Receiver Wattage Needed?

[robster]

I have an 18x28 room, a three channel soundbar (8ohm, 175W RMS, 92db) and a 150W RMS powered subwoofer. I might put some rear speakers in some day but they will be small. I use the sound primarily to listen to the sound for TV & Movies. Sometimes music via iPod. I do turn it up loud for football games and movies. My question is how many watts do I really need for this size room and limited speaker wattage draw. My initial thoughts are 50-100W per channel. I have a 115W per channel receiver now that has issues. I am considering as low as 50W. Your thoughts?

[moves]

I would think it would depend on your listening position. If your seating is at the back of the room, 18 or 28 feet back is quite far.... I don't know how to do the math to figure out the wattage needed to acheive whatever SPL you are looking for but I know there are a lot of people on here who can do this. Maybe they will help you out.

[FirstReflection]

That's a big room! How far away are you sitting from the TV and your sound bar?

The normal way that speakers are spec'd out, what you will see is an efficiency rating of something like 86dB / 1 Watt / 1 meter. In your case, your soundbar appears to be rated at 92dB / 1 Watt / 1 meter. I'm going to go ahead and assume that's the normal marketing BS that's used with most soundbars. They're probably playing all three channels at once, and probably only at a single frequency of 1 kHz in order to get that reading. So the REAL efficiency is probably closer to 86dB / 1 Watt / 1 meter, which would be typical.

So, what that means is that if you are listening to that soundbar from a distance of only 1 meter (roughly 3 feet away), it only takes 1 Watt of power to generate (supposedly 92dB), I'm guessing 86dB of sound output.

Every time you double your distance from the speaker, the output drops by 6dB if you're in a wide open field. In a real room, it drops more like 3 or 4 dB every time you double your distance.

Every time you double the number of Watts being used by the speaker, the output goes up by 3dB. And if you increase the Wattage 10x , you get a 10 dB increase in output.

So basically, you can calculate exactly how many Watts you need in order to get the loudness of output that you want at your seating distance. We just need to know that distance

Basically, I would assume that your soundbar can produce a maximum of about 106-109dB from 1 meter away, using the maximum amount of power that the soundbar can handle (listed as 175 Watts). The soundbar is probably distorting like crazy and begging uncle at that sort of output level, but it's only meant for short peaks of output, like a big explosion, or a sudden crescendo in music.

If you're 12 feet away, that's roughly 4 meters, which would mean that you've doubled your distance, and then doubled it again. That means the output would be somewhere in the range of 6-8dB quieter than if you were just 1 meter away. So I'm guessing the loudest your soundbar can possibly play (if you're 12 feet away) is about 100dB, which is very loud, but not hitting the 105dB peaks of "reference volume".

"Normal" listening levels are only about 75dB though. 85dB is "loud". And 95dB is "very loud". So it doesn't take a lot of Watts to hit those typical loudness levels. Even being somewhat conservative, from 12 feet away, it only takes about 1 Watt per channel to play at 75dB with your soundbar. And only about 10 Watts per channel to play at around 85dB. 100 Watts per channel should let you hit 95dB. And like I say, I'm guessing your soundbar will start to scream uncle if you try to play it much louder than that

You might be farther away than 12 feet though, given your room size. So the power requirements and limits of your soundbar's output start to increase dramatically as you move farther away.

So you might want to consider a speaker upgrade at some point. More efficient, higher output, higher power handling speakers that can really handle your large room size. And you could definitely use a big subwoofer upgrade. A pair of really high output subwoofers, like some Rythmik FV15HP or something wouldn't be out of line in a room size such as yours. But that's only if you want to genuinely hit "reference level" with its 115dB peaks in the LFE and low bass

For now, a receiver that can genuinely deliver 100 Watts/ channel (which means the peak output will be higher) should be able to drive your soundbar literally as loud as it can go.

Since it looks as though you're aiming to keep the cost down a bit, the model I'd recommend to you would be a fully authorized, refurbished (still comes with a full 1 year warranty, no 90 day warranty BS ) Onkyo TX-NR709 from accessories4less

The big reason why I'd recommend that specific model is because it is the least expensive (by quite a margin) AVR that includes full pre-outs for all speaker channels. Having those pre-outs means that you can add separate, higher power amplification in the future if you want to. If you get some higher output, higher power handling speakers in the future, you're gonna want some big, beefy amplification to go along with them. So the TX-NR709 will allow you to add that separate amplification in the future, whereas there are no other receivers at this $450 price point or lower that will allow you to do that!

