How Many Watts do my Speakers Really Need?

 

This entry was posted by Preston Smits On 02/10/2012

 

The amount of watts your PA has probably doesn’t matter nearly as much as you think it does.

We often get questions from our customers asking “Should I get the higher wattage amp to power my loudspeakers?” or “This studio monitor only has 40 watts, is that going to be enough?”. Many people assume that the higher wattage system is going to sound better or give them significantly more volume. Given this common misconception, I thought I’d write a little about how the output from your power amplifier relates to the performance of your loudspeaker or studio monitor.

Doubling the wattage of your amp will not double the perceived volume of your sound system!

When I was a teenager browsing my local music shop for a PA, I remember looking at the 1000w amplifiers and thinking, “man, if I could just afford to get that instead of this 500w amp, I could really blow em away!” What I didn’t know then was that for most common applications, that extra power was going to make a very minimal difference in how loud I could crank it. Essentially, doubling the power output from the amplifier increases the sound pressure level (SPL) output of the speaker by 3dB. For reference, a 3dB increase in output is like turning up your car stereo by a 2-4 clicks, or something said to be ‘slightly louder'; not exactly a huge difference.

Without going too far into the details, power (wattage) doesn’t translate well to our ears and perceived loudness, which is specifically why the decibel (dB) was created. The dB is a ratio that works on a logarithmic scale. It translates the world of electricity and acoustics into units that we use to better represent differences in volume. While the human ear can hear something as quiet as a 0.000001 pascal (sound pressure units, abbreviated as Pa) to perceiving pain at about 20 pascals, this huge set of numbers is not very practical for simple communication to each other. Rather than saying “Turn up the PA another 35,000,000 micro pascals” its a lot easier to say turn it up by 6dB. Not to mention if we wanted to turn it up by 6dB again, this time we’d have to say “Now turn it up 70,000,000 micro pascals.” Put another way, as the power output increases exponentially, our ears hear a steady linear increase in volume!

So how does this relate to your the watts of your PA? Well, since watts increase exponentially for a linear increase in perceived volume, doubling the power output makes only a small notch up the scale our ear hears. So for most consumer level PA systems (i.e. any system that is not installed in a large venue or reinforcing for more than 400 people), the extra cost in purchasing amplifiers with 150%-200% more power is rarely worth it.

So when do the watts make a difference?

If you turn your amplifier up to just before it starts clipping, and you still don’t have enough volume, then at that point yes, you might need an amplifier with higher wattage. But in this scenario, as we mention above, you’re not going to get much from going from your 500 watt amp to a 1000 watt amp, you’ll likely need to upgrade to at least 2000 watts before hearing much of a difference (providing your loudspeakers can handle this increase in wattage). In fact for a perceived doubling in volume, you’ll need 10 times the power (10dB increase), or 5000w!  At this point, you’d likely be better off just buying speakers with a better sensitivity rating. Loudspeaker sensitivity ratings tell you how many dB-SPL you can get out of a speaker using 1 watt measured from 1 meter away. These sensitivity ratings can often swing a good 3-6 dB between different models and manufacturers. A loudspeaker that has a sensitivity of 96dB 1watt/1meter, is going to give you 6 more dB of output than one with a sensitivity rating of 90dB 1watt/1meter, given both are receiving the same wattage. These speakers may be in a very similar price range, and could buy you that 6dB increase in volume for much cheaper than upgrading your amp from 500w to 2000w.

On a similar note, if you aren’t getting enough volume out of your PA due to not enough wattage, and you try to force it out of your amp by increasing the amp’s input sensitivity (volume knob), you may clip the input, causing a square wave to pass on the amp’s output. These square waves can quickly destroy a loudspeaker’s compression driver or “tweeter”. Keep in mind that in this situation, it is not that the amp had too little power that blew your speaker, its that you turned the input sensitivity up until it clipped the signal it was receiving, thus sending out a square wave and frying the tweeter. The only reason this is less likely to happen with an amp with more power is that you are less likely to turn up the input sensitivity that high because its already putting out more volume. I may write a post soon clarifying this misconception of how under powering your speakers can fry them.

What about watts and how it relates to my studio Monitors?

When it comes to active (powered) studio monitors, checking out how many watts it’s on board amplifier has is in a large part useless. Given the two topics we talked about above (3dB increase in volume when doubling wattage, and speaker sensitivity ratings) you really can’t gather much about the monitor’s performance from a watts specification. If fact, here at our office there is a pair of monitors rated at 140w that get significantly louder than a pair we have rated at 180w. I also know that I like the sound of that 180w monitor much more than one of its competitors that is rated at 225w. If you are concerned with how loud the monitor is capable of getting, take a look at its maximum spl rating, or SPL at 1 meter rating. These specifications will provide you much more insight into how loud the monitor will get than will the wattage spec.

