Loudness SPL vs. Number of Speakers

#21
agedhorse said:
The easier of the two is that when presented with a difficult environment, you want to avoid exciting that space with frequencies that decay slowly.
Click to expand...

I agree wholeheartely, and there's Lots of advice after that about what to do when hearing yourself through your amp, which is all good. I'll just add that the ultimate solution to being able to hear yourself really well is to go in ear and ampless. That way, the only source adding low frequencies to the space is the PA - that helps the house sound good. For you, having only what you want in your ears is a great thing, provided you can get a good mix.

Although it shouldn't have surprised me, a couple months ago I was located in the middle of a huge stage, well away from the drummer (Actually, there was noone within 20 feet of me - it was a bit lonely out there) At first, being 40 feet from the drummer seemed like a bad idea, but since I could hear myself well, and with the right mix, I could hear him clearly, not having the wash from the drums being next to me actually made it easier to keep he and I synced up. Eye contact was a bit more difficult, but not as much of an issue as I would have thought, and the whole thing flat out worked. It takes all the right conditions for this to work, but if you have those, it can be fabulous.
 
  • Like
Reactions: Gatlin, Stumbo and agedhorse
#22
Thanks for the great replies. So if I understood correctly, it can be broken down this way.

1 speaker: 0db coupling + 0db increased amp load = baseline SPL, baseline amp load
2 speakers in parallel: 3db coupling + 3db increased amp load = +6db SPL and increased amp load
4 speakers parallel/series: 6db coupling + 0db increased amp load = +6db SPL and baseline amp load
4 speakers including power compression benefit = +8db SPL and baseline amp load

From some reading I've done, power compression varies but is generally 3-5db at max rms power, depending on voice coil cooling. So best case scenario, cutting the per-speaker power by 75% (by going to 4 speakers) will get an additional 2db minimum.
 
#23
fast_frank_d said:
Thanks for the great replies. So if I understood correctly, it can be broken down this way.

1 speaker: 0db coupling + 0db increased amp load = baseline SPL, baseline amp load
2 speakers in parallel: 3db coupling + 3db increased amp load = +6db SPL and increased amp load
4 speakers parallel/series: 6db coupling + 0db increased amp load = +6db SPL and baseline amp load
4 speakers including power compression benefit = +8db SPL and baseline amp load

From some reading I've done, power compression varies but is generally 3-5db at max rms power, depending on voice coil cooling. So best case scenario, cutting the per-speaker power by 75% (by going to 4 speakers) will get an additional 2db minimum.
Click to expand...

Kinda-sorta rule of thumb.

Power compression occurs due to electro-magnetic effects, not just thermal. Thermal is easier to understand, therefore it seems to be the only thing the internet experts seem to focus on. In many cases, the electro-magnetic losses can be as great or greater than thermal losses depending on the application.
 
#24
I like the "Kinda-sorta rule of thumb" notion. Maybe add two footnotes. First, if I understand @agedhorse correctly in post #6, coupling is not linear with regard to frequency. In the two driver system, at low frequencies the system acts as a single-point sound-source with +6dB coupling and at higher frequencies transitions to a multi-point sound-source yielding +3dB coupling. The second deals with "+3dB increased amp load." It depends on the amp. If an amp can deliver twice the power into a 4Ω load as compared to an 8Ω load, then +3dB is correct. If the amp rates at say 500w/8Ω and 700w/4Ω, that is less than +1.5dB.
 
#25
Typically coupling yields ~3dB at low frequencies, somewhat decreasing (and becoming less uniform) at higher frequencies.
 
#26
fast_frank_d said:
Thanks for the great replies. So if I understood correctly, it can be broken down this way.

1 speaker: 0db coupling + 0db increased amp load = baseline SPL, baseline amp load
2 speakers in parallel: 3db coupling + 3db increased amp load = +6db SPL and increased amp load
4 speakers parallel/series: 6db coupling + 0db increased amp load = +6db SPL and baseline amp load
4 speakers including power compression benefit = +8db SPL and baseline amp load

From some reading I've done, power compression varies but is generally 3-5db at max rms power, depending on voice coil cooling. So best case scenario, cutting the per-speaker power by 75% (by going to 4 speakers) will get an additional 2db minimum.
Click to expand...

A couple of observations:

1. It would be good if you distinguished between tube and solid state. If a 100 watt tube amp has 8 and 4 ohm taps, adding a second 8 ohm speaker only gains 3db. Additionally, if the amp only has an 8 ohm tap, it will not make the full 100 watts at 4 ohms due to the impedance mismatch (improper loading of the output tubes), so the gain will be less than 3db.

2. The idea that a solid state amp doubles power when impedance is cut in half assumes the amp can hold voltage constant without frying the output devices. According to the specs of various amps I have reviewed, this is rarely true as it relates to the max power the amp can produce when the added (parallel) speaker takes the amp to its lowest impedance rating.
 
#27
Wasnex said:
The idea that a solid state amp doubles power when impedance is cut in half assumes the amp can hold voltage constant without frying the output devices. According to the specs of various amps I have reviewed, this is rarely true as it relates to the max power the amp can produce when the added (parallel) speaker takes the amp to its lowest impedance rating.
Click to expand...

This is usually true, but there are some amps that use PWM regulated power supplies that do actually double their rated power with halving of the load, and this is becoming a little more common with better technology. The Subways are an example of this, rated at 400 watts RMS into 8 ohms and 800 watts RMS into 4 ohms.
 
