Passive audio filter for voice.

[FirstSpear]

Hello.
I regularly use 3 portable radios, all DAB/FM, two of which are mine. All 3 have two things in common; a boomy bass-heavy sound, and no tone controls whatsoever - not even a simple bass/treble facility; (when did that cease to be important?). I only ever listen to Radio 4; and, rarely, 4 Extra, but then only spoken programmes, never music stations; I save music for decent audio equipment.
I'm looking for a passive circuit that will cut out the boomy bass bits to help make the voice parts clearer; a circuit that I can internally incorporate into the speaker wiring.
I have searched long and hard, but can only find active real-life-component-values filters. Those sites dealing with passive circuits I have found tend to be tutorials, and circuits presented are generic, containing no real-life-component-values, much jargon, and oodles of equations.
Each radio has only one speaker. The rear of the speaker in one offers 2 Ohm, 5 Watt, and the speaker is a little under 4 inches in diameter. This radio has a 5.5V mains adapter (not USB) and no battery.
The other offers 8 Ohm, 1.5 Watt; the speaker just under 2 inches in diameter. This radio has an internal battery, and a USB charger.
As both cases are devoid of any filling, I'm wondering if it might also be useful to place some wadding inside to perhaps help reduce the boominess. Suggestions as to materials appreciated. (Edit: Before closing the larger of the two, I did bung a folded fluffy microfibre cleaning cloth (Asda) around the speaker, and it did make a noticeable difference. I'd still like to reduce the bass further, though.)

Can anyone supply such a circuit, complete with real, voice-preferential component values, or point me to site(s) which provide circuits that do contain real values? I'd be very grateful.

Thanks, Bob.

[David]

This is an interesting query. In essence, you want to incorporate a high-pass filter which will attenuate frequencies in the sub 400-500 Hz band. Experimentation will be necessary; what you do will depend very much on what access you have to the details of the internals of the radios.

The simplest filter will consist of a capacitor in series with the signal at some point, from the input to the AF amplifier, and up to the speaker itself. The cap's reactive impedance together with the input impedance of the stage following will form a frequency dependant potential divider. Try for a potential divider where the cap's reactance equals the input impedance of the following stage at about 500Hz. The reactance calculator on this site will be very useful if you are going to try this process.

If you want to try an attenuator at the speaker, a cap with a reactance of a few ohms at (say) 300 Hz will be of the order of 100 uF, which will usually be a polarised electrolytic, and therefore not suitable for connecting in series with speakers, as they will be reverse-biased for half the time. If you have access to the internals of the radios, and the volume controls are straightforward potential dividers (circular or straight) then you could connect a non-polarised cap (its value will be much smaller than the value in the speaker feed) in series with the feed to the electrical top end of the pot. Knowing the value of the pot and with the aid of the calculator, just experiment with the values of the caps. I would think that that would be a better position for the attenuator.

I think you will be better served by looking for an electrical solution rather than packing the speaker box. The speaker box is probably small, and I doubt if you could put in enough sound absorbent material to make enough difference.

Give it a try, and post results here.

[FirstSpear]

Thanks for replying. Mulling that over.
Bob.