On the topic of multi channel calibration is mainly focus on Home Theater although there are multi channel audio as well for hifi audio. In my post will be focusing on Home Theater and will be doing the 11 channels plus subs for calibration. That will be 7.1.4 configuration in the standard Home Theater setup. Picture as below to illustrate the Dolby or DTS recommendation of setup. The good thing about this standard is the flexibility to reduce the number of channels or speakers based on space available and also room configuration. Although most modern receivers or processors for Home Theater can easily compensate on the placement of speakers and subwoofers but is still recommended to follow the guide line as per picture to maximized the result of the audio. Distance, angle, height, etc for any speakers or subwoofer will impact the end result.
For the calibration there are some basic tools needed. Most receivers and processors do come with some microphone for calibration but are not too accurate unless the processors are some high end brands. The microphone use in this calibration is the umik-1 from a company in Hong Kong (Minidsp). Very affordable and accurate as the company of Minidsp calibrate each microphone individually before it is send out to the customer. So a calibration file will be included for individual microphone. This will be use with the calibration software during measurement.
Will be recommended for some microphone stand which is easier to swing around the microphone especially for multi seating calibration. Some will just use camera tripod which will work as well. As long as it can hold the microphone 90 degrees pointing up upward. This is important as surround sounds comes from multi direction and having a microphone pointing up will have the best possible receiving of the signals for accuracy in measurement. Umik-1 microphone has 2 different calibration files provided by the manufacturer for 90 degrees and 0 degrees angle measurement.
For the start and most important basic to get right is the volume or SPL output. Each speaker and subwoofer in most standard receivers or processors will be able to dial in to the right volume. The simple reason why this is critical , when multiple audio sources playing together or one after another will be pretty noisy simply. More over, different sizes of audio devices will produce different output. It can overwhelm one another. To avoid all this, the balancing act of all channels with the right volume is crucial. The typical example of conversation in a movie can be very soft and explosive scene become too loud.
To perform this step, simply turning on the test tone for the receiver or processor. Measure the test tone in term of SPL. The guidance from Dolby will be using 0 db master volume as the reference level for testing out the SPL. To measure the SPL , it can be done with the Umik-1 and the REW software which provided the SPL tool. Another way is using third party SPL meter. Picture as below
Dolby or DTS reference is 75db reading on the SPL with master volume set at 0db. Simply, this is the guideline use for sound engineers in the movie industry in their mix and recording of the movie. This reading will be across all channels including the subwoofer. This important point to take note of will be LFE (Low Frequency Extension ) or bass on the low frequency are boosted by 10db according to Dolby and DTS standard. So to achieved same 75db level , the subwoofer need to be 65db at 0db on master volume. This is the only exception for all multi channel calibration.
Once levelling is done, the next step will be setting the cross over from low frequency to high frequency. In Home Theater context, it will be 0hz to 250hz as low and above 300hz to 20khz will be high frequency. To work on this Dolby and DTS has determine to be best at 80hz to cross between the high and low frequency. This is a general guide line as the dependency will be on the speaker and subwoofer capability to reproduce the certain range of frequency. Typically a subwoofer will be able to handle 20hz to 100hz. For speaker generally 50hz to 15khz range. Taking the upper limit of the subwoofer and the lower limit of speaker, therefore 80hz is the ideal point to crossover.
The next critical step will be bass management. This is referring how to smooth out the bass in the Home Theater setup. In any Home Theater setup, bass is very important aspect after the vocal of the movie. In any audio setup, bass is something cannot be miss. Using the REW and Umik-1 the measurement of the subwoofer from 0hz to 200hz frequency range will help to determine the ideal output. Again the reference here will be 75db. The basic idea is to get frequency range of 0hz to 200hz as close as possible to 75db. This is the flat response ideally to get the audio frequency spectrum not to be miss out. Below graph to illustrate on trying to stay close to the flat line.
The placement of the subwoofer is important to get the best out of this. There is no scientific way of placement but will have to move around the subwoofer and each placement take a measurement to understand what is the result. The average will be to avoid peak and dip in the result. Peak can create boominess and dip will be totally not hearing the sound. Other than placement of the subwoofer, the tuning on the delay or distance of the subwoofer in the receiver or processor will also help in getting the flat measurement. In real world scenario, is impossible to get to the ideal respond but will be as close as possible. Multiple subwoofers in a room will also flat out the response as each subwoofer will be able to compensate each other. The basic steps are something very doable for most home users. Spending some time for this will definitely bring out the potential of the setup. After this final step is done, you are all ready for enjoying the movie.
