Why Does an Empty House Sound Different?

Anyone who has moved house knows the moment. The furniture has gone, the boxes are in the van, and you walk back inside for one last check. You say something, and your own voice comes back to you strangely: brighter, louder, a little hollow, as though the rooms have grown larger overnight. Footsteps ring on the bare floor. A door closing sounds like it happened in a church. The house looks the same shape as ever, and yet it sounds like a completely different place.

The explanation lies in what sound does after it leaves your mouth. It spreads out through the air and, sooner or later, meets a surface: a wall, the floor, the ceiling, a window. Some of that sound energy is absorbed by the surface, and some is reflected back into the room. The reflected sound then travels on, hits another surface, and bounces again. What you hear in any room is a mixture of the original sound and all of those rapidly overlapping reflections.

Acousticians measure how long those reflections hang around. According to the acoustics consultancy Allegro Acoustics, reverberation time, often written RT60, is the time it takes for sound pressure in a room to decay by 60 decibels. The noise-control company eNoiseControl explains that the first equation for calculating reverberation time was developed more than a century ago by Wallace Clement Sabine, a physics professor at Harvard, and that his formula is still used today.

What decides that time is largely what the room is made of and what is in it. Allegro Acoustics explains that hard materials such as glass, plasterboard, tile and concrete reflect sound strongly, creating echoes and extended reverberation. Soft, porous things, including curtains, carpets and upholstered furniture, reduce reflections and shorten reverberation. eNoiseControl adds that materials are rated by how much sound energy they absorb, and that materials with higher absorption reduce reverberation time.

Now think about what leaves a house on moving day. Out go the sofa, the armchairs, the rugs, the curtains, the beds, the shelves of books and the coats in the hall, nearly all of them soft and absorbent. What remains is mostly bare walls, bare floors and glass. Allegro Acoustics describes exactly this: an empty room produces strong echo and flutter, while once it is furnished, the acoustic signature becomes more controlled. Your voice has lost its sponges.

So the empty house has not changed shape, and your ears are not playing tricks on you. Sound simply has far fewer soft places to land, so it keeps bouncing for longer, and you notice it straight away. The good news is that the effect is easy to undo. Hang some curtains, lay a rug, bring in a sofa and a few full bookshelves, and the echo quietly retreats. Slowly, the house stops sounding like an empty building and starts sounding like a home again.