Another Watts to Volume level question

First of all mixing random dissimilar cabs is not a good recipe for outstanding performance. It could be a polarity problem, or it could simply be the phase and group delay characteristics of the two cabs are so different they don't work well together. Your best results, IMHO, will be pairing identical cabs.

That's not to say you can't get good results mixing cabs, but the odds are lower unless the dissimilar cabs are specifically designed to work well together like the Mesa Subway series.

The next thing to consider is the sensitivity rating. If the two cabs have significantly different sensitivity ratings there will not be much benefit in pairing them.

You also need to evaluate the amp's power and speaker sensitivity when you are considering the benefits of using one or two speakers. When you double power with one speaker you get 3dB. If you can double the power and also add a second identical speaker you get ~6dB from acoustic coupling. This acoustic coupling only occurs within the frequency range where the drivers are within about 1/4 wavelength.

So you need to consider how much power your amp makes at 4 ohms and the sensitivity rating of your 4 ohm speaker. Sensitivity typically tells you how many dB a speaker makes with 1 watt. Since you know the speaker will make an additional 3dB each time the power is doubled, you can figure out approximately how loud the speaker will be with the available power. For example say the amp makes 200W at 8 ohm and the speaker is rated 95dB 1w/1m. 1,2,4,8,16,32,64,128,256 Count the number of time the power can be doubled (7) and multiply by 3. 7x3=21. Add this to 95. 95+21=116dB. You can't quite double the power a 8th time but it's fairly close, so you could add an estimated extra 2dB. 116+2=118dB.

Then you need to consider how much power your amp makes at 4 ohms and the sensitivity rating of your two identical 8 ohm speakers. For example say your amp makes 300W at 4 ohms and and your speakers are rated at 96dB. Since you have two speakers 300W will be divided evenly between them. 300/2=150W. 1,2,4,8,16,32,64,128,256. Count the number of time the power can be double (7) and multiply by 3. 7x3=21. Add this to 96. 96+21=117dB. You can just barely double the power 7 times. 150W is actually 21.8dB above 1W, so not even an extra dB * (see note below)

So each speaker will make approximately 117dB with 150W. Now for the frequencies where near perfect coupling occurs you get ~+6dB. 117+6=123dB.

So with one 4 ohm cab the rig makes 118dB and with two 8 ohm cabs the rig makes 123dB.


* Note. I used this calculator where 1W = 30dBm and 150W = 51.76dBm. The dB difference is 21.8 (rounded up) 51.76-30=~21.8. Watts to dBm conversion calculator


When comparing the sensitivity rating you need to make sure you are comparing apples to apples. Some drivers use a voltage rating instead of a wattage rating. For example the sensitivity may be expressed as 2.83V/1m which will equate to 1W at 8 ohms or 2W at 4 ohms. In order to compare you need to convert all drivers to a 1w/1m sensitivity rating. Easy enough to do since we know reducing the power by half will reduce the SPL by 3dB.

 

You also could have your cabs out of phase.

 

Sensitivity rating is provided by the manufacturer and should be on the spec sheet for the cab.

 

The possibility that there is something wrong with the 210 (or the speaker cable) should not be ignored.

Play the 115, at a good volume. Now, changing nothing, add the 210. The volume should get significantly louder as well as the low frequency response should get deeper and rounder sounding. If this doesn't happen, that's a key symptom of something being wrong. Either polarity of the 210 cabinet, polarity of the cable to the 210, or polarity of one of the drivers in the 210 or possibly a bad 10" driver in the 210 cabinet.

There is also s slight possibility that something is damaged in the amp, but not as likely.

 

Note that IME, neither of those sensitivity numbers are average sensitivity based on 1W RMS over a 50Hz - 5khz bandwidth. It's not likely that sensitivity is the root cause.

Are you 100% sure that neither cabinet is 4 ohms, and that they haven't been monkeyed with? Follow my troubleshooting procedure, It's a proven way to get to the bottom of what is causing this.

 

Yes, polarity issues can be easily corrected once it's identified. It could be a reverse wired cable, or issues within the cabinet.

 

What does this mean?

 

Which cabinet? Did that cabinet do this with both cables?

 

You mentioned one of the cabs is second hand. You might want to open it up and make sure it has stock speakers and the ohmage match what the cab says.

 

BTW if you have a multimeter that does resistance you can do an approximate check for impedance. The DC resistance of a cab will be significantly below the impedance, but rarely as low as the next value down, eg an 8 ohm cab will normally have DC resistance somewhere between 5 and 7 ohm. If you get under 4 ohms you are almost certainly looking at a 4ohm cab.

 

wait what?

sounds like you had bigger problems than mixing cabs!

to echo @agedhorse, adding the second cab even if it wasn't an ideal match should still make the rig louder and bigger sounding, if not then something wasn't working right

which is why little 4Ω cabs are usually a bad idea, despite "getting all the watts" they're still not loud enough but now you can't add another one.

 

You need to check for intermittent connections.