Introduction

This blog article will teach you about home theater calibration - in particular how to calibrate the audio side of your home theater pre-processor (pre-pro) or audio visual receiver (AVR). I'm not going to cover the video side.

The three basic elements of audio calibration in home theater setup are:

• Speaker and subwoofer levels
• Speaker and subwoofer distances
• Subwoofer phase

Whilst many modern AVRs provide automated setup routines, such as those by Audyssey, the results can be variable and in many cases better sound quality can be achieved by manually setting up your system. For the purposes of this discussion we will assume that you are using the ‘industry standard’ THX 80Hz crossover between your main speakers and your sub. Sometimes you can get better results by using a different crossover frequency such as 60Hz, particularly if your satellite speakers are physically large, but exploration of this is beyond the scope of this article.

For best results we are going to use some special home theater calibration software – the XTZ Room Analyzer II – to setup your system. The XTZ is a versatile acoustic measurement package that contains both the hardware (microphone, soundcard, cabling) and the software required to calibrate your home theater.

There are four pieces of functionality that XTZ Room Analyzer provides that we will use to complete the calibration:

• Sound Pressure Level (SPL) meter with C weighting and slow averaging to set speaker and subwoofer levels
• Delay alignment tool to set speaker and subwoofer distances
• Real time analyzer (RTA) to set subwoofer phase / polarity
• Frequency response ‘Room Analyzer’ with 1/3^rd octave smoothing to check subwoofer phase / polarity

Until recently we would have had to use an expensive collection of hardware to calibrate your home theater audio, and even then we wouldn’t have been able to do as good a job as we can today with the modern acoustic measurement packages. In particular the proper setting of subwoofer distance is not as straightforward as just measuring the physical distance from listening seat to sub as we will explain.

Speaker & subwoofer levels

The first step is to set the sound pressure level ‘trim’ for individual speakers in the AVR. We need to do this because quite often each speaker - even from the same manufacturer - has a different sound output or sensitivity for a given electrical input. I often find that surround speakers are less sensitive than left, center and right (LCR) speakers.

To setup the trims you will need to find out how to get into the speaker level setup screen in your AVR. It is quite often called ‘Speaker Levels’ or ‘Channel Levels’. There will be a command to start a pink noise type test tone signal. Once this is playing you should open up the XTZ Room Analyzer software on your computer and look at the left of the screen where you will find the the SPL meter section. The XTZ does not need to be connected to your audio system at this point. Set the controls to ‘C-Weighting’ and ‘Slow (1s)’ (see screenshot for the location of these controls). The process is then to go through each of the speakers (including the subwoofer) in turn on the AVR and change the trims such that the value next to the ‘LCS’ (i.e. Level with C weighting and Slow averaging) is 75dB for all speakers. The SPL readings from the XTZ are quite accurate. I checked them against my calibrated IEC Type II SPL meter and they were within 1dB and so can be relied upon.

Speaker and sub distances

The next step in the process is to set speaker distances. For all speakers except the sub this can be done quite accurately by using a tape measure or laser distance meter. So go ahead and measure the distances and put these into your AVR – they are often found in a menu called ‘Speaker Distance’ or just ‘Distance’.

What we will do now is check all of these settings acoustically using the unique Delay Finder capability in XTZ Room Analyzer. With your left, right and surround speakers there will typically not need to be any additional delay changes – the physical distance measured is accurate enough to align the distances. With the subwoofer, however, that is generally not the case. Nearly all subwoofers introduce some additional acoustic* delay that must be compensated for – i.e. the physical distance is not the same as the acoustic distance. Using the Delay Finder you can compare the sub to a reference channel - I use the center - and determine the additional delay that needs to be added.

Typically you will find that all of the main speakers need to be compensated between 3ms and 10ms to align them with the sub. Remember that sound travels 1.1ft in 1ms. My JL Audio F112, for example, has 4ms of additional acoustic delay relative to my mains. In an AVR this is accomplished by setting the distance for the sub to be 4.4ft GREATER than the mains. The AVR will then delay the mains to align them with the sub.

Open up the XTZ Room Analyzer software and go to the ‘Full Range’ tab. Connect the output of the XTZ to the center channel analog input (or whichever input corresponds to the reference speaker you are using) of your AVR. TURN OFF THE SUB, so you are just getting a measurement of the speaker. Take a measurement of the center speaker and store the result as an 'Overlay' section by hitting ‘1’ (see screenshot below).

The next step is to take a measurement of the sub only. TURN ON THE SUB, AND UNPLUG THE REFERENCE SPEAKER, so you are just getting a measurement of the sub. Once you have taken this measurement you will see the frequency response of the center and sub channels in the graph. You can now use the Delay Finder to check and adjust acoustic delays as necessary.

To use this function press ‘Align Delay’ and then ‘Auto Align’ (see screenshot below). Room Analyzer compares the two measurements and determines the delay that needs to be applied to put the two in alignment. 99% of the time you will find that delay needs to be added to the ‘Overlay 1 Measurement’ i.e. the center channel. Since we can pretty much always rely on the physical and acoustic measurements being the same for all channels except the sub we can add this delay to the center, left, right and surround channels.

Screenshot from XTZ Room Analyzer II Standard – the unique Delay Alignment tool automatically identifies the best time delay to apply to the main speakers to align properly with the sub

Subwoofer phase

The process we will use to correctly set subwoofer phase is the same as that covered in our article Subwoofer Integration for Stereo Systems.

Many people get confused by polarity and phase. The thing to remember is that both controls are adjusting the same thing. The phase control is normally variable between 0 and 360 and the polarity has two settings 0 and 180. Both controls are changing the phase at the crossover frequency. If you set polarity to 180 and phase to 0 you'd get 180 degrees of phase shift at the crossover. If you set polarity to 0 and phase to 180 you get exactly the same result. So they can be used interchangeably.

The method we use to set phase involves using a RTA. Go to the RTA tab in the XTZ Room Analyzer sotware and press the 'Measure' which is found towards the top right of the screen (see screenshot). Now, with the RTA playing pink noise, slowly adjust the phase controls on the sub to get the maximum SPL at the crossover frequency. When you find the phase setting that produces the most SPL at the crossover frequency (80Hz in this case) it means that the sound waves from the sub and mains are properly in phase and not causing any phase based cancellation.

Once you've found this point a quick check should be done using a 1/3rd octave frequency response measurement. Go to the 'Room Analyzer' tab, and take a measurement of the sub and center channel measured together. Once you have this save it as an overlay by hitting ‘1’ in the 'Overlay' section. Now flip the polarity switch on your sub which will change the phase relationship 180 degrees. This should result in a nice deep null at the crossover point. So with the polarity flipped take another measurement and compare the two. If the new measurement does not have a symmetrical or deep null then keep on tweaking that phase control! The chart below shows the mains and sub measured together with the phase set correctly (blue line) and 180 degrees out of phase (green line).

Conclusion

In this blog article from Acoustic Frontiers we've shown you how to use an acoustic measurement package such as the XTZ Room Analyzer II Standard to calibrate and setup the audio side of your home theater system. The process described in this article is what I think of as a 'basic calibration'. Acoustic Frontiers has developed a much more indepth calibration process which is available as a consulting service within the SF Bay Area and beyond - you can find details on our calibration process here. Thanks for reading, and if you have any comments or suggestions on how to improve the process please leave a comment! Nyal / Acoustic Frontiers