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You find speakers that read... 8" 200W , 10" 200W, 12" 200W. What is the difference in these speakers, same manufacturer. Or you find Subs that read, 15" 500W, 18" 500W,15" 1400W, 18" 300W,18" 800W. I'm confused. I don't have much experience with sound/audio.

I was looking at a powered speakers catalog here, http://www.soundz.co.za/c792-1/Powered-Speakers.aspx

What do the inches mean in terms of sound? What do the watts mean in terms of sound?

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Little tidbit: watts is another way of saying joules per second. –  Cole Johnson Sep 17 '13 at 13:03
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4 Answers

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When you have a measurement of inches referring to a speaker it is the diameter of that speaker, so a 12" 400W speaker is physically 12" wide and is rated at a maximum of 400 Watts.

The wattage is so you know how much power you can push through the speaker without damaging it.

  • More Watts = more volume

The physical size is very closely related to the sound, for example to get a good bass response you need to be able to physically move a lot of air - so the only way to do this is with a large speaker. If you want a high frequency response you need a small speaker cone that can be moved very fast. These two differing requirements are the reason you have bass, midrange and treble speakers in any hifi system, and you split the frequency ranges to the appropriate speakers.

  • More inches = better bass response (potentially)
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Thank You for a complete answer, The haze has been lifted of my eyes. If you can, may you please help me with this question as well avp.stackexchange.com/questions/8976/… –  Stoan Sep 15 '13 at 12:10
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As you still seem to wonder about differences in sound between different speaker sizes:

The speaker diameter is not directly related to how much bass you can get. Most important for that is the cabinet size: without a cabinet, for any speaker most of the air will just "flow around" from front to back vice versa, if given enough time; so some distance away you basically get only high-frequency ripples. It's the cabinet's job to prevent this, which roughly speaking requires that any free-air path from from to back of the speaker is longer than the wavelength of the sound you'd like to hear. In principle, you can then achieve a fine bass response by mounting an 8" speaker in a wardrobe-sized cabinet.

The reason this isn't usually done is that it's much harder to build a small speaker with high wattage than a larger one (for similar reasons to why it is easier to build a truck engine of a given power than it is to build a race car engine with the same nominal power); the numbers you've quoted reflect this. And since music has most of its energy in the bass range, you need a lot of power there, so you'd need a lot of small speakers. A large cabinet will fit a lot of small speakers, only, these together will generally be more expensive and heavier than a single large one.

Still, this route is actually taken for some applications; for instance, most electric bass players prefer their "refrigerators" with eight 10" speakers, rather than two 18" ones of the same power. And the reason is that small speakers themselves generally have a much more precise response: a large speaker takes some time to even get moving. It's just not able to produce high-frequency vibrations, too much mass. And the larger size means the cone is more flexible to vibrate in eigenresonances (much like a drum head), which affects the overall sound.

PA and HiFi systems solve this problem, as already mentioned, by splitting the signal in different frequency ranges. In the deep bass range, an 18" will generally work fine and cheaply provide the large power you need down there. Above somehing like 100 Hz, you switch over to perhaps 12" speakers with lower power, but more even response to get a clean mid range. And above 5 kHz again you switch to piezo drivers with comparedly tiny power, but very low mass and thus response easily reaching the audible limit.

In addition, you will nowadays typically use equilisation to make up for any remaining response unevenness.

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What an awesome answer,Thank You, the picture just got more clearer. If I understood correctly, in a PA system, it would be better to have a 18" Sub for the bass, a 12" for the mids and 10" or 8" for a tweeter, for the best vocal output? Please comment on my understanding, this can help me also answer my linked question. Thanks. –  Stoan Sep 17 '13 at 18:16
    
Those would be possible sizes, yes. For a small PA 12", 8" and 2" might also do the trick. –  leftaroundabout Sep 17 '13 at 19:50
    
2"? I haven't came across it, I guess this would be best for the highs? may you link me to any 2" you know? Thanks. –  Stoan Sep 18 '13 at 4:44
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@Stoan: it's just a made-up example. The point is that you can also make each component substantially smaller and still get a full, if quieter sound. — 2" is actually pretty big for a tweeter, for instance the Mackie 300z has a 12" for the midrange and 1.75" compression tweeter. Most professional boxes seem to have in fact 1" tweeters, possibly multiple of them. –  leftaroundabout Sep 18 '13 at 9:27
    
Point taken ;-) correct if I'm wrong, between a 8",10" and 12" mids, the main difference is the cone for the bass, the tweeter on average will be be the same size? –  Stoan Sep 18 '13 at 17:07
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Suggestive Specifications

Both the inches and the Watts are suggestive specifications of the following:

  • Inches - The approximate diameter of the bass driver cone (if there is more than one driver, like in multi-way design). It is an approximation since some designs spell 8'' for what is really 7.8''. This spec vaguely correlates to the lowest frequency the speaker can produce.
  • Watts - The power rating of a speaker/amp is a measure the safety power threshold of the device. Roughly correlates to the maximum output level it can produce.

The reason these are suggestive, is that they don't really tell us the important figures - they just suggest what these may be.

