Acoustic curtains for windows: custom-made vs. standard, what's the impact?
A custom-made acoustic curtain, manufactured to the exact dimensions of the window, offers 3 to 5 dB of additional performance compared to a standard size. For a 22 dB attenuation, losing 3 to 5 dB due to poor sizing represents 15 to 25% of the total performance. Concretely, this is the difference between "the street noise has almost disappeared" and "the noise has decreased a little but remains annoying."
This guide explains why every uncovered centimeter reduces acoustic efficiency, how side leaks cancel out part of the investment, and how to measure correctly for each type of window.
Why curtain size affects acoustic performance so much
Sound behaves like air: it passes through the slightest opening. In building acoustics, this rule is formalized: an opening representing only 1% of a wall's surface can let through up to 50% of the sound energy. Applied to a curtain, this means that a few centimeters of gap on the sides or bottom are enough to halve the effectiveness of the insulation.
A standard-sized acoustic curtain never exactly matches your window dimensions. Typical discrepancies:
- Sides: a 140 cm curtain on a 130 cm window only extends 5 cm on each side (instead of the recommended 15-20 cm). Sound bypasses the fabric at the sides.
- Bottom: a 230 cm high curtain on a rod-to-floor configuration of 245 cm leaves a 15 cm gap. Cold air and sound rush in from the bottom like under a door.
- Top: if the curtain is too short to reach 10 cm above the frame, sound passes over the top.
Figures: performance loss according to type of leak
| Type of leak | Gap dimension | Estimated loss | Actual attenuation (22 dB* curtain) |
|---|---|---|---|
| No leak (perfect custom fit) | 0 cm | 0 dB | 22 dB* |
| Lateral overlap of 5 cm (instead of 15-20) | 10 cm cumulative on sides | -2 to 3 dB | 19-20 dB |
| Curtain 10 cm above the floor | 10 cm at the bottom | -3 to 4 dB | 18-19 dB |
| Curtain too short at the top (no overlap) | 5 cm above the frame | -1 to 2 dB | 20-21 dB |
| Cumulative of 3 leaks (typical standard size) | Sides + bottom + top | -5 to 8 dB | 14-17 dB |
A 22 dB curtain with cumulative leaks only delivers 14 to 17 dB in real conditions. This is the level of a mid-range curtain (300-500 g/m²) that costs 2 to 3 times less. In other words: paying for high-performance fabric and then installing it in a standard size is wasting part of the investment.
Standard size vs. custom-made: scenario comparison
| Scenario | Standard size | Custom-made |
|---|---|---|
| Window 120x140 cm | Curtain 140x230: width overlap OK (10 cm/side), but 230 cm often too long or too short depending on rod-to-floor height | Curtain to exact dimensions + 15-20 cm overlap + floor length. Full coverage. |
| Window 95x160 cm | Curtain 140x230: too wide (bulky, excessive pleats), unsuitable height | Curtain 130x240 (with overlaps). Adjusted, aesthetic, performing. |
| Bay window 240x220 cm | No standard size covers 240 cm. 2 curtains of 140 = central gap | 2 custom panels with 10-15 cm central overlap. Zero leaks. |
| French door 90x215 cm | Curtain 140x230: width OK but extends 15 cm past the floor (risk of tripping) | Curtain 130x235 (with side overlap + precise floor length). Smooth handling. |
| Attic window | No standard size adapted to the inclination | Wall dimensions under the Velux, custom-made is mandatory. |
Result: in 5 scenarios, the standard size only works correctly for very common window dimensions (120-140 cm wide, 200-230 cm from rod to floor). As soon as the window falls outside these dimensions, custom-made is the only option for maximum acoustic insulation.
How to measure for a custom-made acoustic curtain
Measuring rules for an acoustic curtain are stricter than for a decorative curtain. Every centimeter counts for sound insulation:
Width
- Measure the outer width of the window frame (not the glass, the entire frame).
- Add 30 to 40 cm (15 to 20 cm overlap on each side). This overlap is mandatory for acoustic insulation, not optional.
- If you have an existing rod: measure the rod from finial to finial. The acoustic curtain does not require a fullness coefficient (pleats reduce acoustic efficiency by creating gaps). Curtain width = rod width.
Height
- Measure from the rod's position (minimum 10 cm above the frame) to the floor.
- Add 2 to 3 cm so that the curtain brushes the floor or breaks slightly.
- If eyelets: the top of the curtain extends 2-3 cm above the rod. Adjust the measurement accordingly.
Check before ordering
- Does the curtain extend at least 15 cm beyond each side of the window? If not, sound will bypass the fabric.
