Øyvind Kvålsvoll 20.08.2015 Download PDF
How to set up a Home Theater Sound System
26.03.2014: Minor updates for Room acoustics.
06.01.2015: Info for new immersive sound formats Dolby Atmos, Auro3D, DTS:X.
20.08.2015: Revised version. Updated content, more images, more on acoustics.
Calibration means to adjust all settings for speakers and sound so that level, distance and frequency range is reasonably correct for all speakers. The goal is to achieve a smooth frequency response and proper sound level across all seats, from all speakers, so that the movie will sound similar to as it did when the soundtrack was produced. Proper level means that the master volume on your receiver now will adjust the volume according to reference level, so that a given setting on your system always relates to a specific sound loudness.
The settings are adjusted on the receiver. Most receivers will have an automatic setup procedure, which at least will get you started with some reasonable initial settings.
|Setting||Affects||How to adjust|
|Speaker distance||Perceived location of sounds||Distance adjustment on receiver|
|Speaker level||Balance between speakers, playback sound level calibration||Trim level adjustment on receiver|
|Frequency response||Tonal balance, perceived sound quality||Equalizer in receiver, DSP with equalizer, equalizer and crossover frequency on subwoofers, speaker selection, speaker location, room acoustics|
|Crossover frequencies and distance for subwoofer||Subwoofer integration, punch and clarity in bass||Crossover frequency and distance settings on receiver|
A correct frequency response of the system is required for good and balanced sound. Decades of experience with sound reproduction have learned us how the response should be to give a perceptually neutral and good sounding balance to the sound; a tilted in-room response so that bass is louder and higher tones are softer.
Flat anechoic on-axis response from the speakers means that the direct sound - the sound from the speakers without room contribution - is flat. The room contribution will add more at lower frequencies and less at higher, so that the overall response measured at the listening position will be tilted downwards. How large the tilt is, depends on room acoustics, size of the room and loudspeaker radiation pattern.
Generally, smaller rooms have a larger tilt, more damped rooms have less tilt.
It is important to note that this tilted response is not a result of personal preference to make it sound "better but not neutral" - the tilted curve is the neutral sound.
There should be no large holes or peaks on the response, this would indicate a problem with speakers or room acoustics. Generally, the smoother the better, part from the desired tilt.
The reasons behind the tilted response lies in how we perceive sound. The balance will mainly be judged from the direct sound from the speaker - before any room contribution is added. The human hearing operates in both time and frequency, something that is very easy to forget when working with frequency response measurements.
Bass-lift means to add a little lift in the very lowest bass, to give sound effects more weight and impact. This is a customization from a strictly neutral response, and is chosen according to personal preference - some like no lift, some like much. The important thing is how the lift curve looks - it should only amplify the lowest frequencies, so that full level is attained at around 20Hz and below, while gradually sloping down in level up to around 50-100Hz.
The combined target correction curve is often referred to as House-curve.
Tweaking the frequency response requires acoustic measurement equipment and a proper equalizer. Some room correction systems can improve the response by automatic adjustment, but if the correction is not optimized for a tilted response that matches your room and speakers, the result will not be good.
Luckily, it turns out that good, reasonably linear speakers in a reasonably good room will give a decent frequency response, with flat on-axis direct sound, and tilted in-room total response. Then the only adjustment required is the bass-lift which is done on the subwoofers.
Checking L, R speakers calibration for 3 seats using pink noise. Frequency response is smooth and shows the desired gradual tilt, approx. 10dB down at 20KHz from 20Hz. All 3 seats show very good consistency. No eq, no room correction, just decent speakers in a decent room.
The receiver automatic set-up will usually find usable values for speaker trim levels, so that main speakers and surrounds are balanced, and reference level is reasonably correct.
Most important is the balance between front main and surround speakers, so that surround sounds and front sounds have correct relative levels.
The reference level calibration is not very critical, as you will of course adjust the master volume setting to your preference when watching a movie, regardless of what the master volume setting displays.
Currently it seems the most widely used method for level calibration is to use the Dolby -20dB RMS Pink Noise signal. The adjustment is done by playing the pink noise signal and adjusting the level of speakers using a RMS sound pressure level meter. This method ensures your system will be calibrated to the same standard as all others using the Dolby signal as reference. This signal has full frequency range and will result in a much more accurate calibration than a typical receiver which uses narrow-band noise.
|Dolby Pink Noise -20dB RMS calibration|
|Loudspeaker||Receiver master volume setting||Adjust trim level to measured SPL|
|Surround||0dB||85dBC / 82dBC (*)|
|Surround||-10dB||75dBC / 72dBC (*)|
The calibration can be done at lower level as well, just subtract the master volume setting from the measured level, master at -10dB means 75dB on the measured SPL.
