Loudspeaker and Room System Analyzer (LARSA) Instructions

LARSA is a powerful impulse response measurement and analysis tool. It is ideally suited for measuring loudspeakers in rooms, multi-speaker setups, car audio systems, home theater systems or larger, installed loudspeaker systems. It is an easy way to profile and equalize your loudspeakers in your listening space. Quickly make an impulse response measurement in your listening space, generate frequency curves and energy decay curves. Then optionally generate filters to help match the room response to the target curve and generate a file ready to import into many popular DSP platforms. (Coming Soon as an in-app purchase)

LARSA helps to:

* Measure and profile the frequency response and energy time curve of your loudspeaker system at numerous positions in your room.

* Quantify the presence of room mode interactions in your playback system.

* Match subwoofer systems to full-range mains speakers.

* (Optional, coming soon) Automatically generate filter settings ready to be imported into compatible hardware devices.

* Quantify magnitude response differences between speakers or difference channels of a multichannel speaker system.

* Aids with sound field management, EQ and bass blending.

LARSA does not:

* Correct for large anomalies in magnitude response due to improperly set up systems, or deficiencies in playback hardware (i.e. it will not allow you to get 20 Hz bass extension from a small monitor loudspeaker.).

* Correct loudspeaker rattles, extended or inappropriate room reverberation or audible echoes.

* Correct for improper loudspeaker placement, wiring or signal flow.

 


Sections

The Basics

Setup

Basic Measurement

Spatial Average Measurement

Measuring Multiple Loudspeakers in the Same Location

IR Windowing Options

ETC Graph Options

Automatic Filter Design

Home Theater Loudspeaker Delay Measurement


The Basics

LARSA is an acoustic measurement tool that mathematically compares a reference log sine sweep signal to that same signal measured through a transducer and room. This is useful, in that by subtracting the measured signal from the reference signal, what remains is just the impact of the loudspeakers, other devices in the signal chain and any room interactions.

There are a number of ways to record this signal. With our iAudioInterface2, you will plug in the line output of the interface into the input of the loudspeaker you wish to test. You will plug in your measurement microphone into the microphone input of the interface and set up the microphone in the location where you wish to measure the response.

With our iTestMic, the configuration will be the same, except you will use the line output of your iOS device to feed the measurement signal into your system.

Finally, you can use the internal microphone to capture the measurement signal, although for best results, a measurement microphone should be used.

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Setup

The first step you need to perform is to take a reference measurement to determine the latency of your system. Tap the wrench icon and in the ETC Graph setup screen you will see a latency box. Set this to 0.0s to start.

Tap done and place the microphone as close as you can to the transducer you wish to measure. Set the gain of your microphone preamp by tapping the gear icon, and then microphone setup. For complete calibration and gain adjustment instructions, please refer to the instructions in the settings screen.

Tap the measure button to take your first measurement. A log sine sweep signal will be recorded and through a process known as deconvolution, the resulting impulse will be generated.

To set the latency, switch from the analysis curves screen to the ETC screen by tapping the second grey icon on the bottom middle of the module. The ETC, or energy time curve, shows how energy is decaying in your room. First, assure that the 1.00s option is selected in the ETC graph. Use a single finger drag to slide the cursor to the peak of the impulse response and observe the time readout. This is the reference latency of your system. Tap the wrench icon again, and type the time value you observe into the latency field.

Now take another measurement in the same reference location. This time, tap the 0.020s button on the ETC curves graph. Your new measurement’s peak should be 0 ms. If it is not, adjust your latency settings until your close mic is measuring 0 ms delay and 0 ft distance. Once that is set, as you move around your room and make other measurement, the delays and distance differences will be meaningful.

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Basic Measurement

There are two basic ways to use this module. The first is to make individual measurements of different loudspeakers or measurement locations around your room. The other is to make multiple spatially-averaged measurements of the same loudspeaker.

We will start with the first case where you wish to measure, for example, a small studio monitor in 1) a close microphone configuration (as close to the transducer as possible), 2) at the listening location and then 3-8) at multiple locations in the diffuse field. You have two options for sweep lengths, which can be selected in the setup screen. The 0.5s sweep gives you good resolution and a quick measurement time. The 1.5s sweep gives you a bit more resolution in the lower frequencies, which is useful if you are measuring large loudspeakers that have an extended low frequency response or subwoofers.

The sweep measures from 10 Hz - 22 kHz, so please be careful of the signal level sent to your loudspeakers. Protect your hearing and your equipment. If you are measuring for an extended period of time, please use ear plugs. In the audio section of the setup pane, there is a test tone option, which will toggle a sine tone for level setting (this signal is also available on the main screen). Assure that the signal is intense enough, but not too loud that it will distort your microphone input. (about 75-85 dBA at 1 m is a good starting point)

In addition to the sweep length setting, you can also select if you are using a balanced out or a mono out and select the input source.

