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How to Measure Sound Transmission Class (STC)

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REW is a software package that measures the transfer functions of acoustic systems and displays the corresponding frequency, phase and impulse responses and various quantities derived from them. If that sentence made you ask yourself what on earth is he talking about? It would be worth taking a few minutes to read through the introduction to Signals and Measurements, which explains the basic concepts. Even if you are already familiar with the terminology a quick browse through that introduction may be helpful.

REW uses a logarithmic sine sweep method and a basic explanation follows. If you'd rather not know, you can skip the explanation.

To make a measurement we need a sound source (a loudspeaker or subwoofer) and a microphone (SPL meters contain a microphone and many can be used instead of a mic). A logarithmic sweep signal is sent to the source, starting low and steadily increasing to a higher frequency. A logarithmic sweep is one that takes the same time to double in frequency. It takes the same time to go from 40 to 80Hz or 4kHz to 8kHz as from 20 to 40Hz. The mic picks up the sweep, sensing the sound that travels directly from the source and all the sound that first bounces off the room surfaces. Some sound paths may bounce off several room surfaces before reaching the microphone.

Once the sweep completes, analysis starts. A process called "Fast Fourier Transform" (FFT) is used to calculate the phase and amplitude of the frequency spectrum that made up the sweep. By comparing the amplitudes and phases of the spectrum of the received microphone signal with those sent in the sweep we can work out how each frequency has been affected by the loudspeaker and room we are measuring. This is called the "Transfer Function" of the room from the location of the source to the location of the mic. Note: A different source position or different mic position will have a different transfer function. In other words, our measurement is only valid for one specific source and mic position. Having worked out the transfer function we can use an "inverse FFT" to get from the frequency amplitude and phase information to a time signal that describes the way any signal is changed when travelling from the source to the mic. That time signal is called the "impulse response" - like the transfer function it is derived from, it is only valid for one specific source and mic position.

The impulse response is actually exactly the same signal we would see if we could emit a very short but loud click at the source position and record what the mic picks up afterwards ("very short" meaning lasting just the time of 1 sample at the sample rate we are using for our analysis, so for a 48 kHz sample rate that would be just 1/48,000 of a second which is 21 millionths of a second). You might ask why we don't just use a click then. One difficulty is that the click, because it is so short, needs to be extremely loud for us to be able to pick up what happens after the initial click over the background noise of the room. We could no longer use a speaker to generate that, we would need something like a starting pistol or to pop a balloon. We would also need a mic that could cope with both the extremely loud click itself and the much quieter echoes of the click produced by the room. You are likely to find that your family and neighbours are not that keen on you repeatedly firing a pistol to figure out what your room is doing, and even if they put up with it your results would not be as good as using a sweep. To be technical about it, you can achieve a much higher signal-to-noise (S/N) ratio by the sweep method. The S/N is determined by the background noise level and by how much energy is in the test signal, which in turn depends on how loud the signal is and how long it lasts. An impulse is extremely short, a few millionths of a second, so to get any significant energy it needs to be extremely loud. A sweep can last for many seconds, so even at a modest volume its total energy can approach as much as a million times more than an impulse.

Once the impulse response has been obtained, it can be analysed to calculate information about how the room behaves. The simplest analysis is the FFT, to show the frequency response between the source and mic positions. However, we have some control over it. Altering which part of the impulse response is analysed by the FFT changes what aspect of the room's response we see. The early part of the impulse response corresponds to the direct sound from the source to the mic, the shortest path between them. Sound that has bounced off the room's surfaces has to travel further to reach the mic, which takes longer, so the later parts of the impulse response contain the contributions of the room. "Windowing" the impulse response to look at only the initial part shows us the frequency response of the direct sound with little or no contribution from the room. A window that includes later parts of the response lets us see how the room's contribution alters the frequency response. The ability to separate the contributions of the direct and later (reflected) sound is a key difference between the frequency response derived from an impulse response and one from a Real Time Analysis (RTA), which can only show the total combined response of source and room.

Other information we can get from the impulse response include a "waterfall" plot, which is generated by moving a window in steps along the response and plotting the various frequency responses in order to produce a 3D picture of the way the response changes over time, and the room's "RT60" data, which is the time it takes sound in various frequency bands to decay by 60dB (or 1,000 times smaller than the original level).

