Testing an 8-ohm speaker

I have been trying to test an 8Ω speaker, but I am having some difficulties. I have tried to test the circuit using multisim using the circuit below:

I used the 8Ω resistor in the place of the 8Ω speaker in the circuit. When simulating, I get about 106mA, which should be enough to power the speaker. However, when I built the circuit on a breadboard, I did not hear anything from the speaker. Should I be using an AC voltage source instead of a DC to power the circuit? Am I doing something else wrong? Can somebody help me troubleshoot this circuit please?

• Can you describe what sound you expected to hear?
– Kaz
Commented Dec 24, 2012 at 22:11

Speakers are AC devices, the cone will move with the waveform to make it audible. However, like Matt says, you should hear a click when you connect a DC supply, and another one if you remove the source. That doesn't contradict the AC behavior; when you connect the power supply the voltage steps up and for a very short time you get a alternating current. Striking a power wire against the connection may give you something more audible.

• This is not really the best answer, as traditional magnet/coil/cone speakers themselves are not really AC devices, but will in fact hold position in response to DC. It is rather sound propagation and hearing which require change with respect to time. Commented Nov 19, 2011 at 5:35
• @Chris - Strictly speaking true. But a speaker only does what it's supposed to do when you apply an AC voltage. In that sense it's an AC device. Commented Nov 19, 2011 at 8:35
• @stevenh, no, to first order the speakers responds to DC the same as it responds to AC. It's just that this isn't very useful for sound generation. Effectively, it's air that is the "AC device". Commented Nov 19, 2011 at 15:18
• @Chris - wow, tough crowd! 1. Speakers are made to produce sound. 2. DC means no sound. From 1. and 2.: Speakers aren't DC devices Commented Nov 19, 2011 at 15:59

Sound waves are inherently AC. The frequency of voice is usually between about 300 Hz up to 3400 Hz.

If you look up the frequency response of the speaker you are using, you will find that it will have some defined range that will be different for every quality of speaker. Usually the lowest you will find a cheap speaker can produce is 300 Hz or so. DC is 0 Hz so of course is below the range of the speaker, but it is also below the human audible range and really wouldn't be called audio at all.

So if you want to keep using this speaker you will need to use an AC function generator of some sort. If all you want is to have a buzz without having to find a AC function generator, you can look into buying a buzzer that can be powered with DC directly.

You also might want to look into how to properly power a speaker (I believe there are question on this site that go over this, just can't find them right now). I assume you put the 39 ohm resistor in series to be a current limiting resistor, however you will need to understand that the speaker actually doesn't act like an 8 ohm resistor.

• Ok, so should I model a speaker in multisim using a resistance and an inductance in series? Commented Nov 18, 2011 at 13:42
• Speaker models can actually get quite complicated if you transform the acoustic radiation impedance back into the electrical domain. But for many everyday purposes such as estimating the power delivered by an amplifier, an 8 ohm resistor is a useful first-cut model. If you start designing crossover networks, then perhaps you need to use a better model. Also note that on the low frequency end, it isn't that the speaker doesn't move, but that it can't couple enough energy into the air. Commented Nov 19, 2011 at 5:39

With that circuit you should hear a click when you turn it on, and the speaker cone should move either in or out.

If you want to make a noise then you will need an oscillating waveform, like an AC power source. The frequency of the oscillations defines the pitch of the noise.

• Thanks, that makes sense. Still can't hear the click though. Commented Nov 18, 2011 at 13:32
• It may be too soft to hear - most of the power is wasted heating the resistor. Commented Nov 19, 2011 at 5:43