Speaker Ohms. - Page 1
Forum section: electronics / projects

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: February 09, 2013, 03:56:59 pm
content: Hey guys i have a question about speaker ohms.I have two speakers one is 8 ohms 2 Watt and the other is 8 ohms 3 Watt and i would like to know what the ohms on a speaker are because i am not sure if i can use a good speaker i found (8 watt 14 ohm) for my Lm386 project which i supply with nine volts dc.So please can u explain me what ohms are (i searched about it but i did not got a thing.)and what ohms have to do with wattage (if there is any connection between the two)Thank you.

author: PStevenson
author_rank: Regular Contributor
timestamp: February 09, 2013, 05:02:58 pm
content: ohms are a unit of resistance - in the case of a speaker it is the resistance of the coil that "drives" the cone of the speakergenerally with speakers rated Watts tends to indicate what power a speaker can handleoff the top of my head - and depending which one you have the LM386 gives out about 500mW with a 9v supply to an 8ohm speakerso use the 2 watt speakerwatts, ohms, current, voltage are all connected but there are definitely threads about that stuff on here that explain better than I wouldwith my poor verbal skills.

author: ElectroElvis
author_rank: Regular Contributor
timestamp: February 09, 2013, 05:54:27 pm
content: But also be carefull since there is also version LM386n-1 which gives only 250mW output, I made once mistake and bought it instead of 500mW version one.

author: grumpydoc
author_rank: Super Contributor
timestamp: February 09, 2013, 05:59:29 pm
content: QuoteThismight help a little but I'd brush up on your internet search skills. Also if you are taking any sort of physics class, even at high school, it should contain the basics of ohms law and the relationship between volts, ohms, amps and watts (and joules and coulombs and a few other things).With loudspeakers the ohms value is the coil DC resistance and the power figure is the maximum power that the 'speaker can handle without damage. Do not bother particularly with the relationship between volts, ohms and watts here because the loudspeaker is driven with a varying voltage and, hence, an alternating current and the calculations get somewhat more complex.Instead look at the amplifier - it will have, as part of its specification, the lowest loudspeaker resistance it is designed to be used with. In the case of a LM386 this is 4ohms (or so) - as long as the 'speaker has ahigherresistance than this you will be OK. It will also have an output power. In the case of an LM386 and an 8ohm 'speaker this is, as PStevenson says about 500mW (0.5W) - as long as the loudspeaker can handle more than this it is suitable. So, any of your 'speakers will be fine on that basis.All things being equal if you connect a 'speaker with a higher resistance less current will flow and there will be less power so the sound will be quieter. However for the LM386 all things arenotnecessarily equal - if you use the 14 ohm speaker you will actually be able to get up to about 1.2W from the LM386N-4 with a 16V supply. So I'd disagree slightly and suggest that your 8 watt 14 ohm speaker could be the best to use if you can increase the supply a bit and have the -4 part.The LM386 datasheet ishere.The relationships are:V = VoltsI = AmpsR = ohmsW = WattsI=V/RW=IVso.W=I2RW=V2/RRead the ohms law wikipedia page and play around with the equations until you understand why the last two are correct.Aside: Why does pasting the ohms symbol show OK in preview but not in the post??

author: IanB
author_rank: Super Contributor
timestamp: February 09, 2013, 06:05:24 pm
content: The speaker ohms is how a speaker is balanced with an amplifier that drives it. An 8 ohm impedance is very common and "normal" for most speakers out there. You may sometimes find 4 ohm speakers in high power applications, and you may find that some headphones have a higher impedance like 30 ohms. But an 8 ohm speaker is a very common, standard kind of speaker.For your application, which is a small amplifier, there is something more important than impedance. What you really care about is sensitivity, which is how loud a speaker will be for a given input power. A sensitive speaker can be driven with a small amount of power and produce a lot of sound. An insensitive speaker will need a lot of power from a big amplifier to work.As you might guess, you need a sensitive speaker. You should look for a small speaker for this, such as the one you might find in an old transistor radio, or in a USB speaker for a computer, or something like that. A big speaker such as you might find from a music or hi-fi system is probably going to be too big to drive effectively from a 9 V battery.

author: grumpydoc
author_rank: Super Contributor
timestamp: February 09, 2013, 06:18:18 pm
content: QuoteIan is right but often even quite large 'speakers can be driven OK from small amps. I've driven 100W speakers from a 1W amp and it's sounded fine.As any of the 'speakers you have will work with the LM386 why not just try each and see which is best?

