All new, ridiculously over-engineered engine sound

The engine sound is particularly important when it comes to creating an engaging racing experience and the engine sound in Rad Trails 2 never felt quite right.

It has a sort of dull, monotone ringing that gets annoying after listening to it for more than a minute at a time.

And it lacks the dynamics of a real motocross engine, with its much wider rev range and stronger attack when twisting the throttle.

Take a listen for yourself.

Before the overhaul

So, I finally got around to replacing it with a more realistic engine roar that doesn’t get too painful to listen to while playing for extended periods.

Building The Sound

Starting With A Single Combustion

An engine sound consists of many combustions firing in rapid succession. After failing to find a good audio sample online and exhausting the sound effects software options, I decided to build a custom combustion sound generator.

Basically, a small program that simulates the acoustic environment inside a cylinder and uses it to shape the sound of a combustion. On its own, the result is somewhat underwhelming:

A single combustion

Repeating The Combustion

In a real engine, there are thousands of combustions firing per minute, so the next step is to repeat the combustion sound.

The same combustion, repeated

Adding Variation

The main problem at this point is that the sound is too repetitive. In a real engine, the cylinders don’t produce exactly the same combustion sound every time they fire. Small timing differences and variations in the combustions make the sound less synthetic.

So I generated a few different combustion samples and added random timing variations to make a more pleasant base for the engine sound.

Idle loop with combustion and timing variations

This is the engine firing at its idle rate, around 1,400 RPM.

Building The Rev Range

I repeated the process to pre-generate loops for different RPM ranges, up to a maximum of 12,000 RPM.

Mid-RPM loop

High-RPM loop

Twisting the Throttle

On the game side, the bike simulates an engine by increasing its RPM along a curve while the throttle is held down.

This is then mapped to the RPM loops, creating an engine sound like this:

The base loops following the throttle

Adding Afterfire

This still sounds a bit too synthetic, so I introduced afterfire: when some of the unburnt fuel ignites sporadically inside the exhaust, creating lower pops and crackles when revving up and down.

The base loops with afterfire

The Final Roar

The final processing adds drive, reverb, and noise to simulate how an exhaust pipe shapes the engine sound and give it a rawer character. An extra layer of the high-RPM loop comes in at a higher volume to give the sound a stronger attack when you twist the throttle.

The final sound with afterfire and exhaust processing

Engine Sound In Action

Here’s how it sounds on the track. Turn up the sound and take a listen.

See You In The Beta

The new engine sound is in the latest beta. Rev it up and let me know if you think it makes the game feel more engaging to you.

Sign up for beta access here.

Daniel - AKA Crazy D