Under-powering cabs

[lug]

My bandmates attempt to get me to underpower my cabs all the time! I keep it dimed to protect my gear!

 

[agedhorse]

+1 this. It's really about the sensitivity rating of the drivers because in practical circumstances, under powering a cabinet may NOT work.

I have these bass cabinets built by a legit custom builder. But him being mainly a pro audio subwoofer designer, he used faitalPro high end sound reinforcement woofers which sound awesome but the sensitivity is way too low. You just can't move much air from the get go unless you slam a huge amount of wattage at it. Thus, you really CAN underpower them for the task they were designed to do.This may be an uncommon example but in the bass world, with so many different speakers/cabs types out there, there is nuance to always consider when matching amp to cab.

The voicing of drivers intended for pro audio are also not very suitable for use in bass guitar applications. There's a rough trade-off between low frequency extension and mid-high frequency response, this may be just as much of a factor as the appearance of lower sensitivity. This gets back to real world experience in the bass guitar world and why it's important.

You aren't going to hurt this type of driver from underpowering either.

 

[dalahorse]

Its pretty much a dead cert that anyone talking about square waves in this context doesn't have a thorough understanding of the topic.

Indeed. If square waves alone were a problem, my Moog synthesizer would have blown apart my amplifier years ago.

 

[MegaSwing]

I'm intrigued with the notion of running ANY equipment full-out. It's just hard to imagine needing that much volume. And I've been on every point on the spectrum at some time or another. Whether it was a low-powered head to high-output speaker/s or a high-powered head to modest-output speaker/s, the quantity of the sound was never too little, because 'enough' is almost never the maximum, at least for me. The QUALITY of the sound is quite a different thing, and something that only taste and hard experience will teach you. Some like it rough; others like it smooth. Not everyone figures it out that you don't need to dime out to be playing too loud to sound good to normal ears.

An appropriate analogy might be that you can drive too fast for city streets in a Kia the same as you can in a Ferrari. It happens every day. The Ferrari is just sweeter to look at. Both will kill you just as dead if you aren't an experienced driver—or pedestrian.

Power is not the same as tone. Power is merely an ingredient for tone.

 

[dabis]

JBL with a white paper that was greatly misunderstood then and still is to this day.

This may have been the start of the misunderstanding. Unfortunately, the details have been taken out of context so badly as to create such urban myths.

If folks were to really understand the paper, they would discover that JBL actually recommends DECREASING the power to speakers to 50% of rated power for musical instrument applications to account dis distortion and compression increases in signal density. Tweeters are handled differently and are responsible for some of the misunderstanding.

Please gentlemen, go on with this class. These threads have to have an utility at least...

 

[Redbrangus]

Some amps' volume knob controls the wattage, some control the input gain into the power amp that is running at full rated power.

I don't think this is true...I believe all power amps have input level controls and I'm not aware of any in which the volume controls do anything other than attenuate the input signal. I could be wrong though... can you provide examples of the other type?

 

[5StringBlues]

 

[dalahorse]

I don't think this is true...I believe all power amps have input level controls and I'm not aware of any in which the volume controls do anything other than attenuate the input signal. I could be wrong though... can you provide examples of the other type?

I believe that is how the SVT-VR head works: The volume control attenuates the input level and the power section always runs at full. I believe that's also true for many (most?) pro audio amplifiers.

 

[HolmeBass]

AMPS FAQ!! Info on OHMS, Allsize RIGS-OverUnderPowerCabs DIY TechTalk-Links

It is a TOTAL myth that makes absolutely no sense at all. Please see the forum sticky linked in the post I quoted.

 

[whero]

All amps drop to 0 watts when you stop playing.

If your theory were correct then our cabs would explode every time there is a rest in the music!

Kind of like the musical version of the movie "Speed"?

 

[Redbrangus]

I believe that's also true for many (most?) pro audio amplifiers.

AFAIK, that's how all audio amplifiers work... this what I'm sayin'...

I guess the issue could get slightly more complicated with guitar/bass amps that include a preamp, but it's still the same thing -- the volume or gain controls are still just input attenuators.

 

[Nuage420]

I see a lot of rigs, the other way round... a 500w head (or more) into a cab that's rated @ 300w RMS. Obviously, it just takes a little common sense, with the volume knob. I rarely see rigs where the cab capacity is significantly above the output of the amp.

i see this scenario often. Usually followed by negative reviews and brand bashing because the cabs flapping out the low end. Completely ignoring the fact they're slamming 500watts through their 300 watt cab.

 

[Nev375]

un·der·pow·ered
/ˌəndərˈpou(ə)rd/

adjective

1. lacking sufficient mechanical, electrical, or other power.
   "the turbo diesel engine is underpowered"
2. misleading term salesmen use to sell musicians amplifiers they don't need.

 

[Pilgrim]

Just thought I'd throw this one out there, and see what the general consensus is. I've heard many differing and often conflicting ideas on this topic, over the years. Do you see any issue in under-powering a cab ?

(big snip)

What say ye ?

Not intending to be rude, but you evidently neglected to search this topic, which is an old turkey that has been discussed ad nauseam.

This is an old, redundant, debunked issue. No such problem exists.

 

[Gustopher]

Search for, e.g., "under powering". This has been covered ad-nauseum.

But this is Talkbass not SearchDatabass

 

[Pilgrim]

But this is Talkbass not SearchDatabass

It's also not failtodoanyhomeworkwhateverbass. When you post something for the 150th time, expect the responses to reflect that.

I have a 1967 Blackface Bassman head, 50W. It sounds great through my 2x10 Genz-Benz cab rated 300W. Never a problem; not gonna be a problem.

