conclusion

When a supersonic aircraft flies at low altitude, its shock waves travel a shorter distance before reaching the ground and generally experience less atmospheric attenuation.

The result may be a sharper and more intense pressure disturbance, perceived as a powerful sonic boom and sometimes accompanied by vibration.

The sonic boom is not produced only when the aircraft crosses Mach 1. As long as the aircraft remains supersonic, it continues generating a system of shock waves along its flight path.

During a low-level supersonic pass, the reduced distance between the aircraft and the ground can transform a distant aerodynamic phenomenon into a much more intense event for people, buildings and the surrounding environment.

Marcuss Silva Reis
Economist | Commercial Fixed-Wing Pilot | Aviation Expert Witness
Postgraduate Specialist in Aeronautical Sciences, Civil Aviation Safety and Higher Education Teaching
Former Flight Instructor and Founder of Instituto do Ar