Trying to make sense of the rules for matching amps to speakers

[jronnquist]

Hello,

Can anyone explain the general rule of thumb to be employed when matching amps to speakers (or vice-versa) to me? From the dozen or so YouTube videos I have watched, the consensus seems to be that the amp should have a rated RMS output per channel about 1.5 to 2 times higher than the RMS rating of the speaker. This is all well and good, but in practical terms it doesn't seem to work.

For example, if you go onto any of the major car audio retail websites in the UK, even most 1000W 4-channel amps will only have a RMS rating per channel of 50-60 Watts at 4 Ohms. If I was to go with the 2x rule above, it would mean matching it with speakers rated at 25-30W RMS, and I have yet to find a half-decent speaker below 45W.

I appreciate this question betrays the depth of my ignorance, but I'm determined to get my head around the science before I begin spending any money. Any help would be greatly appreciated.

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[KicktheSDbasshead]

Hello,

Can anyone explain the general rule of thumb to be employed when matching amps to speakers (or vice-versa) to me? From the dozen or so YouTube videos I have watched, the consensus seems to be that the amp should have a rated RMS output per channel about 1.5 to 2 times higher than the RMS rating of the speaker. This is all well and good, but in practical terms it doesn't seem to work.

For example, if you go onto any of the major car audio retail websites in the UK, even most 1000W 4-channel amps will only have a RMS rating per channel of 50-60 Watts at 4 Ohms. If I was to go with the 2x rule above, it would mean matching it with speakers rated at 25-30W RMS, and I have yet to find a half-decent speaker below 45W. For example I have a kicker zx 350.4 here's its rms ratings 4-channel car amplifier

I appreciate this question betrays the depth of my ignorance, but I'm determined to get my head around the science before I begin spending any money. Any help would be greatly appreciated.

First make sure the amp you buy does its rated power. I believe when you say a 1000 watt amp only does 50- 60 watts per channel its not doing its rated power even at 2 ohms.i have a kicker zx 350.4 here's its rms ratings 4-channel car amplifier

60 watts RMS x 4 at 4 ohms (90 watts RMS x 4 at 2 ohms)

175 watts RMS x 2 in bridged mode (4-ohm stable in bridged mode)
They're are a ton of amps that do rated power the amps you are seeing are what they call max power wich isn't used in the rule of thumb on any measure in real set ups. Kicker, jl audio, and fosgate are mainstream amps that will give you decent numbers that are real.

 

[ThxOne Premium Member]

Hello,

Can anyone explain the general rule of thumb to be employed when matching amps to speakers (or vice-versa) to me? From the dozen or so YouTube videos I have watched, the consensus seems to be that the amp should have a rated RMS output per channel about 1.5 to 2 times higher than the RMS rating of the speaker. This is all well and good, but in practical terms it doesn't seem to work.

For example, if you go onto any of the major car audio retail websites in the UK, even most 1000W 4-channel amps will only have a RMS rating per channel of 50-60 Watts at 4 Ohms. If I was to go with the 2x rule above, it would mean matching it with speakers rated at 25-30W RMS, and I have yet to find a half-decent speaker below 45W.

I appreciate this question betrays the depth of my ignorance, but I'm determined to get my head around the science before I begin spending any money. Any help would be greatly appreciated.

Assuming reputable components are being discussed...

A speaker rated at 50w rms will/should have NO problem handling peaks/burst of music well above its rms rating.

An amplifier rated at 50w rms should have no problem peaking at a little above its rms without major distortion.

In reality, that 50w rms rated speaker should handle 75w rms from an amp all day or even 100w rms because music is dynamic so it will not demand full power all the time as sound is being produced by the speakers. It will bounce around from needing just a few watts to needing many. The problem is distortion. Distortion (clipping) makes the speaker pause at the top and bottom of its movements which allows the voice coil to heat up and eventually it will break.

So matching a 50w rms amp to a 50w rms speaker is not the ideal as it will distort quicker when the speakers needs more power for those peaks in the music. Since a speaker of good quality can handle burst of power (watts) above its rating it is a good idea to use an amplifier with an rms rating that is more than the rms of the speaker. This allows that headroom and moves the distortion further away.

My opinions are:

25w rms speaker - 50w rms amp
50w rms speaker - 75 -100w rms amp
75w rms speaker - 100 - 125w rms amp

And so on. Too much power can fry the speaker too. I would not suggest a 250w rms amp on a 125w rms speaker for too long.

Hope this helped.

 

[1aespinoza 10+ year member]

the consensus seems to be that the amp should have a rated RMS output per channel about 1.5 to 2 times higher than the RMS rating of the speaker.

Just stick to matching the RMS of speakers and amp. RMS is what the amp will play all day long with absolutely zero issues. So if there are high peaks in the music, the amp will play them with no issues for they are short, and will not strain it.
Some like to have excess power for headroom, but I think that tactic is better suited for high power systems. Another important specification is the speaker's sensitivity. Look that it is 90 decibels (db) and above. Sensibility is the speaker's ability to convert electrical signal to sound.

 

[ThxOne Premium Member]

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[HardofWhoring]

The problem with that question is that you're asking it on a car audio forum. There are far too many people on here, who know a lot more than what's needed to just answer your question, that's just irrelevant FOR WHAT YOU ARE ASKING.

