# Does RMS convert to dB?



## Phil Indeblanc (May 27, 2015)

We all often look at the RMS wattage and often select speakers based on more watt handling and how that it will be powerful enough to satisfy our ears to the bleeding end. So then there are freq differences and how much these different frq require to keep up.

Is this so? Does the wattage dictate the volume level I'm gonna get? And does this in turn convert to dB level I can measure from a specific distance or my seating position??

Is there a break down of the freq of high, mid high, to mid, mid bass, to bass, sub that we can look at in a "ratio" manner to say that i.e. If you have a 100watt 6.5 mid bass, your 3-4" mid should be about X, and your tweeter in the 2500+freq or something should be X wattage to keep up with the system?

Is there a rule of thumb to have this relation?


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## Alrojoca (Oct 5, 2012)

When it comes to output levels, maybe only approximate estimates.

I Would think that if we had anything like that, there should be also a formula or calculation to determine the result after you provide your cabin size, listening position from the drivers and angles they point at when they are installed.

There are too many things to deal with that play a roll, including deadening, reflections and who knows what else. Based on driver size, what they are, and specs, we know a range to play them, no rules for power other than get as much as you can get or enough based on the numbers we know


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## Phil Indeblanc (May 27, 2015)

yes, that is what I'm asking for some, approximation. of course maybe an average to cars placement to driver position I would roughly say 2.75 ft. and yes, maybe the deadening would be a factor enough to average out perhaps the 30-45% ? or something to round off the estimate.


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## Seraphim38 (Aug 7, 2015)

NO, RMS is root mean square, in a reference to the measurements of wattage capability applied to a system, whereas dB is decibels, being a measurement of sound pressure level (SPL), or the result of the movement of a mechanical system that alters air pressure in plus/minus oscillations.

The amount of volume (sound pressure level, SPL measured in dB) that is created from a system is determined by a variety of factors, not just the wattage consumed by the system. These variables include the efficiency of the drive motors, loss in cabling, frequency bandwidth, as well as the amount of space that you are trying to move. You can put a boom box in a car and make a lot of noise, but that doesn't mean that it will sound good. 

Also, different frequencies require different amounts of power to reproduce. If you put a 100 watt RMS amp up to a tweeter, it can be extremely loud, but 100 watts to a 15" motor might not be as loud. 

Put simply, there is no way to calculate resultant volume based off just system power. Just because the physics is challenging, I once attempted an install where the object was to get system resonance from 2 12 inch woofers at 18Hz with just an 80 watt Alpine amplifier. Result: I blew out my windshield seal with that setup. People think that you need 1,000 watts or 2,000 watts to get loud, or get low, but often times that is just to make up for inefficient designs.


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## Alrojoca (Oct 5, 2012)

It's a good question, I'm just not sure how accurate it could be if ever developed because I doubt it exists or could be developed as a tool, maybe just in a case by case using a cobination of calculations that not the average guy with basic college math can do.

And it may not be something that will reinvent the wheel either, with the sound processors and current tools available. A good DSP has powerful tools to get good results.

I'm sure Maybe some of the engineers may have better answers, Andy M or PB, Erin. My mind simply says, tune with what you have, ha ha


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## DDfusion (Apr 23, 2015)

Simple answer. NO


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## squeak9798 (Apr 20, 2005)

You can calculate theoretical maximum SPL based on power handling or power input, frequency in question, sensitivity/efficiency, excursion/displacement limitations, etc. So in theory you can do what you are asking. It won't take into consideration the environment but gives you the ability to compare (in theory) different driver capabilities compared to your goals or limitations. It also does not take into account real world issues such as power compression, distortion attributes, etc. So while no you can't get 100% real world definitive answers without extremely intensive measurements and models it is possible to "gestimate" or approximate.

I'm not arguing for or against this type of theoretical analysis, just mentioning what is theoretically possible and briefly mentioned just a few of the issues. Whether or not it's needed or a good idea is a whole different conversations.


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## Seraphim38 (Aug 7, 2015)

Let me put this another way:

I work every day in an office with a loudspeaker design team sitting outside my door. That team includes mechanical engineers, acoustic engineers, PCB layout electrical engineers, firmware coders and software designers. 

We have software systems that model out the volume in a space based on loudspeaker system/driver arrays. However, these tools were designed for our various speaker systems, to work with our DSP engine to create digitally steerable line arrays. Basically, a computer model simulates the interaction between different point sources and uses modulation of degrees in polarity to alter the directionality and nodal patterns in a 3 dimensional space, without physically changing the orientation of the drivers. If that topic interests you, read up on these digitally steerable arrays: EAW's Anya or Martin Audio MLA. 

For those who are lazy, try this youtube link: https://www.youtube.com/watch?v=yioEgV0kmE8

Sorry if that was too technical. The answer is yes, it is possible to create such a tool, but they are extremely complex, and I can't imagine someone would create it with inherent accommodation for multiple manufacturers of drivers, and with all the different cars on the road. 

Put simply, there is no easy way that you can create a simple software tool that will predict the loudness (dB output) of a system in a car with only a few basic parameter inputs. Sure, there are simple tools that can predict frequency response curves of simple, single driver systems (box modeling software).


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## DDfusion (Apr 23, 2015)

Way to many varibles to get remotely close to any accurate measurement. It just don't work that way. You can't predict how the vehicle will act as SPL increases. You can't predict power compression. Many things you can't predict. 

Rockford puts SPL numbers on their woofers, I want to punch them in the face. Marketing machines.


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## Phil Indeblanc (May 27, 2015)

These are great responses. All of them as they are all valid.



> Also, different frequencies require different amounts of power to reproduce. If you put a 100 watt RMS amp up to a tweeter, it can be extremely loud, but 100 watts to a 15" motor might not be as loud.


The freq are also a break down I was hoping to weigh in on the appraoch to driver selection.

If this can be approximated/"gestimate" I think it can a good indication for wattage.

But even using the most general assumptions, like the car space as a medium size sedan, then working with non-commercialized components, those with measures that give usable hz, and 90+dB of the 2.83v measure, and so on. These SHOULD help....at least I thought should be good info to adapt the "TYPICAL"
6.5" in door, 3-4" in dash, 1" in sail, 8"-12" (this with a few variables, which the sub is the least of concern in the mix of things) configuration.



Very interesting of some of the insights you guys have. Some cool situations!


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## jode1967 (Nov 7, 2012)

simple answer is yes, with caveats. It would have to be in the same enviroment. every car will have its own nulls and peaks and cabin gain, if you measure a cars volume with given gear and measure db's and power input. at that same power input it would equate to the same db's on the same part a the same song


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## 14642 (May 19, 2008)

So long as the speaker manufacturer is providing accurate sensitivity numbers, yes. 

Outside the car (measured anechoically, which is how speakers are measured --or should be), the difference between 87dB 1W/1M and 90dB 1W/1M indicates that half the power is required to make the 90dB speaker play at the same output level as the 97dB speaker. 

Transducer engineers often rate speakers using voltage rather than power. Those would be something like 2.83V/1M. They do this because most amplifiers are voltage sources and adjusting the DC resistance of the speaker is one way to match the sensitivity of speakers that will be driven by the same amp. For voltage ratings, 6dB is equal to double the voltage.

2.83V is one watt for an 8 ohm driver. If you need to convert to 1W/1M, then power is voltage squared divided by resistance.


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## gstokes (Apr 20, 2014)

I'm going to say no based on the variables in speaker sensitivity..


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