Acoustics 101 | What is Reverberation?

What is Sound?

Sound is our perception of a specific type of physical event. This event is a change in air pressure that is detected by our ears. A sound pressure wave (SPW) is essentially a cycle of compressed and de-compressed air. SPW's develop over time, therefore there must be time (and air) for there to be sound. The quantity of compression and de-compression cycles over time indicates frequency. i.e 100Hz = 100 cycles per second. 1000Hz = 1000 cycles per second. 'Hertz' (Hz) is the unit of measurement for frequency (cycles per second). Human beings can hear frequencies that range from 20Hz to 20,000Hz and our optimal hearing range is between 1000Hz and 4000Hz. The frequency of a sound will affect how it is sensed by the human ear. For example, a 20Hz sound at 70 decibels is perceived to be quieter than a 1000Hz sound at 40 decibels. This is due to the 1000Hz sound (though lower in decibel value) being in the optimal hearing range.

The Fletcher Munson Graph (FM Graph) above shows us that our ears hear differently at different levels. In general, our ears don't pick up extreme high or low frequencies as well as the mid-range frequencies, particularly the 1000Hz to 4000Hz range. The FM Graph links points of equal loudness i.e a 50Hz sound at 65dBSPL and a 1000Hz sound at 25dBSPL will be perceived to be of equal loudness.

Perception of Space

In addition to detecting the direction of a sound source, our ears are also capable of determining what type of physical space/room we are in without actually seeing it. When a sound is made in a room, sound pressure waves spread out from the source in all directions. A small percentage of the sound will reach your ears directly without bouncing off anything first. A greater percentage will hit one or more surfaces of the room before reaching your ears. These reflected sounds will be lower in volume than the non-reflected sounds and will also take longer to reach your ears. The sounds that we hear in an enclosed space can be broken down into four aspects.

Direct Sound:

This is the part of the sound that reaches our ears first before hiting anything else.

Early Reflections:

These first reflections will hit your ears after approximately 10-40 milliseconds after bouncing off 1-2 surfaces. The reflected sound will be slightly lower in volume than the direct sound and will have fewer high frequencies due to absorption into surfaces it hits and reflects off.

Pre-delay:

The time gap between the direct sound and early reflections is known as the 'pre-delay'. The larger the room, the longer the pre-delay.

Reverberation time:

The exact frequency content and level of the early reflections will be determined by the types of material and surfaces in a room. The sound waves that reach our ears after the early reflections and bounce of several surfaces are known as 'reverberation'. Reverb will also be lower in volume and have fewer high frequencies. Reverberated sound reaches our ears in an almost constant stream (often making it difficult for us to clearly hear and understand what a person in a room is saying). This is due to the near constant reverb stream that has raised the noise floor. The time it takes for these reflected sounds to drop in level by 60dBSPL (to the point were you can't really hear the sound anymore) is known as the 'reverb time' or 'RT-60'. The larger the room, the longer the reverb time will be.