Power draw by frequency: what am I missing?

JerryLove

JerryLove

Audioholic Samurai
It's been asserted, basically as a truism, that demands on the amp go up as the frequency goes down.

This has been cited repeatedly as a reason to add a subwoofer to a given speaker setup (moving the most demanding loads to a dedicated amp).

But I can't seem to figure out where I can find that value.

Speaker sensativity is frequency independent (that is to say: the crossovers level it). So on, say, a 90db@1w speaker, the tweeter might be intrinsically more sensitive than the woofer; the crossover makes both 90db sensitive.

So [for a non-builder] where does this "extra demand" play out? 110db@40hz will require 100w, as will 110db@20khz. The only exception will be after roll-off starts; but doing custom EQ to compensate for that is really beyond the scope on my question.

To rephrase: does it really require any more power from the amp to generate Xdb at the bottom end of a speaker's flat-response than at the top? If so: what is the math? How is that draw expressed?
 
Irvrobinson

Irvrobinson

Audioholic Spartan
You are missing nothing. It takes as much power at 5KHz to produce 90db at one meter as it does at 50Hz. I think the only thing you're missing is that human hearing sensitivity varies by frequency, and human hearing is a lot less sensitive at bass frequencies, so bass tones have to be much louder to be heard, hence most power demands in loudspeakers are in the bass octaves. See the following as one of many example references:

https://biology.stackexchange.com/questions/53062/why-is-the-human-ear-most-sensitive-to-4000-hz-tones
 
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JerryLove

JerryLove

Audioholic Samurai
So.

Would an accurate conclusion from your post be that, if I looked at the SPL-by-frequency for a given movie or musical piece; I would expect to see that the LF noise would be higher SPL than HF?

Where do drums fall in this? I've heard, and tend to believe, that the majority of the sound in an orchestra is from the drum line.
 
Irvrobinson

Irvrobinson

Audioholic Spartan
So.

Would an accurate conclusion from your post be that, if I looked at the SPL-by-frequency for a given movie or musical piece; I would expect to see that the LF noise would be higher SPL than HF?

Where do drums fall in this? I've heard, and tend to believe, that the majority of the sound in an orchestra is from the drum line.
Since my wife is a percussionist, I've spent some time recording drums of various sorts. My wife's 22" bass drum produces a lot of energy in the 40-80Hz range, but my spectrum analyzer registers important output in the 25-40Hz range too. Jamming on her "rock music" kit, which is a full-custom DW Collector Series kit with the 22" drum, produces massive output over a wide range of octaves. The loudest sounds by decibel measurements are rim shots, which exceed 110db in a residential venue. In normal rock music the kick drum is not the loudest sound, but the power of a strong pedal hit is in the 45-60Hz range, and that's mostly what people perceive as the pressure on their chest. Snare drums sound very loud too, and I've measured her enthusiastic playing at over 100db, and snare sound has quite a bit of upper midrange content, making that 100db seem very loud. Her metal snare is really like that. My Revel/Velodyne system can't realistically reproduce this kit played aggressively. Close, but there's no problem discerning the real thing.

For most orchestras, from the seats we usually buy, the average sound level is only in the low 80s decibels, and often much less, and the power I see on my spectrum analyzer phone app is in the 80-200Hz range, but it really depends on the material. Some orchestras use 42" concert bass drums, which produce a lot of low frequency power, but tympani and such don't really produce much in the way of deep bass in a large venue. (In a smaller room a tympani can be impressive, but that doesn't really count.)
 
WaynePflughaupt

WaynePflughaupt

Audioholic Samurai
So.

Would an accurate conclusion from your post be that, if I looked at the SPL-by-frequency for a given movie or musical piece; I would expect to see that the LF noise would be higher SPL than HF?
That is correct.

Regards,
Wayne A. Pflughaupt
 
highfigh

highfigh

Audioholic Slumlord
So.

Would an accurate conclusion from your post be that, if I looked at the SPL-by-frequency for a given movie or musical piece; I would expect to see that the LF noise would be higher SPL than HF?

Where do drums fall in this? I've heard, and tend to believe, that the majority of the sound in an orchestra is from the drum line.
The low frequencies NEED to be louder, if we're to perceive them as loud as the frequencies we're most sensitive to, with this disclaimer: we don't 'hear' low frequencies in the same way we hear other frequencies. Low frequencies excite the inner ear via vibrations coming through the skeleton, rather than through the air and into the ear canal.

