Why is bass so boomy, and what can I do about it?

April 18, 2008

Dear Cecil:

One of the joys of apartment living is the thrill of hearing your neighbor practicing bass guitar at five in the morning. When he blasts his stereo, it’s specifically the bass that I hear and feel and stew over. What is it about bass that makes it impossible to block using conventional methods? Are there any practical ways to stop this unrelenting wall of sound from driving me to insanity (besides asking the neighbor to turn it down, which doesn’t always work)?

Cecil replies:

It could be worse. If your place is built flimsily enough, it’s not just the bass you’re subjected to — long ago I briefly resided in a building where I could hear the guy next door cranking Zeppelin as clearly as if the speakers were mounted inside my mouth. But even when the walls are doing a decent job, yep, it’s those bass lines that tend to burrow unmolested through all intervening barriers and straight into the deepest reaches of the brain.

What makes the low end of the audio frequency spectrum especially invasive? There are a few key concepts in play here:

  • Attenuation. Everyone knows intuitively — it barely counts as an observation — that sounds get weaker (i.e., become attenuated) as they travel farther from their source. But why? Much of it is scattering and absorption. A sound wave passing through any medium — air, say, or drywall — does so by causing the medium’s molecules to vibrate. Scattering is the extent to which the wave gets fragmented and redirected upon striking an obstacle in its path. (Picture ripples on the surface of a pond breaking into subripples when they encounter a rock or stick poking out of the water.) Absorption is the drop in volume caused by energy loss in the form of heat — the result of making all those molecules move around. And the effects of both scattering and absorption increase with the frequency of the wave — the higher the frequency, the greater its tendency to die out. Thus, the treble and midrange sounds coming from your neighbor’s apartment get scattered and absorbed more thoroughly as they pass through the various matter surrounding it, leaving the big, dumb low-end waves to lumber along till they find you.

  • Directionality. Another property of low-frequency waves is that they’re less directional — they’re more prone to spreading out uniformly from wherever they originate. Though your neighbor may have his speakers pointed straight at his head, it’s mainly the higher frequencies that actually go where they’re aimed; the low ones are more likely to leak all over the place.

  • Resonance. If the frequency and volume are right, sound waves can cause entire objects to vibrate sympathetically — surely you’ve heard those souped-up car stereos that turn their host vehicles into gigantic joy buzzers. Because of their relatively large mass, things like walls and floors resonate more at low frequencies than at high ones, and thus can help to pass the bass notes along, particularly if the speaker is touching the potentially resonant surface. Long-term resonance can be pretty destructive: Thanks in part to vibrations caused by the wind that regularly swept over it, in 1940 the old Tacoma Narrows Bridge shook itself to pieces and collapsed into Puget Sound.

The good news is there are plenty of ways to cut down on the low-end sound oozing into your unit. The bad news is they all involve some fairly serious rehabbing. You could pump the empty spaces inside the walls full of state-of-the-art insulation; you could mount your wall panels on flexible spacers, or try multiple plasterboard layers glued together with resin adhesive; hell, you could put in new walls made of eight-inch concrete block. Assuming you’re in no position to proceed along such lines, though, the options quickly dwindle to band-aid remedies. You might consider some tweaks in d�cor — e.g., cultivate a fondness for thick textile wall hangings.

Low-frequency noise is weird stuff. Years back I noted that infrasound — sound pitched below the hearing range of most humans, which stops at around 20 hertz — can cause dizziness. Some recent research suggests it may do more than that. After taking spectrum analysis readings at a couple of U.K. sites repeatedly described by visitors as "haunted," Vic Tandy and Tony Lawrence of Coventry University have argued that the presence of 18.9 Hz infrasound is responsible for the creepy feelings described. (In one case they concluded that a terrifying, seemingly paranormal experience of Tandy’s had likely resulted from the whirring of a laboratory extractor fan causing his eyeballs to resonate.) And in 2003, the use of 17 Hz infrasound at London concerts of experimental electronic music correlated with audience reports of "unusual experiences" including nausea, momentary anxiety, tingling, and a sense of coldness. Ideally, Derek, by the time your neighbor has traded his bass for an ultra-low-end tone generator, one of you will have found someplace else to live.

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