Please feel free to leave a comment below if you have any other questions or thoughts related to the topic.

-Preston Smits

Check out all studio monitors compared by Sonic Sense.

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About Preston Smits

I got my start in audio technology in high school while playing music in a variety of different groups. Whether it was choir, jazz band, or a four piece rock group, I found integrating my love for music with my inquisitiveness in technology exhilarating. The more I integrated the two, the more I found it driving me towards a career in audio. I received my BS degree in Audio Engineering and started working in the commercial Design/Build industry where I designed and installed audio video and lighting systems for performance venues (such as Theaters, Churches, Arenas, etc). I also continue to play in local music groups and run Wave Stable Studio, a commercial facility located here in Denver, CO. I recently joined the team at Sonic Sense where I’m excited to be sharing my experiences and audio knowledge to all who care to hear! Through this blog I hope to provide tips and tricks in all aspects of the performance audio world whether it be in the studio, as musician on stage, behind the console at FOH, or for the not-so-audio-enthusiast who just needs to get a decent recording of his interviews. I’ll be conducting equipment shoot outs, writing up real life experiences on audio gear, answering specific questions, and anything else I can find to help the world strive for great audio! -Thanks for checking in, feel free to contact me with your comments and questions, Preston Smits

  • Sean

    Preston,
    I hate to ask such a generalized question, but here goes-
    The auditorium at my school was just renovated, and the old speakers are some very large, completely unlabeled yamaha’s that are run using speaking cabling. I have a mixer with a built in 250/250 amp. I’ve been asked if I could use those speakers occasionally on our soccer field for the games. The speakers would only really need to cover 50 people sitting in an open space. The speakers can face them. My question is this- will 250 watts likely be enough to power a single (24 inch ?) speaker that was previously used as an auditoriums main house speaker? Like I said, very general question, but I’m don’t have much to go on either :-/.
    Thanks for the consideration!
    Sean Grady

    • http://www.sonicsense.com Preston Smits

      Hi Sean – Like you said, it’s hard to know since we don’t have all of the information, but I would suspect that you’ll be just fine. Especially since it would mostly be used for announcements & not foreground music. Save some cash & give them a shot! BTW, if you ever need additional equipment for your systems, feel free to reach out to us; Sonic Sense gives really great discounts to educational facilities. :-)

  • gman

    Hi Preston ~
    Being aware of the potential to cause clipping if pushed to hard (which I’m not in the habit of doing) is there enough power in a Yamaha 473 A/V unit @ 85 -115 watts, to effectively run JBL ES80 speakers rated at 400 watts? The JBL spec sheet says yes, even claiming 25W is enough. What’s your thinking on this, curious minds need to know.
    Thank you, gman

    • http://www.sonicsense.com Preston Smits

      Yeah, I agree with their spec sheet -you’ll be just fine and get plenty of SPL out of them with your Yamaha 473. Just make sure to watch the clipping like you mention.

  • Josh rogan

    Hi mate. This is also a generalised question. I want to put together a cheap solo music stage monitor. Can I just buy a cheap amplifier from radio shack and attach it to a hifi speaker which is 8 inch. I’m not after the greatest sound just want to hear vocals and acoustic first.

    • http://www.sonicsense.com Preston Smits

      Thanks for getting in touch. This thread is really more focused on appropriate power for loudspeakers and we aim to keep it as such for the good of our members & readers. All the same, we recommend professional equipment for professional applications. There a many high-quality powered stage monitors available that would deliver better performance for probably not much more money. Best!

  • Arnie

    Hello Preston,
    I have a 4-piece smooth jazz band and we often play small rooms. In those venues, I use my portable Yamaha stage pass 300 system (it’s actually a pretty good PA system). It has 2-passive 150w speakers, and I use a active/powered monitor. My board has an additional line for another powered monitor. My question is this, can I use the second powered monitor as a third speaker for the audience and get a sound compatible to that of the system speakers? I don’t know if it would sound weird.

  • Chris

    Can I run one receiver to run 6 zones in my house? And if so how powerful of an amp do I need (What specs do I look for)?
    5.1 in my family room (Front Artison 55 custom non powered sound bar 89db, 75-150w)(Rears Artison mezzanine 8 (87dB, 50-150w)
    and 2 stereo Martin Logan electromotion ic speakers in the other 5 rooms (90db, 20-200w)
    Please help
    Thanks for your time!

