What RT60 and C50 Tell You
RT60 (reverberation time) is the time it takes sound in a room to decay by 60 dB after the source stops. C50 (clarity) compares the sound energy arriving in the first 50 milliseconds with the energy that arrives later and blurs speech. Together they show whether a room is too live for its use and how clearly speech carries within it.
There is no universal target. ANSI/ASA S12.60 sets a maximum reverberation time for unoccupied, furnished classrooms of 0.6 seconds for rooms up to 10,000 cubic feet and 0.7 seconds for rooms up to 20,000 cubic feet. Rooms used for music, such as worship spaces, are commonly designed with longer reverberation. Memtech Acoustics measures the room and sets the target from how it is used.
In classrooms, courtrooms, conference rooms, and worship spaces, RT60 and C50 are two of the main measures of how well speech carries. Background noise is the other, and we measure it at the same time.
There is no universal target. ANSI/ASA S12.60 sets a maximum reverberation time for unoccupied, furnished classrooms of 0.6 seconds for rooms up to 10,000 cubic feet and 0.7 seconds for rooms up to 20,000 cubic feet. Rooms used for music, such as worship spaces, are commonly designed with longer reverberation. Memtech Acoustics measures the room and sets the target from how it is used.
In classrooms, courtrooms, conference rooms, and worship spaces, RT60 and C50 are two of the main measures of how well speech carries. Background noise is the other, and we measure it at the same time.
A room with the wrong reverberation causes:
- Speech that is hard to follow
Reflections blur the words, so listeners strain, mishear, and tune out. - Listener fatigue
A live room wears people down over a long class, meeting, or service. - A PA that fights the room
Turning up a sound system in a reverberant room raises the reverberant level along with the speech, so clarity does not improve. - Spaces that miss their criteria
Classrooms have maximum reverberation times under ANSI/ASA S12.60, and many other rooms have design criteria set by the owner or the project.
Measuring RT60 and C50 gives you the data to treat the room precisely instead of guessing at panels.
How We Measure It.
In the Field.
Memtech Acoustics measures RT60 and C50 from the room's impulse response, captured with an omnidirectional source and a Class 1 analyzer across multiple source and receiver positions, following the ISO 3382 series (Part 2 for ordinary rooms).
Impulse Response Capture
We excite the room with an omnidirectional source, using an impulse or interrupted broadband noise, and record the full decay. That recording is the basis for both metrics.
RT60 by Octave Band
Reverberation time is evaluated from the measured decay (T20 or T30) in each octave band and reported as RT60, because a room can be controlled at speech frequencies and still be live at low frequencies.
C50 and Speech Clarity
The same impulse response yields C50, the early-to-late energy ratio, and the Speech Transmission Index (STI, defined in IEC 60268-16) where a single intelligibility value is needed.
Multiple Positions
We measure several locations across the room, because reverberation and clarity vary with where a listener sits.
Reporting Against Target
Results are reported per octave band against the design target for the space type, with the treatment direction where a target is missed.
Our Process
STEP 1
Discovery & Qualification
Memtech Acoustics starts with a collaborative process to understand the room, how it is used, and what people are experiencing, whether that is echo, poor speech clarity, or a space that is simply too loud.
We confirm the target RT60 and C50 for the room's purpose, identify the surfaces driving the problem, and align on whether the goal is a measurement baseline or a full treatment plan.
We confirm the target RT60 and C50 for the room's purpose, identify the surfaces driving the problem, and align on whether the goal is a measurement baseline or a full treatment plan.
Here are the types of questions we typically ask:
How is the room used, and is the main complaint echo, understanding speech, or overall loudness?
What are the room's hard surfaces (glass, drywall, exposed deck, hard flooring), and roughly how large is the volume?
How many people use the space at once, and is speech reinforced with a sound system?
Has any acoustic treatment been tried before, and what was the result?
How will the results be used: a design target, LEED or WELL credit documentation, or resolving a complaint about an existing space?
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STEP 2
On-Site Measurement
Memtech Acoustics measures the room as it performs, during a site visit, not from a drawing.
The room's impulse response is captured with a calibrated source, and the decay is recorded across octave bands from 125 Hz to 8 kHz.
All measurements use a Class 1 sound level meter (IEC 61672-1), with a field calibration check before and after the session.
Readings are taken at multiple source and receiver positions so the result represents the whole room, not one spot.
Background noise is measured in octave bands at the same time and rated against NC or RC curves (ANSI/ASA S12.2), because background noise reduces speech intelligibility even in a room with the right reverberation time.
The room's impulse response is captured with a calibrated source, and the decay is recorded across octave bands from 125 Hz to 8 kHz.
All measurements use a Class 1 sound level meter (IEC 61672-1), with a field calibration check before and after the session.
Readings are taken at multiple source and receiver positions so the result represents the whole room, not one spot.
Background noise is measured in octave bands at the same time and rated against NC or RC curves (ANSI/ASA S12.2), because background noise reduces speech intelligibility even in a room with the right reverberation time.
STEP 3
Analysis & Reporting
Memtech Acoustics calculates RT60 by octave band, along with C50 and STI values, from the measured impulse responses.
