| Cinema Cynergy February 2001
Audio Calibration of Your Home-Theater System
Last month we talked about the general placement of
speakers in a home-theater system. This month we will consider how to calibrate the levels
of these speakers to get the best possible sound from your system, and we will even look
at a few tricks to further enhance the quality of the audio presentation.
Why do I need to calibrate the audio levels of my home
theater?
When a multichannel movie soundtrack is mixed, the sound
coming from each channel is set to a specific volume to create an overall effect. If the
level of each of the channels is not set correctly, then the effect that was intended will
be diminished. For example, if a scene in a movie calls for crickets to be quietly
chirping in the background while an important conversation takes place onscreen, setting
the surround channel levels too high might cause the sound of the crickets to become too
prominent and detract from the onscreen dialogue.
One reason why different channels in a home-theater system
play at different volumes is because not all speakers are equally efficient, meaning that
they play at different volumes when the same amount of power is applied to them. Also,
speakers that are further away from you or mounted above you (such as surrounds) will not
seem to be as loud as speakers that are closer to you or that are aimed directly at you.
What do I need to calibrate my system?
While it is possible to attempt to calibrate a home-theater
system by ear, you really do need a sound pressure (SP) meter to do it properly.
Fortunately there is an inexpensive one available from Radio Shack (Cat. No. 33-2050) that
will do the job quite nicely for around $35. Nearly all surround-sound processors and
receivers will generate a test tone that can be used to check the speaker connections and
to set the levels of each of the channels. However, I use a test DVD for calibration
because I like to have a signal from an actual source and sometimes the tones generated by
processors are not adequate to set the subwoofer level. Discs such as Video Essentials,
Avia Guide to Home Theater, the Ultimate DVD (Platinum Edition), and DVDs
featuring THX Optimode signals all have the required test tones.
How do I actually go about calibrating my system?
Audio calibration of your system is quite simple and can be
broken down into the following steps:
- Set the SP meter to "C Weighting", "Slow
Response", and a range of 70dB (decibels).
- Place the meter in the ideal seating position at ear level
and tilted slightly forward and towards the speaker that is being measured. Make sure that
your body or other objects are not blocking the sound from any direction.
- Adjust the master volume level so that the tones register
roughly in the middle of the range of the meter (70 to 75 dB).
- Cycle through the test tones for each channel and adjust the
trim levels of the processor or receiver so that each channel measures as close to the
same level as possible.
It really is that easy. However, if you do not perform this
simple procedure, you are probably not hearing film soundtracks the way that the
filmmakers intended and are not getting the most out of your home-theater system.
A few notes about subwoofers
Nearly all subwoofers are powered (meaning that they have
their own internal amplifiers) and have at least a volume and crossover control. These
types of subwoofers should be connected using the "Subwoofer" output located on
the back of the processor or receiver. Be careful when you initially set the subwoofer's
own volume as most have controls that range in level from "no volume whatsoever"
to "loud enough to stun small animals!" Once the volume control on the subwoofer
is set to a reasonable level, the processor’s controls can then be used to fine tune
the output to match the levels of the other channels.
Surround sound receivers and processors normally have only
one output for the subwoofer. However, there are usually several RCA connectors on a
subwoofer for left and right stereo inputs and outputs, and sometimes even an additional
one to bypass its internal crossover and volume control. Be sure to consult the
owner's manual of your subwoofer to ensure that you use the correct input for the
typical single subwoofer output of your receiver or processor. The crossover setting of a
subwoofer determines the upper limit of the frequencies that it will reproduce. The
subwoofer will then reproduce frequencies below the crossover setting and down to its
operationing limit. Set the subwoofer's crossover to its highest point so that it
reproduces all the bass that it receives from the processor or receiver.
For a more in-depth look at serious home-theater
subwoofing, refer to Doug Blackburn’s January, 2001
"Video Noise" column. As Doug mentioned in his column, home theater requires
a lot of bass to achieve that room-shaking, skull-crushing effect that you get in movie
theaters. Thus you might want to try goosing the output of your sub by increasing its
level by a few dB so that it really energizes the room. If your subwoofer has a
"Video" setting, you might experiment with that as well. These types of settings
usually increase the output of a subwoofer in the midbass range where film soundtracks
have most of their low-frequency energy.
Subjective adjustment of your speakers
You may find that after setting the levels of your home
theater that you are not satisfied with the sound. For instance, the center or surrounds
may sound fatiguing or too loud or they may be laid back and not involving, which can make
dialogue difficult to understand or surround effects that are too prominent etc. This
could be due to a variety of reasons, such as the speakers having uneven frequency
response or room interactions, causing other such anomalies.
Increasing or decreasing the output of the center or
surround channels may help to alleviate these problems. Remember that no one speaker or
set of speakers should be prominent in a home-theater system. Of course, most of the sound
will usually come from the front three speakers, but all of the speakers should work
together in harmony with none of them drawing undue attention to themselves, which may
require a few subjective adjustments to their levels.
One aspect that we have not addressed is the imaging of the
front speakers. Audiophiles go to great lengths to achieve precisely the imaging they
desire by fussing with the distance between speakers, the distance to room boundaries,
degree of toe in etc. I like to adjust the imaging of my left and right front speakers by
using a stereo CD such as the Delos Surround Spectacular [Delos DE 3179], which has
several imaging tests. You can also use a familiar CD to experiment with the position of
the left and right front speakers until you achieve the imaging that you desire. Once the
center channel is engaged for surround sound, the front soundstage often collapses
somewhat and loses some of its imaging capabilities, but properly mixed multichannel film
soundtracks will still benefit from this attention to left and right front speaker
positioning for optimal imaging.
All things being equal
There is really no substitute for the proper audio
calibration of your home-theater system. While it may sound like an unnecessarily
complicated and technical process for only hardcore videophiles, it is actually quite
simple and absolutely necessary to obtain the maximum enjoyment from your home theater.
...Roger Kanno
roger@hometheatersound.com |
