May 2019 Newsletter: Low End Theory

With the rise of musical genres such as hip-hop and EDM, low frequency sound reproduction is vital to delivering the music the artist intends, and subwoofers have becoming the norm in residences. It’s clear that culturally we’re all about the BASS. Whether it’s transmitting through the wall between you or your neighbor or generated by a mechanical unit on a lightweight roof, all that low frequency sound presents interesting acoustic challenges. Read on to learn more about how low frequency sound propagates.

The frequency range of human hearing before any loss is between 20 to 20,000 Hz. This range is typically broken up into 3 parts: low (20 Hz – 160 Hz), middle (160 Hz – 500 Hz), and high (500 Hz – 20,000 Hz). As frequency decreases, the wavelength, or distance between crests of a wave, increases.  Those bass sounds we’re so fond of fall into the low frequency range, which have longer wavelengths than higher frequency sounds.  A 20 Hz pure tone has a wavelength of roughly 56.5 feet, while a 160 Hz tone has a wavelength of approximately 7 feet. Those long wavelengths are responsible for interesting acoustical phenomena.

A common strategy to block sound from traveling from one point to another is to build a barrier; however, sound waves can diffract, or bend, around obstacles: a large obstacle is needed to effectively block a large wave.  A good rule of thumb is that a barrier’s minimum dimension should be at least as big as the wavelength the barrier is intended to block.  Low frequency sound effectively doesn’t “see” a barrier smaller than itself and diffracts around it as if it weren’t there. That means to block a 20 Hz sound wave, you need a wall over 50 feet tall! This phenomenon also applies to walls inside a building. If the wavelength of a sound is larger than the dimensions of a wall, it will pass through fairly easily as it doesn’t “see” the wall as a barrier to the next room.

Diffraction also plays into the directionality of sound. High frequency sound is typically highly directional, mostly due to the fact that sources of high frequency sound are at least as large as the wavelength of the sound produced. Conversely, low frequency sound is often omnidirectional, propagating more or less equally from all sides of the source. A common example of this directionality can be experienced when a car with an upgraded audio system is nearby. When the car is far away from the listener, say around a corner, the listener can hear the bass produced by the subwoofer quite well. It isn’t until the car turns the corner, however, that the listener can see the car and hear the crisp highs and punchy mids generated by the system. There are clever ways to apply signal processing to make a relatively small speaker produce directional low-frequency sound that were discussed in previous newsletters.

Whether it’s designing an appropriate building shell of a concert venue, improving the sound isolation between your office and the 110 dB rehearsal space next door (true story!), or evaluating rooftop equipment above apartments, low frequency sound propagation is a mighty force that can’t be ignored. With Metropolitan Acoustics it is all about that bass, no treble!
April 2019 Newsletter: Acoustic Metamaterials