スペースに必要な音響ライトの数を計算する方法

You walk into a meeting room that looks perfect on paper. Clean design, modern finishes, glass walls, polished concrete floor. But five minutes into a conversation, something feels off. People start repeating themselves. Voices overlap. On video calls, words blur together. By the end of the meeting, everyone is slightly drained, even though nothing physically demanding happened.

That discomfort is not visual. It is acoustic. And it is measurable.

This article shows you exactly how to calculate how many acoustic lights your space needs to fix that problem. You will understand RT60, Sabins, and NRC in practical terms, not theory. You will walk through a real example room step by step, calculate the exact absorption gap, and translate that directly into a Feltlite fixture count you can act on.

Does Your Space Have an Echo Problem? Start Here

The easiest way to diagnose an acoustic issue is not with a meter. It is with experience.

The first sign is overlapping speech. Two people begin talking at the same time, and instead of one voice naturally standing out, both blur together. This happens because sound reflections arrive at your ears milliseconds after the direct voice, creating a layered effect. Your brain works harder to separate them, and clarity drops.

The second sign is the “repeat loop.” Someone says something simple, like a number or a name, and it gets repeated two or three times. The reason is not volume. It is the persistence of sound in the room. When sound lingers too long, new words mix with previous ones, making speech less intelligible.

The third sign is fatigue after short conversations. You leave a 30-minute meeting feeling like it lasted an hour. This is cognitive load. Your brain is constantly filtering reflected sound, trying to reconstruct clear speech from noise.

There is a fourth pattern that makes all three symptoms worse. When background noise increases, people unconsciously raise their voices to be heard. This causes others to raise their voices further. Researchers call this the Lombard Effect. The room itself is driving everyone to speak louder, which increases the noise level, which causes more raising of voices. The cycle does not stop on its own. It stops when the acoustics change. 

All three symptoms come back to one balance: room volume versus total sound absorption. Picture it like a scale. On one side is the amount of air in the room where sound can travel. On the other side is how much of that sound is being absorbed by surfaces. If volume outweighs absorption, sound keeps bouncing. If absorption balances volume, sound settles quickly.

If conversations feel harder than they should, your room likely has an RT60 imbalance, not a communication problem.

Acoustic lighting addresses this problem at the source  and it is increasingly becoming the standard approach in Modern office design. 

The Simple Version: Three Steps Before Any Formula

Before the formulas, here is the simplest way to understand what is happening in your room acoustically. Think of your room like a glass of water with a hole in it. Sound is the water being poured in constantly. The hole is your absorption. If the hole is too small, water overflows  that is echo. The bigger the hole, the faster sound drains away, and the quieter the room feels. Right now, most office spaces have a hole that is too small. That is all this calculation is fixing. 

Step 1: How Big Is Your Space?

Room size is not just a number. It defines how far sound can travel before hitting a surface. In a small room, sound reflects quickly and returns to the listener almost immediately. In a larger room, sound travels longer distances, creating delayed reflections that stack on top of each other.

This is why a 12-foot ceiling feels very different from an 8-foot ceiling, even if everything else is the same. The vertical dimension increases the total volume of air, which increases the time sound remains active in space.

When you calculate room size, you are not just measuring dimensions. You are defining the environment sound operates in. Larger volume means more energy needs to be absorbed to achieve the same clarity.

Step 2: How Much Sound Is Your Room Currently Absorbing?

Not all materials interact with sound the same way. Hard, dense surfaces like concrete, glass, and painted drywall reflect most sound energy. Soft, porous materials like carpet, fabric, and acoustic panels absorb it.

A glass wall might look clean and modern, but acoustically it behaves like a mirror. Nearly all sound reflects back into the room. A carpeted floor absorbs some sound, but only a fraction unless it has a thick underlay.

This is why many modern offices struggle acoustically. They are designed visually with hard finishes but lack sufficient absorption. The result is a space where sound keeps bouncing without being reduced.

