RTA Analyzer

Sections

Introduction

About The RTA Algorithm

GUI

RTA Setup Menu

Using Gestures

Calibration

Room Tuning

Transmission Loss


Introduction

RTA, or Real Time Analyzer, is a standard module included in the base price of AudioTools. RTA is also available as a standalone application. The information in this manual applies largely to the RTA module available within AudioTools, although the two apps are nearly identical. You will find RTA in the Acoustics section of AudioTools. To quickly see RTA in action, make sure to check out our RTA Demo Videos in the Support -> Video Center of AudioTools.

RTA is a precision audio measurement instrument, and its accuracy is limited only by the choice of input device. The algorithm and filters used in Studio Six Digital's RTA meet or exceed ANSI Class 1, and provide the same professional quality measurement results that you would find in any other hardware- or software-based RTA.

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About the RTA Algorithm

The RTA is a full-featured one-third octave Real Time Analyzer that takes a broadband audio signal input, filters it into multiple bands, and reports the amount of energy in dB SPL present in each of those bands in real time. RTA uses filters conforming to ANSI and IEC Class 1 specifications.

All calculations are done in 64-bit floating point for the best accuracy and to get the lowest possible noise floor.

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GUI

Let's take a quick tour of the RTA's interface:

This section describes the elements of the main RTA screen (iPad interface is shown).

A -- Audio Input: This text will show the input that RTA is acquiring audio from. The options include Internal Mic, iAudioInterface, iAudioInterface2, iTestMic and USB Audio if using a USB audio interface and the camera connection kit.

B -- Graph Display: This area shows the current energy in each RTA band of the incoming audio signal. The X-Axis displays the center frequency of each band (either octave or 1/3 octave wide) and the Y-Axis displays the dB SPL of the given band.

C -- Overall SPL: This is the overall dB SPL of the broadband signal being analyzed. The SPL readout can be set to Flat, A-Weighted or C-Weighted from the RTA Setup.

D -- Capture Image: Taping the photo thumbnail icon will save a screen capture of the current analysis to your iOS device's photo roll.

E -- Band Size Selector: Taping this select box will bring up the band size selector. Octave- or 1/3 Octave-bands are available. Tap the adjacent Select button to change the band size being displayed on the screen.

F -- Start Analysis: Taping the play/pause button will begin or stop the RTA.

G -- Save/Recall Screen: Taping the folder icon will bring up the Save/Recall RTA screen, where measurements can be stored, shared or recalled for comparison with the current measurement.

H -- Generator: Taping the sine wave icon will bring up the Signal Generator (seen below). The generator will play back test signals from the output of the connected audio device or internal speakers of the iOS device. The generator can create sine or square wave tones from 5 Hz to 20,000 Hz, white noise or pink noise. When the iAudioInterface2 is connected and the Surround Generator module has been purchased (Utilities -> Surround Generator), the option to play back the signal from any of the surround channels over the optical output of the iAudioInterface2 will also be available.

Pink noise is the typical test signal used with an RTA. Pink noise has equal energy in each octave (or 1/3 octave), so in theory if your system is flat, you will see a flat line on the RTA display.

I -- RTA Setup: Taping the wrench icon displays the configuration options of the RTA, which will be described in detail in the RTA Setup section.

J -- Decay Mode: This selector will adjust the decay time being used for analysis. The decay times apply to the graph dB values. A decay time of one second will cause a point to decay at the rate of 20dB/second. Peak Hold holds the highest value received, and Average is a true linear average of all readings over the time of the average. To clear Peak or Average mode, tap the Reset button on the Average or Peak box (Average shown below), which appears when these modes are selected.

Clipping: If clipping of the input occurs, the words "Clipping Detected" will be displayed above the graph in the upper left-hand corner in red for 1.5 seconds.

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RTA Setup Menu

Tapping the wrench icon will bring up the RTA Setup menu. From here, you can customize how the RTA operates. Tapping the Done button at any point will bring you back to the main analyzer window. Let's take a look at what each option available does to the analysis.

A -- SPL: Adjusts the filter used for the broadband SPL reading on the main RTA screen. Choose between Flat, A, and C-weighting filters.