- Does the curtain reach the floor (max 2-3 cm gap)? If not, sound will pass underneath.
- Is the rod at least 10 cm above the frame? If not, sound will pass over the top.
For a detailed measurement guide with all configurations (classic window, bay window, front door, Velux), consult our complete guide to measuring custom curtains and the Kurtens size guide.
Fullness coefficient: useful for decoration, useless for acoustics
In decoration, the rod width is multiplied by 1.5 to 2 to obtain beautiful waves. In acoustic insulation, this is counterproductive. Pleats create spaces between the waves through which sound infiltrates. The more pleated the curtain, the more micro-openings there are.
For an acoustic curtain, the optimal configuration is a flat or very slightly pleated curtain (coefficient 1 to 1.2) that hugs the window surface as uniformly as possible. The fabric must form a continuous barrier, not a series of pleats and gaps.
This is an important point for choosing the finish. Kurtens fixings (eyelets, pleating tape, rod pocket) allow you to control the level of fullness:
- Rod pocket: the curtain slides directly onto the bar. Flat rendering, optimal for acoustics.
- Pleating tape: adjustable pleats depending on the number of hooks. Can be stretched for an almost flat rendering.
- Eyelets: create regular pleats. Less optimal for pure acoustics, but the dense 620 g/m² fabric largely compensates.
Key takeaways
Curtain size is the #2 factor in acoustic performance after fabric density. A high-performance curtain (620 g/m², 22 dB*) poorly sized delivers 14 to 17 dB in real conditions: the level of a mid-range curtain. Custom-made is not a luxury; it is the condition for utilizing 100% of the fabric's insulation potential.
Kurtens custom-made acoustic curtains are manufactured to the centimeter, up to 270 cm in height and in free width. 620 g/m², 22 dB of measured attenuation*, 100% blackout* and up to 7 °C of thermal gain*. 7 colors, free delivery in 2 to 3 weeks. Exact-Fit Guarantee (29.90 €): if your measurements are incorrect, Kurtens remanufactures the curtain to the correct dimensions.
*Data from tests conducted under optimal conditions.
Frequently asked questions
How many dB are lost with an acoustic curtain that is too small?
A standard-sized curtain with side, bottom, and top leaks loses 5 to 8 dB compared to the same perfectly fitted custom fabric. On a 22 dB* curtain, this is a loss of 25 to 35% of performance. The curtain then delivers 14 to 17 dB in real conditions, which is the level of a much cheaper mid-range model.
How much should the acoustic curtain extend beyond the window?
Minimum 15 cm on each side (30 cm more than the frame width in total). Ideally 20 cm per side for windows exposed to intense noise. In height, the rod should be positioned at least 10 cm above the frame and the curtain should extend to the floor (maximum 2-3 cm gap). These overlaps are not optional: without them, sound bypasses the curtain.
Should an acoustic curtain be pleated to be more effective?
No, it's the opposite. Pleats create micro-openings between the waves through which sound infiltrates. An acoustic curtain is more effective flat or very slightly pleated (coefficient 1 to 1.2) than with wide waves (coefficient 2). The fabric must form a continuous barrier in front of the window. This is a key difference between a decorative curtain (aesthetic pleats) and an acoustic curtain (uniform surface).
Can a standard acoustic curtain be effective?
Yes, if and only if the standard size exactly matches your window with the recommended overlaps. The 4 standard Kurtens sizes (100x150, 150x230, 250x230, 300x270 cm) are suitable for the most common windows. But if your window + rod configuration does not match any of these dimensions, custom-made is necessary to avoid leaks that reduce performance by 5 to 8 dB.
Which finish to choose for an acoustic curtain?
The rod pocket offers the flattest rendering (optimal for acoustics). The pleating tape allows for adjusting the pleat and can be stretched for an almost flat rendering. Eyelets create regular pleats, slightly less optimal for pure acoustics but compensated by the density of the fabric (620 g/m²). In practice, all three finishes work well with a Kurtens fabric because the mass of the fabric is the dominant factor.
How to measure an acoustic curtain for a bay window?
Measure the width and height at three points (walls are not always parallel). Take the largest measurement. Add 30-40 cm of side overlap. For bays over 200 cm, two panels with 10-15 cm of central overlap prevent the gap through which sound would pass. The overlap is crucial: two panels edge to edge leave a sound leak in the center.
What happens if I make a mistake in my measurements?
Kurtens offers the Exact-Fit Guarantee (29.90 €): if your measurements are incorrect, the curtain is remanufactured to the correct dimensions at no extra cost. This is a particularly useful safeguard for acoustic curtains, where dimensional precision directly impacts insulation performance. Measure twice, order once.
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