Talk to me about resistance

[saustindavis]

I recently bought a new Acoustic B200H and B115 and really like the tones it makes. I saw a review that said since the head sends the output at 4 ohms, and the cab is 8 ohms, you are only getting half of the volume compared to a 4 ohm cab. Is that true? Can someone explain resistance as it relates to amps/cabs (in plain English). Is there anything I can do about it? Should I even worry about it?

[GrowlerBox]

Resistance is futile.

[Selta]

Where's the facepalm emote?

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[GrowlerBox]

Sorry, someone was gonna say it, I thought it might as well be me.

Yes, you will get half the available power into your 8 ohm cabinet, and that doesn't matter for at least 2 reasons. If the cabinet were unchanged in every way except that it provided 4 ohms of impedance, you would gain somewhat less than 3 dB of extra volume from the rig, which you would hear if you were concentrating. Your current configuration allows you, should you need more volume at a later date, to add a second 8 ohm cabinet (ideally a copy of the current one), doubling both the power output from the head and the number of speakers available to use it. There will also be other advantages from the coupling of the cabinets giving better low-end response, for example, but these are icing on the cake.

[will33]

Originally Posted by Ray Salamon

Where's the facepalm emote?

AMPS FAQ!! Info on OHMS, Allsize RIGS-OverUnderPowerCabs DIY TechTalk-Links

+1

Twice the power does not equal twice the volume, etc., etc., etc.

[Selta]

Note: my facepalm isn't directed at the OP, but rather, the person who told him that you only have half the volume etc. At least the OP has the smarts to go asking questions, not just believing what he's been told.

[Downunderwonder]

Two guys are standing at the top of a ladder and a pipe. That's your amp with an 8 ohm speaker. They each have a bucket of water but only one of them can access the pipe.

Make the pipe bigger ( a four ohm driver ) and the one guy can get a bit more water down the pipe. Not a lot more. But give both guys a pipe ( two cabs ) and now you're in business.

It's not thiele small with water but hopefully it's useful.

The problem is intuition doesn't always work with audio. Two times power doesn't equal two times volume. It takes TEN times power to do that with power.

But luckily 2x cabs with 2x power = a hell of a lot louder. Your amp can do that if you want to add the matching extension cab.

Don't worry about it. You'll be gassing for something else entirely soon enough.

[chadds]

No.

[john m]

resistance is not equal to impedance

[BurningSkies]

[nysbob]

[billfitzmaurice]

Originally Posted by saustindavis

I saw a review that said since the head sends the output at 4 ohms, and the cab is 8 ohms, you are only getting half of the volume compared to a 4 ohm cab. Is that true?

No. The reviewer is an idiot, pure and simple. The difference is no more than 2dB, if that. Half volume is 10dB.

Should I even worry about it?

Follow this guys motto:

[dincz]

Originally Posted by billfitzmaurice

No. The reviewer is an idiot, pure and simple.[/IMG]

Let's not mince words Bill. What do you really think

[billfitzmaurice]

Originally Posted by dincz

Let's not mince words Bill. What do you really think

I was being charitable.
There are a few adjectives that I could also have employed, but this isn't an adults only site.

[Bassamatic]

A couple of quick things -

1. As said above - resistance is not impedance. When it is audio, the interaction with the speaker and other items is very complex, and it is called impedance. It is ABOUT the same as resistance but not exactly.

2. When the impedance is halved, you do get twice the power in watts, but not twice the volume. For example a 100 watt amp is not 10x as loud as a 10 Watt amp. In reality, you need about 10x the power to be 2x as loud to your ear. The reasons for this are complex, but that's the way it is.

3. A 3dB difference (Twice the volume) is about the minimum difference that you can perceive. When you think that something is twice as loud, it is probably much more than that in reality.

4. An important thing to remember is that the dB scale is logarithmic - every 3dB is double. So 6 dB = 4x, 9 dB is 8x, etc. So a speaker or cab that is 6dB more efficient for example, puts out 4x the volume at the same power input.

[dincz]

Originally Posted by Gio S

3. A 3dB difference (Twice the volume) is about the minimum difference that you can perceive.

Whoa! Twice the power. And 3dB is not a lot louder but definitely noticeable. 1dB is just noticeable.