For those who do not know what a notch filter is, I will explain it briefly: the purpose of a notch filter is to reduce the strength of an audio signal within a certain frequency range. Such filters are often used in single-driver (also known as broadband or full-range) speaker designs. Because they have only one driver, it is often very difficult, or even impossible, for such speakers to achieve a perfectly flat response across the entire frequency spectrum. Usually, these speakers emphasise certain frequencies, which leads to tonal colourations.
For the correction of a single frequency range, a typical notch filter consists of a group of three elements (a resistor, an inductor, and a capacitor), whose role is to suppress the so-called “notch” or peak in the amplitude response of the signal. The idea is to flatten these frequency peaks in order to avoid coloration. Such peaks can degrade the reproduction of the original recording because studio monitors with a relatively flat frequency response are usually used by audio engineers during the mastering process.
By choosing components with appropriate values, one can design a notch filter for a particular frequency range and a specific amount of amplitude reduction. The original notch filter solution conceived by Günter Damde from Art of Sound for the Fostex FE-108 Sigma full-range speaker in the BK108 enclosure consists of a 0.22 mH inductor, a 4.7 μF capacitor, and a 12-ohm resistor connected in parallel. This configuration is intended to reduce the sound pressure level produced by the speakers by around 5 dB in the frequency range from approximately 2 kHz to 8 kHz.
Compared to the sound of the speakers without the filter, it was easy to notice that the notch filter brought a more relaxed and natural presentation. After reading some interesting reports about modifying Damde’s original solution, I decided to experiment myself and replaced the 0.22 mH inductor with a 0.47 mH one. This moved the starting point of the affected frequency range down from 2 kHz to approximately 1.2 kHz.
I have to say that for several months I was quite satisfied with the results this modification had brought. The stereo image became slightly deeper, the musicians appeared to move a few steps backwards, and vocals were no longer so forward in the mix. However, after installing proper speaker and interconnect cables, the overall presentation became too laid-back.
So I restored the notch filter to its original state with the 0.22 mH inductor, and what a change, dear fellows! I was rewarded with the most realistic presentation I had ever experienced. Part of the success of this restoration was also due to the newly constructed filter enclosure, for which I used thicker material and better insulation, making it heavier as well.
The conclusion is clear: never be afraid to experiment and fine-tune the components in your audio chain. However, it can also be useful to occasionally click the “undo” button and return a component to its original state.
Thank you for stopping by!