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 09, 2013, 06:26:44 pm
content: Depends on the battery. A 9V NiMH battery designed for a model car can supply a lot of current.If you're looking for a speaker that is very loud and compact but don't need good audio quality, take apart an old cell phone.

author: ciccio
author_rank: Frequent Contributor
timestamp: February 09, 2013, 07:49:24 pm
content: Quote from: grumpydoc on February 09, 2013, 05:59:29 pmIn fact speakers are rated for nominal impedance (they are used for a.c. signals), not resistance.The moving coil resistance of an 8 ohm rated speaker has a d.c. resistance of about 5 to 6 ohms.Please look at a speaker data-sheet, such as this one:http://www.bcspeakers.com/product.php?id=382: the nominal impedance is 8 ohms, the minimum impedance is 6.5 ohms and the d.c. resistance (Parameter Re, scroll down the table) is 5.8 ohms.Best regards

author: grumpydoc
author_rank: Super Contributor
timestamp: February 09, 2013, 07:53:00 pm
content: QuoteIndeed but for someone who is new to ohms law I thought I'd just keep things simple.

author: ciccio
author_rank: Frequent Contributor
timestamp: February 09, 2013, 08:59:58 pm
content: Well, sometimes things are not really so simple, but i'll try to be simple (those who already know please forgive me):resistance is for d.c. signals: ohms = volts/amperes - Resistance is indicated with Rimpedance is for a.c. signals: it's still ohms = volts/amperes, but it's value is frequency dependent, that means that it may change with frequency. Impedance is indicated with Z.If Z increases with frequency, it is because the component is inductive (e.g. an inductor, maybe combined with a resistor)If Z decreases with frequency, it is because the component is capacitive (e.g. a capacitor, maybe combined with a resistor).If Z is constant with frequency, it is because the component is resistive (e.g. a resistor). In this cause the impedance is in every aspect a resistance, and it has the same value even at d.c. (zero hertz).If Z changes with frequency, decreasing and increasing, it is because the component is both inductive and capacitive, e.g. made by an inductor and a capacitor, and maybe a resistor.The last one is the case of a speaker: the voice coil has a resistance (it's made by wire) and an inductance (it is a coil) and the mechanical effect of cone movement results in it acting as a capacitor.With all respect due to novices.

author: IanB
author_rank: Super Contributor
timestamp: February 09, 2013, 10:17:52 pm
content: And the reallysimpleranswer is that it doesn't really matterIf you have a small amplifier with a maximum output of about ½ W it's highly unlikely you can damage any normal moving coil loudspeaker. Similarly, if the loudspeaker has a regular impedance of 8 ohms it's highly unlikely you can damage the amplifier.So just experiment! Experimentation is the life blood of the hobbyist, the scientist, and the budding engineer

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: February 09, 2013, 11:33:55 pm
content: Quote from: ElectroElvis on February 09, 2013, 05:54:27 pmAs for that is the pin connections the same between these chips (can you change it if you did a mistake or you must desolder everything?)

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: February 09, 2013, 11:40:55 pm
content: Quote from: grumpydoc on February 09, 2013, 05:59:29 pmThank you for this.I know the ohms law as good as my name(i would suicide if i did not) but it did not occurred to me that the speaker ohms would be the same as the resistor ohms i thought it was one engineering thing that was too complicated for me.

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: February 09, 2013, 11:46:49 pm
content: Quote from: IanB on February 09, 2013, 06:05:24 pmThank you for that but i will order the speaker and judging by its size (5.5Cm) i would say that it is pretty good for my project.

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: February 09, 2013, 11:48:27 pm
content: Quote from: ciccio on February 09, 2013, 07:49:24 pmGot it .Thanks.

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: February 09, 2013, 11:49:47 pm
content: Quote from: IanB on February 09, 2013, 10:17:52 pmThank you that i am going to do my friend i will order the 14 ohm speaker and see what we can accomplish with it.