  • Like
Reactions: Wasnex
#28
Which leads me to the question. In the past I've thought of amplifiers as power devices and used the paradigm of bridging, matching and loading. With class D, am I right to think of amplifiers more as voltage devices? It seems the knee of bridging, matching and loading is much more narrow.
 
#29
agedhorse said:
This is usually true, but there are some amps that use PWM regulated power supplies that do actually double their rated power with halving of the load, and this is becoming a little more common with better technology. The Subways are an example of this, rated at 400 watts RMS into 8 ohms and 800 watts RMS into 4 ohms.
Click to expand...

Yep, Subways are notable exceptions.

IMHO, checking the amplifier specs is a good practice rather than blindly assuming you will get a free +6db from doubling speakers. For example with the Genz Benz GBE1200, power is almost doubled when moving from an 8ohm to a 4 ohm load; power is respectively 550W VS 1000W, so the gain will be close to +6db. When moving to a 2ohm load the amp only puts out an additional 200W (1,200W at 2 ohms)...no +6db gain this time.
 
#30
Wasnex said:
Yep, Subways are notable exceptions.

IMHO, checking the amplifier specs is a good practice rather than blindly assuming you will get a free +6db from doubling speakers. For example with the Genz Benz GBE1200, power is almost doubled when moving from an 8ohm to a 4 ohm load; power is respectively 550W VS 1000W, so the gain will be close to +6db. When moving to a 2ohm load the amp only puts out an additional 200W (1,200W at 2 ohms)...no +6db gain this time.
Click to expand...
Correct. due to a reconfiguration of the power supply. The alternative would have been to make it ~400 watts @ 8 ohms, ~700 watts @ 4 ohms and ~1200 watts @ 2 ohms.

You will notice that the Subway shares the same characteristic.
 
#31
Boundary Effect said:
Which leads me to the question. In the past I've thought of amplifiers as power devices and used the paradigm of bridging, matching and loading. With class D, am I right to think of amplifiers more as voltage devices? It seems the knee of bridging, matching and loading is much more narrow.
Click to expand...

No, not really, and it's not at all a class D thing. It's a special category of switchmode power supply that includes regulation which compensates for supply sag by increasing the PWM duty cycle. This is why the rated output doubles with a halving of impedance. If you looked at what's happening before clipping, you will se a doubling of power with a halving of impedance all the way up to the point that the amp clips.

Bridging is a different thing, many class d amps are internally bridged (not in the traditional audio sense but in output stage topology), but other than that they behave very much like class AB/G/H amps. They are all voltage sources.
 
#32
Okay. I think we may have a miscommunication. My use of 'bridging' is that the load impedance is above the source impedance. 'Matching' is what is says. 'Loading' means below the source. Be that as it may, it is your statement, "It's a special category of switchmode power supply that includes regulation which compensates for supply sag by increasing the PWM duty cycle" that has me wishing to pick your experience. Damn.
 
#33
agedhorse said:
Correct. due to a reconfiguration of the power supply. The alternative would have been to make it ~400 watts @ 8 ohms, ~700 watts @ 4 ohms and ~1200 watts @ 2 ohms.

You will notice that the Subway shares the same characteristic.
Click to expand...
To make it easy for everyone to see here are the specs from the Subway manual:
400 watts rms @ 8 ohms, 10% THD (includes preamp O/D THD) [note 1]
800 watts rms @ 4 ohms, 10% THD (includes preamp O/D THD) [note 1]
800 watts rms @ 2 ohms, 10% THD (includes preamp O/D THD) [note 1, 4]

[note 1]: area under the curve, factoring preamp harmonics management.
[note 2]: measured approx. 1 octave from knee
[note 3]: chassis dimensions include controls, handle and feet
[note 4]: impedance selector switch set to 2 ohm position
 
  • Like
Reactions: agedhorse
#34
To clarify, and I have mentioned it in the past a few times when this topic comes up.

There are indeed two different meanings of the term bridge. The one that's far more common these days is also called a "bridge tied load" (BTL) where each terminal of the speaker is driven by the opposing phase (or polarity). As one terminal rises above ground, the other terminal falls below ground.

The other meaning relates to the ratio of load to source impedance. The it's commonly accepted that when the source impedance below 1% of the load impedance, any loading effect on the source can be ignored and the load is said to bridge across the source.
 
#36
Aged, I can't tell you how much I (and no doubt all of us) really, really appreciate you taking your time to be here in TB. Thank You !

One last question if I can: I was always convinced I heard single driver cabinets better than standing in front of 410's in particular. Is this me, or is there any advantage (was it comb filtering I was hearing?) to single driver cabs over multiple driver cabs? I know 'it depends' on the cabs in question, but let's say we restrict it to the Subways or the Powerhouse range . . . . . would I hear 2 single 12 cabs better than a single 212 for example?
 
#37
Part of the difference might be the multiple sources, a 410 will typically have 4 lobes of non-uniformity and 4 nulls. It might sound better to you if you moved to the side a foot or two, angled the cabinet up a bit, or even moved away from the cabinet a few more feet. This effect is not the number of drivers in the cabinet but the number of drivers (each in their own cabinet or all in one cabinet, doesn't matter) and their orientation. The orientation or relative positions define the polar pattern of the system (how the sources combine acoustically on and off the polar axis)

Now beware of the folks who will jump in and say that a typical 410 (in 2 x 2 configuration) can't possibly work, or an 810 (in 4 x 2 configuration) can't possibly work because there are tens of thousands of players (or more) who have no problem whatsoever with them and they suit their needs perfectly. Vertical stacking might help, but might not either. The answer really is "it depends"
 
You must log in or register to reply here.
Top Bottom
Back