If you have calibrated measurement equipment with a real-time spectrum analyzer you can verify the level and also check the frequency response at the same time.
Note that the Dolby pink noise signal does not have a RMS value of -20dB, it is closer to -18dB. The standard calibration in AV-receivers use a narrow-band -20dB RMS signal, which means an AV-receiver calibration will usually end up around 2dB hot compared to using the Dolby signal.
Most modern receivers have some kind of room correction - a digital processing filter making adjustments to the sound trying to minimize effects of room acoustics and speaker response.
The result of an automatic room correction will depend on how the correction system is implemented, speakers radiation pattern and frequency response, and room acoustics. If the correction equalizes to a flat target frequency response, the end result will most likely be too bright sounding.
If you find it difficult to achieve a perceived neutral frequency balance with the room correction, it is always possible to turn it off.
I recommend getting a better room correction system if this is something you desire very much. Such a system should let you customize the target frequency response and how corrections for reflections are done. Note that using an equalizer, whether manually set or automatic, is not the same as time corrective room correction.
If you decide to not use the receivers built-in room correction, you can now do manual adjustments to get the best sound.
After running the automatic set-up on the receiver, I recommend checking and tweaking the settings:
- Set level and distance for L, R to same
- Set level and distance for SL, SR to same
- Set level and distance for SBL, SBR to same
- Set subwoofer crossover frequency to suitable values for all speakers
- Set subwoofer distance for best impulse response combined with L, R
- Disable room correction
Verify and adjust the trim levels if necessary, as described above in "Level calibration".
Note that if you change level or distance settings this will render the room correction configuration faulty, as the calculated filters were made using the automatically configured values.
If your speakers have a reasonably flat anechoic on-axis frequency response, and your room has reasonable acoustics, the system may now have a good frequency response - one with flat direct sound from the speakers, and a total in-room response slightly tilted downward with increasing frequency.
Some crude adjustments on frequency response can be done if the receiver has a graphic equalizer. On many receivers this equalizer can only be used when room correction is disabled. Measurements are required to make useful adjustments, as it is difficult and unreliable to do it without actually seeing the effects.
Proper calibration of the subwoofer system is required for good seamlessly integrated sound in the lower frequencies, from lower midrange and down.
The receiver usually has an additional subwoofer trim level that can be used to set different bass level for different sources, or to change the level if a movie has too soft or too much bass. For this to work properly, the subwoofer system has to be set up with the right frequency response, so that the overall response is reasonably smooth for different subwoofer level settings.
The bass-lift is added to the target frequency response. Even with no bass-lift it will usually be better to set the target to a downwards tilt with increasing frequency, so that the lowest frequencies are louder than the higher up to the crossover. Generally, a target curve with no additional bass-lift will be perceived as more balanced on a system with a good bass system.
It is recommended to use acoustic measurements as an aid in the set-up process, it is very difficult to get it right without seeing the actual response and effects of adjustments. But ordinary acoustic measurements do not show a complete picture of how sound is perceived at bass frequencies, so listening evaluation is still necessary.
When using a DSP with parametric equalizer, be careful not to add much gain at holes in the response, focus on taking down peaks by adjusting q, center frequency and gain on filters.
Subwoofer calibration steps:
- Place subwoofers in the four corners of the room, if you have proper 40cm damping on the back wall place subwoofers along front wall
- Adjust DSP equalizer, delay, phase so that frequency response of subwoofers alone is reasonably smooth.
- Never time-align 4 corner-placed subwoofers to the listening position, try adding 2ms delay on the front right and back right units.
- Choose crossover frequency - often higher than usually recommended - try 120Hz, do not run mains as "large"/ full-frequency range.
- Time align subwoofers and main speakers by setting Subwoofer Distance, remember to add delay of DSP and additional acoustic group delay in subwoofers and low pass filters - in total this may end up being several meters more than the physical distance to the subwoofers.
To be able to do a proper calibration where you check the frequency response it is necessary to measure using a microphone and acoustic measurement software.
I recommend a calibrated acoustic measurement microphone, a professional USB I/O device, and REW room acoustic measurement software, you run it on a standard laptop computer.
With measurement capability in place, you can now see the effects of tweaking the configuration settings and changes in room acoustics.
Getting the subwoofer system properly integrated is now possible, as you can see what is happening with the combined response when you make adjustments.
If you do not want to put in the required effort to find out how this works, you can try to find a friend or enthusiast in the neighborhood to help you out with some basic measurements. A competent dealer may also be able to help, but be prepared to pay for the service.