newbie: 2 cabinets = more volume?

[cordura21]

hi guys. I have a Trace Elliot combo with 2 speaker outputs. It has 150w. If I add a second cabinet to the second output, will it be louder? Is there a difference in volume between 1 and 2 speakers, even if the amp total power is still the same? Why?

Sorry for my lack of knowledge and thanks in advance for all the help.

Cheers, Andrés

 

[Khompewtur]

Here's a neat article i found from links on this site regarding decibels.

Article about decibels

In this case would this follow the rules?

Combining two different sounds of the same level results in an increase of 3 dB.

The smaller the difference in level between two sounds, the greater the increase in their combined SPL will be (to a maximum of 3 dB).

The greater the difference in level between two sounds, the smaller the increase in their combined SPL will be.

 

[guhlenn]

correct...

to answer your question; yes but not noticeably... although 3 db in theory is quite a lot... the decibel thing is really confusing sometimes...

guhlenn

 

[twist]

Well, two cabinets can handle more power than one, so you can now turn volume up higher on the amp, so yes, then it will be louder.

Twist

 

[cordura21]

that sounds right.
But my reasoning is the following:
since it's a combo, let's suppose that the speaker inside it can already handle the 150w (I hope so).
What you say seems better if I had, say, to 75w speakers. But I'll hope this one inside is matched to the preamp that it's wired at.
See where I am going to? Am I very wrong?

 

[guhlenn]

a speaker is never wired with a preamp...

and a combo always has a speaker that can handle the power the POWERAMP can produce.

and twist, if i'm not mistaking adding an extra speaker to a combo is in parallel fashion, therefor the combination cannot take more power. you would have to rewire the speakers to act in series in order to get your scenario... i think...

guhlenn

 

[guhlenn]

ok i thought about it some more...

preamp -> poweramp

poweramp has two outputs to speaker, one is being used for combo speaker.

then both outputs will be 150 watts at a certain ohmage...

adding a speaker will increase volume ONLY by 3db by the above mentioned physics...


guhlenn

 

[twist]

Yeah, I think I understand what you're saying, but.....

to clarify my train of thought, according to Ohms Law, with two speakers in parallel, the power would be divided between the two.

If you have a single speaker capable of handling 50 watts, and you add another 50 watt speaker in parallel, you now have a load capable of safely handling 100 watts. that doesn't automatically make it louder, but you could turn the volume up thus delivering more power to your load (speakers).

Twist

 

[guhlenn]

Yeah, I think I understand what you're saying, but.....

to clarify my train of thought, according to Ohms Law, with two speakers in parallel, the power would be divided between the two.

no. the ohmage would drop ( 1/ohmage a + 1/ohmage b = 1/ohmage total = ohm's law in parallell chain)


the power is divided between the two when used in a serial loop (don't know the exact term here, bare with me) but the ohmage will increase times 2

guhlenn

 

[cordura21]

I think you're right, guhlenn.

In a paralel circuit the voltage remains constant, and the resistance diminishes by the equation you wrote , so the current has to go up (v=c/r). So if the power equation is p=v x c, then you have more power since v is the same and c is greater.

In a serial one, I think that you have to sum the resistances, and the current remains equal. So in that case V is lower, due to the greater resistance.

This amp has two speaker outputs, each labeled @ 8 ohms. I am not sure about this, but I'll guess that those two outputs are serial, and not the outputs of 2 different amps.

Does this sound ok to you? Please correct me if I'm wrong, cause I don't know much. Cheers.

 

[guhlenn]

the power is a constant as in the power that the amp delivers. lowering the resistance will have an effect but not increasing volume much. people rather tend to describe it in terms of "the sound gets looser or tighter..."

i don't think it is a series out put however, i'm guessing here cause i'm no technician but i think the transformer just gives two outputs of 150 watts at 8 ohm. just stick to that type of speaker and you'll be fine... after all it's made to do just that!

guhlenn

 

[cordura21]

I'm gonna ask the guys at Trace Elliot. If what you say is right, then buying an extra cabinet will turn it into a 300w amp (150w per speaker output). That'll be great, and much better than buying a new 300w amp.
Although I don't think it will work, I have the feeling that it's a parallel output for the same 150w. Who knows...

 

[ausrock]

Two 150watt outputs doesn't equate to a 300 watt amp. As others have said you may get a slight db increase but the most obvious benefit is that two speakers will be "pushing" twice the volume of air as one speaker (so long as they are in phase)........the end result being better projection and audability.

ChrisO

 

[Tekker]

I have a feeling that it's going to be 150 watt max (but of course I could be wrong as I've only had one college semester of electronics so far ). The outputs "should" be in parallel and although it is true that voltage is constant in a parallel circuit (so if we were dealing with a 150 volt amp then yes it would double) but the power is going to be devided between the two speakers. Therefore each speaker will receive 75 watts.

However like ausrock said, both speakers will be pushing air, which will result in a louder sound.