"majority of sound..."- do you mean SPL? If so, there's some truth to that and it definitely applies to a band playing through a PA- the instruments that are electric or more quiet acoustic instruments need to approach the SPL of the drums- more or less, depending on the drummer's ability/inability to use dynamics in their playing. For the drummers who have no idea what "Play quieter!!!!!!!!!!!!!!!!!!!!!!" means, they can use some kind of baffle to prevent the sound being too loud and hopefully, make the drummer pay attention to what the rest of the band and the sound guy are saying. Electric guitar players are just as guilty but in many cases, the sound guy can turn them down in the mix.

Q- How do you make a guitar player stop playing?
A- Put sheet music in front of them.
 
Swerd

Swerd

Audioholic Warlord
… Speaker sensativity is frequency independent (that is to say: the crossovers level it). So on, say, a 90db@1w speaker, the tweeter might be intrinsically more sensitive than the woofer; the crossover makes both 90db sensitive.

So [for a non-builder] where does this "extra demand" play out? 110db@40hz will require 100w, as will 110db@20khz. The only exception will be after roll-off starts; but doing custom EQ to compensate for that is really beyond the scope on my question.

To rephrase: does it really require any more power from the amp to generate Xdb at the bottom end of a speaker's flat-response than at the top? If so: what is the math? How is that draw expressed?
To address your questions, I have a few comments.

Speaker sensitivity is expressed as a single number, as if it were constant. But it rarely is. As an example, I chose the Canadian National Research Council's (NRC) measurements of the B&W 704 s2 speaker. There is no widely accepted standard for measuring sensitivity. It varies significantly among different speaker manufacturers. (Please understand, I am not singling out this speaker as a poor performer. I haven't heard it and I've avoided saying anything about how it sounds.)

The NRC does clearly state their own standard as "averaged 300Hz-3kHz on Listening Window, 2.83V/1m". With the B&W 704, sensitivity was estimated at 87.2dB. A quick glance at its frequency response curve shows a wide range of SPL in that range. This graph shows only the on-axis, 15° and 30° off-axis measurements. The NRC defines its Listening Window as the average of five measurements: on-axis, 15° left and right off-axis, 15° up and down off-axis. So, its clear that sensitivity isn't a constant value across the audio frequency spectrum.
1546103111210.png

All this is about how speaker sensitivity is measured. It doesn't address how we hear it. We listen to speakers in different locations than where speaker test microphones are placed. We hear a combination of on-axis sound as well as off-axis sound and its room reflections. In the test measurements, the test gear (microphones and software) is set up to maximize sound from one angle while minimizing the others. That's why the NRC defines sensitivity as an average over its listening window.

The NRC's sensitivity numbers average sound from 300 to 3,000 Hz. They avoid speaker response below 300 Hz where room reflections play a large role in how we hear loudness, and where many woofers drop off in loudness becoming more difficult to drive. So when it comes to bass loudness, all bets are off when it comes to speaker sensitivity (at least the way the NRC defines it).

If we can't use speaker sensitivity to judge bass loudness, how can we decide if a speaker is difficult for an amplifier to drive? We have to look at a speakers impedance magnitude in ohms (next graph below),as well as the electrical phase angle of impedance in degrees (below the impedance magnitude graph).

The impedance magnitude shows this speaker's lowest impedance value is below 4 ohms, in the range of 100-200 Hz. In that same frequency range, the phase angle (next graph) shifts quickly from below -45° to above 0°. As the frequency climbs to 1000-2000 Hz, that phase angle continues to increase, reaching as high as +45°. In a speaker with impedance below 4 ohms, this large a phase angle shift of about 90° over a range (100-1000 Hz) that includes the impedance low value, can make it hard for all but the most robust amplifiers to drive.

To summarize, large phase angle shifts when the impedance is low make it difficult for amps to drive speakers loud at those bass frequencies below the NRC's 300-3000 Hz definition of speaker sensitivity. I don't know how to explain in plain English just why this is so (others are welcome to try),but I do know how to look for it in these graphs.

@JerryLove – I hope this helps.
1546103755995.png

1546103771799.png
 
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JerryLove

JerryLove

Audioholic Samurai
Swerd,

The output doesn't so much matter in this case as the input. [yes, the desired output informs the input].

Indeed: in your sample speaker, your worst performance is right at the roll-off (by definition) and 2KHz.

