Replacement Speaker for a BA-115 (4 ohm instead of 8 ohm)?

Make sure you e-mail Ampeg and make sure it's okay and the amp won't fry.

There may not be enough cooling in the BA-115's amp to run with a four ohm load.

 

yeah 1st and formost make sure the amp can take a 4 ohm load

next up, assuming it can, you won't get much of a volume increase. even if the amp puts out twice the watts at 4 (it won't) that only gives you a 3 dB increase, which is just a lil' bit.

if you're lucky the amp will be able to take a 4 ohm load, have an internal 8 ohm speaker, AND have an extension speaker jack for an additional 8 ohm speaker cabinet. adding a second cabinet will give you much more of a volume increase than swapping a 4 ohm driver for the 8 ohm internally.

 

www.steelsound.com I got a really nice replacement 18 from them. might not be ampeg quallity, but it might also be exactly what you are looking for.

 

Wait...someone in another speaker discussion made the (undisputed) claim that an increase of 3dB was double the volume, i.e., that a speaker with sensitivity of 103dB would be twice as loud per watt as one with a sensitivity of 100dB.

If not, how many dB=2X the volume?

Thanks for any definitive answer or links.

 

I just looked and much to my surprise, you're right.

Four Ohm drivers are relatively rare, and I have read that there's some downside to them (I can't remember the specific assertion; if anyone does, I'd be grateful to hear it).

Curiously, my SWR "Black Beauty" combo also has an internal four Ohm 15" speaker.

Hmm.

 

3dB is barely noticable to the human ear.

To double the percieved volume, you need 10dB.

 

So as an accepted acoustical constant, +10dB = twice the volume?

This completely screws up some "definitive" advice I got a couple of weeks ago about the desirability of adding cabs to increase amp volume.

[sigh!] I guess I'm going to have to take that one to rec.audio.pro.

 

Yes, BA115 has a 4 ohm speaker. User manual mentions that in the specifications.

As far as volume for power increase, anywhere between 6dB and 10dB may be perceived as "double" the volume. That's a fairly subjective decision, what "sounds like double" to you may not to me, and may not correspond to an actual measurable 2x in SPL.

I think it is closer to 6dB, the "average person" may be somewhere between 6 and 10.

What that says is that you need between 4x and 10x the power to double apparent volume. That is significant.

 

I've been meaning to bother you with a related question I've been trying to get to the bottom of, namely the effectiveness (or not) of adding speakers to increase an amp's volume, given a fixed output wattage into the same impedance.

I've asked about this in several fora and have gotten much heat (and infinite digression) but little light.

If you have a boilerplate on this question somewhere, I'd be very grateful to see it.

As far as I have been able to determine from published specs, doubling the number of drivers -- all other things being equal -- seems to add about 2db to 3dB to the net sensitivity. This would not seem to me to account for the huge increase in sound that is claimed for adding cabs to one's amp while maintaining the same net impedance load.

Many thanks for any help in clarifying this very common question once and for all.

 

Here y'go!

If this is true, I can think of several possibilities, but I'll defer to an authority for the time being!

In any case, they appear to have the identical speaker, judging from the specs on each amp.

 

On the two amps.....they have completely different power amps, so I can easily see them sounding different. As far as why, well I have suspicions, but no facts............

On adding speakers...

The different thing about adding speakers, is that you are, well, adding more cones.

That adds radiating area.

For one thing, the coupling between the speakers can increase effective output, particularly at low frequencies.

Then also, as per the discussion on lobing a while back.....two cones are more directional, tending to produce a stronger lobe directly in front of the speakers, and less off axis.

So you have doubled the power (approximately) by adding a second speaker which is producing equal SPL in-phase. There is 3 dB.

Then in the sweet spot, you have increased the power due to greater directionality. Less power went off-axis, and more striaght ahead. Same power in smaller area of space is higher SPL. That can easily be 1 or 2 dB, added to the other 3 dB.

Any better coupling at low frequencies probably has no big directional improvement, because the two are still way smaller than a wavelength in size. But they may still have a dB or two of improvement due to increasing the cone area and effective radiating area. There is a tendency for multiple cones to act somewhat as a single large cone would if it filled all the area between the cones. (works only if the cones are pretty close).

Bottom line is that if you add a second 8 ohm speaker, and do double power due to the (now) 4 ohm load, you may get MORE than a straight 3 dB difference. The balance is mixed between better coupling and improved directionality.