Bass through walls

Why can you hear a neighbour's bass through the wall but not their conversation?

Through a party wall at eleven at night you get the thump of a kick drum and the shape of a bass line, and nothing else. Not the melody, not the vocal, certainly not the conversation in the same room. Yet in the neighbour's flat the vocal is far more prominent — it is what they are listening to.

The folk explanation is that low frequencies "travel further". That is not doing what people think: over a few metres air absorbs almost nothing at either frequency, so both sounds arrive at the wall in roughly the proportion they left the speaker. What filters them is not the journey. It is the wall.

So ask what a wall must do to stop sound. Airborne sound is a pressure wave; to pass into the next room it must set the wall in motion, and the wall must push the air beyond. Its resistance to being moved is its inertia — its mass — and that is the basis of the first useful rule in the subject. Think what inertia does to a push. Sound is an oscillation, so the panel must be reversed at whatever rate the sound demands. At a low frequency the push has a long time to act, and the panel achieves a real displacement. At a high frequency it reverses long before the mass has been got moving, and the panel barely stirs. Inertia defends far better against a fast wiggle than a slow one.

That is what the mass law states quantitatively: for a single limp partition, transmission loss rises about six decibels each time you double the surface mass, and six decibels each time you double the frequency. The second half answers the question.

Work it out. A bass note might sit near 63 hertz, while the consonants carrying intelligibility — the differences between "cat", "cap" and "cash" — live mostly between two and four kilohertz. From 63 hertz to 2 kilohertz is five doublings, so the mass law alone predicts about thirty decibels more attenuation for the consonants. That is not a nuance; it is the difference between conversational level and the threshold of a quiet room.

Now stack two things on top. The frequencies with most energy in recorded music are the low ones — kick drum and bass are the loudest elements, often deliberately boosted — so the wall's weakest region and the source's loudest coincide. And in your room the surviving low frequencies form standing waves, since a wavelength at 63 hertz is around five metres and a bedroom is a resonator at those sizes: stand in one place and the thump is unbearable, move two paces and it halves.

There is also a mechanism the mass law does not cover, which is why the obvious fix so often fails. A modern separating wall is two leaves with a cavity, and two masses with a springy air gap between them form a mass-spring-mass system with a resonance of its own — typically in the tens to low hundreds of hertz. Near it the leaves move as one and the wall performs worse than the mass law predicts for its total weight. Double-leaf construction is excellent above that resonance and has a hole below it, and the hole is where bass lives.

Then there is the path avoiding the wall entirely. A loudspeaker on a floor is coupled to the building, and low frequencies travel through concrete and joists exceptionally well. Structure-borne sound flanks the partition, under it through the slab and around through the shared ceiling, and no work on the wall touches it.

Four contributions, then, calling for different remedies. Can we tell them apart? Yes, with a test a tenant can run: lift the speaker off the floor onto a compliant pad, and change nothing else. The refuting observation: if the thump next door drops substantially, the dominant path was structure-borne and the wall was never the problem — and if decoupling makes no audible difference, the sound is arriving airborne through the partition, where mass and cavity design are the answer. A second test checks the mass law: adding dense board to one leaf should improve the mid and high range clearly and the bass much less, because it also drags the mass-spring-mass resonance down into the very band you were trying to fix.

The folk account is pointed at the wrong object. "Bass travels further" describes a property of the sound; the real property belongs to the barrier. Two sounds leave the speaker together and arrive at the wall together, and one is filtered out going through. Nothing about the air did that.

A related confusion: this is not reverberation. A room's acoustic character — the richness a concert hall adds — is sound bouncing within an enclosure, treated elsewhere in this collection. The question here is sound crossing out of one enclosure into another, a different quantity with different governing properties. A room can be acoustically dead and still transmit bass magnificently.

Two qualifications on how far the mass law can be pushed. It idealises a single limp panel, and real walls depart from it both ways — upward where a cavity helps, downward at the cavity resonance and again at the coincidence frequency, where bending waves in the panel match the airborne wave and it transmits far more than its mass suggests. And the relative weight of airborne and structure-borne paths in a given building cannot be settled from outside; it is a measurement, of which the decoupling test is the cheap version.

Hence the design consequence. Every countermeasure effective at low frequencies is expensive in the currency buildings are short of — mass, cavity depth, mechanical separation — while the cheap remedies, foam panels and curtains and bookcases, absorb frequencies already being stopped. That is why neighbour-noise complaints are overwhelmingly complaints about bass.