The first requirement is a way to capture the test signal. There are a few options:

A tripod to support the mic or meter (hereafter just called "mic"). Small movements of the mic can result in large variations in the measurements, for repeatable results a means of supporting the mic for a prolonged period is essential. For low frequency measurements (below a few hundred Hz) the mic can be set pointing straight up. This avoids having to move it to measure different speakers and makes it easy to read the display on a meter. Use a "90 degree" mic calibration file if pointing the mic upwards. To make measurements at higher frequencies it is best to point the mic directly at the speaker being measured. Use a "0 degree" mic calibration file in that case. In both cases the mic should be placed at ear height in your usual listening position.

If you are using a USB mic your computer's headphone output can generate the test signals REW uses without an external interface. If you are using an SPL meter or a mic with a preamp then an audio interface (internal or external) with line inputs and line or headphone outputs is required. Note: Older PC and laptop mic-only inputs are NOT suitable and should not be used as they had too much gain and most suffered from high noise levels and limited bandwidth. Combination mic/line inputs can be used successfully. Inexpensive or built-in interfaces are typically adequate, a reference measurement of a loopback connection can be used to remove the interface's frequency response from the measurement.

Cables to connect from your SPL meter or mic preamp's output to your interface (if you are not using a USB microphone) and from the interface's line or headphone output to an input on your AV processor or equaliser. The leads need to be long enough to reach from your computer to your listening position (where your mic will be placed) and to your AV processor or equaliser. If your interface has phono (RCA) connectors phono-phono leads will be needed, if the interface has 3.5mm (1/8") sockets you will need a pair of stereo jack plug to stereo phono plug leads (also called Y adaptor cables) or stereo audio adaptors , see images below.

If you use male to male Y adaptor cables you will also need two phono socket-to-phono socket adaptors (also called RCA phono plug couplers, see image below) to connect to the leads that run to your SPL meter and AV processor. (RCA Y-cables come in all formats: M->2M, M->2F, F->2F, F->2M and a selection often comes in handy when making measurements.)

If connecting to a BFD Pro DSP1124P or FBQ2496 you will need phono to mono 1/4" jack plug adaptors like that pictured below.

An interface to your equaliser if using REW's RS232 or Midi communication capabilities to set up the equaliser's filters. Requirements for communicating with TAG McLaren AV processors are detailed here, for BFD Pro DSP1124P or FBQ2496 (Midi interface) lookhere.

The overall setup for measuring when using an SPL meter is shown below. If you are using a USB microphone you do not need to make any connections to the interface inputs, just connect the mic to a USB port on your computer.

When not using a USB mic, one of the interface's input channels is used to measure the sound pressure signal from your mic or meter. It must be connected to the meter's or mic preamp's analogue output. The default is to use the Right input, but either input can be used. A control in the Soundcard Preferences tells REW which input to listen to.

One of the interface's output channels (typically the Right) must be connected to an input channel on your AV processor or to the input of your equaliser. Connecting to your AV processor allows you to make measurements that will show the response of main speakers as well as the subwoofer, and to view the integration between subwoofer and main speakers. The effects of your AV processor's bass management can be included in the measurements. Connecting to the left or right channel of an analog input will allow the corresponding main speaker and the subwoofer responses to be measured. Turn off or disconnect the main speaker or the sub to exclude them from a measurement.

If your computer and AV processor both support audio over HDMI you can also use an HDMI connection to carry the audio. The HDMI audio driver needs to be selected as the output device under Preferences -> Output Device. If you are on Windows and want to access multichannel outputs over HDMI you will need to use an ASIO wrapper like FlexASIO or ASIO4All to access the outputs and inputs.

The other input and output channels do not need to be used for basic measurement. The response of the interface itself can be compensated for by taking a reference measurement with the output connected directly to the input and configuring REW to subtract that measured response from subsequent room measurements. However, it is also possible to use a loopback connection from the interface's other output to its other input as a timing reference for REW to automatically compensate for the time delay in the interface and operating system when it makes a measurement. A timing reference is required to compare phase or time delays between measurements or for getting speaker delay settings correct in multi-channel systems. If you wish to do this you may need an additional RCA phono plug coupler to make the loopback connection. Whether REW uses one channel as a timing reference is controlled by a selection on the Measure dialog. When the input is via a USB microphone instead of an interface an acoustic timing reference can be used. If an acoustic timing reference is used REW will generate a timing signal on the output that has been selected to act as the reference before it generates measurement sweeps on the channels being measured. The timing signal is a high frequency sweep to allow accurate timing so a subwoofer cannot be used as the reference channel. Measurements will have a time delay that corresponds to the difference in their distance from the microphone compared to the distance of the reference speaker - if the reference speaker is further away the delay would be negative. Note that multiple sweeps cannot be used with an acoustic timing reference.

