Sound insulation and sound absorption: the difference, explained with figures

Sound insulation and sound absorption answer two separate questions. Sound insulation asks: how much sound passes from one room into the next? Sound absorption asks: how much does it reverberate within a room? The first question belongs to building acoustics, the second to room acoustics.

Sound insulation describes the obstruction of sound propagation into adjacent rooms, essentially through the inertia of a massive, heavy building element. Sound absorption describes the conversion of sound energy into heat at a porous or vibrating surface in the room itself. A heavy concrete wall insulates very well and absorbs hardly at all. An open-cell foam absorbs very well and insulates hardly at all. That follows directly from the physics of the two effects.

In practice this means: against noise from the flat next door, mass and structural separation help, an absorber panel on the surface achieves nothing. Against reverberance and poor speech intelligibility in your own room, absorption helps, a heavier wall achieves nothing. Anyone who mixes the two up buys the wrong product for their problem.

The airborne sound insulation of a building element is described by the sound reduction index R in decibels. From the frequency-dependent curve, a standardised procedure to ISO 717-1 forms a single figure: the weighted sound reduction index. Measured in the laboratory without flanking paths it is called Rw; on the finished building, with sound transmission via flanking elements, it is called R'w, the weighted apparent sound reduction index. Because of flanking transmission, R'w usually comes out lower than the Rw of the same element. R is measured in the laboratory to ISO 10140 and on site to ISO 16283-1.

Decisive for the insulation of single-leaf elements is Berger’s mass law. In the theoretical ideal case the sound reduction index rises by 6 dB per doubling of the mass per unit area and likewise by 6 dB per doubling of frequency (octave). In building practice the gain per doubling of mass is often closer to 4 to 5 dB because of resonance and coincidence effects. What matters is the order of magnitude: insulation comes from mass and stiffness. Stiffness cuts both ways, though: it lowers the critical frequency and can worsen the insulation in the mid range.

The mass law only holds within a limited frequency range. At the coincidence frequency (also called the trace-matching frequency) the insulation dips. The critical frequency fc is the lowest frequency at which coincidence occurs. The bottom of the dip lies at roughly 2·fc, and only above roughly 3·fc does the insulation follow the mass law again. This is why thin, bending-stiff panels often have an audible weak spot.

As a guide to requirements: DIN 4109-1:2018, the German standard for sound insulation in buildings, requires an R'w of at least 53 dB for walls separating dwellings in apartment buildings. The weighted normalised impact sound pressure level L'n,w was tightened in the revised edition from 53 dB to 50 dB. These are minimum values; enhanced sound insulation to the German guideline VDI 4100 or DIN 4109 Supplement 2 is higher. These values are achieved through construction and mass, surface finishes play no part in them.

Sound absorption is described by the sound absorption coefficient α, a dimensionless value between 0 (complete reflection) and 1 (complete absorption). The sound absorption coefficient αs of plane absorbers is measured in the reverberation room to ISO 354. From the one-third-octave band measurements, practical sound absorption coefficients αp are formed per octave band, and from the octave bands 250 to 4000 Hz the weighted sound absorption coefficient αw follows to ISO 11654. In the reverberation room, αs can arithmetically exceed 1 because of diffraction effects.

Similar to insulation, the procedure works with a reference curve. It is shifted in steps of 0.05 until the sum of the unfavourable deviations is at most 0.10. The value of the shifted curve at 500 Hz gives αw. If the practical absorption coefficient in a band exceeds the curve by 0.25 or more, a shape indicator is appended: L for low (250 Hz), M for mid (500 and 1000 Hz), H for high frequencies (2000 and 4000 Hz). From αw, the sound absorption classes A to E are formed, from class A (αw 0.90 to 1.00) to class E (αw 0.15 to 0.25). Values below 0.15 are not classified.

The effect in the room works through the equivalent absorption area A. It is the product of area S and absorption coefficient α. A panel with α = 0.8 on 10 m² therefore delivers 8 m² of equivalent absorption area. The sum of all A in the room determines the reverberation time via Sabine’s formula T = 0.161 · V/A. More absorption area means shorter reverberation.