Understanding what your surfaces are made of tells you how much sound is already being controlled and how much is still uncontrolled.

Step 3: What Is the Gap Between Where You Are and Where You Need to Be?

This is where the concept becomes actionable. Every room has a current acoustic condition and a target condition. The difference between them is the gap.

You are not trying to eliminate all sound. You are trying to reduce how long sound lingers. That means calculating how much additional absorption is required to reach a comfortable RT60.

This gap becomes your design input. It tells you exactly how much acoustic material needs to be added. Without this step, decisions become guesswork, leading either to under-treatment or unnecessary cost.

The One Number That Explains Everything RT60

Clap your hands once in your room. Listen carefully. How long does the sound take to fade away? That duration is your RT60, the time it takes for sound to decay by 60 decibels.

In a noisy office, you might hear the clap linger for over a second. That is around 1.1 to 1.2 seconds. It feels echoey, slightly chaotic. Speech overlaps.

At around 0.8 seconds, the room feels controlled but still natural. Conversations flow comfortably. This is typical for open offices.

At 0.5 seconds, the room feels tight and precise. Speech is crisp. This is ideal for meeting rooms and classrooms.

Different spaces need different RT60 targets because they serve different purposes.

Room Type	Target RT60	What Happens If Exceeded
Open Office	0.6 to 0.8s	People raise voices, focus drops
Meeting Room	0.5 to 0.6s	Words overlap, calls muffled
Classroom	Under 0.6s	Students miss words
Restaurant	0.7 to 1.1s	Noise becomes uncomfortable
Lecture Hall	Under 1.0s	Rear seats struggle to hear

A restaurant can tolerate more reverberation because energy and liveliness are part of the experience. A meeting room cannot. It needs clarity and precision.

Two Numbers You Need: NRC and Sabins

What Is NRC  And Why Zero and One Are Extremes You Will Never See in Practice

NRC, or Noise Reduction Coefficient, tells you how much sound a material absorbs. It ranges from 0 to 1.

A concrete floor has an NRC of around 0.02. That means 98% of sound is reflected back into the room. Almost nothing is absorbed.

An open window has an NRC of 1.0. Sound passes through and does not return. This is the theoretical maximum.

Most real materials fall somewhere in between.

材料	NRC Value	What It Does to Sound
Concrete floor	0.02	Reflects almost everything
Glass wall	0.02	Reflects almost everything
Painted gypsum wall	0.05	Very little absorption
Carpet on concrete	0.20 to 0.35	Some absorption
Carpet on foam	0.35 to 0.55	Moderate absorption
Acoustic ceiling tile	0.50 to 0.70	Good absorption
Feltlite PET panel	0.75 to 0.95	Excellent absorption

The reason soft materials absorb more sound is their structure. They trap air and allow sound waves to dissipate as heat energy rather than reflecting back. This is also why acoustic lighting fixtures are not rated using NRC the same way wall panels are; the measurement methodology is different for three-dimensional fixtures. 

What Are Sabins: The Unit That Tells You Total Room Absorption Power

Sabins translate NRC into something usable. One Sabin equals the absorption of one square foot of a perfectly absorbing surface.

The formula is simple: NRC multiplied by area equals Sabins.

If you have 100 square feet of material with an NRC of 0.5, that gives you 50 Sabins of absorption.

Think of Sabins as the total “absorption capacity” of your room. It combines all surfaces into one number you can work with.

Now consider this: a room with 63 Sabins and a volume of 1440 cubic feet will always sound noisy. There simply is not enough absorption to control the sound energy in that volume.

The Full Calculation: Real Room Walked Through Step by Step

This example uses a real room: 12 feet by 10 feet with a 12-foot ceiling.

The math here looks intimidating written out. It is not. You are doing four multiplications and two subtractions. A calculator on your phone handles all of it in under two minutes. Walk through it once with this room then try it with your own dimensions. 

Step 1: Calculate Room Volume

Volume is calculated as length × width × height.

12 × 10 × 12 = 1440 cubic feet.