    • http://www.sonicsense.com Preston Smits

      Hi Chris, that’s probably not going to work out very well. Those Martin Logan speakers are probably 8 or 4 ohms. If you daisy-chain all of them to 2 channels of amplification from a 5.1 receiver with an extra zone you’ll have some major issues. Most home receivers don’t take loads below 4 ohms per amp channel very well. And if each speaker is say 8 ohms, you have to divide the impedance in half each time you add another 8 ohm speaker to the chain. IE: 8ohm + 8ohm = 4ohm. 4ohm + 4ohm = 2ohm.

      While most home receivers will operate at 2ohms per amp channel, they tend to have worse distortion at that impedance (although often barely noticeable difference). Maybe more importantly is the fact that the receiver will heat up much more quickly and possibly burn out or even catch fire if running at too low an impedance!

      You’re probably going to need one amp channel per 4ohms total speaker load (Such as two 8ohm speakers). Or if your speakers are 4ohms each, then find an amp comfortable living at 2ohm loads and put two of those speakers per amp channel.

      In commercial settings, celling speakers are ran via a “70v” system with transformers at each speaker location in order to get around this impedance problem.

      Hope this helps!

  • ashik

    i have a JBL 400 watts speaker and now i’m planing to buy an amp for it because the output from the player is not at all good.
    I concluded a 2 channel amp(600 watts).
    will it blow up the speaker or will work fine?

  • kski

    Just like to mention, little things matter alot. Clean cable ends and speaker inserts, using high quality 12 ga speaker wire. Also getting those speakers up in the air about head level and speaker placement too. I only run 250 watts per side for a standard local bar gig and have no issues. Also remember not to mic up too much gear in these smaller venues since they also eat up power. Especially bass and drums. Sometimes less is more.

  • Ssr

    I own a B&W 685 bookshelf speaker. Manufacturer recommended amp is between 25-100 watts. If I am given an option to choose between 125 watts and 200 watts per channel amp, which one should I choose?

    Thanks in advance

  • Steve

    Hi.I have a Behringer pmx2000 250w@ 4 ohm. 180w @ 8 ohm. I currently have JBL JRX100 speakers 250-500w 8 ohms. These work great for the band setting.The speakers are a bit heavy an overkill powerwise for a solo performance in a smaller venue.I was thinking a smaller lighter less wattage pair of speakers while still using my Behringer unit.My question is can I use a pair of say 120w speakers with the more powerful Behringer as long I don’t crank i up? Ay suggestions for A decent pair?

  • Fredy

    Hi, I am in the process of building a sound system for my Magic show. Don’t have but around $600-650 to play with. It must also be portable. Average size audience 300-400 people. Would you make some budget-minded suggestions that could get me started? I can upgrade in the future. By the way I have some wireless mics that were given to me. And what do you think of a mid sized Fender Passport system?

    • http://www.sonicsense.com Steven Leccese

      Hey Fredy, thanks for contacting us! You’d probably want something along the lines of an all-in-one PA system like the Fender Passport. I’ve used that before for some smaller shows, and it’s worked great. Depending on what type of venue you’ll be doing shows at, the all-in-one PA should be fine. There’s some other units such as the Behringer Europort and the Yamaha Stagepas series that may fit what you need as well. For a cheaper option, you could even look at getting some individual powered loudspeakers, some speaker stands, and a tiny mixer for your microphones. With that second option, it depends on how many inputs your system will need. If you just have one or two microphones running, some powered loudspeakers have a built-in 3-channel mixer in them, so that would remove the need for a dedicated mixer. Check out the loudspeakers here and it should help you budget a little more. There’s a lot on the list, so you should have some nice options. Let me know if you have any more questions and I’d be happy to help!

  • marionel

    Hi Preston,

    i was so happy to see and read your blog i was just wondering that maybe you can help me because i was planning to put up and start a basic sound system setup for business. anyway i have already prospect which is Behringer EPS500MP3 Ultra-compact 8-channel PA system with dual speakers and i want to use Behringer EUROLIVE B212D Active PA Speaker System as monitor speaker is possible to combine all of this things.

    thank you and i am hoping to get your feedback as soon as possible.

  • Trenton

    Hey quick question so if I have three 12s Mofos and they are 1500 rms that equal up to 4500 watts max. will a 6000 watts amp will be enough to push them three 12s to their max?

    • Roldan Padlan Paul

      he said, doubling the power of the amp required by the power of the speaker is better than under powered amp to protect the speakers or maybe the amp. . i think active speakers are better so there’s no need for us think bout this power ratings and requirements.. in my opinion, passive speaker need an amp which is, let say, from from it’s speakers. thinking of distance we know that there is a drop voltage or power in every meter maybe thus requiring greater power of amp for the passive speakers. . i hope this helps. . To God be all t
      he Glory. .