Basic Audio & Video Set-Up Guide - page 2
A lot of money can be spent on a professional audio calibration, but for most people that's just not practical. You can conduct your own Home Theater calibration for a minimal investment of about $60.
You'll need the following tools:
- Avia: Guide to Home Theater (or similar audio calibration DVD)
- SPL meter from Radio Shack.
A calibration should be performed a couple of times a year or anytime new equipment is introduced into your Home Theater system. The Avia DVD has a variety of tests you can perform some of which can be found on THX certified movie DVDs. The basic tests you'll need to perform on your Home Theater's audio system include the following.
Speaker Volume/Channel Identification: Ensure all speakers in your system present audio for the correct channels. THX certified DVD movies will contain this test as do calibration DVDs like Avia. Your speakers receive a pink noise tone one at a time corresponding with the highlighted speaker in a graphic representation of all the channels in your audio system.
The correct speaker should not only be identified by the graphic (i.e. identify the correct channel) but all speakers should present the tone at the same volume level. If you find any speaker is sounding out of sync with the graphic either a speaker or component is wired incorrectly. Also listen for the pitch of the noise, if one channel is noticeably lower than the rest it could be out of phase so you'll need to re-check your speaker wires' polarity. With the master volume level properly adjusted, your system should be able to put out an even 75db of volume from each speaker to the SPL meter according to the standards for THX Home Theater optimization.
Using the SPL meter: Sound Pressure Level meter is a small handheld device you can buy at Radio Shack for around $35. This device will take the guesswork out of the volume of each speaker in your system.
It's critical to get the volume from each channel right from the prime listening spot and the human ear isn't always capable of deciding if two sounds are at the same sound pressure level especially when different speakers have their own timbre. There are some simple rules to help when you're using the SPL meter to calibrate your system.
- Check Battery . It might have been months since the last test, use the meter's own battery check to ensure it tests "good" before you proceed.
- Switch to 70db range. You don't have to listen to movies that loud, but 73 - 75db is widely considered optimal for Home Theater. The 70db setting on the large circular dial will correspond to 0dB on the needle indicator scale. This will give the needle an effective 60dB (-10dB on scale) to 76db range (+6dB on scale).
- Response. Set response to fast to read changes in sound pressure.
- Weight. Set weight to C for full range sounds such as music and most test tones.
- Holding Meter. Hold the meter at about head level in your Home Theater area's sweet spot keeping the microphone pointed diagonally upward and toward the source of the sound.
- Frequency response. When weighting is set to C, frequency response of the meter is flat from ~100Hz to 10kHz, below 100Hz above 10kHz, response of the meter will fall off as per ANSI S1.4-1971 response curve for C-weighed networks.
- Below 100Hz the built-in C-weighting network will cause the scale to read the bass response as lower than it actually is. For instance, if your subwoofer is truly capable of playing flat to 20Hz in your room the Radio Shack meter would indicate -6dB.
- Subwoofer. The sub's measurements should be conducted with pink noise in its range 20 - 120 Hz. Because it is known that the calibration of receivers and processors can vary as much as 4dB, using a test disc of known, accurate test tones such as Avia which already accounts for properly scaling subwoofer levels with +10dB more volume than the other (single) speakers in your system is recommended.
Low frequency sweep. The purpose of a low frequency sweep is to measure the transition from speakers to sub woofer or crossover. A test tone of 200Hz is where low frequency sweep will start. Use the SPL meter to measure the SPL of frequencies from 200Hz all the way down to 20Hz. Select the right combination of crossover frequency, and subwoofer phase to minimize the dips and peaks in response.
For more information on properly configuring and calibrating system bass response, the reader is advised to review the following article:
A system that suffers from dips in lower frequencies could be afflicted by any number of problems. The system itself is the first place to look, although not always directly proportionate to cost, a cheaper system with satellite "woofers" smaller than 4" is not likely to be able to reproduce frequencies down to 80Hz. So there will almost always be a dip in the overall system's frequency response between when the subwoofer reaches it's upper 80Hz limit (by a filter inserted with Dolby Digital and DTS software) and the small satellite reaches it's lower bass limit which may be well above 80Hz.Speaker / subwoofer placement with reference to the primary listening positions is also critical and the reader is advised to read more about this in our Get Good Bass section of the website.
Full range frequency sweep. Using test tones from a DVD like Avia starting at 200Hz, progressing higher, measure the SPL of sound at each frequency. Since the pink noise test has already given you balanced full frequency level, you're looking for the same balanced SPL to be reproduced at each frequency. This is the flat frequency response highly desired but nearly impossible to attain in most Home Theater systems.
It's good to know the strengths and weaknesses of your system, but there is little you can do about fluctuations, which may be inherent in the speaker system itself, outside of using a parametric EQ, replacing speakers and/or manipulating the room's acoustics. Experiment with speaker positioning and room acoustics using acoustic damping and reflective materials (noted above). The science of room acoustics is complex and will require patient experimentation.
Special thanks to Home Theater Focus.