Infrasound less than 20Hz filter

Hi all,

Typically my first post is a question, anyway, I looking to filter out signals over about 30Hz from a speaker being reversed and used as a microphone.
I want to detect frequencies under 30Hz without anything above interferring with it.

Essentially I have a large speaker, it has a frequency range starting from 20Hz and up.

I have the output of the speaker hooked up to a mono amp
**broken link removed**

I have connected the output of the speaker straight into the amp, I have not used an audio transformer ( said to be required for impedence matching) but I have not seen the need for it yet, but if you guys advise I'll hook one up.

The output of the amp is hooked up to a scope that I have connected via a USB connection to my PC. The frequency of the input matches the frequency of the output exactly (as expected) the addition of the amp makes the unit much more sensitive and it will pick up me speaking from about 3M away. My voice registers at about 800 Hz

I've been messing around with capacitors at the speaker side trying to filter the sound, ideally I'd like to have a selection of filtering , so i could see it it was working, as we can't hear below 20Hz. Then use the device to look for >20Hz

The response on the scope looks great. It's been 12 years since I have worked on electronics, I have an office job now, but at one stage maybe I could have done this, but I need some help now please, having said that I am enjoying getting back into it.

Don't laugh!!! but i want to use this device when ghost hunting. Not to prove the extance of ghosts, but to note if there is any sound from 15 to 20Hz. I know the amp only caters for 20Hz and up as does the speaker, but I picked the lowest Hz rated components I could. I also aim to have two speaker back to back detection and using both channels on the scope, I'm going to house the unit in a large plastic pipe.

Anyway as you may or may not know infrasound effects some people and at around 19 Hz (according to NASA) the eyeball can resonate and make people see shadows, or at least feel unwell, I have never seen this happen, I'm just talking about the issue, I only want to see if these sounds are present. Trucks etc can cause these sounds, and I want to be able to eliminate infrasound as a cause of "haunting" as that is my role in an investigation team, to rule out causes etc as best as possible.

Anyway, any help would be greatly appreciated.

Regards

Just a low-pass filter, or a series of them, depending how sharp you want the filter to be - there are plenty of examples on the net.

A Panasonic WM-61A electret condenser mic (they're about $2 at Digikey) would probably be better than a speaker.

A Behringer ECM8000 mic (which probably uses a similar if not the same condenser mic as above) runs about 50 bucks and you'd need a mic pre with phantom power, but it would also likely be better than a speaker.

But then again, maybe a speaker will be adequate for your needs.

Anyway, just hit it with a 4th order lowpass. Sallen-Key equal component topology makes it cheap and easy. There are good calculators on the net. To get 4th order just cascade two second order filters. The result won't be a butterworth filter but will likely be adequate. If you want a perfect 4th order butterworth I think Analog Devices has a good component value calculator on their site for active filters and will tell you what you need to get that.

Oh, and you'll definitely want to do this with active filters, just in case you were just trying to strap a series cap in the circuit or something.

Also, your voice showing up at 800Hz is probably a component of your voice exciting the primary cone resonance. I'm guessing you're using at least a 10" or 12"?

Yeah, a speaker has a big and heavy cone that resonates strongly unless it is driven from the extremely low output impedance of a power amplifier, so it makes a lousy microphone.
A microphone has a small and lightweight diaphragm so it barely resonates.

Why not use little electret mics with a preamp?

thanks lads, it is a low pass filter I was going for, I just wanted to know the best way to approximate the value of the cap.
I see you have recommended a on line calculator, I will have a look for one. I tried to build the circuit inc the amp in circuitmaker 6, but it doesn't seem to like the caps i have in, as the source i put in what i assumed would be the output form the speaker, i was hoping to try a trial an error approach, but I'd rather have some caluclations to back it up.

speakerguy79 you are correct it is a 12" speaker starts at 25Hz and up, I have a 15" ready to go with 20Hz and up.

thanks again everyone, I'll look at those different approaches too, maybe I should have some here for ideas first before jumping in.

audioguru said:

Why not use little electret mics with a preamp?

Click to expand...

I've located an electret mic in the UK, they'll send it to Ireland.

would the Amp circut I have do for this , it's also rated for 20 Hz and above? is that what you mean by a preamp?

and once again I guess I'll still need a filter at the far end?

thanks again

The output level of a microphone is about only 5mV. The input level requirement for a power amplifier is about 500mV. A preamp amplifies the output of the mic to about 500mV. the preamp also powers the FET transistor inside the electret mic.
Then a lowpass filter circuit can be added between the preamp and the power amp.

Hi again lads, I started off looking for a filter but I've ended up further away from a solution then ever!!! (not really i know)

anyway the little amp I have was correctly identified as a Velleman preamp.

I have replaced the speaker with a WM-61A mic.

the mic is accross the input of the amp and I have the scope on the output (i know the amp works as i have tested it)

I have placed 10Vdc across the amp but I am getting noting from this set up.

these mic has a max operational current of 9V, but my amp has a range from 10 to 30Vdc so I kept it at 10Vdc.

I've been trying to get the parts to build a speaker testing kit from www.madaboutsound.com but I can't get all the bits, maplins dont have them and some are discontinued by digiKey with no replacement. If I could do this without help I would, thanks.

Anyway I'm not getting a pip out of it, I'm wondering if i connected the mic correctly, but it only has two connections that I can see, one is the ground (the whole case) so I dont know where I'm going wrong,

I have not looked at the filter yet. Once again thanks for all the help.

Velleman have hundreds of kits. If you have the K1803 preamp then it is the wrong one:
1) Its input impedance is too low becuse it is made for a low impedance dynamic mic. Your mic is a high impedance electret mic that needs a high impedance load and needs to be powered.
2) Its frequency response is 3dB down at 72Hz.
3) It uses a lousy old 741 opamp that produces lots of noise.

audioguru

It is a K1803, is it useless to me. I have other ampplifier options, like

https://cpc.farnell.com/jsp/search/...search_003&Ntt=hk00378&Ntx=&_requestid=531321

or
even this with a mic already on board

https://cpc.farnell.com/jsp/search/...search_003&Ntt=hk00627&Ntx=&_requestid=531373

but in each case they start at 30Hz the Velleman one is the only one that starts as 20Hz that I can find.

Can you recommend one? if not and if I am too raw to be posting here, thank you guys for your help to date, the info on not using a speaker as a mic was great

Those preamps don't show their schematic so we can't see if the input capacitor can have its value increased for it to work at lower frequencies.

The Velleman preamp doesn't power an electret mic and has an input impedance too low for one.

This is the preamp you need:

Thanks again audioguru, thank you for the circuit diagram, I appreciate the time you spent on this.

I have everything I need bar that amp, I'm going to order this one, what do you think?


https://cpc.farnell.com/jsp/search/browse.jsp?N=411+1001871&Ntk=gensearch_003&Ntt=TL071&Ntx=

The TL071 single, TL072 dual and TL074 quad opamps are low noise, low distortion and wide bandwidth. They have been used in stereo equipment for years. They have FET inputs so are easy to bias. They were invented by Texas Instruments but many manufacturers make them.
Their minimum total supply voltage is 7V.