Sensitivity?

[MDS]

Sooo....how does one go about calculating sensitivity?

You have to have an anechoic chamber and a lot of expensive equipment...

If you have two speakers, one 4 ohm and one 8 ohm and both listed as x dB/1w/1M then subtract 3 dB from the 4 ohm rating to get the equivalent 8 ohm rating. That, I believe, is what mtyrcrafts was referring to for 'doing the calculations'.

 

[zumbo]

Mac said 87@4 = 84@8

jgarcia said, using my specs, 89@4 = 89@8 (same spl)

MDS agrees with MAC.

So, going with Mac and MDS, if the two speakers in question are 89@4 and 89@8, the 8ohm speaker will be louder.

I just don't agree with either of these.

I believe the 4ohm speaker will be louder at the same volume level if both are rated @ 89dB.

Anyone else care to give an opinion?

 

[Jack Hammer]

I believe the 4ohm speaker will be louder at the same volume level if both are rated @ 89dB.

Anyone else care to give an opinion?

My 4 Ohm speakers, which replaced 8 Ohm speakers, play louder at the same volume. However, it is not an apples to apples comparison. The 8ohm speakers were much, much smaller than the 4ohm ones.

I have some speakers from the same series that are similar in size and are from the same manufacturer, but they're still not the same model, close though. One is a 4 ohm and the other is an 8 ohm speaker. Sometime this week, I'll make some time and play with them and measure spl's at the same distance with the same test tone and see if there is a difference, and if so, which is louder.

Of course, this really won't be definative, it'll just show how those two are. But I'm curious now to see for myself if there is much of a difference.

 

[j_garcia]

So, going with Mac and MDS, if the two speakers in question are 89@4 and 89@8, the 8ohm speaker will be louder.

If both of them measure 89dB, then they are producing exactly the same SPL. Period, end of line. What that doesn't tell you, is how much current they are consuming, but that doesn't factor into the sensitivity measurement. That measurement is how loud they will actually play with a known input, which is why it is the standard measurement. So if you switch from one to the other on an amp that can handle the 4 Ohm load, theoretically they should produce the same SPL in room.

 

[Buckeyefan 1]

Sensitivity isn't an end all. Keep in mind this is 1 watt at 1 meter, or about 3 feet away. Now perform a test at 10 watts at 5 meters, and you'll notice a difference IF the amp isn't stable at 4 ohms. If it is, the speakers will still require more current and crap out sooner compared to driving 8 ohm speakers. It's a matter of distance and volume. Also keep in mind larger drivers are usually more sensitive, all other things equal.

With 1 watt (all receivers can do this into very low loads)
Ex 1: 92dB 4 Ohm speakers = 89dB 8 Ohm speakers
Ex 2: 89dB 4 OHm speakers = 86dB 8 Ohm speakers
Those speakers should have the same output at 1 meter.

 

[j_garcia]

If you assume anechoic chamber for measurement so there is no reinforcement from the room, then the measurement would have account for that distance and should still yeild the same results (theoretically).

I found this at Secrets:

Some speakers have a nominal impedance of 4 Ohms, and others, 6. This is important, because, as the impedance drops, the demand for current to be delivered by the amplifier increases for a specific output level. While the current goes up, the voltage stays the same, and the power requirements at the same signal level go up. This is defined by "Ohm's Law" which states that E = I R, or Voltage (Volts) = Current (Amps) x Resistance (Ohms), and the Power Function which states that P = E I, or Power (Watts) = Voltage (Volts) x Current (Amps).

You can see from Ohm's Law that, for a specific amount of voltage, the current will go up if the resistance drops. So, if you change from an 8 Ohm speaker to a 4 Ohm speaker, the current doubles when the voltage is held constant. Then, by the power function, if you want to deliver 100 watts to the 4 Ohm speakers as you did when you had 8 Ohm speakers, the voltage would drop by a factor of Voltage/1.414, while the current would increase by a factor of Current x 1.414, in order for the power to remain at 100 watts. Therefore, if you choose a low impedance speaker (4 Ohms nominal), make sure that your amplifier can supply the current that will be necessary to drive the speaker at high listening levels. On a side note, a since a 4 ohm speaker will draw twice as much power as an 8 ohm speaker at the same signal (voltage) level, even if the efficiency is the same, the sensitivity increases by a factor of 3 dB, so that in a back to back comparison, the 4 ohm speaker may seem more efficient, even though it's simply just drawing more power.

So, I believe I am mistaken, and Zumbo's thoughts were correct, and that would also seem to confirm what Mac said. There is a 3dB gain for the same sensitivity going from a 4 Ohm speaker to an 8 Ohm with the amp at the same level. It also said you should use sensitivity as a guideline, since impedance varies with frequency, so the overall capability of a speaker is not completely defined by it's sensitivity rating.

I had to turn up my system slightly when I went from 4 Ohm to 8 Ohm with just the receiver (which says 6 Ohm min), but when I added my amps, which are 4 Ohm stable, I had to turn it back down a few dB also.

 

[zumbo]

Now we are getting somewhere. I look at it like this.

You hook the 8ohm speaker to the 200w amp, 100w at half volume (theoratically).

Now hook the 4ohm speaker to the 200w amp that becomes 350w, so 175w at half volume (theoratically).

Now factor in the sensitivity rating of 89dB for each speaker, and it becomes appearant to me the 4ohm speaker is (theoratically) 75% louder, once again given the amp is 4ohm stable.

 

[j_garcia]

I suppose so, but for that 75% increase in volume, you are also using a 150% increse in amplifier power, which yeilds both more heat byproduct and leaves you with far less headroom. This is assuming the amp can deliver 350w of power at 4 Ohms also.