The 709 also has some nice features, such as THX Select2 Plus and Audyssey MultEQ XT, which give you some really nice listening modes and some really great room EQ and auto-setup that you don't often find at this price point either.

For now, the 709 can power your soundbar literally as loud as it can go, so no worries there

Hope that helps!

[moves]

So I got Totem Forest speakers http://totemacoustic.com/en/hi-fi/co...forest-model-1

I am not sure if I understand this....

My speakers are 87db sensitive which means that at 1 Watt it plays 87db.

If I go 2 meters away... I would need 2 Watts to play at the same volume...

This doesn't make sense to me... if you only need a few watts to play 87 db then why are we focusing on 200 watt amplifiers?

I am clearly missing something here...

[FirstReflection]

So I got Totem Forest speakers http://totemacoustic.com/en/hi-fi/co...forest-model-1

I am not sure if I understand this....

My speakers are 87db sensitive which means that at 1 Watt it plays 87db.

If I go 2 meters away... I would need 2 Watts to play at the same volume...

This doesn't make sense to me... if you only need a few watts to play 87 db then why are we focusing on 200 watt amplifiers?

I am clearly missing something here...

Nope, you're not missing anything. I think you're maybe just underestimating how quickly the power demands can get up into the 200, 500 and 1000 Watt range!

Most of the time, we're using very few Watts. 10 Watts per channel or less in most cases, most of the time. That's why you can have your TV, receiver and DVR all plugged into the same power strip without tripping a breaker! And that's why way back in the day, little 10 Watt, 5 Watt or even 2 Watt tube amplifiers were still able to drive speakers in most rooms just fine! Most of the time, we don't need very many Watts at all

But the required Wattage goes up 10 fold every time we call for 10 dB more output. And 2 meters is pretty close. Many people are farther away than that.

Reference volume calls for an average output level of 85dB at the seat, but with 105dB peaks.

So let's say you've got your 87dB / 1 Watt / 1 meter speakers. But let's say you're a very common and reasonable 10 feet away. At that distance, you'd be getting around 81 or 82 dB of output from just 1 Watt in a typical room. Double the power to 2 Watts and you're hitting close to reference volume at 85dB average. So yeah, the majority of the time, you only need about 2 Watts of power!

But now comes a very loud section at 95dB. 10x the power to hit that. So 20 Watts are needed. No biggie. Pretty much any modern receiver can supply that!

But then come the 105dB peaks. 10x more power. Boom! Suddenly you're calling for 200 Watts! That didn't take much, did it?

And let's say you're a little farther away. Maybe 15 feet. You've just dropped another 3dB at least. Gotta double the power again to make up for that 3dB drop due to distance. Now you're at 400 Watts to hit reference level 105dB peaks!

So that's what the big 200+ Watt amps are all about. Unlimited headroom for those big peaks. And it doesn't take insane distances or ridiculously inefficient speakers to make it happen.

There is also the matter of low impedance speakers, which demand a lot more current from the amps, which is harder to deliver than just increasing the Wattage by increasing the voltage into a higher impedance speaker. Low impedance speakers can be difficult loads for an A/V receiver. So part of the appeal of big, separate 200+ Watt amps is that they are also able to deliver more current, and thus, handle low impedance speakers more easily than an AVR.

But you're right. People often see the spec on their speakers that says they can handle 150 Watts or 200 Watts or whatever, and they mistakenly think that they need an amp that can continuously deliver that much power. And that just isn't the case! People don't realize just how little power and how few Watts we really need the vast majority of the time. That's why it's so foolish to think that an amplifier upgrade is going to drastically improve the sound quality most of the time. Contrary to what some publications and some "audiophiles" will tell you, amps do not sound audibly different from one another for the most part. They have a fairly simple job, which is to take the signal from the pre-amp section, which are just a few milliwatts, and boost those up to a few, or a few hundred Watts when called for. So the audible differences don't show up at all most of the time, because we're only using 1 or 2 Watts! It's only during the very loud sections and the big peaks that the differences really show up

[moves]

what a great post thank you for the info!

[moves]

Is it right to say that if the impedance was lowered to 4 ohms then it would take twice the power to produce the same SPL of a speaker rated at 8 ohms?

[PENG]

Is it right to say that if the impedance was lowered to 4 ohms then it would take twice the power to produce the same SPL of a speaker rated at 8 ohms?

This question needs more specifics, even then it may still be impossible to answer without stating a lot of "assuming....", and/or "it depends.....".

[mtrycrafts]

Is it right to say that if the impedance was lowered to 4 ohms then it would take twice the power to produce the same SPL of a speaker rated at 8 ohms?

It is about speaker sensitivity but what PENG said.