Home Theater Calibration 101: Speaker Levels, Distances and Subwoofer Phase

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 Standard – 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 1/3rd 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.

Screenshot of XTZ Room Analyzer II Standard edition showing SPL meter with averaging and levels set correctly and where to read the SPL level

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 i.e. I need to delay my reference speaker 4.4ms more than a pure distance measurement alone would tell me.

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. Now 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. You will need to plug the output of the XTZ into the sub input to do so. 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.

RTA screenshot from XTZ Room Analyzer II Standard – we are aiming to get the 80Hz SPL as high as possible i.e. most in phase

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).

In phase / out of phase – blue line is response of mains and sub when they are in phase. Flipping the polarity switch for a check gave the green line. The deep symmetrical null is what we expect when the phase control is set correctly.

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 the full home theater calibration checklist here and 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

12 thoughts on “Home Theater Calibration 101: Speaker Levels, Distances and Subwoofer Phase”

  1. Hi Nyal

    1.Which graph in the XTZ Full Range tab does the ear hear in the listening position?

    2. Which tab is best used for EQ full range in XTZ?

    Thank you
    Peter

  2. Hi Nyal

    To use the Align Delay for time aligning the left main speaker + left sub with a receiver crossover set to 80hz and the subs crossover disabled (letting the receivers control the crossover).

    Would it be advisable to leave the receivers crossover at 80hz and measure, then measure the sub with the crossover disabled or set the subs crossover to 80hz as well.?

    Appreciate any comments

  3. Hi Peter, the crossover should ALWAYS be disabled on the sub if you are using the crossover in your AV receiver / pre-pro, otherwise your sub will have a 'double rolloff', since there will be a rolloff applied in the AV receiver / pre-pro and another by the sub.

  4. Hi Nyal thank you.

    Phase vs Distance settings

    As i bellieve phase setings effect the crossover frequency so what does distance effect?

    Appreciate your help to understand

  5. Hi Nyal

    When setting up dual stereo subs (one sub for each main speaker) is it strange to have extremes for each channel e.g. left sub and main, to have a distance of 0 meters for the sub and the left main to add 2 meters to the the already 3.50 meters, but with the right is suggesting to add 7 meters to the already 3.45 meters and leaving the right sub at 0 meters.

    Doesn't this now through out the mains distance delay?

    Thank you

  6. Hi Peter

    The left and right should always have identical delays, the subs can have different delays if your room is asymmetrical or placement is asymmetrical. XTZ tries to find the flattest frequency response overall.

    I'm not really a believer in stereo subs, and prefer to sum to mono and use a lower crossover point for two channel such as 50Hz. This allows you to place the subs separately to the mains for beneficial overall smoothing of the bass response and potential room mode cancellation.

    Thanks

    Nyal

  7. Hi Nyal thank you.

    I am trying stereo subs currently.

    All the original settings were set by the receiver in auto setup.

    I re-did the measurement and this time i ended up adding to the mains (left main original 3.5meters, now 5.55 meters, right main original 3.45 meters, now 5.70 meters).

    The subs were originally set to 7.25 meters, (i changed this back to 0 for the measurements) and XTZ suggested only a change to the mains. Does this sound right?

    When i measure both speakers combined with the Room Analyzer there is a null, but when i measure the left main and sub seperately the null isn't there as much. I change the phase of the left sub to reverse and the null appears, but when measured combined the null appears. Which phase is correct?

    For mono subs do i measure the 2 x mains combined, then the 2x subs combined, and then what do i add to e.g. 2 meters to both subs?

    Thank you very much

  8. Hi Peter

    1) Yes, adding delay to the mains rather than the subs is normal.
    2) Nulls can appear when both speakers are played together, though typically this is at a different frequency than the sub crossover.
    3) For mono subs, measure the L/R combined, then the subs combined.

    Thanks!

    Nyal

  9. The pink noise played back by most AVRs and pre-pros is not suitable for
    subwoofer integration. The signal is most often band passed pink noise
    (essentially pink noise with the top and bottom octaves filtered out) that
    is designed to be used for setting levels, not for integrating subs. You
    would need to find a source of pink noise to use for the calibration. I am
    not familiar with the calibration discs on the market but a search might
    reveal one that has the pink noise on it. If not then the XTZ Room Analyzer
    Standard edition provides an inbuilt signal generator that produces pink
    noise. You can plug the XTZ into your AVR or pre-pro via a two channel
    analog input and set the surround mode to Dolby Pro Logic II. With this
    done the sound will come out of the center speaker and subwoofer. A two
    channel input should generally be used rather than a multi-channel input as
    most multi-channel inputs bypass the DSP section and hence any level or
    distance adjustments. Using the two channel input you will be able to see
    changes in the subwoofer integration as you adjust the distance setting in
    the AVR or subwoofer phase control.

  10. I have a quick question. I’ve read a couple of you articles regarding
    getting the sub and mains in phase with each other. Do I need to play pink
    noise from the mains and sub at the same time to measure the best response
    at the crossover frequency, or do I just measure the sub. My amp (Onkyo
    Thx) plays pink notice per Chanel, not simultaneously. Do I need a special
    disc to play pink noise thru my system for this?

    I’m using an iPad spectral analyzer to measure SPL

    1. The pink noise played back by most AVRs and pre-pros is not suitable for
      subwoofer integration. The signal is most often band passed pink noise
      (essentially pink noise with the top and bottom octaves filtered out) that
      is designed to be used for setting levels, not for integrating subs. You
      would need to find a source of pink noise to use for the calibration. I am
      not familiar with the calibration discs on the market but a search might
      reveal one that has the pink noise on it. If not then the XTZ Room Analyzer
      Standard edition provides an inbuilt signal generator that produces pink
      noise. You can plug the XTZ into your AVR or pre-pro via a two channel
      analog input and set the surround mode to Dolby Pro Logic II. With this
      done the sound will come out of the center speaker and subwoofer. A two
      channel input should generally be used rather than a multi-channel input as
      most multi-channel inputs bypass the DSP section and hence any level or
      distance adjustments. Using the two channel input you will be able to see
      changes in the subwoofer integration as you adjust the distance setting in
      the AVR or subwoofer phase control.

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