Airborne vs Impact Sound: Ratings & Part E Explained

Airborne vs impact sound — the two problems, and how they're rated.

Before you can fix a noise problem, you have to identify which type it is. Airborne sound (voices, TV) and impact sound (footsteps, dropped objects) travel differently and need different solutions — and Building Regulations rate them with separate metrics. This article explains both and what the law requires.

The first question in any soundproofing project is: what kind of noise is it? The two fundamental categories — airborne and impact — behave differently and are tackled differently, so misidentifying the problem leads to spending money on the wrong solution. Many real-world complaints involve both at once.

Airborne sound

Airborne sound originates in the air — speech, television, music, a barking dog, traffic — and then vibrates the building structure, which re-radiates it into the neighbouring space. It's the 'I can hear them talking' problem. Airborne sound is tackled primarily by mass, decoupling, cavity absorption and sealing — building a barrier that's heavy, isolated and airtight, since even small air gaps let airborne sound straight through.

Impact sound

Impact (structure-borne) sound is created by direct mechanical contact with the structure — footsteps on the floor above, a dropped object, dragged furniture, a slammed door. The vibration is injected straight into the structure and travels through it efficiently. It's the 'I can hear them walking about' problem, and it's especially common (and intrusive) between flats. Because the energy enters the structure directly, impact sound is tackled at source — resilient layers, floating floors and soft finishes that cushion the impact — and by decoupling the structure, rather than by adding mass alone.

How they're measured

Sound insulation ratings (UK)

Metric	Measures	Better performance
DnT,w (+ Ctr)	Airborne sound insulation between rooms	Higher dB = better
L'nT,w	Impact sound transmission between rooms	Lower dB = better
Rw	Airborne insulation of an element (lab)	Higher dB = better

Airborne insulation is a 'how much is kept out' figure, so higher is better; impact transmission is a 'how much gets through' figure, so lower is better. The Ctr term added to airborne ratings is a low-frequency correction — it makes the rating reflect performance against bass-heavy noise (music, traffic), which is the harder, more realistic test.

Building Regulations Part E

In England and Wales, Approved Document E sets minimum sound insulation between dwellings — separating walls and floors must meet airborne and impact performance standards, with somewhat less demanding figures for conversions than for new build, reflecting what's achievable in existing structures. New separating floors must meet both an airborne minimum and an impact maximum; separating walls an airborne minimum. Part E also requires, in many cases, pre-completion testing (or the use of approved 'Robust Details') to prove the built performance — recognising that sound insulation, like airtightness, must be verified, not assumed.

What the numbers mean in practice

Because the ratings are in decibels on a logarithmic scale, it helps to translate them into lived experience. For airborne insulation (DnT,w, higher is better), a separating wall in the low 40s dB lets you hear raised voices and television fairly clearly; the mid-to-high 40s — around the Part E minimum for conversions — means normal speech is muffled but loud TV and music still come through; the mid-50s and above, the territory of good new-build separating walls, means normal conversation is inaudible and only loud bass is faintly noticeable. Each 10 dB improvement roughly halves the perceived loudness, so the difference between a 45 dB and a 55 dB wall is dramatic, not incremental. For impact sound (L'nT,w, lower is better), the lower the figure the quieter the footsteps above; a poor floor in the 60s dB transmits footfall intrusively, while a well-treated floor pulling the figure down into the high 40s or low 50s makes walking above far less noticeable. Knowing roughly where a building sits on these scales sets realistic expectations for what a given treatment will achieve.

The Ctr penalty and bass-heavy noise

The Ctr term that appears alongside airborne ratings (as in DnT,w + Ctr) is the single most useful number for anyone troubled by music or traffic, and it deserves explaining. The basic weighted rating is calibrated against a 'pink noise' spectrum that is fairly even across frequencies; Ctr is a correction that re-weights the rating toward low frequencies, reflecting the spectrum of real bass-heavy sources like amplified music, home cinema and road traffic. It is always a negative number, so a wall rated, say, 45 dB might be 45 (−8) Ctr, meaning its effective performance against bass-heavy noise is closer to 37 dB. A large Ctr penalty is the signature of a construction that relies on mass without decoupling — it does well against speech but poorly against bass. When assessing or specifying soundproofing for a music or traffic problem, the Ctr-adjusted figure is the honest one to judge against, because it is exactly the low-frequency content that ordinary mass-based treatments leave behind.

Common questions from clients.

What sound insulation figure do I actually need?

It depends on the noise and your tolerance, but as a guide: a separating wall in the mid-to-high 40s dB (around the Part E conversion minimum) muffles normal speech but lets loud TV and music through, while the mid-50s and above makes normal conversation inaudible with only loud bass faintly noticeable. Because each 10 dB roughly halves perceived loudness, the gap between a 45 dB and 55 dB wall is dramatic. For footsteps, a lower impact figure (L'nT,w) is better — pulling a poor 60s-dB floor down into the low 50s makes a big difference.

What does the 'Ctr' in a sound rating mean?

Ctr is a low-frequency correction added to airborne ratings (e.g. DnT,w + Ctr). The basic rating is judged against fairly even 'pink noise'; Ctr re-weights it toward bass, reflecting real sources like music and traffic. It is always negative, so a wall rated 45 (−8) Ctr performs closer to 37 dB against bass-heavy noise. A big Ctr penalty signals a mass-only construction that handles speech but not bass — so for a music or traffic problem, judge against the Ctr-adjusted figure, which is the honest one.

Is my noise problem airborne or impact?

Airborne sound is voices, TV, music and traffic — noise that starts in the air. Impact sound is footsteps, dropped objects and dragged furniture — noise from direct contact with the structure. If you mainly hear talking and TV, it's airborne; if you hear walking and thumps from above, it's impact (very common between flats). Many situations involve both, which is why diagnosis comes before specifying a solution.

What does Part E require?

Approved Document E sets minimum sound insulation between dwellings — separating floors must meet an airborne minimum and an impact maximum, and separating walls an airborne minimum, with somewhat easier figures for conversions than new build. It often requires pre-completion sound testing (or the use of approved Robust Details) to prove the built performance, because sound insulation is very sensitive to workmanship and flanking.

Why is impact sound so hard to stop between flats?

Because the impact energy (footsteps, drops) is injected directly into the structure and travels through it efficiently, often via flanking paths through the surrounding walls as well as the floor. Adding mass to your ceiling does little, because the energy is already in the structure. It's best tackled at source (a resilient layer or floating floor in the flat above) and by decoupling — which is why it often needs cooperation or a ceiling isolation system.