Deep bass serves as the foundation for any high-performance audio system. For both music and movies, the ability to play low is a crucial capability. Whether you use full-range speakers or separate subwoofers to get there, the lowest notes require ever-increasing amounts of power to reproduce properly.
A few nights ago, my wife Danya and I had dinner with Keith Yates and his wife Hanne at Philly’s a.kitchen restaurant, where we discussed deep bass among other things. At one point, I asked, “What is the lowest frequency someone should aim to reproduce at reference levels in their system?” As is typical with Keith, the answer was not simple—but it was thorough and scientific. It also included a number, but I’ll get to that later. First, it’s worth discussing what it takes to achieve great bass response.
The deeper you go, the less sensitive your ears are to bass. Below a certain point—right around 20 Hz—bass morphs into a tactile experience, and the ears play an ever-diminishing role. Yet, infrasonic bass is precisely the stimulus that makes things seem real. When you “feel” a door slam in another part of the house, infrasonic bass carries the sensation to you. In a movie, the tangible nature of deep bass makes things like gunshots and explosions feel real. At it’s best, bass produces goosebumps.
Subwoofers come in all sorts of shapes and sizes—and capabilities
Great bass response encompasses more than just digging deep. Because bass waves are prone to bouncing around a room, resulting in a matrix of peaks and valleys—just think of a wave pool at a water park—you must take great care to achieve a room response that is as smooth as possible. Often, the best solutions involve the use of multiple subwoofersalong with careful placement. Of course, buying more than one sub multiplies the cost and takes up additional space.
When shopping for a subwoofer, it’s hard not to notice the correlation between bass extension and price—digging deep costs money because it requires a lot of power and displacement (the amount of air a driver can push). Quite a few high-end full-range speaker designs are essentially a subwoofer with a bookshelf speaker on top—the iconic Wilson Watt-Puppy and B&W’s 802D come to mind. The problem with the full-range speaker approach is that it’s rarely ideal to have deep bass emanate from the same spot as the rest of the audio spectrum—hence the popularity of separate, dedicatedsubwoofers.
Smooth response (using multiple subs) is a good thing, and so is bass extension. How low should you go, given the budget-busting nature of digging really deep? Let’s look at some numbers:
32 Hz – It’s deep, but not that deep. In musical notation, it’s known as “pedal C” or C1. 32 Hz aligns with what commercial movie theaters and concert venues define as the bottom end. By design, many “pro” subwoofers can’t play much lower than 32 Hz—instead, the focus is on efficiency from that frequency on up. Subwoofers designed for the home that play down to 32 Hz (but not deeper) are usually compact and relatively affordable. The problem is that a lot of movies contain a lot of audio information below 32 Hz. Also, 32 Hz is one octave above the lowest note used in classical music—again, there’s a real chance that you’ll miss out on part of the performance if you only get down to 32 Hz. On the other hand, some movies (e.g., Star Trek, The Hobbit) filter out everything below a certain frequency, and often that frequency is 30 Hz or higher.
Subs that only go down to around 30 Hz are relatively compact and/or inexpensive
20 Hz – This is commonly considered the lowest frequency humans can hear, and 20 Hz is deep. Unfortunately, deep doesn’t come cheap—the leap from 30 Hz to 20 Hz requires more power, larger cabinets, and larger drivers. Many movies use sound effects around 20 Hz to add visceral sensation to the experience. Sometimes it’s a super-low drone that induces a sense of dread, sometimes it’s an explosion that sucks the air out of your lungs. Other times, it’s the subtle thud of a car door or the crunch of a horse’s hoof that feel real thanks to deep bass between 20 and 30 Hz.
16 Hz – This is the frequency of the lowest note used in classical music—sub contra C. In my view, fans of 2-channel music listening can’t claim to have a full-range system unless it is capable of playing at reference levels from 16 Hz up to 20 kHz. While it’s rare to find notes that deep in music, film is another story. For movies, the extra depth (below 20 Hz) translates to even-more visceral audio.
Subs with extension to 16 Hz tend to cost more and ported designs that dig deep require large enclosures
8Hz — This is true ULF (ultra-low frequency) territory. Few commercial subs claim to dig this deep. Still, content exists to take advantage of a system that can play down to single-digit Hz. For example, the “F****** Irene” scene from Black Hawk Down provides a 7Hz pulse that convincingly emulates standing under the spinning blades of a Black Hawk helicopter. The power and displacement required to achieve bass that low makes it a rare experience; I got my first taste less than one year ago when I visited the (first) AVS Forum Home Theater of The Month, “Popalock’s Bassment.” That theater featured sixteen 18-inch subs crammed into a standard-sized room, and the effect was literally scary—if I recall correctly, my fight-or-flight instinct kicked in several times during that visit. 8Hz also happens to be the frequency Keith Yates suggested as a target.
Sixteen 18-inch subwoofers handled infrasonic content with ease
A chart of the bass response in Black Hawk Down’s “F***** Irene” scene
Is infrasonic bass a worthwhile pursuit? What matters more to you, deeper bass or flatter response? Do like using discrete subwoofers, or do you prefer full range speakers that integrate subwoofer capability? Ultimately, how low should you go?