But here's a little experiment you can try:
Go over to your stereo and unplug one of your speakers, obvserve the level. Then plug it back in, notice a difference?


3dB doubles the volume. So if you had 5 watts, 3db more would be 10 watts and if you had 1,000 watts, 3dB more would be 2,000 watts.

-tkr

(This would be a good question for Skippy )

 

[cordura21]

why just + 3 dB ? In the Article about decibels at Prorec, it says that it holds true if the sounds are different. In this case, however, it's the same sound. So I think you can get a 6dB increase (if they are in phase). Or not?

I also think that the speaker outputs are in parallel, I don't believe that there is another "hidden" 150w just waiting for me to plug another cabinet.

I think in stereos speakers it's a different story, cause they have a per-channel power specification, something like 300w+300w. This amp (Trace Elliot GP7-SM) has two speaker outputs, each one has an 8 Ohm sign, and the 150W sign is above both of them. So I think that means that it has 150W overall.

I still don't get why will two speakers push more air than one, if that one is capable of pushing the whole amp power rating. I mean, I can understand it if it is parallel wire and then resistance is lowered, but if it is not the case then why would that happen?

Sorry guys for my lack of knowledge, I am getting a little embarassed.

Cheers, Andrés

 

[Harvey Gerst]

Lemme see if I can clear some of this up:

Adding speakers increases the output in several ways:

The first is called "mutual coupling" (better coupling to the air - the added speaker cone area couples more efficient;y to the air in the room). Assume the amplifier is designed to deliver maximum output into an 8 ohm load. Replacing the single 8 ohm speaker with two 16 ohm speakers in parallel (or two 4 ohm speakers in series) decreases the power to each speaker but doubles the radiating area, effectively doubling the output (i.e., the sound level goes up 3 dB).

Going to four 2 ohm speakers in series, four 8 ohm speakers in series/parallel, or four 32 ohm speakers in parallel again decreases the power to each speaker but doubles the radiating area, effectively doubling the output again (i.e., the sound level goes up another 3 dB compared to the 2 speaker level).

The second method involves solid state amplifiers. While the amplifier may be designed for most stable output at 8 ohms, lowering the impedence of the load can also increase the available wattage. You will often see amplifiers labeled as:

100 watts@8 ohms,
200 watts@4 ohms,
400 watts@2 ohms.

The second output jack on most solid state amplifiers is usually just wired in parallel with the first output jack. If you have an 9 ohm load on the first jack, and you plug in a second 8 ohm load, the load is in parallel, you double the amplifiers output, but split it between two speakers, so the net gain is about 6 dB (3 dB from acoustic "mutual coupling", and 3 dB from doubling the actual power output).

Any of this make sense now?

 

[cordura21]

I didn't know a thing about "mutual coupling". Is it like if you fill a box with water and you hit one side with one hand, it will make a wave. But if you hit it with both hands with the same strength, the wave will be bigger, because it was hitted more efficiently? Something like that?

I suppose that it is not just a matter of adding speakers, but also the distance between them and their positioning in the room. I'm sure there will be a relationship between the separation and the size of the room or the wavelength that you want to send.

This seems to be a good way to upgrade my rig onstage, instead of bigger amp I-ll buy another cabinet.

Is there any good link to read about "acoustic coupling"?

PS: there's something I didn't understand on the configuration examples. I thought 4 speakers at 8 ohms in series or parallel were a different ohmage, 32 and 2 respectively. I'm not good at math, so maybe I am doing something wrong here, or maybe you tried to change the load in the example to combine both coupling and resistance change.

Thanks, it's a great explanation.
Cheers, Andrés from Argentina

 

[Harvey Gerst]

Andrés,

Think of "Mutual coupling as poking a big bale of cotton with a stick. The air in the room is the cotton, and the speaker is the stick. The object is to try and move the whole cotton bale, but the stick simply pokes into the cotton and it doesn't really move the bale very well. Two sticks work better, and four sticks work better yet. The added speaker cone area couples to the air in the room better.

The four 8 ohm speakers are wired in a combination of series and parallel connections.

Two of the speakers are wired in parallel to drop the resistance to four ohms total. The second two speakers get the same treatment.

You now have two sets of two speakers, wired for 4 ohms total (each set). You then wire these two 4 ohm sets in series to get back to the original 8 ohm load.

 

[cordura21]

thanks, Harvey. Now I get it.

 

[pchorman]

love those analogies, especially the one describing the cardioid pattern as a balloon pressed up hard against a microphone over on the mic forum. it doesn't get any more vivid than a picture painted by Harvey.

pardon this interjection but I think the term "mutual coupling" refers to the amount of leakage from one radiator into those nearby, like what occurs with radiating elements of a phased array antenna. one element couples energy into its neighbors resulting in higher SWR or increased reverse travelling power. same goes with speakers even though we're dealing with longitudinal waves rather than transverse electromagnetic waves.

believe that and I'll tell you another.