WRITTEN BY DAVID CARFAGNO
SOUND EFFECTS EDITOR, BOOM BOX POST
As you might have read in our previous blog post How Do Ears Work?, our brains use our ears to derive sounds from detected frequencies. These frequencies are natural occurring vibrations that enter our ears where they are then processed into what we perceive as sounds. But what exactly are these frequencies? And how do they work?
Basics
First and foremost, frequency is a form of measurement described as the number of repetitions of a periodic process, or a cycle, in a unit of time. The standardized unit for frequency is the Hertz, abbreviated Hz. Named after Heinrich Hertz, 1Hz is the equivalent of 1 cycle per second. The faster a cycle occurs, the higher the frequency, and thus the higher the perceived pitch.
Frequencies and You
As a human, your ears and brain can process frequencies from about 20 Hz - 20,000Hz, or 20kHz. Although this may seem like a large range, when compared to other animals, it’s nothing impressive. Dogs, for instance, can hear in a range from about 67Hz - 45kHz. That is why a dog whistle can be used to get a dog’s attention while the whistler can’t hear it.
The perceived loudness of these frequencies, however, is not consistent. The Fletcher-Munson Curves were first developed by Harvey Fletcher and Wilden A. Munson in 1933. There have been revisions to it since then, but it shows how frequency affects loudness. A good way to hear an example of this is listening to white noise and pink noise. White noise and pink noise are made up of all frequencies, but while white noise projects all frequencies at equal power, pink noise adjusts the power as the octaves become higher so the perceived volume is the same. That is why an engineer will use pink noise over white noise to tune a room.
Applications in Sound Design
Filling up the frequency spectrum is a very important aspect of creating a professional sounding mix. Creating a lush and full sounding background for a scene can help sell elements that are not included in the picture. If a scene takes place on a mountain top, your first instinct might be to add a high whistling wind. While this will help establish your location, the mix might seem thin. By layering a more meaty roaring wind, you would fill out the frequency spectrum in the mix and give it a more full feeling. Punches are another sound design element that can benefit greatly from a full frequency spectrum. A nice slappy sounding punch (meaning it contains largely high frequencies) can convey the impact of the hit well, but adding a nice low end thump will help the audience to feel more pain behind the hit.