Back in the main screen, we are ready to take a measurement. Tap the 1 test number and position the microphone in the close microphone configuration (as close as you can get to the transducer). Now tap measure. In the analysis curves graph, you will see the frequency response of your measurement. The currently selected measurement in the test number will be displayed as a blue trace. Test numbers that have a stored IR will be displayed in green text, and open locations will be displayed in red text. The AVG shows the average frequency response of all measurements. In the setup screen there is an option to turn on show all plots. This will toggle being able to view all measurements you have made in a single graph (with the currently selected measurement shown in blue) and looking at just the current measurement.

The frequency response graph can be smoothed. In the setup screen there are options for the frequency graph. Curve smoothing options from none to full-octave smoothing are available. You can also manual set the ranges for the dB (Y) axis, or frequency (X) axis.

Next, move you microphone to the listening location (usually located in the sweet spot between your stereo loudspeakers). Repeat the measurement process. Notice how the intensity of the frequency response has decreased because you are further away from the loudspeakers? In LARSA, none of the responses are normalized, so you can see the level differences and time difference in multiple measurements around your room.

Move over to the ETC curves, assure that 0.020s graph time option is selected and notice how you can see that the second IR that has been measured is further back in time than the close mic IR. This is because there is a delay introduced by the distance from the loudspeaker to the microphone. Drag your finger onto the plot the measure this delay. Line up the vertical cursor with the second peak and you will see the ms delay and the delay expressed in distance.

Move your microphone to the third measurement location and repeat the process. You can repeat this process for up to 8 measurement locations in the room. To store your results, tap the folder icon. For complete save / recall instructions, please refer to the info in this screen.

If you have inadvertently made a spurious measurement, or wish to clear the memory in the test number boxes, highlight a test, and tap the garbage can icon. The module will prompt you the clear this test (the one selected), all tests, or cancel. All tests will remove all of your measurements from memory (1-8), no matter which one is highlighted.

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Spatial Average Measurement

A useful technique for characterizing how a loudspeaker sounds in a room is to measure a spatial average around the sweet spot.

The technique here is to measure IRs around the listening location and take an average of all 8. This helps to smooth out modal behavior and gives you a reasonable impression of the frequency response that you are actually hearing from your loudspeaker. This technique is useful for setting tone controls on a pre-amp or receiver, or general equalization of your system. This is also a useful technique for setting EQ setting present on many active studio monitors or DSP equalizers.

Start again by setting your latency as instructed above. Tap test number 1 and place the microphone where the close to the listening position, at ear level with the microphone pointing up. Continue the same process for the remaining 7 measurements adjusting your measurement in a random fashion around your listening location. If you listen in a chair, keep the measurements within a 1 foot radius in all dimensions around your listening position. If you listen on a couch or similar, the radius can be 3 feet or so. After you are done, tap the AVG button. This should be a reasonable approximation of the frequency response of the loudspeaker in your room. Note, you are not looking for a flat response here. Most systems that sound perceptually "good" have a slight elevation of the response from 20 Hz - 200 Hz, are flat to 1 kHz and then have a gradually dropping high frequency response towards 20 kHz. To see just the average measurement, and not the individual ones, enter the setup screen and turn off the show all curve switch. Then tap done and tap the AVG Test Number.

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Measuring Multiple Speakers in the Same Location

Another measurement technique that you can use LARSA for is to make measurements of multiple loudspeakers at the same location. We will take the case of measuring a 5.1 surround sound system in the sweet spot of a home theater.

Again, you will start by making the reference latency measurement. Then place the microphone in the sweet spot with the diaphragm pointing up, positioned at ear level. Tap test number 1, and route the signal to the left loudspeaker. Measure the response. Tap test number 2, route the signal to the center channel and measure the response. Continue measuring the right, right surround, left surround and subwoofer in the same way.

Now you can compare the frequency response of each loudspeaker as well as the arrival time by inspecting the ETC curves screen.

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IR Windowing Options

After an IR is recorded, it is windowed in the time domain to reduce errors in the frequency response due to the abrupt stopping and starting of the IR sound file. Windowing simply smoothly removes some of the beginning and end of the IR. Since most loudspeaker systems are being measured in rooms with relatively short reverberation times, this has minimal impact on the resulting ETC analyses. There are three options for time windowing. They can be adjusted in the setup screen. The options for adjusting the time of the window are available for short, medium and large.

You also have the option to disable the time windowing, which can be useful for inspecting the noise floor and signal to noise ratio in your room.

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ETC Graph Options

In the setup menu there are options for adjusting the display of the ETC graph. You can manually set the dB scale (y-axis) and time scale (x-axis). Here, you can set your system latency, as discussed in the basic setup section. Options are here is well to express the x-axis in units of time or distance. Finally you can turn on ETC Smoothing. This applies an exponential moving average smoothing function to the time response. You can turn this off, or have a time window of 10-40 for the smoothing.

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Automatic Filter Design

Filter design is available as an in-app upgrade in the LARSA module. It is accessed through the setup menu. There is a switch to turn on the generation of equalization filters. The idea of the filter designer is to take the measured frequency response of your loudspeaker and design inverse filters to flatten the response at the listening location by attenuating room modes. There are a number of commercially available digital and analog devices, which can perform this filtering.

