I'm sure this is a stupid question, but those are the type I like to ask.

I have a device that needs a little speaker. My question about wattage is: do I need an amp rated higher than the speaker? Or a speaker rated higher than the amp?

 

Generally you want a speaker rated higher than the amp to avoid blowing the speaker.

 

Generally you want a speaker rated higher than the amp to avoid blowing the speaker.

Generally of course that is spot on. In this case though where the op is saying he needs a Tiny speaker, it may be that the "amp" is so weedy that it could cause no damage and possibly any speaker whose physical size fits in the project would be quite ok.
In the world of PA the situation may also not so clear cut. Some amps of insane power ratings may be used with lower powered speakers if the final sound level the speaker can safely produce exceeds what is required. In other words the amp is coasting. In that case it's up to the operator to not overdrive the speaker(s) It's usually painfully obvious when distortion sets in !

 

Ok, thanks. I wasn't sure if it worked like electricity in general, where the speaker could be seen like a lightbulb and the amp like the voltage regulator, and it was the lightbulb that determined how much power was used... or if it was the other way round. If I understand you, it is the other way round and the speaker needs to handle what the amp is putting out.

W.r.t. cornishlad's comment... if everything is under design's control (not user's), then it doesn't really matter because we just make sure we don't output enough to blow the speaker. In that case, does it make sense to "oversize" (in terms of wattage) both? Does straining either/both cause distortion, or just over-loading the speaker?

FYI - we're just talking about voice, like a GPS sort of. Pre-recorded messages playing at certain events. The user will have digital volume control but, of course, we can constrain that.

Sorry, I know nothing about audio at all.

 

The speaker does look like a resistor, so the power it draws is proportional to the square of the voltage across it, as determined by the sound source and the amp volume setting.

It's not uncommon to have an audio amp that's rated at a power higher than the speaker.
Normally that's not a problem since the speaker will start to sound very distorted before it reaches the burn-out point so the user will not go past that when adjusting the volume.

Both amplifiers and speakers will sound distorted when their respective power limits are reached.

How loud a sound do you need for these "events".

 

Not that loud. It is a handheld device that you'd hold similar to, let's say, a geiger counter or r/c airplane controller. The voice needs to be easily heard in an environment that can range from dead-quiet to noise like someone mowing their lawn down the street.

 

The fastest way to destroy a speaker is to apply DC voltage to it. DC can not be converted into movement and will be converted directly into heat instead. This occurs when a too small of an amplifier is used. When an amplifier "clips" it is the same as applying DC to the speaker. It is always better to have an amplifier rated for higher power than the speaker.

In other words, a sine wave that exceeds the speaker power rating, is less damaging then a square wave within the power capability of a speaker.

 

OK, I now have to exact opposite recommendations

 

Hey Lestraveled, BRAVO!!....My sincere appreciation Sir for your simple, spot on ‘non-techno-speak’ (thank you!) explanation!!
You are a gifted communicator, able to simplify technical concepts for the many ‘lay folks’ in this greater novice audience who benefit greatly by simplified and concise answers.... more of THIS is GOOD!!

I know posters mean well, but often ‘assume’ some of us (dummies like me - lol) already have a greater understanding of electronics....

Your explination of WHY speakers cannot tolerate straight DC has elluded me for quite sometime - A hearty Thanks for clearing up this DEEP mystery !!

Bunkysdad

 

Unfortunately, what Lestraveled stated contradicts what others have already stated.
See if you can figure out what is right and what is wrong by your own reasoning and what you have learned in the other threads.

 

Well as I’ve learned on the topic of speaker signal (AC or DC context defined....), I’ve found that there can be more than one correct answer...So I now believe that a ‘non-alternating’ form of DC is harmful to speaker voice coils...Is this incorrect (if so, please clarify!), thank you

Bunkysdad

 

Any over voltage, AC or DC is harmful to a loudspeaker if it exceeds the loudspeaker's rating.

Think about this.
What would happen if you send 120VAC to a light bulb designed for 240VAC?
What would happen if you send 240VAC to a light bulb designed for 120VAC?

 

My specific statement (assumption...) was this :

“So I now believe that a ‘non-alternating’ form of DC is harmful to speaker voice coils...Is this incorrect (if so, please clarify!), thank you”

My question is centered around non-alternating signal current vs alternating signal current, not voltage...More confusion is creeping back in - UGG!!

 

Yes, I do realize that voltage is present in the signal but current is dominate at the amps output stage....

 

If we consider the resistance of the voice coil to be constant (for the moment) then it doesn't matter if we discuss voltage or current. The two are interrelated by Ohm's Law.

 

Of course always a relevant point, but in my view somewhat off my topic, the concept of dc doing damage to a voice coil seems readily apparent, JMHO

 

AC will do just as much damage as DC if the amplitude exceeds what the speaker can handle.

 

My copy of ‘The Illustrated Dictionary of Electronics” (fourth edition) states that a fluctuating current is also referred to as a composite current, “a current containing companants of both AC and DC, i.e. an alternating current superimposed on a direct current”..... does this not describe the signal to a speaker??....

Just read your last - yes of course a voltage componant IS always present, but not the dominate componant via the output stage.... Correct? (Yes -No??)

 

DC in a speaker produces no sound, only heat.
The rated power of a speaker is with a sinewave input, not DC and not a squarewave.
If an amplifier can produce a sinewave with a RMS level of 4V into an 8 ohm speaker then the power in the speaker is 2W. But if the amplifier is overdriven and clips badly producing a squarewave with the peak-to-peak voltage (11.3V) the same as the 4V RMS sinewave then the power in the speaker is doubled to 4W because the distortion harmonics produce the same amount of power as the fundamental sinewave. The speaker is still being cooled because the voice coil is moving back and forth but with DC it does not move.

 

The signal to a speaker averages as AC. Occasionally there is some positive DC added to the AC but it is cancelled by the same amount of negative DC added very soon after.