Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Ivan Beaver]

When I stated that all things were equal, I meant that the Cabinets were the same, Rating the same and Amp Output the same and both speakers doing the same amount of work. It is not a real life scenario but it can very well be.
I will explain why and the reason for my asking the original question.  I have noticed that many Speaker/Enclosure Manufacturers are now promoting Wattage instead of individual specs like they used to do. Some are also touting 4ohm impedance instead of  what used to be the norm of 8ohms. Why are they doing this, is it because the larger numbers would attract these new flock of DJs, buyers etc? Is there some real merit or explanation to it?

The impedance has nothing to do with the max output-but rather the sensitivity of a speaker system to a particular voltage.

One reason 4 ohms is becoming more popular is that amps have changed and amps that are stable at 2 ohms is more common than it used to be.

It also allows you to get more "power" out of an amp-this can be an issue with the higher power rated speakers.

Wattage is one of the last things you should be looking at (although it is the only thing that some people "think"-which they DON'T-they know).

Things like actual MEASURED freq response and sensitivity (based on that measurement-NOT some unusable peak in the response) are far more telling than a simple "wattage" number-that could be VERY VERY VERY wrong and totally misleading and inaccurate.

Just look at the cheap DJ or car audio gear if you want to see some stupid ratings that are totally wrong.

They ARE NOT the same as used by real companies providing real gear and are simply there to fool the buyer into a purchase.  Put them against some real pro gear and see what they actually do.  NOWHERE near the published specs.

[Phil Graham]

Canute,

There are two systems at play in a loudspeaker, the mechanical system (i.e. cone mass/area, spider, suspension, former mass, air load, etc.) and the electrical system (i.e. magnet, voice coil windings, magnet return structure). For the sake of looking at your question, we are going to ignore the mechanical system and its interplay with the electrical system.

The force applied to the cone by the motor structure is:

F = I*l x B  or  Force equals Current * Wire length in magnetic field x Magnetic field strength  [1]

Here the "x" means the "cross product" but you can think of it as multiplication (ie. "*") for the case of speaker.

---

Ohm's law is:

V = I*Z  or  Voltage equals Current * Impedance  [2]

---

Re-arrange Ohm's law [2] and insert in [1] gives

F = (V/Z)*l x B  [3]

---

Now our pro audio amplifiers are what as known as "voltage sources." The functional result of this is that V in [3] is a constant if you are swapping three different speakers to the same amp. Similarly, for your "all things being equal" case, the magnet strength (B) and length (l) are the same. This means that only variable left to control force is "Z" or the loudspeaker impedance.

Now "Z" for real drivers is a complicated quantity, it is frequency dependent, dependent on the temperature of the voice coil, and dependent on the excursion of the driver. For the sake of education, though, let's assume that the driver impedance is a single value for the whole driver operating bandwidth.

---

For this simplified case, and subbing in all the other values as constants:

Force = 1/Z  [4]

This [Z] indicates as Z increases, the force applied to the cone from a constant voltage source will decrease. This represents the answer to your hypothetical case. But your hypothetical case is not all of reality, as we shall discuss below:

---

Now, for real drivers, we see some variation of the sensitivity, which is proportional to the force. There is the variation between 4, 8, and 16 Ohm nominal impedances by [4] for constant voltage input. There is also some additional variation in the motor strength of each of the different impedances. Why is this?

The resistance of the wire that is wound around the voice coil former is influenced by controlling the wire's cross-sectional area. Higher resistances lead to higher impedances in the finished driver. In the space surrounding the voice coil former, there is a given amount of volume to contain voice coil wire. In a given volume for wire, one can fit a longer length of wire with a smaller cross-section in the same volume. Thus "l" in [1] is changing for the different nominal impedances.

Because the voice coil wire has to physically fit in the magnet structure's voice coil gap width, and the wire cannot be a fractional cross-section, one of the three impedances typically will have the highest overall "B*l" product.

There are some subtleties for magnets and thermal behavior based on the effects of multiple layers of wire overlapping each other. There is also influence on the driver behavior based on the total mass of the wire on the voice coil. But overall the nominal driver impedance that enables the most wire to fit in the driver's the gap will exhibit the highest force output for a given input current.