To start, tap the Target EQ Curve text and load in a house curve. This is the curve that you want to match your measured response to. The target EQ curve uses the same protocol as the reference curves available in AudioTools. Full details about generating and loading in reference curves are found in the reference curve info section. Tapping use default curve will load a standard reference curve.

LARSA supports automatic filter generation for the miniDSP 2x4, generating a text file of filter coefficients that can be loaded directly into miniDSP's desktop plugins. If you have a 2x4, tap the miniDSP 2x4 text. You can also choose a generic 4 or 5-band filter generation option, which will give you a text file of the frequency, bandwidth (Q) and gain for each of the filters. These can be input manually into a digital or analog equalizer.

You have options to specify a maximum boost and cut in dB of the designed filters. Generally, it is a good idea to avoid large boosts. This is a good way to introduce distortion, reduce headroom and damage your loudspeakers. Good starting points might be +3 dB and -10 dB. Also increase the volume of your system carefully after adding equalization filters.

When perform filter design is turned on, addition traces are shown in the analysis curves graph. Tapping the text will turn on or off the traces. The before trace is the raw frequency response of the system. The target is the reference curve that the filters are trying to turn the raw response into. Predicted shows what the response should be after being processed by the equalization filters. Finally the filters trace plot the magnitude of each of the peak filters being used in the equalization. Tapping the magnifying glass icon will bring up a table, which shows the gain, Q and frequency of each of the filters.

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Home Theater Loudspeaker Delay Measurement

LARSA Loudspeaker Delay Test (LDT) provides a solution for obtaining the real-world delay time of the loudspeakers in a surround system, compared to the reference loudspeaker. To use it, you will need the LARSA module and the Apple TV Surround Generator app, with either the 5.1 signals or the Atmos test signal package installed. LDT is a free feature, once you have the LARSA module installed. 

LDT requires the special LARSA dual-sine chirp signals that are in the Apple TV Surround Generator app.

LDT works by analyzing the dual-swept sine signal, which sends one chirp to the reference (left) loudspeaker, and the other to the loudspeaker chosen for testing, offset by a fixed amount. LARSA uses the offset time to pick out which signal comes from which loudspeaker, and run them both through the same analysis, and determine precisely the difference in the initial delay time. 

This process ends up being able to determine the loudspeaker delay difference, including all electronic processing delays, DSP delays, or any other sources of delay, since we are using the actual acoustic waveforms. This process is very repeatable, noise immune, and can resolve differences with a resolution of 0.5ms.

Setting up and Calibrating Loudspeaker Delay Mode

Turn on Delay Mode on the LARSA settings page, and select External Chirp, and 0.5s Chirp.

We recommend setting the Left channel delay in your receiver to the largest distance that you expect to see, based on the distance to each of the other loudspeakers. This will usually give you the best results, as then the other channels will be delayed relative to the Left channel.

Make sure that the sub will not be engaged while you are testing the main speakers. If you cannot turn off bass management, or are unsure, just unplug any subs during the main speaker tests.

Start the Surround Generator app on Apple TV 4k and select ATMOS or 5.1 and the Delay signal. Pick a the Left loudspeaker so that we can check the delay calibration.

Tap Measure, then start the test signal on Apple TV. It may need to loop to engage the processor. Both chirps will be output from the Left channel. 

Now you should see 2 plots on the main screen. Switch to the ETC screen (tap the > button on the screen to switch plots), and you should see the delay time, which should be 0ms, or very close to it. The two plots on the screen are showing the two deconvolutions which have been computed. If the time is not very close to 0, open the settings page and scroll down to the +/- control that shows the Delay Trim Samples. Adjust this value to increase or decrease the calibration time and go back to the ETC plot and check the delay time. Repeat this until you get a value close to 0ms. In many cases leaving the trim value to 0.0ms will be the correct setting.

Measuring Loudspeaker Delay

For these tests, make sure the test microphone is centered in the room, at the main listening position. Remember that moving the microphone 1 foot right or left can make a 2ms change in delay. You want to optimize the delay for the main listening position for the most immersive experience.

Now change to another loudspeaker, for example Center, on the Apple TV Surround Generator. Tap Measure on the LARSA app and click the selected speaker to start the signal. Note that if you are using this function in the HAA app, you can link to the Surround Generator and control it directly from the HAA app.

You will see two plots, and a new delay number which is the delay to the loudspeaker under test. It may be quicker to write this number down and collect all the delay times before changing the delays in the receiver or processor. 

Repeat the test for the rest of the loudspeakers and write down the results.

When you get to the subwoofer, follow the same procedure. The sub should be able to reproduce enough of the sine chirp to get good results, although there are other methods that may be used for sub time alignment.

Now you can adjust all of the delay times in the receiver by adding or subtracting the numbers and go back and run the test again and observe the results. If needed, go back and again adjust the receiver delay times.

Tips for Best Results

Use a fairly loud test signal, since we need to be able to clearly find both of the sine chirp signals.

If you have multiple subwoofers that have independent delay time controls, disconnect all but one and test them separately.