It might be worth noting that this was a sub-portion of a discussion about the advantages of subwoofers. One asserted advantage was "you can play the speaker louder because of reduced amp load" which I disputed on two grounds. The first being "then you should have gotten an amp as large as the one in the sub", but the second was "the power required for 110db@10khz is the same as the power required for 110db@50Hz (assuming you aren't below the roll-off).

...and if you are below roll-off, that's not more demand on the amp; but rather less output from the speaker (turning it up to compensate would deafen you with HF sound)

As an example: here's the chart on one of my speakers:
fr_merged.gif


It might be worth noting, before there's a derail here, I was actually an advocate of subs... and I agree that (in theory) adding a second amp means more power... I was, however, disputing the belief that adding a sub offered significant real-world advantage to a sufficient amp.

Which lead me to my "LF takes all the power" vs "efficiency is [broadly speaking] equal across the performance envelope of the speaker"
 
ryanosaur

ryanosaur

Audioholic Warlord
I was, however, disputing the belief that adding a sub offered significant real-world advantage to a sufficient amp.
I don't have a lot of experience in the more technical aspects you guys are discussing, but I want to agree, based on my recent experiences, and without any measurements...
So perhaps there is something more that I don't get, which is why I'm here, now... Please if I'm not getting something, correct me. :)
To my understanding, the AVR amp or external power amp will not miraculously perform better(by increasing power output) because you make its work load lighter. My analogy as a professional chef: if pureeing spinach in a Vitaprep, I stick my spoon in the blender and stir (the added subwoofer) while the motor (amp powering example speaker) is working. I am not increasing the power of the motor, but merely allowing that motor more efficiency in doing its job. That's the basic argument, yes?
To me, I see this as, perhaps, extending the useful life of the amplifier. Maybe it will run cooler if not working so hard?

(Full reveal... I only just plugged in my very first external amplifiers today... like I said... experience. Just here to learn! Thanks All!)
 
ryanosaur

ryanosaur

Audioholic Warlord
As I've been considering this... does adding a Sub to takeover some of the low frequency work from the main Amp equate to creating (perhaps a false sense of) dynamic headroom? This is another concept which has been somewhat elusive to me, and seems to be misunderstood by many others. As I subscribe to the epistemic function of language... since I can't define, I don't know it. Again, my thanks!
 
Swerd

Swerd

Audioholic Warlord
Indeed: in your sample speaker, your worst performance is right at the roll-off (by definition) and 2KHz.
I chose that B&W 704 as an example mainly because it's two impedance graphs showed how it could be harder to drive in the bass frequencies below 300 Hz. I was surprised to see the rather large suckout in frequency response between 1 & 4 kHz, centered a bit higher than 2 kHz. This is quite a lot more than the usual polite BBC dip.

The frequency response graph you posted looks remarkably flat. Did you produce that graph? Is it of your Salk Supercharged SongTower? Salk publishes a remarkably similar looking graph of it (see below),except it does not show frequencies below 200 Hz, as yours did. How did you eliminate the floor bounce peaks & valleys?
1546146008049.png
 
Verdinut

Verdinut

Audioholic Ninja
As I've been considering this... does adding a Sub to takeover some of the low frequency work from the main Amp equate to creating (perhaps a false sense of) dynamic headroom? This is another concept which has been somewhat elusive to me, and seems to be misunderstood by many others. As I subscribe to the epistemic function of language... since I can't define, I don't know it. Again, my thanks!
If you have not already calculated how much amplifier power you actually need to get to the maximum SPL at your listening position, based on the sensitivity of your speakers, I suggest that you check this link fot getting an answer: http://myhometheater.homestead.com/splcalculator.html

Then, if the rated power of your amplifiers is borderline, it wouldn't be a bad idea to get one or two powered subs in order to obtain some power reserve and prevent your amplifiers from clipping, and risking damage to your speakers.

Let us know what you found after calculation if you need further guidance.
 
ryanosaur

ryanosaur

Audioholic Warlord
Hi @Verdinut. I've been all over that SPL calculator and have both used it a lot and shared it around several times. ;) Its a great tool!
I recognize my thinking and comment is probably on the remedial side, but I do want to understand the equipment better: how it works and interacts in the bigger system. @JerryLove's original post, and the subsequent comment about the subwoofer struck a chord with me...
Because of several comments I've seen where a claim is made about "gaining extra power" when you take the low frequencies away from the main amp being confusing, I was drawn in here. It doesn't make sense to me that such a thing would occur. Hence my question.
Thanks!
 
