Frequency Sweeps


Low Frequency Sweep

The purpose of a low frequency sweep is to measure the transition from front and center speakers to sub woofer, also called crossover. A test tone of 200Hz is where low frequency sweep will start. Use the SPL meter to measure the volume of frequencies from 200Hz all the way down to 20Hz.

On a graph, the preferred result is a "flat" frequency response, meaning all frequencies measured are about the same volume free of significant dips or peaks. Use the results of the test to look for drops in volume as the front speakers crossover to the subwoofer, this should alert you to your system's optimal crossover setting.
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 is less likely to provide smooth frequency response than a more expensive system. The room itself is the next most common problem and could have problems with certain frequencies. Try to reposition speakers in relation to walls and furniture. Reflections from smooth objects on walls could cancel out certain frequencies. To avoid frequency cancellations first try moving objects around in the room and repositioning speakers, some might go so far as to place "bass traps" in certain corners where out of phase frequencies might converge. The final option is your subwoofer's "phase" switch or dial, only use this as a last resort to try and smooth frequency response.

Full Range Frequency Sweep
Using test tones from a DVD like Avia starting at 200Hz, progressing higher, measure the volume of sound from each frequency. Since the pink noise test has already given you balanced full frequency volume, you're looking for the same balanced volume to be reproduced at each frequency. This is the flat frequency response highly desired but nearly impossible for most home theater systems. Adding to the difficulty of a full range sweep is volumes of higher frequencies (10kHz or greater) are difficult to measure. Results with the SPL meter shouldn't be taken too literally at these frequencies so you'll have to rely your ears.

It's good to know the strengths and weaknesses of your system, but there is little you can do about fluctuations outside of using a Parametric EQ, replacing speakers and manipulating the room. Experiment with Speaker Positioning and room acoustics using acoustically dampening and reflective materials (noted above). The science of room acoustics is complex and will require patient experimentation. A Parametric EQ is an audio component that can help you level off fluctuations in frequencies.

Always remember as you undertake your audio calibration that your ears represent the final truth; if you like how it sounds you've made the right choice. Readjusting speaker positioning and listening for subtle differences in sound should be the most enjoyable aspect to the home theater hobby. There is no perfect room acoustics, a huge room with a cathedral ceiling and theater curtains might be the ultimate for some tastes but could lack lively sound and intimacy of a smaller room with wood paneling desired by others. It's all about personal tastes and most of all fun with home theater.