Resonance and dB

[Hero999]

An LC filter would be big and also needs its frequency to be tuned. How?

It itsn't hard to calculate the nurber of turns on a suitable core and it doesn't have to spot on. Huge 100kW transmitters have filters so I don't see how a little 400W unit is hard to produce.

A lowpass filter won't work well.

Class C audio amplifiers seem to do alright.

Maybe a series resonant circuit in series with the speaker?

What about the huge voltages that would develop across the capaciitor and inductor?

 

[Marks256]

I'm ok with buying parts, guys.

What should i buy that would give me a crap load of power? Remember, i only know how to use NPN, PNP, and POWER transisors/regulators.

 

[audioguru]

Hi Hero,
We are talking about making 1.6kHz, not 1MHz. The filter's inductor and capacitor will be huge and will need to be tuned.

Hi Marks,
Plug your speaker into 120VAC from the mains if you want to destroy it. There is a 1.6kW amplifier making the rounds on this site. You will need about 20 of them to feed hundreds of ordinary speakers to make enough sound pressure to break a little glass.

 

[Hero999]

Hi Hero,
We are talking about making 1.6kHz, not 1MHz. The filter's inductor and capacitor will be huge and will need to be tuned.

A 1.6kHz coil designed for a small 400W inverter (because that's what it is) will still be much smaller than a 1MHz coil used in a huge 100kW AM transmitter, it'll sill be much smaller than a 400W mains transformer.

Tuned?
I don't think so, you only need to filter out the higher frequency harmonics.

 

[Dr.EM]

I think it was best tuned so it would resonate and create a higher output?

 

[audioguru]

An LC lowpass filter won't filter out very well the very strong 3rd harmonic from the square-wave. A tuned circuit would be an excellent filter because it could have a high Q.

 

[Marks256]

Plug your speaker into 120VAC

I may be a newb, but I am not stupid.

 

[Hero999]

An LC lowpass filter won't filter out very well the very strong 3rd harmonic from the square-wave.

Class D audio amps have them and they don't seem to have any problems.

 

[audioguru]

Class D audio amps have them and they don't seem to have any problems.

If a class-D audio amp had its switching frequency at only 4.8kHz like this circuit would have its 3rd harmonic then the loud whistle would drive you crazy. They switch at hundreds of kHz so their LC lowpass filter is very effective.

 

[Marks256]

Yeah, so anyways....

You said a "horn speaker"? Care to explain?

 

[audioguru]

PA systems at stadiums need to project sound far and use very high sound pressure levels to travel far. They have a horn to focus the sound and make the speaker very efficient. They can produce 132dB at 1m at frequencies above 800Hz. They are fairly expensive compared to ordinary speakers.

 

[Nigel Goodwin]

A horn is a mechanical transformer, it matches the imedance of the speaker to the impedance of the air (both MECHANICAL impedance, not electrical) - this gives MUCH greater efficiency - quite possibly between 5 and 10 times more than a conventional speaker. The drawback is much greater complaxity and size!.

 

[Hero999]

If a class-D audio amp had its switching frequency at only 4.8kHz like this circuit would have its 3rd harmonic then the loud whistle would drive you crazy. They switch at hundreds of kHz so their LC lowpass filter is very effective.

The sound will already drive you crazy. Providing most of the power is at the frequency you need then it simply doesn't matter. I challange you to prove to me that an ineffieient class AB design will provide an over all greater efficiency than an h-bridge connected to a low pass filter. Yes I know an h-birdge will waste power in unwanted harmonics but no way near as much power as the class ab system will waste in heat.

You don't really need fidelity here so a high powered PA or siren speaker will do.

 

[audioguru]

Hi Hero,
Transistors in an inefficient but distortion-free amplifier can be cooled with heatsinks, speakers that are also being heated with unwanted harmonics cannot.
The harmonics of a square-wave have the same amount of power as the fundamental sine-wave frequency when they have the same p-p voltage.

If the inductor is low loss, a 2nd order Butterworth LC filter would reduce the power of the fundamental sine-wave to half (-3dB) the total output power, and will reduce the power of the harmonics to about 1/4 (-6dB) of the fundamental's power. Therefore more speakers, h-bridge drivers and LC filters would be needed to produce max sound pressure at a single frequency, when compared to a sine-wave signal.

 

[Hero999]

How about building a low quality class-D amplifier, it's a more complex circuit but will probably work out cheaper because the transformer and heatsinks are a the most expensive parts?

 

[Nigel Goodwin]

Funnily enough, I was watching a repeat of 'Brainiacs' on TV yesterday, it's one a I saw a year or two back - and they tried the glass breaking trick.

Initially they tried an opera singer, who determined the glass resonated at C#, but was unable to break it. They then tried the rock band Status Quo, who also determined it was C# - they continually played the note on their guitars, through a BIG stack of Marshall cabs - and after about 10 seconds or so the glass smashed to pieces!

So you need an accurate note, VERY high power, and keep it going until the glass breaks!.

 

[Marks256]

Funnily enough, I was watching a repeat of 'Brainiacs' on TV yesterday, it's one a I saw a year or too back - and they tried the glass breaking trick.

Nigel, i am not one to hold a grudge, but you spelt "two" wrong.... Lol.


Anyways, i know for a fact that i have a VERY accurate tone generator, i have all the time in the world, but i am not sure about finding enough power.... Oh well, were there is a will there is a way.....

 

[Nigel Goodwin]

Nigel, i am not one to hold a grudge, but you spelt "two" wrong.... Lol.

Not any more!

Anyways, i know for a fact that i have a VERY accurate tone generator, i have all the time in the world, but i am not sure about finding enough power.... Oh well, were there is a will there is a way.....

The frequency isn't the problem, just the power - get yourself a big heap of 4x12's and a couple of thousand watts!.

 

[Sceadwian]

For effectiveness, a single tone may not be the answer, just about any physical object has different harmonics in more than one dimension, and even if the core resonance of the object is known that might not be the best frequency to hit it with, unless it's a tuneing fork shaped glass... Considering the actual amount of power delivered to an object is related to frequency you may want to use a frequency several harmonics above the objects resonant frequency, perhaps two speakers at the same distance from the object on opposite sides, one being fed it's signal through a basic inverting amplifier (doubling the effective delivered energy) If both speakers are generating the base frequency plus a few odd harmonics at resonance you might get some results.

 

[audioguru]

Haven't you wet your fingers then gently rubbed around the top of a crystal galss to make it ring? The sound is a pure sine-wave without harmonics. That sound is needed very loud to break the glass.