 

[zumbo]

As far as the power, I am just taking a manufacturer at their specs. There are big $$ amps that double their power.

My amp only yields 175w@4ohm, and 125@8ohm. So a 4ohm speaker on this amp only yields a 40% gain (theoratically).

 

[zumbo]

I suppose so, but for that 75% increase in volume, you are also using a 150% increse in amplifier power, which yeilds both more heat byproduct and leaves you with far less headroom. This is assuming the amp can deliver 350w of power at 4 Ohms also.

So, now the big question. Given the statement above "far less headroom" and "more heat", wouldn't there be alot to gain bi-amping with bi-amp capable 4ohm speakers?

 

[j_garcia]

My monoblocks are about the same - 125w @ 8 Ohms and 180w @ 4 Ohms. Not all amps "double down", but that doesn't mean they aren't good amps. 180w is plenty for my speakers

So, now the big question. Given the statement above "far less headroom" and "more heat", wouldn't there be alot to gain bi-amping with bi-amp capable 4ohm speakers?

More or less, yes. That headroom is definitely where I see biamping being a benefit. The total impedance most likely goes up when you biamp, so you aren't going to end up with two 4 Ohm loads, each will likely be higher impedance and each are already easier to drive because each amp is doing less work.

 

[Eric Apple]

Here's what I meant by louder

What I meant when I said the 4 ohm speaker will be louder in this case, is that the amplifier stated in the first post will supply more power to the 4 ohm speakers.

Since the 4 ohm speakers are the same sensitivity of the 8 ohm speakers, and the 4 ohm speakers will be supplied more power, the 4 ohm speakers will play louder.

P.S. Thought I'd edit this post and just add in the formula for what "louder" is in this case. 10log(350/200)=2.4db=not much louder

 

[zumbo]

What I meant when I said the 4 ohm speaker will be louder in this case, is that the amplifier stated in the first post will supply more power to the 4 ohm speakers.

Since the 4 ohm speakers are the same sensitivity of the 8 ohm speakers, and the 4 ohm speakers will be supplied more power, the 4 ohm speakers will play louder.

Yep. That's the point I got from your statement. And, I agree.

 

[zumbo]

More or less, yes. That headroom is definitely where I see biamping being a benefit. The total impedance most likely goes up when you biamp, so you aren't going to end up with two 4 Ohm loads, each will likely be higher impedance and each are already easier to drive because each amp is doing less work.

That is what I was thinking.

http://i19.photobucket.com/albums/b152/tpc3416/9-4-05041.jpg

http://i19.photobucket.com/albums/b152/tpc3416/9-4-05042.jpg

http://i19.photobucket.com/albums/b152/tpc3416/9-4-05040.jpg

 

[j_garcia]

That is what I was thinking.

That is what I would expect with the typical speaker. They are wired in parallel so you get a 4 Ohm load when they are connected with the jumper and two 8 Ohm loads individually. Thus, when biamping, you are LOSING 3dB, but gain a lot of headroom for each portion that is being powered. I went with monoblocks for this reason. My speakers are not biwirable, but each speaker has it's own power supply that is sharing no load for any other speaker.

 

[zumbo]

Now, lets take it farther.

With my amp, it's 175@4ohm and 125@8ohm.

So, bi-amped would be 250w. A 42.8% increase. (theoratically)

So, a less expensive amp bi-amped on 4ohm speakers looks like a good thing.

A more expensive amp rates 200@4 and 125@8. So, bi-amping with this amp only yields a 25% increase.

 

[Jack Hammer]

Oh maaannnn...

...I think I'm getting a headache.

It all seemed so simple in the beginning: buy a better reciever and maybe a set of front speakers...
...that way movies will sound a little better...

...Oh, my poor aching wallet, uh head I mean...

 

[mtrycrafts]

Thanks. I couldn't remember which thread it was in.

Me neither but learned it from other sources some time back.

 

[mtrycrafts]

You have to have an anechoic chamber and a lot of expensive equipment...

If you have two speakers, one 4 ohm and one 8 ohm and both listed as x dB/1w/1M then subtract 3 dB from the 4 ohm rating to get the equivalent 8 ohm rating. That, I believe, is what mtyrcrafts was referring to for 'doing the calculations'.

You have to be careful how the sensitivity is published. If it specifies 1 watt for a 4 ohm speaker, then, that is it but the applied voltage would be 2. and a small fraction volts, not 2.83V. If the rating is specified at 2.83V for both a 4 ohm and 8 ohm speaker, then the 4 ohm speaker is getting 2 watts and that is when that 3dB is subtracted.

 

[mtrycrafts]

So, going with Mac and MDS, if the two speakers in question are 89@4 and 89@8, the 8ohm speaker will be louder.

I believe the 4ohm speaker will be louder at the same volume level if both are rated @ 89dB.

Anyone else care to give an opinion?

Well, if the fixed position on a volume setting delivers 2.83V at a speaker terminal, then, depending on sensitivity what a 4 ohm or 8 ohm speaker will put out. If both speakers are rated with the X dB, 2.83V/1m, if both speakers will have 89dB spl at 2.83V, they will sound the same volume level as you hear the same spl.
BUT, one speaker will have 2 watts, the other will have 1 watt applied.
The amps capability into 4 ohm load will determine when that 4 ohm speaker will run out of gas If the amp doubles in power to a 4 ohm load, they will sound the same in the end at a given volume setting. If less, it will run out of gas before the 8 ohm speaker.