Creating a system you love shouldn't be difficult. The Acoustic Frontiers blog is here to help.
No industry standard crossover frequency. The vast majority of home theater systems are setup using an 80Hz crossover. 80hz was chosen to be a good compromise between the need to support the main speakers and localization. In a home theater setup the main speakers are most often smaller 'monitor' types with limited low frequency extension and power handling which require support from a subwoofer if the system is to reproduce bass with any degree of fidelity. At frequencies of 80Hz and below there is a simple rule defined from psychoacoustic research that shows that humans cannot localize sound. In essence we cannot tell where a sound at these frequencies is coming from. This allows the sub to be physically separated from the main speakers. This simple rule does have some complications, however. For one the crossover between mains and sub is not a brick wall. With a 12dB per octave crossover the sub will only be 12dB down at 160Hz. That is why more progressive calibrators are using either steeper crossover slopes, moving the crossover down to 50Hz or both. In a two channel system we have no 'industry standard'. Furthermore most two channel systems have no line level crossover. The choice of crossover frequency must therefore take into account the acoustical characteristics of the left and right speakers and in particular the frequency at which the sound pressure they output starts to fall off.
With these two points in mind let's look at how to integrate a sub with a stereo system.
without the sub.
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Screenshot from XTZ Room Analyzer II Standard showing how to set SPL levels and take a measurement[/caption]
The Room Analyzer will now take a frequency response measurement and display it to you in the main window. Analyzing this measurement will allow you to set the crossover slope and frequency on the sub. You will see that it is not an exact science, and multiple iterations are often necessary to find the best sub to mains integration. But before we look at your system's measurement, let's briefly discuss crossover slopes.
Crossover slope. This control on a sub, most often called the 'Low Pass' filter, is specified as xxdB per octave and comes most often in 12dB and 24dB options, although sometimes you find 6dB and 18dB. The slope defines the rolloff above the crossover frequency. In a situation where there is no line level crossover in the system, as will be the case in most audiophile two channel setups, the appropriate crossover slope to choose will match the acoustical rolloff of the left and right speakers. To find the acoustical rolloff you need to try and fit a straight line to the frequency response measurement. This isn't an exact science as quite often there are few different ways you can fit the straight line to the measurement. Room modes and speaker boundary interference effects also create peaks and dips in the measurement that are not speaker related. Our advice is to focus most on the section of the frequency response after rolloff begins as this is the part that will dictate the smoothness of integration with the sub.
The chart below shows the results of a straight line fit for our measurement. To find the crossover slope you need to find two points on that line separated by one octave. One octave represents a doubling of frequency. Looking at the chart below we can see that the rolloff of the main speakers is around 24dB per octave. The chart makes this easy to see because each set of three squares represents one octave e.g. 31.5Hz to 63Hz is one octave, 63Hz to 125Hz is one octave, 125Hz to 250Hz is one octave. If we start at the square at 25Hz (the square just below 31.5Hz) and count three squares up we get to 50Hz which is one octave. The difference in SPL is 24dB (40dB at 25Hz and and at 64dB at 50Hz).
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Frequency response measurement of main speakers taken with XTZ Room Analyzer II Standard - the 25Hz and 50Hz points on the rolloff slope have been annotated[/caption]
Crossover frequency. Choosing the most appropriate crossover frequency is complicated by the fact that the point to choose depends on the crossover that is built into the sub. Most subs use either Butterworth (BW) or Linkwitz-Reilly (LR) filters. BW filters sum to +3dB at the crossover point whilst LR filters sum flat. The crossover point for a BW filter is the -3dB point and for a LR it is the -6dB point. The diagram below, from this Wikipedia article, shows this clearly.
sub only. Have a close look at the chart, because it will tell you if the crossover frequencies and slopes marked on the sub actually measure that way in real life! In our case (we used a JL Audio Fathom F112) they were spot on. You'll see there is a bump in the frequency response at 75Hz or so, which is a room mode. If you see things like this you should try and see 'through them' by extrapolating the straight line above the problematic frequency.
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Frequency response of sub taken with XTZ Room Analyzer II Standard - extrapolate the straight line and ignore bumps and dips due to room modes and boundary interference effects[/caption]
Once you've selected the crossover slope and phase on your sub the next step is to use the RTA function to examine the frequency response of the whole system playing together i.e. Left / Right speakers and sub. So open up the XTZ software again and go to the 'RTA' tab and click on 'Measure'.
Level Part 1. The level of the sub should be 'eyeballed' by looking at a RTA measurement of the whole system playing together and approximately matching the level below the crossover point (i.e. the sub level) with the level above the crossover point (i.e. the left and right speakers). Remember that there might be big dips in the frequency response at the crossover since you haven't set phase properly yet. You will fine tune level after setting phase correctly.
Polarity and Phase. 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 are interchangeable. Theoretically speaking a 6dB filter has 90 degrees of phase shift at the crossover frequency and each extra 6dB adds 90 degrees, so a 12dB filter is 180 degrees out of phase and so on. Theory goes out of the window at this point, however, since the phase of the main speakers at low frequencies is very hard to pin down since not only is there the main rolloff of the speakers, there is also ports and other things that cause strange phase shift effects at low frequencies. Therefore the method we use involves using a RTA. What you do 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 it means that the sound waves from the sub and mains are properly in phase and not causing any phase based cancellation.
In the XTZ software take another RTA measurement, but this time slowly change the phase control on your sub whilst watching the RTA bar corresponding to the crossover frequency you have chosen.
RTA taken with XTZ Room Analyzer II Standard showing the frequency band we care about in this example - that of the crossover point between the sub and main speakers
In phase / out of phase - green line is response of mains and sub when they are in phase. Flipping the polarity switch for a check gave the blue line. The deep symmetrical null is what we expect when the phase control is set correctly.[/caption]
I know what you are thinking! You are looking at the graph and wondering why it isn't a perfectly flat straight line. The chart above is actually very good - it's within 5dB over most of the range below 300HZ. You have to remember the other major factors at play - room modes and speaker boundary interference. Even if you get your sub and mains properly integrated there will still be peaks and dips in the frequency response caused by room acoustics. To mitigate these effects you need to start looking at subwoofer placement relative to room modes / boundaries, using multiple subwoofers in mode canceling arrangements and using parametric EQ. Those aspects are beyond the scope of this blog post but if you want to learn more follow the links above.
Level Part 2. This is the point at which you play some music and find the best level for the sub. It's best to play a variety of music. Aim for the point at which the sub doesn't draw attention to itself but rather provides a solid foundation for the presentation. It will take a little while but soon you will find the best level and once you've found it you shouldn't need to move it.
After this process you should note down and live with your sub settings for a few days. It's often worth experimenting with different slopes and crossover points as these can sometimes lead to slightly improved results. If you don't have patience then don't worry, following the instructions above should result in a well integrated sub.
JL Audio, the manufacturer of the superb Fathom F110, F112, F113, F212 and Gotham subwoofers showed their new E112 subwoofer at CES. I have seen this sub before, at CI Expo, a custom integrator event that was held in Berkeley this year. But this is the first time I had my camera with me to take some photos!
Soundstage Network have published a photographic tour of the JL Audio facilities in Miramar, Florida. JL Audio make nearly all parts of their fabulous home audio subwoofers in house and have fanatical attention to detail, as can be seen from the shots taken in different parts of their factory.
Nyal Mellor, Founder, Acoustic Frontiers
Nyal Mellor
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