Verdinut

Verdinut

Audioholic Ninja
Hi @Verdinut. I've been all over that SPL calculator and have both used it a lot and shared it around several times. ;) Its a great tool!
I recognize my thinking and comment is probably on the remedial side, but I do want to understand the equipment better: how it works and interacts in the bigger system. @JerryLove's original post, and the subsequent comment about the subwoofer struck a chord with me...
Because of several comments I've seen where a claim is made about "gaining extra power" when you take the low frequencies away from the main amp being confusing, I was drawn in here. It doesn't make sense to me that such a thing would occur. Hence my question.
Thanks!
When you take the low frequencies away from an amplifier, the saved power is available for the mid and high frequencies and may help preventing power demands beyond it's rated capacity.

If your amp has a power rating which is amply sufficient ( 6dB more than the required value, or 4 times the required power) to obtain the maximum SPL that you need, provided that your speakers can handle the required amp output for that SPL and also produce adequate bass response for your needs, you would not have to get a subwoofer.
 
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ryanosaur

ryanosaur

Audioholic Warlord
When you take the low frequencies away from an amplifier, the saved power is available for the mid and high frequencies and may help preventing power demands beyond it's rated capacity.
If I follow correctly, then it is a function of creating more available headroom through more efficient usage of the amps available power. Am I getting it?
 
TLS Guy

TLS Guy

Audioholic Slumlord
It's been asserted, basically as a truism, that demands on the amp go up as the frequency goes down.

This has been cited repeatedly as a reason to add a subwoofer to a given speaker setup (moving the most demanding loads to a dedicated amp).

But I can't seem to figure out where I can find that value.

Speaker sensativity is frequency independent (that is to say: the crossovers level it). So on, say, a 90db@1w speaker, the tweeter might be intrinsically more sensitive than the woofer; the crossover makes both 90db sensitive.

So [for a non-builder] where does this "extra demand" play out? 110db@40hz will require 100w, as will 110db@20khz. The only exception will be after roll-off starts; but doing custom EQ to compensate for that is really beyond the scope on my question.

To rephrase: does it really require any more power from the amp to generate Xdb at the bottom end of a speaker's flat-response than at the top? If so: what is the math? How is that draw expressed?
There continues to be huge miss understanding about all this.

There are really three relevant issues. For most music the most output occurs above sub range especially from 80 Hz to 400 to 500 Hz, with power demands declining fairly rapidly above 2.5 KHz and below 80 Hz The power intensity divide for most music is centered around 400 Hz, with half the radiated power below and half above that point.

Now this is the second point. Most people listen to speakers that are monopole radiators. Now there comes a point that is higher in frequency the narrower the baffle, that the speaker transitions from being a half space radiator to a full space radiator. So it transitions from being a forward radiator to an omni directional radiator. This is known as the baffle step. Now the take home is that the power to the speaker must be increased by 6 db below the baffle step. Now a passive network can not boost only cut, so the only way to increase the power is to lower the impedance and therefore increase the current draw which will increase the power.

Now the third point is that to reproduce bass efficiently requires a large enclosure. As you reduce enclosure size then bass sensitivity/efficiency drops. This is Hoffman's Iron Man's Law. So if you put a driver in a small sealed box it will take lots of power. Far more than my large transmission lines which take very little power to produce very accurate room filling bass.

So a sub does not actually off load much power from a receiver, very little in fact. What it does do is offload the the other speaker drivers. These days they are narrow speakers with small diameter drivers that do not have the excursion to produce bass at a realistic spl. So subs offload speaker drivers and power amplifiers hardly at all. It is on this last point there is huge ignorance and miss understanding. So the oft repeater phrase that a sub relieves a receiver from doing the heavy lifting is absolute nonsense.
 
L

Leemix

Audioholic General
There continues to be huge miss understanding about all this.

There are really three relevant issues. For most music the most output occurs above sub range especially from 80 Hz to 400 to 500 Hz, with power demands declining fairly rapidly above 2.5 KHz and below 80 Hz The power intensity divide for most music is centered around 400 Hz, with half the radiated power below and half above that point.

Now this is the second point. Most people listen to speakers that are monopole radiators. Now there comes a point that is higher in frequency the narrower the baffle, that the speaker transitions from being a half space radiator to a full space radiator. So it transitions from being a forward radiator to an omni directional radiator. This is known as the baffle step. Now the take home is that the power to the speaker must be increased by 6 db below the baffle step. Now a passive network can not boost only cut, so the only way to increase the power is to lower the impedance and therefore increase the current draw which will increase the power.