B -- Screen: Adjusts the background color of the main analysis window. Choosing bright (default) will display a white background behind the RTA bars, and choosing dark will display a black background.

C -- Input Source: This is the standard AudioTools input source selection screen, where you can choose microphone gain, or change settings.

D -- Graph: Adjusts the weighting filter applied to the individual RTA bands. Choose between Flat, A, and C-weighting filters.

E -- Overlay Curve:  Several industry standard noise curve overlays are available in RTA. To display a noise curve overlay, tap the the blue text (default text: None) underneath the Overlay Curve text. This will bring up an additional setup screen (shown below). Select one of the overlays: NCB -- Balanced Noise Curve, RC -- Room Curve, PNC, NR, or Noise Rating, used in the EU, and the cinema large-room X-curve. The overlay will appear on the graph. The noise curve value will be displayed on the top left corner of the analysis screen. Note: Noise curves are only valid when the RTA is in octave mode.

An example of the noise curve overlay is shown below. The single number is derived from the lowest curve that does not have any bars touching it (a green dot will be displayed in the center of each octave band, which is the point where the curve is measured from). The overall noise curve number interpolates between defined curves and is calculated with a minimum step size of 1. (i.e. NC 30 and 31 can be derived).

F -- Transmission Loss: Transmission loss calculation is available as an in-app upgrade to RTA. Also known under the names STC, NIC, OITC, and Rw, depending on the standard that is being used, the purpose of Transmission Loss measurements in any case is to determine how much acoustic attenuation a wall or divider will provide. Detailed instructions about how to take Transmission Loss measurements are provided in the setup screen once purchased. You can view a demo of Transmission Loss measurement in the Support -> Video Center of AudioTools.

The full instructions for Transmission Loss testing appear at the end of this document.

G -- Hearing Threshold Line: Superimposes a line chart, which displays the threshold of hearing on the main RTA graph. For normal hearing individuals, any bar that is below this threshold line is inaudible, even though significant noise energy may be present.

H -- Audio Monitor: Turning this switch on will allow you to monitor that audio signal that the RTA is actively analyzing through the output of your audio device.

I -- Difference Mode: Turning difference mode on will allow you to compare the frequency spectrum of an incoming signal to the currently measured signal. Recall a reference signal from the Save/Recall screen (by tapping the folder icon from the main window) and then tap the Play button. Notice the graph is green and the name of the recalled file will be displayed as shown below. The recalled graph is being subtracted from the current measurement. Difference mode can be useful if you'd like to determine the equalization curve required to match a new measurement to previously measured response. Tapping the Invert button within Difference Mode simply flips the curve so you are looking at what the required EQ curve to flatten the response would be.

By turning the Difference Mode switch off, you can still compare the current measurement to a recalled measurement visually. When a measurement is recalled from the Save/Recall screen, it will show up on the main display as thin horizontal green lines displaying the spectrum. The name of the recalled file will be displayed in blue under the Real Time Analyzer title text. The figure below shows a current and recalled measurement with difference mode off (A) and with difference mode turned on (B).

J -- Max and Min Bars: Turning this switch on activates the Max and Min bars on the main RTA screen (shown below). When active, the maximum and minimum levels of each bar are stored, and shown as blue and cyan bars on the screen. To reset the bars, tap the Max/Min Reset button, which is displayed in a box in the lower left of the screen. Dragging a single finger across the screen will bring up the vertical cursor, which displays the dB of the Min, Max and current band dB.

K -- Normalize Plots: Turning on the Normalize Plots button does 2 things, it pins the difference plot near the 1000 Hz point on the graph, rather than letting it float, and in non-difference mode it pins recalled plots to the live plot near 1000 Hz, making it easier to compare them.

L -- Peak Decay Bars: Turning on this switch turns on the ballistic peak meters that will appear as thin, red lines.

M -- 3dB Scale: Turning this switch on will display the major ticks of the Y-axis in 3dB steps as opposed to the default 5 and 10 dB steps.

N -- Graph Scales: This area will let you manually set the max and min of the dB Scale, and also lock the graph scale, which will turn on and off the fixed X- and Y-axes of a given measurement. When lock scales is turned on, this lock icon will be displayed on the main window analysis window. Any subsequent measurement or recalled measurement will be relative to this fixed scale.