If using an equaliser(such as BFD Pro DSP1124P or FBQ2496) to optimise your subwoofer's response it should be connected between your AV processor's LFE/Sub output and the subwoofer's low level input. For a BFD Pro the operating level switches on the rear panel should be pressed in to select the -10 dBV range.

If the AV processor input being used has an anti-clipping feature (automatically reducing its sensitivity if it detects large signals) this should be turned off, as it could shift the measurement levels. The sensitivity of the input should ideally be set to 0.5V, though this is not critical.

The TAG McLaren AV32R DP and AV192R allow the test signal input to be routed to any speaker output via the Test Signal entry within the TMREQ filter menus for each speaker, which is handy for measuring other speakers (see this note for details). They seem to be the only AV processors with such a facility, other processors may have 5.1 or 7.1 analog inputs that can be used to similar effect, but in some cases bass management will not be applied to such inputs, limiting the ability to check sub/main speaker integration.

If you are using an SPL meter as the input the meter's range should be set to the value normally used for speaker level calibration and must not be altered while using REW. If you are using the Radio Shack meter, select the 80dB range if you calibrate your system at 75dB (this is the standard level recommended by DolbySelect the Input device is a C weighted SPL meter option in TM). Set your meter to C weighting and "slow". Preferences -> Cal Files.

Having assembled the equipment required we can look it how REW itself is organised for making and analysing measurements in the REW Overview.

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REW is a software package that measures the transfer functions of acoustic systems and displays the corresponding frequency, phase and impulse responses and various quantities derived from them. If that sentence made you ask yourself what on earth is he talking about?

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When REW is started it looks like this:

The main window is blank until we either make a measurement or load some existing measurements. The SPL Meter, Signal Generator, Scope and Level Meters can be used without loading any measurements,

as can the RTA window, EQ window and the Room Simulator,

After making or loading some measurements the main window looks like this:

The measurements appear on the left, in overlapping panels. The currently selected measurement has a white background, the others are grey. On the right is the graph area for the current measurement.

All the toolbar buttons are now enabled. They are in 3 groups, firstly the buttons related to measurements:

These buttons allow a new measurement to be made, existing measurement files to be opened, all the current measurements to be saved in a single measurement (.mdat) file, all the current measurements to be removed, and an Info panel to be opened that shows additional information about the current measurement.

The next group has the various tools

The IR Windows button opens a window that allows the type and extent of the Impulse Response windows for the current measurement to be changed. Next to that are the SPL Meter, Signal Generator, Scope and Level Meters buttons. Then there is a button to open the Overlays graph window, which allows any or all of the loaded measurements to be plotted on the same graph. The last three buttons are for the RTA window, the EQ window (which is used to study the effects of EQ on the current measurement) and the Room Simulator.

The final toolbar button brings up the Preferences panel

In the graph area there is a button to capture the current graph as an image

a selector strip to pick the graph type

and buttons to turn scrollbars for the graph area on/off, toggle the frequency axis between log and linear, set the graph limits and show the graph controls menu.

Below the graph is a legend area that shows the trace values at the cursor position

If smoothing has been applied the octave fraction (1/48 in the image above) appears between the trace name and its value.

The first step in getting REW running is to set up the audio input and output and calibrate the soundcard.

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When REW is started it looks like this: The main window is blank until we either make a measurement or load some existing measurements. The SPL Meter, Signal Generator, Scope and Level Meters can be used without loading any measurements, as can the RTA window, EQ window and the Room Simulator, After making or loading some measurements the main window looks like this: The measurements appear on the left, in overlapping panels.

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Room EQ Wizard: REW Room Measurement Tutorial

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Measurement Microphone Recommendations

USB Microphone Recommendations

For convenience and ease-of-use, a USB-connected measurement microphone can't be beat. Many of these mics come with a calibration file that ensures proper frequency response for measurement purposes.

The UMIK-1 is a 'Plug & Play' omni-directional USB microphone designed for acoustic measurement. This microphone provides low noise and accurate results you can rely on. Forget about driver installation, OS compatibility and un-calibrated mics. The Umik-1 is a USB Audio class 1 device automatically recognized by the Mac Operating System. After purchase, you can download a calibration file specifically for your UMIK-1 ensuring an accurate measurement.

Ships Worldwide
How To Use The UMIK-1 With Amarra Symphony with iRC
Analog Microphone Recommendations

Many analog microphones are useable with iRC Measure. Most all measurement - worthy analog mics require a pre-amp with phantom power and a female XLR (from pre-amp) to male 3.5 mm stereo adaptor (to your Mac line input) to operate. Contact your mic's manufacturer to find out if your mic is suitable for measurement use.

