Why a vacuum cleaner's power stays the same as you walk away

Music · Published 19 October 2026 · Updated 25 October 2026 Swipe more like this →

In short: If you move away from a speaker, the volume drops, but the source's actual power remains constant. Understanding this distinction is the key to acoustic engineering. It is the difference between the energy a device produces and how that energy behaves as it travels through space.

Sound power (or acoustic power) represents the total rate at which sound energy is emitted, reflected, or transmitted by a source per unit of time. Measured in watts (W), it is an intrinsic property of the source itself. Unlike sound pressure, which is a property of the field at a specific point in space and decreases as you move away, sound power is neither room-dependent nor distance-dependent. It represents the total energy output in all directions.

To calculate this, engineers integrate sound pressure and particle velocity over a surface enclosing the source. For example, in a free field positioned over a reflecting plane like the ground, the sound power level (LW) can be derived from the sound pressure level (SPL) by accounting for the surface area of a hemisphere. This relationship is mathematically expressed as LW = Lp + 10 log10(2πr² / A0), where r is the distance from the source and A0 is a reference area of 1 square meter.

In practical applications, such as regulating industrial noise or labeling household devices like vacuum cleaners, measurements are often taken in hemi-anechoic spaces. These environments use hard ground and large open spaces to simulate free-field conditions. Regulations often use the A-weighting scale to ensure these power measurements reflect how the human ear actually perceives loudness. This distinction is vital for converting noise into usable energy, as it allows scientists to account for losses in capturing devices independently of the source's inherent output.

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