I'm going to glue an aluminum plate across the top of a small speaker to create a sealed chamber and attach a small port to the plate (1/8" diam) for a tube that will lead to a pressure transducer.

I want to create pressure signals (square, sine, etc) in the range of 0 to 4 psi and this is one possible way to do it.

I can use a function generator to create the signals I need, but I'm going to need an amplifier that has a low frequency response down to 1 hz or less (ideally - DC) into a 4 or 8 ohm load. Probably a few watts at most. Any circuit ideas?

 

Hello,

This amp is about 40 Watts.
The circuit on this page will go down to 4 Hz (with a 1uF input cap, even lower with a larger one).

From the page:

Conclusion
There's not a great deal to say about the amp. It is simple, and as such may appeal to some readers. The three diode string sets the bias, and it will be found to be quite variable in real life. Ideally, the diodes should be mounted in contact with the heatsink, but if the sink is adequately sized this should not be necessary. The input capacitor is much larger than it needs to be, and there is a real benefit if it is reduced to 1uF. Since input impedance is about 50k at the base of the emitter follower (Q1), a 1uF input cap will still provide a -3dB frequency of just under 4Hz - provided the output cap is increased to 4700uF.
With an open loop gain of only 180 (45dB), there is not as much feedback as we are used to with modern amps. However, the amp remains essentially flat to 10kHz open loop, and this is dramatically better than most amps having a massively high open-loop gain. Because of the small amount of feedback, it may be thought that output impedance would be somewhat higher than we have come to expect. This is not the case though, and Zoutwill normally be less than 100 milliohms (a damping factor of >80 for an 8Ω load).
Speaking of feedback - because the input stage creates an inherently stable amp, there is no reason to expect that TIM (Transient Intermodulation Distortion - assuming that you believe it exists of course *) will be a problem, since feedback is simply applied to the base of the input amp, and very little frequency "compensation" is needed. Although a 68pF cap was specified for the original, it should be possible to reduce this if wider open loop bandwidth is your goal.

http://sound.whsites.net/project12a.htm

Greeting,
Bertus

 

Is there a low-frequency audio amp that can drive a speaker directly, without a capacitor in series on the output stage?

 

If you run a lot of DC current through a small speaker, you'll burn it up.

Since you're going for low frequencies, and fidelity isn't much of a concern, you might consider using an L272 or L2722 dual power opamp. They're cheap, and you could make a simple push-pull amp or voltage follower with it.

There are plenty of other single-IC amps that are designed to be used in automotive audio systems that would do it, too.

 

I have learned that LF at certain frequency and certain amplitude would harm human body

 

Is there a low-frequency audio amp that can drive a speaker directly, without a capacitor in series on the output stage?

Yes, many top end audio amps are direct connected. IMHO, any really good audio amp should be direct connected to the speaker. It does mean that the amp must use dual power supplies (positive and negative) and have the output stage adjustable to "zero" the DC bias set point. It's definitely doable.

You might want to refer to the data sheet for a TDA2030 or similar part to get some ideas.

 

I have learned that LF at certain frequency and certain amplitude would harm human body

Maybe if it's really strong signal level. Subsonic signals applied to the body make people feel queasy but don't hurt them. I read that in the 50's, some movie theaters had them attached to the seats for horror movies to make the people feel more scared.

 

The story so far:

I bought a cheap 6x9 car speaker, rated for 30 watts, with a mylar cone. I connected it to an Akai audio amplifier and fed a sine-wave signal from a signal generator. At frequencies between 1 to 10 hz, I observed the cone of the speaker move, with an amplitude of about 1/4 inch.

I then sealed the top of the speaker with 1/4" plexiglass and attached a small tube (1/8") running from the sealed chamber to a pressure transducer.

With the top side of the speaker now sealed up, the speaker now seemed to barely move at all at those same frequency and amplifier settings. The pressure transducer did respond by generating a pressure signal, but the amplitude of the pressure signal is only about 1/10 to 1/20 of what I need (I need a signal that ranges from 0 to 2 or 0 to 4 psi).

It now seems that a sealed, air-tight speaker (of the size and power that I am restricted to using) will not give me the kind of pressures I need.

A 12-volt aftermarket windshield-washer pump will give me the peak pressures I need, but it seems hard to modulate it's rpm to give me a 1-hz to 10-hz pressure signal, and they don't seem to be designed for continuous operation (many minutes to several hours continuous is what I need).

I might go back to the aquarium air bubbler and use a 3-way medical-grade stop-cock. A reduction-gear motor would drive the stop-cock at a variable rate (1 to 10 rpm) and the bleed-off from the open port should give me something approximating a square wave pressure signal.

If anyone has any other ideas how to generate a low-pressure signal (0 to 4 psi) of variable frequency (1 to 10 hz) please let me know.

 

OK, so the entire sealed speaker idea wasn't so great.

A 6x9 speaker has a surface area of pi*a*b (where a is 1/2 the width and b is 1/2 the length), so total surface area is 42.4 square inches. To get 4psi out, you'd need a force of 169.6lbs from the voice coil. Not too likely that's going to happen without a really big amplifier.

Can you remove the plate from the speaker without destroying it?

Perhaps if you could attach some kind of small diapragm or piston/cylinder to the voice coil of the speaker you'd be able to get the pressure you need. Maybe an old piston-type windshield washer pump?

There's also the ratio of your plumbing's total interior volume to the swept/displacement volume of any kind of pump/diapragm you'll use. The greater your plumbing's interior volume, the greater sweep/displacement volume you'll need to reach the pressure you need.

 

1/4" plex is not really compliant. A thin plastic diaphragm like mylar glued across the speaker opening should transmit pressure waves better. A 4" circular speaker might work better. Some for automotive sound systems have big power ratings.

 

I would think about some type of solenoid. It's very easy to get a solenoid that will push/pull 2 pounds of force and has a 1" area. hence 2 PSI.

If you can get a small airtight piston+cylinder off something (maybe one of those little $20 car 12v air pumps?) and attach the piston directly to a large solenoid that should do it.

If you had access to a lathe you could make a small iron piston (say 1/2" diam 1.5" length) with o-rings in a non-ferrous cylinder, then just wind 2 windings around it to make it a push-pull solenoid that will generate LOTS of PSI.