The CEntrance MicPort Pro is an easy way to incorporate your analog mic into the digital realm. Simple plug-and-play integration with your Mac, studio-quality specs, phantom power for condenser mics and appropriate sample rate support, this pre-amp is perfect for use with with iRC measure.
Click here for MicPort Pro Information
Praise for iRC
Copyright © 2001 Sonic Studio, LLC. All rights reserved.

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For convenience and ease-of-use, a USB-connected measurement microphone can't be beat. Many of these mics come with a calibration file that ensures proper frequency response for measurement purposes. The UMIK-1 is a 'Plug & Play' omni-directional USB microphone designed for acoustic measurement. This microphone provides low noise and accurate results you can rely on.

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Acoustics - Measurement of room acoustic parameters - Part 1: Performance spaces

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Room sizes compliance with international standards and recommendations

Room sizes compliance with international standards and recommendations

* initial sizes have to be given in meters

Copyright © 2009-2010 Acoustic Ukraine

Developed by Andrey Smirnov | Powered by Webtime

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Room sizes compliance with international standards and recommendations

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Room sizes compliance with international standards and recommendations Room sizes compliance with international standards and recommendations * initial sizes have to be given in meters Copyright © 2009-2010 Acoustic Ukraine Developed by Andrey Smirnov | Powered by Webtime

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Calculator Use

The ratio calculator performs three types of operations and shows the steps to solve:

Simplify ratios or create an equivalent ratio when one side of the ratio is empty.

Solve ratios for the one missing value when comparing ratios or proportions.

Compare ratios and evaluate as true or false to answer whether ratios or fractions are equivalent.

This ratio calculator will accept integers, decimals and scientific e notation with a limit of 15 characters.

Simplify Ratios:

Enter A and B to find C and D. (or enter C and D to find A and B)
The calculator will simplify the ratio A : B if possible. Otherwise the calculator finds an equivalent ratio by multiplying each of A and B by 2 to create values for C and D.

Compare Ratios and Solve for the Missing Value:

Enter A, B and C to find D.
The calculator shows the steps and solves for D = C * (B/A)

Enter A, B and D to find C.
The calculator shows the steps and solves for C = D * (A/B)

Evaluate Equivalent Ratios:

Enter A, B, C and D.
Is the ratio A : B equivalent to the ratio C : D? The calculator finds the values of A/B and C/D and compares the results to evaluate whether the statement is true or false.

Convert Ratio to Fraction

A part-to-part ratio states the proportion of the parts in relation to each other. The sum of the parts makes up the whole. The ratio 1 : 2 is read as "1 to 2." This means of the whole of 3, there is a part worth 1 and another part worth 2.

To convert a part-to-part ratio to fractions:

1. Add the ratio terms to get the whole. Use this as the denominator.
1 : 2 => 1 + 2 = 3

2. Convert the ratio into fractions. Each ratio term becomes a numerator in a fraction.
1 : 2 => 1/3, 2/3

3. Therefore, in the part-to-part ratio 1 : 2, 1 is 1/3 of the whole and 2 is 2/3 of the whole.

Related Calculators

To reduce a ratio to lowest terms in whole numbers see our Ratio Simplifier.

To simplify a fraction into a reduced fraction or mixed number use our Simplifying Fractions Calculator.

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The ratio calculator performs three types of operations and shows the steps to solve: Simplify ratios or create an equivalent ratio when one side of the ratio is empty. Solve ratios for the one missing value when comparing ratios or proportions. Compare ratios and evaluate as true or false to answer whether ratios or fractions are equivalent.

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Build your 3D modeling skills by mastering the basics of SketchUp. In this course, you can get up to speed with this easy-to-use computer modeling application by gaining a foundational understanding of the drawing and design tools offered in the 2021 version of SketchUp. Instructor Tammy Cody covers navigating the interface, manipulating objects, drawing, organizing drawing elements, and working with materials and textures on both Windows and Mac computers. Plus, learn how to apply simple styles and animation to make your 3D projects more polished and presentable. Throughout the course, Tammy provides opportunities for you to practice what you've learned.

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Certificate of Completion

Showcase on your LinkedIn profile under "Licenses and Certificate" section

Download or print out as PDF to share with others

Share as image online to demonstrate your skill

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SketchUp 2021 Essential Training Online Class | LinkedIn Learning, formerly Lynda.com

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Build your 3D modeling skills by mastering the basics of SketchUp. In this course, you can get up to speed with this easy-to-use computer modeling application by gaining a foundational understanding of the drawing and design tools offered in the 2021 version of SketchUp.