Room acoustic treatment

Three problems, three treatments

Before buying anything, you need to know which of the three you want to solve.

A panel placed at random may well act on the third problem while doing nothing for the first two. That is the most common situation.

Thickness sets the frequency

Here is the figure that ought to appear on every package, and never does.

A porous absorber, mineral wool or foam, works by damping the movement of air, not the pressure. Against a rigid surface, that movement is zero at the contact and maximal a quarter of a wavelength away. An absorber therefore only becomes genuinely effective from the frequencies whose quarter wavelength fits within its thickness.

The same arithmetic as for SBIR, and the same quarter wavelength:

Read the first row again. The 5 cm foam panel, the one you see everywhere, does nothing below roughly 1,700 Hz. It treats a treble problem, often non-existent, and leaves untouched everything that was being complained about.

And to act from 100 Hz by porous means alone would take 86 cm of thickness. That is the reason corner bass traps exist, exploiting the diagonal to gain depth, and the reason for resonators, membrane or perforated panel, which obtain action in the bass through the resonance of a system rather than the thickness of a material. They are selective, so they must be tuned to a target frequency.

Thickness is not enough: size matters too

A very thick but small panel remains ineffective in the bass, for a geometric reason distinct from the previous one.

To intercept a reflection, a panel must be large relative to the wavelength concerned. Below a certain size, the wave goes round it as though it were not there. The minimum useful area depends on the target frequency and on the geometry of the bounce, described by the Fresnel zone of the path.

Hence two criteria rather than one: thickness decides from which frequency the material absorbs, size decides which frequencies the panel can actually intercept at its position. Treatment dimensioned on only one of the two disappoints.

Early reflections: finding where to place

The classic method is the mirror: sit in your seat, slide a mirror along a surface, and note every spot where you can see a speaker. Those are the early reflection points for that seat.

What you are after is not silence but time. The later and weaker the first reflection arrives relative to the direct sound, the more clearly the ear attributes the source to the speaker. That is what the gap between direct and first reflection measures, often called ITDG.

Two practical consequences. Treatment goes where the mirror showed a speaker, not uniformly. And the surfaces concerned are few: a handful of well placed square metres beats fully covered walls.

RT60: a quantity, not a quality

RT60 measures the time sound takes to decay by 60 decibels. It is a global, statistical quantity, very useful and often over-interpreted.

It tells you how much absorption the room contains, not where it sits. Two rooms with the same RT60 can sound very different, one having its absorption concentrated on the reflection points, the other spread across the ceiling.

It is read per octave band, and that is where it becomes informative. An acceptable RT60 in the midrange but a very long one in the bass is the typical profile of a room treated with panels that are too thin: exactly what the thickness table predicts.

Reference values exist for small listening rooms, and they vary with intended use and with authors. The most useful goal is not a number but a decay that is even from band to band: it is the unevenness that is heard, more than the absolute value.

Diffusers: what they do, and what they do not

A diffuser is not a substitute for an absorber. It disperses energy in time and space instead of returning it in a single clean bounce. The energy stays in the room: the RT60 does not move.

That is precisely why it is useful where you want to keep liveliness while breaking a specular reflection, typically the rear wall. And it is also why it does not solve a ringing problem in the bass.

Two dimensioning caveats. A diffuser works over a given frequency band, set by the depth of its wells: a decorative relief two centimetres deep diffuses nothing audible. And it needs distance for its scattered field to form: placed a metre from the ear, it does not do what is expected of it. In a small room, absorption is often the safer choice.

The major confusion: isolation is not absorption

These are two different trades, and everyday vocabulary mixes them up constantly.

The concrete consequence: covering a wall in foam does nothing to reduce transmission to the next room, and a highly isolating partition does not improve the acoustics inside. The two jobs are planned together, but they never substitute for one another.

The point no setting can fix

It is the same as in the other pages, in its most general form.

Equalisation acts on level, at one instant and one place. It does not shorten a decay: a resonance that takes a second to die away will still take a second, simply a little quieter. And it is that duration which makes bass muddled, as the page on room modes explains.

Treatment is therefore the only lever that acts on time. That is why it comes before equalisation, not instead of it.

Where the answer is different

You add too much. An over-absorbed room becomes dull and tiring, and voices lose their naturalness. The risk is real as soon as absorption is concentrated in the treble, which is the failing of thin panels used in quantity.

You unbalance the spectrum. Absorbing a lot of treble without touching the bass produces a room that is deadened yet still muddled at the bottom. It is the same failing seen from the other end: the thickness table explains why.

The room is a loft or a complex shape. Reflection points can no longer be found by symmetry, and the mirror method becomes essential, or modelling does.

The surfaces are lightweight. A stud partition already absorbs in the bass, sometimes considerably. The room may therefore need fewer bass traps than a concrete space, and a calculation assuming rigid boundaries will be pessimistic.

The budget is limited. In that case the order matters more than the quantity: early reflection points first, because that is where the ratio of perceived effect to surface area installed is highest.

What HTM computes

From the geometry of your room, HTM predicts RT60 per octave band, ranks the room modes, locates the early reflection points, flags the risk of flutter echo and boundary interference, then proposes a prioritised treatment plan costed in surface area. A QRD diffuser designer lets you dimension one, well by well.

The Pro tier adds per seat metrics including ITDG, C50, C80 and STI, complete modal analysis, the DIY absorber calculator, composing treatment in layers with transfer matrix computation, and the minimum panel size per band via the Fresnel zone, which speaks directly to the second criterion described above.

Several additions are announced for version 1.2, including the membrane bass trap and the perforated absorber in the calculator, with their assembly folded into the RT60. See the Features page for the exact status of each function.

All of this remains prediction, dependent on the dimensions and materials declared. The full approach is to predict, treat, then measure to verify: see the guide on measuring with REW.

Sources

• The quarter wavelength criterion for the effectiveness of a porous absorber in front of a rigid surface is classical building acoustics. The table values are computed for a speed of sound of 343 m/s.
• The insertion loss measurements of stretched fabrics carried out for this site are described in the fabrics methodology, with the protocol and its limits.
• Desirable RT60 values for a small listening room vary with intended use and with authors: this page therefore proposes no target, only a criterion of evenness between bands.

Further reading

The pages on room modes and on SBIR describe the two phenomena that bass treatment seeks to damp, and the one on subwoofer placement covers the free lever that should be exhausted before buying materials. The glossary defines the terms used here.