Now the third point is that to reproduce bass efficiently requires a large enclosure. As you reduce enclosure size then bass sensitivity/efficiency drops. This is Hoffman's Iron Man's Law. So if you put a driver in a small sealed box it will take lots of power. Far more than my large transmission lines which take very little power to produce very accurate room filling bass.

So a sub does not actually off load much power from a receiver, very little in fact. What it does do is offload the the other speaker drivers. These days they are narrow speakers with small diameter drivers that do not have the excursion to produce bass at a realistic spl. So subs offload speaker drivers and power amplifiers hardly at all. It is on this last point there is huge ignorance and miss understanding. So the oft repeater phrase that a sub relieves a receiver from doing the heavy lifting is absolute nonsense.
Very interesting read, this is for most music (2channel) as you wrote, but for movies with separate sub channel, and often a lot of action well below most speakers drop off point, the addition of sub(s) will lessen the load on the main speakers and their amps(?)


Sent from my iPad using Tapatalk
 
TLS Guy

TLS Guy

Audioholic Slumlord
Very interesting read, this is for most music (2channel) as you wrote, but for movies with separate sub channel, and often a lot of action well below most speakers drop off point, the addition of sub(s) will lessen the load on the main speakers and their amps(?)


Sent from my iPad using Tapatalk
Yes, but briefly. I don't know a movie that has continuous low LF. It is just short effects. Most of the movie is dialog and the background music. Most of the time playing a movie does not take a lot of power, but there is intermittent punishing LF effects on a lot of movies now. Continuous loud deep LF would be very irritating and boring. I guess some movies now come close, but not the ones we watch.

However I would say that a lot of music scores require higher average power than movies.
 
Irvrobinson

Irvrobinson

Audioholic Spartan
There continues to be huge miss understanding about all this.

There are really three relevant issues. For most music the most output occurs above sub range especially from 80 Hz to 400 to 500 Hz, with power demands declining fairly rapidly above 2.5 KHz and below 80 Hz The power intensity divide for most music is centered around 400 Hz, with half the radiated power below and half above that point.
This is probably correct for symphony orchestras and baroque chamber music, but it's not true for modern pop music or some fusion jazz. (In my measurements it's also less true for Beethoven than Mozart, for example, but I digress.) I doubt you listen to Kanye West or Cardi B, but the bass line below 80Hz is powerful and nearly continuous. Fusion jazz often has strong synthesizer bass lines. I'm not a fan of popular music either, but you and I are in the minority.

this is the second point. Most people listen to speakers that are monopole radiators. Now there comes a point that is higher in frequency the narrower the baffle, that the speaker transitions from being a half space radiator to a full space radiator. So it transitions from being a forward radiator to an omni directional radiator. This is known as the baffle step. Now the take home is that the power to the speaker must be increased by 6 db below the baffle step. Now a passive network can not boost only cut, so the only way to increase the power is to lower the impedance and therefore increase the current draw which will increase the power.

Now the third point is that to reproduce bass efficiently requires a large enclosure. As you reduce enclosure size then bass sensitivity/efficiency drops. This is Hoffman's Iron Man's Law. So if you put a driver in a small sealed box it will take lots of power. Far more than my large transmission lines which take very little power to produce very accurate room filling bass.
The point is not to confuse sensitivity with efficiency, which Swerd explained well, but that's not what Jerry asked initially. I figured he was knowledgeable enough to already know the difference between sensitivity and efficiency. As for the comment about the inefficiency of small sealed subs, true, but how is that relevant to his question?

So a sub does not actually off load much power from a receiver, very little in fact. What it does do is offload the the other speaker drivers. These days they are narrow speakers with small diameter drivers that do not have the excursion to produce bass at a realistic spl. So subs offload speaker drivers and power amplifiers hardly at all. It is on this last point there is huge ignorance and miss understanding. So the oft repeater phrase that a sub relieves a receiver from doing the heavy lifting is absolute nonsense.
Again, this is true for symphonies and chamber music, but many action movies (which I generally don't watch either) and pop music genres do have strong bass lines, and with a crossover at 80Hz, like most AVRs use, I'm certain powered subs of whatever cabinet strategy are indeed off-loading the primary amplifiers to some significant degree, and it's not "absolute nonsense". I had a few people who want to hear what their music sounds like on my primary system, and out of curiosity I've fired up my RTA software, so I'm not guessing about what's on a Kanye West album.
 
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