The most common mix-up in practice goes: I hang an absorber on the wall, then I hear the neighbours less. Physically, that is not true.

The apparent sound reduction index is determined on site from the level difference and a correction: R' = L1 - L2 + 10 · lg(S/A) to ISO 16283-1. Here S is the area of the separating element and A the equivalent absorption area in the receiving room. In this equation the absorber acts solely through A in the receiving room. The wall remains physically unchanged: same mass, same stiffness, same transmission behaviour.

Now the order of magnitude. If A in the receiving room were doubled, the correction term 10 · lg(S/A) would fall by 10 · lg(2), roughly 3 dB, and the measured R' would arithmetically even get slightly worse. But a single wall panel comes nowhere near doubling A. A receiving room easily has 30 to 50 m² of equivalent absorption area. A 2 m² panel with α = 0.8 adds 1.6 m². Against 40 m², that corresponds to 10 · lg(41.6/40) ≈ 0.17 dB, well below 0.5 dB and not audible. The airborne sound insulation of the wall stays practically constant.

The same absorber at the right scale, by contrast, changes the reverberation clearly audibly. In a 200 m³ meeting room with 40 m² of absorption area, the reverberation time is around 0.8 s (T = 0.161 · 200/40 = 0.805 s). To get down to 0.6 s you need around 14 m² of additional absorption area (A = 0.161 · 200/0.6 = 53.7 m², so about 13.7 m² more), for example as acoustic panels or ceiling rafts. The level in the room obeys the limit of 10 · lg(A2/A1): even doubling the absorption lowers the diffuse-field level by at most around 3 dB. Absorption makes the room quieter and clearer; it does not make the walls any tighter.

Alongside insulation and absorption, a third term keeps appearing: sound attenuation, often also called sound damping. In everyday language and even in product texts it is frequently used as a synonym for absorption, which adds to the confusion.

Sound attenuation denotes the obstruction of sound propagation through the absorption of airborne sound, in other words the conversion of sound energy into non-audible energy. At its core, sound attenuation is therefore the same physical process as sound absorption and belongs to room acoustics. There is no separate standardised figure in decibels for it as there is for insulation; what is measured and stated is the absorption coefficient. In technical acoustics, attenuation also stands quite generally for energy losses, for instance in silencers or as the decrease in level over distance. Here the concern is the confusion with absorption.

Rule of thumb for classification: if the figure carries the unit decibel and an R in its symbol, it is about insulation and about separating rooms. If it carries an α and a value between 0 and 1, it is about absorption or attenuation and about the sound within a room. For product selection this distinction is decisive, because a high αw says nothing about sound insulation and a high Rw says nothing about absorption behaviour.

When selecting material, it pays to classify the problem cleanly first and then to require the matching figure.

Problem: neighbour noise, impact sound, noise from room to room. That is building acoustics. The decisive figure is R'w in dB to ISO 717-1, for impact sound L'n,w, and the requirements of DIN 4109. Solutions come from mass, double-leaf constructions with separation and the avoidance of flanking paths. An absorber on the surface does not help here.

Problem: reverberance, poor speech intelligibility, high noise level within the same room. That is room acoustics. The decisive figure is the absorption coefficient, the weighted αw to ISO 11654 and the resulting equivalent absorption area. Solutions come from sufficient absorption area on ceiling and walls, dimensioned via Sabine’s formula and target values such as those of DIN 18041 (the German standard for reverberation time targets).

When comparing datasheets, a clear separation helps: αw and the sound absorption class A to E describe room acoustics, R'w or Rw in dB describe sound insulation. Two values stated side by side belong to different tests to different standards; one cannot be derived from the other. A material that insulates well and absorbs well in a single layer is rare. Multi-layer build-ups are the norm, for instance a heavy, dense partition as a mass-spring-mass system for insulation plus a porous lining for absorption on the room side. A neutral data basis with real ISO 354 measured values makes it possible to compare αw across frequency bands instead of relying on a single marketing figure.