O -- Reference Curves: Tapping this text will bring up the Reference Curve screen. This option is an in-app upgrade that when purchased is available program-wide in AudioTools. Reference curves are useful if you want to compare live measurements against a pre-defined level, or try to match the curve using EQ, for example, or to test whether a piece of equipment is within spec. Full details about usage are in the Reference Curves info screen.

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Using Gestures

Numerous gestures are supported on the main RTA screen for adjusting the view of the current measurement.

Double-tap: At any time, double-tap the screen to normalize it. This will show the full frequency range, and fit the graph to the screen.

Graph Scrolling: The graph will scroll up and down when you slide one finger up and down on the screen. Use a two-finger vertical pinch to adjust the screen scale, to see more or less dB on the screen. Note: if Lock Graph Scale is turned on in the RTA Setup menu, the two-finger pinch gesture will not function.

The state of the graph is stored as you make changes, so the next time that you start the RTA app, the zoom and scale settings are restored.

Cursor: Swiping left and right will bring up a cursor, which you can dismiss by swiping off the graph to the left.

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Calibration

In AudioTools, all calibration is adjusted in a single location, Setup -> Microphone calibration. We have a video, which describes the calibration procedure in AudioTools. This video can be viewed in the Support -> Video Center of AudioTools.

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Room Tuning

RTA can be used for basic exploratory room response analysis and tuning.

In practice, the RTA is really showing the sum of the direct sound as well as the reflections, so this is not 100% correct. Also, most people roll off the high end somewhat, and many like to boost the low end. So, use the RTA and it's information, but also use common sense, and don't make extreme EQ changes. You're usually better off making conservative changes in EQ, but you can also try moving speakers and subs around and a first step before EQ, and watch the effect in the RTA.

Here are some links to learning the basics on room tuning with an RTA:

Understanding and Properly Using RTA and FFT

Getting You Room In Tune

RTA from Studiotips

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Transmission Loss

Both static and dynamic measurement of transmission loss can done by using this module. We also support several different standards.

Sound transmission field testing of walls and partitions is normally done using ASTM E336 as the standard in the US, and ISO 717-1 in Europe. Since this test does not include RT60 times, it is technically an NIC (Noise Isolation Class) measurement rather than an (F)STC (Field Sound Transmission Class) measurement. We have added one more Transmission Loss variant, OITC (Outdoor-Indoor Transmission Class), another ASTM method using an A-weighted algorithm.

How To Test

First, select the method: STC (US-based E-336 ASTM standard), Rw (European ISO 717-1 based measurement), or OITC (US-based A-weighted ASTM measurement) Next, turn "Compute Sound Transmission Loss" on.

Source Room Measurement

Setup a pink noise source in the Source room. It should be as non-directional as possible, and should be roughly in the center of the room.

Make sure the RTA is set to 1/3 octave mode, and to Average decay mode.

Turn on the noise source, and begin taking measurements 1 meter from the wall, and move slowly along a section of the wall, and when done pause the RTA measurement.

Store this measurement, using the Save / Recall function.

Receive Room Measurement -- dynamic

First, recall the previously stored Source Room measurement. It will show on the screen as a set of green bars.

Select a slow decay mode, turn on the noise source, start the RTA measure and measure any point along the wall, 1 meter back. You may also hold the microphone close to any suspect noise leak area, and get a qualitative measure of the amount of leakage. The Transmission Loss value will be shown in real time on the screen.

Receive Room Measurement -- static

Turn on the noise source, start the RTA measure and slowly move along the wall in the receiving room, 1 meter away from the wall. When done, pause the RTA, and save to another memory position. To compute the Transmission loss, recall the RECEIVE room first, then recall the SOURCE room. Do not start the RTA running. The calculated transmission loss value will be displayed on the screen.

Background Noise

ASTM E 336 specifies that if the background noise during the measurement is within 10dB of the measured signal, it must be taken into account. And, if the noise is within 5dB of the measured signal, the results can only be used as an estimate. Since the app does not compute a noise penalization factor, you should manualy confirm that the background noise during the test is at least 10 dB lower than the measured, for each octave band.

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