This number represents the total air space where sound exists. More volume means more sound energy needs to be absorbed.

If you are working in meters, the formula stays the same, but the RT60 constant changes later.

Step 2: Add Up Current Sabins

Surface	材料	Sabins
Floor	Concrete	0.50
Floor covering	Carpet 3/4 inch	42.13
Walls	Double gypsum	11.70
Windows	Laminated glass	4.20
Ceiling	Concrete and wood deck	4.47
Total		63 Sabins

For a 1440 cubic foot room, 63 Sabins is low. A well-treated room of this size would typically need closer to 80–100 Sabins for balanced acoustics.

Step 3: Calculate Current RT60

RT60 = 0.049 × Volume / Sabins

0.049 × 1440 / 63 = 1.12 seconds

This means sound lingers for over a second. In practice, this creates noticeable echo and reduced speech clarity.

If using meters, the constant becomes 0.161 instead of 0.049.

Step 4: Set Your Target RT60

For this room, assume it is used as a small office or meeting space. The target RT60 is around 0.8 seconds.

Choosing 0.5 seconds would make the room feel overly dampened. Choosing 1.0 seconds would not solve the clarity issue.

The target must match how the room is used, not just what sounds “better.”

Step 5: Calculate Additional Sabins Needed

Required Sabins = 0.049 × 1440 / 0.8 = 88 Sabins

Current Sabins = 63

Gap = 25 Sabins

This means you need to add 25 Sabins of absorption to reach the target.

In practical terms, this is the amount of sound energy you need to remove from the room.

Step 6: Filling the Feltlite Gap

Every Feltlite acoustic lighting fixture carries a certified Sabin value — a number derived from standardized laboratory testing that tells you exactly how much sound absorption that specific product adds to a room. This is not an estimate or a marketing claim. It is a measured performance figure you can use directly in the calculation.

The process is straightforward. Take your Sabin gap — in this room, that is 25 Sabins — and divide it by the certified Sabin value of the Feltlite fixture you plan to use. If a fixture provides 2 Sabins, you need 12 units. If a fixture provides 3 Sabins, you need approximately 8 units. The formula does not change — only the fixture specification does.

This matters because different Feltlite products have different absorption values depending on their size, geometry, and PET material configuration. A larger pendant will provide more Sabins per unit than a smaller one. A linear baffle fixture will perform differently than a circular pendant. Choosing the right product for your space is not guesswork — it is a specification decision based on certified data. 

Feltlite Acoustic pendant lights are designed specifically for this purpose  combining sound absorption with architectural lighting in a single ceiling fixture. 

The ceiling is also the most important surface to treat in most commercial spaces. Unlike wall panels, which only absorb sound that travels laterally, ceiling-mounted acoustic lighting intercepts sound before it completes a full reflection cycle. This makes fixtures positioned at ceiling level disproportionately effective compared to equivalent material placed on walls.

For this specific room — 12 by 10 by 12 feet with a 25 Sabin gap — approximately 12 standard Feltlite panels at ceiling height will bring RT60 from 1.12 seconds down to the target of 0.8 seconds. That is the number that makes this room functional for focused work and clear conversation.

Step	What You Find	This Room
Room Volume	Total air space	1440 cubic feet
Current Sabins	Existing absorption	63 Sabins
Current RT60	How noisy now	1.12 seconds
Target RT60	Comfort goal	0.8 seconds
Sabins Gap	What you need	25 Sabins
Feltlite Fixtures	Fixture count	Approx 12 panels

Step 7: Calculate Lighting Requirements 

The same room that needs acoustic treatment also needs the right amount of light. For this 12 by 10 foot space, the calculation is straightforward: length x width x required foot candles = required lumens. At 30 foot candles  standard for office work,  this room needs 3,600 lumens minimum. Feltlite acoustic fixtures are specified with lumen output so you can meet both requirements with a single product decision.

