Frequency response is one of the most important quality aspects in speakers. Suboptimal frequency response of speakers can be compensated for by an equalizer (EQ). In this article, I explain how to compensate for suboptimal frequency response, using the Super EQ plugin. This method is only effective if your speakers have a bad frequency response. There could be other reasons for bad quality sound produced by speakers.
Before starting, you need a good pair of headphones as a reference. I recommend Sennheiser HD-600, or maybe Sennheiser PX 100-II if your budget is tight, for the reference. You need to use Super EQ when you listen to the music. You also need a good sound card. It would be even better if that card is not affected by the Windows kernel sampling rate converter, which potentially degrade the sound quality. I recommend the M-AUDIO Audiophile 2496 sound card.
First of all, you have to get wav files of sine waves of various frequencies. Download from here < eqtestset.zip >, or make them yourself. Connect your headphones and speakers to your PC. It is better if volumes for the headphones and the speakers can be set independently. You need to install Super EQ plugin in your PC. Play the 880Hz sin wave on speakers and headphones in turn, and set the volumes so that they are heard in same loudness. Do not forget to turn off your speakers when you listen using headphones. Turn on the EQ and load < flat.eq >. Next, turn off the EQ, play 880Hz and 55Hz successively on headphones, and remember their loudness. Turn on the EQ, play the 880Hz and the 55Hz successively on speakers and set 55Hz slider so that 55Hz sounds as loud as the headphones. Do not forget to press the Preview button after you change the slider setting. Repeat the same thing to 7040Hz. As for frequencies higher than 7040Hz, use the parametric EQ, and do the same thing. Always specify negative gain on the parametric equalizer since positive gain may introduce clipping. Note that even if you specify negative gain, clipping can happen. After all EQ parameters are set, your speakers should sound better, or closer to the reference headphones, than previously. If not, it is possible that there remains a very narrow frequency unflatness. You may use a cheap electrostatic microphone and a spectrum analyzer to detect these narrow areas of unflatness. You can use the EQ to compensate for them. Again, frequency response may not be the only reason that speakers sound badly.