The short answer is match em close, within about 10% (amp a little higher if that's an option). Don't look at an amp's total power, match up RMS at the same Ohms per channel (1channel =1 speaker/set of components). You can go bigger and dial it down if you really want. Maybe you get a great deal, or already have the equipment. It's definitely NOT a requirement. For 90% of the people that is all you need to do if you just want a little better sounding, and a little louder.


Listening to music at full volume for hours changes that.
If you might want to upgrade your speakers down the road, it changes that, (and is why people say to oversize an amp). If you really want to spend money on buying bigger you can, it will only take up more space, and cost more. It's like buying a hemi v8 for grandma. Sure it will work, but it's a waste.

P.S. "car audio RETAIL SITES" tell you to buy bigger than what you need.... Of course they do.

 

[hispls 5,000+ posts]

The problem with that question is that you're asking it on a car audio forum. There are far too many people on here, who know a lot more than what's needed to just answer your question, that's just irrelevant FOR WHAT YOU ARE ASKING.

I always say that if you have to ask this question then you have no business running things beyond manufacturer recommendations, but I think you're a little late to this thread and OP will never be back here.

 

[nauc 10+ year member]

youtube says all kinds of stupid ****

 

[jronnquist]

I always say that if you have to ask this question then you have no business running things beyond manufacturer recommendations, but I think you're a little late to this thread and OP will never be back here.

I'm back. Would have returned sooner, but I forgot to turn on email notifications for replies. I suppose the cynicism is justified. I imagine a lot of people jump on here with questions and disappear.

 

[jronnquist]

Thanks for all the replies, guys! Much appreciated.

The amp I ended up with is the Kenwood X302-4. Rated RMS power output: (4Ω, 14,4V) 50W x 4 (20Hz ~ 20000Hz, 1%THD). I've paired this with a set of Focal Auditor ASE 165 2-way components for the front doors and Focal Auditor ASE 130 2-way components for the rear. The plan was to simply match the RMS numbers for amp and drivers, but the 165s are actually rated at 60W RMS, not 50W like the 130s (something I missed when I bought them and only noticed about 5 minutes ago). I hope this isn't going to be a problem.

 

[nauc 10+ year member]

it wont matter. less than 1 db change

 

[jronnquist]

it wont matter. less than 1 db change

Great to hear. Thank you!

This is not a system I have any intention of driving to its limits. What I want is a stable system composed of relatively decent quality hardware that can reproduce the music I listen to cleanly across the full range of sound. I suspect the extensive sound dampening I have planned will be as much a factor in achieving this as the speakers themselves.

 

[jronnquist]

I've started a full project thread here for anyone who is interested.

 

[winkychevelle 10+ year member]

I've started a full project thread here for anyone who is interested.

I would suggest setting the amp to 50w using the dmm method at 1khz sinewave

V(ac)=√(watt×ohm)
14.14(Ac)=√(50×4)
1. Set the dmm to v(ac) on the output terminals

2. Set the radio to the desired max volume usually 75% of the volume

3. Play a 1khz tone recorded at -3db

4. With the tone playing turn the gain up until the dmm reads 14v. +/- .5v

5. Remove the dmm and set the crossovers

6. Plug in the speakers

7. Starting with the volume low turn the volume up to the desired level without passing the previously selected max point.

8. Adjust time alignment and eq (try and cut more than boost)

Having more power on hand isn't really a bad thing since it will more than likely go unused anyway. Even with 50w you will likely have to turn the highs down or the subs up relative to one another since a lot more power is needed to match the efficiency of higher frequency speakers.

90db subs need 2x the power of 93db mids and 2x more to match 96db tweeters

500sub, 250mid, 125tweet

But it's more like 87db subs and 93db mid and 96dbtweeter
500w sub, 125w mid, 62.5w tweeter

thats just for flat response. Most people want subs and midbass 10db higher than everything midrange which means a lot more power is used in the lower frequency low efficiency subs.

It's easy to see why you would use significantly less power for highs once you consider efficiency and then the target response curve.

 

[jronnquist]

I would suggest setting the amp to 50w using the dmm method at 1khz sinewave

V(ac)=√(watt×ohm)
14.14(Ac)=√(50×4)
1. Set the dmm to v(ac) on the output terminals

2. Set the radio to the desired max volume usually 75% of the volume

3. Play a 1khz tone recorded at -3db

4. With the tone playing turn the gain up until the dmm reads 14v. +/- .5v

5. Remove the dmm and set the crossovers

6. Plug in the speakers

7. Starting with the volume low turn the volume up to the desired level without passing the previously selected max point.

8. Adjust time alignment and eq (try and cut more than boost)

Having more power on hand isn't really a bad thing since it will more than likely go unused anyway. Even with 50w you will likely have to turn the highs down or the subs up relative to one another since a lot more power is needed to match the efficiency of higher frequency speakers.

90db subs need 2x the power of 93db mids and 2x more to match 96db tweeters

500sub, 250mid, 125tweet

But it's more like 87db subs and 93db mid and 96dbtweeter
500w sub, 125w mid, 62.5w tweeter

thats just for flat response. Most people want subs and midbass 10db higher than everything midrange which means a lot more power is used in the lower frequency low efficiency subs.

It's easy to see why you would use significantly less power for highs once you consider efficiency and then the target response curve.

Thank you for taking the time to reply in such detail. It's truly appreciated. It's going to take me a week or so to get everything installed (if it ever stops raining, that is), so that should give me enough time to re-read this until it sinks in. I do have a dmm, but I'm probably going to have some stupid questions for you before I attempt to follow the above instructions, if you don't mind.