Why the Ceiling Matters More Than Any Other Surface

Most people instinctively think of wall panels when they consider acoustic treatment. Walls are visible, accessible, and easy to understand as a sound barrier. But in most commercial spaces, the ceiling is where the biggest acoustic opportunity sits — and it is almost always untreated by default.

Sound travels in all directions from a source. In a room with a 12-foot ceiling, sound waves reach the ceiling faster than they reach the far walls. The ceiling is also typically the largest single uninterrupted surface in a commercial space. If it is concrete, plaster, or gypsum with no treatment, it is reflecting sound back down across the entire floor area with every reflection cycle.

Ceiling-mounted acoustic lighting intercepts this reflection at its source. Instead of allowing sound to complete a full ceiling-to-floor-to-ceiling bounce, acoustic fixtures absorb energy on the first pass. This is why a relatively small number of ceiling-mounted acoustic fixtures can outperform a much larger area of wall treatment in terms of RT60 reduction. The position of the absorption matters, not just the quantity.

This is also why Feltlite acoustic lighting is specifically designed for ceiling mounting. The PET material faces downward and outward, intercepting both direct upward-traveling sound and the reflections that would otherwise return from the ceiling surface. Each fixture works in three dimensions, not just as a flat panel.

Before and After What Actually Changes When You Add Acoustic Lighting

Factors	Before Acoustic Lighting	After Feltlite
Auditory sensation	Noisy, voices overlap, hard to follow conversation	Clear, calm, speech feels natural and separate
RT60	1.12 seconds — noticeably loud	0.8 seconds — comfortable and controlled
Speech clarity	People repeat themselves frequently	Words land clearly the first time
Productivity	Focus breaks due to constant background noise	Sustained concentration becomes easier
Video calls	Sound muffled, words missed, callers ask to repeat	Clean audio, no complaints from the other end

What the table does not show is how these changes compound. A room that drops from 1.12 to 0.8 seconds does not just sound better — it changes how people behave in it. The Lombard Effect reverses. People stop raising their voices. The overall noise floor drops. And that drop makes the room feel even quieter than the RT60 number alone would suggest.
How Many Feltlite Fixtures Do You Need: Quick Reference by Room Size

Room Size	Approx Sabins Needed	Feltlite Fixtures Approx
Small office 10×10	15 to 20 Sabins	6 to 8 panels
Meeting room 12×10	20 to 30 Sabins	10 to 14 panels
Open office 20×30	50 to 80 Sabins	20 to 30 panels
Classroom 25×30	70 to 100 Sabins	28 to 40 panels

Find your room size in the table above. The middle column shows the total Sabins your space needs to reach a comfortable RT60. Most standard commercial rooms start with 40 to 70 Sabins, which means the gap is often smaller than people expect. Most people are surprised by how small that gap actually is  and how few fixtures it takes to close it. These are directional estimates. The exact number depends on ceiling height, materials, and the specific Feltlite product used.

For precise results, provide room dimensions and material details to Feltlite for a full calculation.

Conclusion

The calculation is not complicated once you understand what each number represents. Room volume tells you the scale of the acoustic challenge. Current Sabins tell you how much the room already absorbs. The RT60 formula connects those two into a measurable performance figure. Setting a target and calculating the gap gives you a precise brief — not a vague sense that “something needs to be done.”

Most commercial spaces that feel acoustically uncomfortable are sitting somewhere between 1.0 and 1.5 seconds when they should be at 0.6 to 0.8. The distance between those numbers is not an expensive renovation. It is a fixture count and a ceiling installation.

One thing worth noting before you finalize any specification: the people who most often blame a bad microphone, a poor internet connection, or simply “loud colleagues” are usually working in a room with an untreated RT60. The room is the problem. Acoustic lighting is the fix that also happens to light the space.

If you want to see how this process works in a Real Project, read how we took a client from a single reference photo to a fully installed custom acoustic lighting solution. 

Send us your room dimensions, ceiling height, and the primary materials on your floor, walls, and ceiling. We will run the calculation and come back with a specific Feltlite fixture count for your space.

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