Results are compared against the target for the room's use, and the absorption needed to reach that target is modeled using established room-acoustics methods.
We then deliver a clear report that states the measured performance, the target, and what it will take to close the gap.
Results are compared against the target for the room's use, and the absorption needed to reach that target is modeled using established room-acoustics methods.
We then deliver a clear report that states the measured performance, the target, and what it will take to close the gap.
STEP 4
Actionable Guidance
Reports are written to support real decisions, with measured numbers and a defensible method behind every recommendation.
Each report includes:
- Measured RT60 by octave band at each position
- C50 and STI values, with background noise rated against NC or RC curves
- Comparison against the target for the room's use
- A modeled treatment plan and product placement when Memtech handles the full package
STEP 5
Verification & Re-Test (Optional)
After treatment is installed, Memtech Acoustics can return to measure the room again and confirm it now meets the target.
When Memtech controls the engineering, materials, and installation as a full package, the measured result is backed by a written performance guarantee.
When Memtech controls the engineering, materials, and installation as a full package, the measured result is backed by a written performance guarantee.
We provide:
- Post-treatment re-testing to confirm RT60 and C50 reached target
- Before-and-after comparison of the measured room acoustics
- Documentation for LEED, WELL, or owner sign-off
- Performance guarantee on full engineered packages, under Memtech's written guarantee terms
The Memtech Difference
At Memtech, noise is our business. Our engineers oversee every step, ensuring that each project is properly delivered, on-time and within budget. This integrated approach delivers predictable results and protects your investment.
Certified Acoustic Professionals
Testing conducted by certified engineers and acoustic specialists, not general contractors or “safety consultants.”
National Standards Alignment
We follow OSHA 1910.95, NIOSH criteria, and ANSI S12.19-1996 methods for noise exposure assessment. Measurements are conducted via ISO procedures or per academically accepted methods.
Engineering Expertise
Noise measurements, analysis, modeling and installation support are overseen by professionals who understand acoustic theory, as well as the implementation of practical solutions.
High Responsiveness, Proven Reliability
We are your trusted local partner providing on-demand support and dependable project execution. Michigan-based with regional reach and national* shipping capabilities.
Confidential & Detailed Reporting
We don’t just submit a report. You are provided with complete documentation, delivered in easy-to-understand language with accurate data and comprehensive drawings.
*Shipping available across the continental USA, and international; installations are offered in select regions.
NOTE: We do not stock custom products in our U.S. warehouse. Ask us about lead times for your project.
NOTE: We do not stock custom products in our U.S. warehouse. Ask us about lead times for your project.
Frequently Asked Questions (FAQs)
What is RT60, and what does it measure?
RT60 is reverberation time: the time it takes for sound to decay by 60 decibels after the source stops. It is one of the most widely used metrics in room acoustics. A high RT60 means a room is live and echoey with poor speech clarity; a very low RT60 can make a room feel dead. Memtech Acoustics measures RT60 by octave band, because a room can perform well in one frequency range and poorly in another.
What is C50, and how is it different from RT60?
C50 is the clarity index: the ratio of early sound energy (the first 50 milliseconds) to the late energy that arrives after it, expressed in decibels. Higher C50 means clearer speech. RT60 tells you how long sound lingers; C50 indicates how clearly speech carries in that room. Both are derived from the same impulse-response measurement, and Memtech Acoustics reports them together with STI.
What is a good RT60 for my space?
There is no universal target; it depends on how the room is used. ANSI/ASA S12.60 sets a maximum of 0.6 seconds for unoccupied, furnished classrooms up to 10,000 cubic feet and 0.7 seconds up to 20,000 cubic feet. Conference rooms, open offices, and worship spaces have no single code value, so the target comes from the room's use, and spaces used for music are commonly designed with longer reverberation. Memtech Acoustics sets the target, then measures and models to reach it without over-deadening the space.
How does Memtech Acoustics measure RT60 and C50 in the field?
Memtech Acoustics captures the room's impulse response with an omnidirectional source and a Class 1 sound level meter, with a field calibration check before and after the session, following the ISO 3382 series. Measurements are taken across octave bands from 125 Hz to 8 kHz at multiple source and receiver positions. RT60, C50, and STI are calculated from those measurements, and background noise is measured in octave bands and rated against NC or RC curves, because background level affects speech intelligibility.
What spaces need RT60 and C50 testing most?
Classrooms, lecture halls, conference and training rooms, restaurants, worship spaces, gyms and multipurpose rooms, and open offices: any room where people say speech is hard to understand or the space feels loud and echoey. Memtech Acoustics also tests to set a baseline before treatment and to verify the result after.
Will acoustic treatment make my room sound dead?
Not when it is engineered to a target. Over-treating a room drives RT60 too low and makes it feel lifeless. Memtech Acoustics models the absorption needed to reach the target for the room's use, then specifies engineered noise control in the right amount and the right places, rather than covering every surface and hoping.
What do I get in an RT60 and C50 report?
A clear report with measured RT60 by octave band at each position, the C50 and STI clarity ratings, the background noise rating, and a comparison against the target for the space. When Memtech Acoustics also handles the treatment, the report includes a modeled treatment plan and product placement. Assessment reports are typically delivered within about 15 business days of data collection.
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