 

[Bruiser Stone]

If there is such a thing as under powering a cab, why is this the industry standard?

 

[JazznFunk]

This. I've never felt the need to MAX OUT any amplifier I've ever owned in any situation I've ever been in.... maybe I came close with my little SWR Workingman's 12 back in the day when playing with a loud drummer, but even that amp was fine in most situations. If people are diming their 500W amps constantly.... maybe it's time to ask everyone else to turn down? Wattage/headroom are like horsepower - when you really need it in a pinch, it's there... but you don't red-line your car every time you get in it.

I'm intrigued with the notion of running ANY equipment full-out. It's just hard to imagine needing that much volume. And I've been on every point on the spectrum at some time or another. Whether it was a low-powered head to high-output speaker/s or a high-powered head to modest-output speaker/s, the quantity of the sound was never too little, because 'enough' is almost never the maximum, at least for me. The QUALITY of the sound is quite a different thing, and something that only taste and hard experience will teach you. Some like it rough; others like it smooth. Not everyone figures it out that you don't need to dime out to be playing too loud to sound good to normal ears.

 

[vvvmmm]

Last week I blew the speaker in a VHT-AV-RL-10BC cabinet.
It's rated at 75 w. continuous, I blew it playing a P/J thru a 50 watt PF50t.
It's 4 months old w/ a 3 month warranty and VHT did not respond to the seller or me.

Not proof of anything (other than poor customer service and I don't reco the cab), but ...

 

[Wasnex]

Some amps' volume knob controls the wattage, some control the input gain into the power amp that is running at full rated power. The latter can be dangerous especially if you're running a bunch of pedals into the amp. Just understand if your amp and cab is rated at peak / RMS / program watts. You don't want to run a 1000W RMS rated amp into a 1000W peak rated cab. 1000 == 1000, right? Click, magic smoke leaves the speakers.

Speakers ratings are often given in RMS Power, Program Power, and Peak Power. These come from the common way speakers are tested.

I think of it this way:
-RMS Power is long term average measured in RMS volts. The idea is the driver can function at this average power level indefinitely.
-Program is short term average using RMS volts. The idea is the driver can handle bursts of power at this wattage level, as long as the average power does not exceed the RMS rating.
-Peak Power is instantaneous using Peak volts (Vp). This one is a bit misleading (see below).


RMS is the Average wattage the driver can take. The test wave form has a 6dB crest factor. Crest factor is defined as the ratio of peak value to rms value of a current waveform. Although it seems confusing, a continuous state sine wave actually has a crest factor of 3dB. This is because the Peak power of the waveform is double the RMS power.

The other 3dB in the 6dB crest factor comes from variation in the waveform. Program power uses a complex waveform with peaks that are 3dB above RMS power. The envelope of the test waveform essentially varies above and below the RMS rating of the driver. The idea is the driver can take transients that are 3dB higher than the RMS value, but the average RMS power must remains at or below the RMS value or the voice coil will smoke. So Program power tests both the thermal power handling of the voice coil and also to some degree the mechanical power handling if the entire cone. For clarification doublings the power causes a 3dB change in SPL

In the context of speaker ratings: Peak Power is usually equivalent to Program power....but Program power is measured using RMS voltage, while Peak power uses Peak voltage.

To convert from Peak voltage to RMS voltage:

VRMS=Vp/(sq rt 2)

(sq rt of 2) is approximately 1.414 so:

VRMS = Vp/~1.414.

Likewise: Vp = VRMS x ~1.414

I use ~ to mean approximate and I am rounding the figure up to the last digit displayed. The actual symbol for this looks like

. I will use unrounded numbers for calculations.


Less assume we have a speaker rated for 1W RMS, 2W Program, and 4W peak. This power relationship shows a 6dB crest factor.

Here is our AC Ohms Law Formula Wheel:

We will use this formula to determine the voltage:

V=[sq rt (PZ)]

Let assume we have an 8 ohm speaker

1W RMS
V=[sq rt (1x8)]=~2.83VRMS

2W Program
V=[sq rt (2x8)]=4VRMS

4W Peak
V=[sq rt (4x8)]=~5.66Vp

Now let's convert the voltage we get for 4W peak to RMS using:

VRMS=Vp/(sq rt 2)

VRMS = ~5.66Vp/~1.414=~4VRMS.

Note: You get exactly 4VRMS if you use unrounded values.

So 4VRMS = ~5.66Vp. Therefore, 2WRMS Program is equal to 4Wp.

Proofs using:

P = (V^2)/Z

P(peak) = (~5.66^2)/8 = 4Wp
P(program) = (4^2)/8 =2WRMS

Some manufacturers define Peak power a bit different. It appears they call "Program power"" Peak power." It's not necessarily clear in the spec sheet whether they are using RMS voltage or Peak voltage.

The other type of power we have not covered is Mechanical power handling. Mechanical power handling below 100hz is often less than the RMS power handling. The actual Mechanical power handing is determined by both the driver design and the tuning of the cabinet. Unfortunately many manufacturers actually use the driver's RMS power rating and make no mention of the cab's Mechanical power rating. The point is many cabs cannot take even the driver's full RMS power rating at some frequencies below 100hz. So feeding the cab Program or Peak power can be a recipe for destroying the driver.

Mesa is the only MI company that I know of that publishes data related to Mechanical power handling. Check the specs here: MESA/Boogie® (mesaboogie.com)

This is about the extent of my knowledge on this subject, and I am just barely scratching the surface. But hopefully now you see why power handling is sort of confusing.