The Important Figures

In the spec sheets of a speaker you may find what you are really looking for, although these specifications can be manipulated by manufacturers to their advantage. This is either by means of the system used for measurements, or by the way information is presented. The specifications are:

Frequency Response

The frequency response shows the range of frequencies the speaker can produced. This figure is always a measure that involves some deviation from a mean within a particular window of error (whether the error is presented or not). Often we say that a speaker has "flat" frequency response within a specific range, where "flat" means a certain error is allowed (±3dB, for instance).

For example, a speaker may show a spec like this:

50Hz-24kHz, ±3dB

This means that across the quoted frequency range the level of all frequencies are never 3dB more or less a chosen level (alternatively all frequencies are within a 6dB level window).

The speaker will still produce frequencies below (and above) the quoted range, but these will gradually decrease in level and will be lower (very rarely higher) by 3dB from frequencies in the quoted range.

The way manufacturers cheat is that a speaker with:

50Hz-24kHz, ±3dB

response, is given as:

30Hz-25Hz, ±10dB

The specification is still valid, but 30Hz may be produced at a level 10 dB below, say 80Hz.

The error margin is typically:

  • ±3dB - High end and respectable manufacturers.
  • ±5dB - Acceptable error margin.
  • ±10dB - The manufacturer is likely to be cheating. Beefing their spec with a higher error margin.
  • No error margin - The manufacturer is likely to be cheating.

Maximum Sound Pressure Level (Max SPL)

Given in dBSPL, typically at 1 meter away from the speaker, this is an indicator of how loud the speaker can get.

For studio speakers, 100-120 dBSPL is normal. PA speakers are often louder than this.

Again, manufacturers can cheat here. For instance a peak measurement will be higher than an RMS one; it is common practice to show Max SPL in peak, but RMS correlates to our loudness perception more than peak. Also, manufacturers may perform tests using a half-space setup (speaker close to one wall), which will increase the figure by approximately 3dB; so unless the full test specifications are mentioned, it's hard to tell the exact performance or to compare it.

The Science

What is crucial to mention is that all loudspeakers are essentially a calculated compromise between various factors like:

  • Output Power
  • Spectral Linearity
  • Efficiency (how much input power translates into output power, with the rest converted into heat - either electronically or acoustically).
  • Accuracy - For instance, do bass frequencies at the output linger much or little longer than the bass frequencies at the input.

Watts

In simple terms, this is a measurement of how much work the speaker can perform - the more work, the more air mass it can displace and thus the louder the output will be.

But there are many other factors that will affect this. The class of the amplifier design affects the efficiency of the amplifier, thus with some designs you can get higher output levels with less input power.

Cone Size (Diameter)

To reproduce a particular frequency, all that a cone needs to do is oscillate at that frequency, regardless of its size.

Reproducing loud low frequency sounds requires large displacements of air mass. There are two principals ways to achieve this:

  • High Excursion - high amount of cone displacement (from its resting position).
  • Bigger Cone Size - bigger cones will move more air.

There are many tradeoffs between the two. A small cone with high excursion will require less power to move, but it will also exhibit high distortion level and low efficiency (due to reduced acoustic impedance) compared to a bigger cone with lower excursion.

It is important to notice that the task is to produce "Loud Low Frequencies" - so the cone size also relates to output levels. That's why iMax theatres employ momentous subs, whereas the dubbing theatres in which films are mixed employ smaller subs.

But this is an over-simplified explanation. Practically, each manufacturer tweaks these two variables together with many others, like magnet size, cone depth and the cabinet (enclosure) design itself. See this link for more.

But you can say that bigger cones allow a more efficient low-frequency reproduction.

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The dimension of inches more than likely relates to the smallest dimension of the speaker enclosure and basically if you want more power, you need something that is bigger. Here is a section from a PDF document for the XSP_A series you linked to: -

enter image description here

Other manufacturers may also refer to the diameter of the bass driver.

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I understand that inches are size of the speaker, what I want to understand, how is it connected to Watts, for example, 15" 500W Sub vs. 18" 500W sub, In terms of sound, how do these speakers differ? Will one sound louder? Will one have more bass? another example 10" 200W top vs. 12" 200W. –  Stoan Sep 15 '13 at 11:30
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@Stoan the power level refers to how much power you can safely put into the speaker from the amplifier and doesn't easily lend itself to understanding how much sound pressure level is being produced by the speaker. A speaker is a very inefficient device and unless you can compare SPLs you can't really tell from reading the documents. What I can say is that for any given speaker, if you double the power into it you get about 30% more perceived loudness. Ten times the power means doubling SPL. –  Andy aka Sep 15 '13 at 11:50
    
now it's all clear, Thank You. If you can, may you please help me with this question as well avp.stackexchange.com/questions/8976/… –  Stoan Sep 15 '13 at 12:07
    
@Stoan - I use Mackie speakers and would only consider other speakers after a proper listener test with my own ears. Sorry dude, I did read the question an hour or so ago but it's really impossible for me to answer without listening first. Good luck though and if someone makes an answer please add a comment here and I'll take a look to see if what they say makes sense. –  Andy aka Sep 15 '13 at 12:13
    
I understand, will do that. :-) –  Stoan Sep 15 '13 at 12:47
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