[Steve M Smith]

Just look at the cheap DJ or car audio gear if you want to see some stupid ratings that are totally wrong.

Special numbers designed to appeal to sales people!

I can remember in the days before the internet, it was possible to buy lots of things from catalogues on hire purchase.  In the early 1980s they discovered that if hi-fi equipment was quoted in peak music power output rather than RMS, the figures looked much more impressive.


Steve.

[Ivan Beaver]

Special numbers designed to appeal to sales people!

I can remember in the days before the internet, it was possible to buy lots of things from catalogues on hire purchase.  In the early 1980s they discovered that if hi-fi equipment was quoted in peak music power output rather than RMS, the figures looked much more impressive.


Steve.

My favorite was the IPPO spec.  Instantanious Peak to Peak Output.

So if you took a speaker that was a real 50 watts @ 8ohms the RMS voltage would be 20 volts.  But if you figured the "peak" of that you have 28.28V.  Then if you double it for the Peak to Peak you have 56V.  THen if you figure the "Peak" is 6 db HIGHER (DOUBLE THE VOLTAGE) YOU HAVE 112v.  So 112 V into an 8 ohm load is 1568 watts.

Now THAT is a real power rating-it MUST get loud.  Yeah but it is still a 50 watt speaker-so matter what "math" they use.

But sadly- some of that same mentality is still at play today-even at the very top of the professional markets.

They just LOVE people who cannot do simple figuring on their own and are more than happy to take their money.

[Canute J. Chiverton]

This has been very enlightening guys. Great information! That is why you guys are leaders!

[Phil Graham]

This has been very enlightening guys. Great information! That is why you guys are leaders!

Canute,

You're welcome. Some simple math is often the best way to clarify concepts.

The use of wattage as a specification goes back a long way. From an engineering standpoint it has never made sense. Voltage sensitivity specifications make much more sense.

An apples to apples voltage sensitivity comparison would require precise definitions of a number of things, including:

• A fixed stimulus (e.g. EIA RS-426-B)
• A fixed RMS input voltage value
• A defined way to quantify the RMS voltage value, including time constants for measuring RMS and averaging
• Defined measurement condition (e.g. half space free field)
• Defined atmospheric conditions
• Defined pre-measurement conditioning period and stimulus for transducers
• Clear sensitivity definition, including time constants for RMS and averaging in similar fashion to measuring the amp output
• Clarity on what fraction(s) of input stimulus frequency range is used to integrate sensitivity values

To my personal knowledge, companies that do a good job of encompassing at least some of these concerns with good engineering practice currently include Community, Danley Sound Labs, and Fulcrum Acoustic. Dave Gunness from Fulcrum has at least one presentation on this topic.

As Ivan is fond of saying, when you dig deeply enough, it is not a simple thing to measure accurately and with uniformity. Even within a company's own product line, let alone across manufacturers.

[Steve M Smith]

As Ivan stated earlier, some of the claims from car audio manufacturers are obviously hyped up for sales too.

If you took some of the output power claims then worked out the current draw needed from a 12v battery, it would be flat in minutes.


Steve.

[Taylor Hall]

As Ivan stated earlier, some of the claims from car audio manufacturers are obviously hyped up for sales too.

If you took some of the output power claims then worked out the current draw needed from a 12v battery, it would be flat in minutes.


Steve.

That's where high output alternators and cap banks come into play. And while, yes, most manufacturer claims are quite ridiculous and not at all true, there are companies that properly rate (And sometimes) underrate their equipment.

[duane massey]

Back in the late 70's we would get a lot of clients who worshiped the "watts" god. We took a cinder block and mounted two speaker terminals on it, and then attached an anodized label that said "5,000 watt capability". Someone later took a marker and wrote "Bose" on it.

[Luke Geis]

It is my understanding that the higher the impedance the more closely the speaker will follow the incoming voltage. Something to do with it's resistance to electrical movement that it will track the incoming voltage with more accuracy? Can't find any immediate literature on it at the moment, but perhaps others can elaborate? 

[ProSoundWeb Community]