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Multi-Way Conventional loudspeakers with crossovers

Relation between Power and Frequency
Relation between Power and Frequency
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Old 11th July 2018, 07:41 PM   #1
Ghyduart is offline Ghyduart
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Default Relation between Power and Frequency

Hello all,
It痴 my first post here, so let me know if I知 doing something wrong.

I知 planning to create my first DIY active speakers. I intend to replicate something like the Model M of Steinway Lyndorf with to boundary woofer. And I want to calculate exactly how much power I need for the mids and the tweeters.

I知 searching for month one 渡ot that simple information : what is the relation between power needed and frequency. I understand that, all parameters being equal, you need much more power to get 100dB at 20Hz than at 80Hz or even more than at 2000Hz.

But what is the equation? The only specific information I found is that we need half the power by octave. But I知 not sure of it.

Is there someone who have this information or a link to it?

Thank you in advance for your help.
Ghyduart
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Old 11th July 2018, 08:06 PM   #2
RobWells is offline RobWells  United Kingdom
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The amount of power needed to achieve a certain spl from a driver depends on the drivers efficiency. For example a high efficiency 15" driver could achieve 100dB @ 80Hz with less power than a 5" driver of low efficiency would need @ 400Hz.

If you are trying to copy a speaker maybe choose your drivers and work out what sort of power they need to get to xmax once their filters are in place. This will give you an idea of power required per driver and then just go slightly over to allow some headroom.
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Old 11th July 2018, 08:54 PM   #3
DougL is offline DougL  United States
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RobWells, +1. SPL for a driver is determined by the efficiency and power applied.

What you may be thinking of is what the power distribution is in music.
Its simplest to size all of your amplifiers the same and ignore this.
If you have a 90 dB efficient woofer and a 90 dB Tweeter, it takes exactly the same power to reach the same SPL, regardless of frequency.
If your still interested in the link, Its in my PM to another member that had a similar question.
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Old 11th July 2018, 09:01 PM   #4
DougL is offline DougL  United States
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I found the reference.

The question was why can a "200 Watt" passive speaker have a 40 watt power handling tweeter?

Here was part of that answer.

"I did find a source that claimed that the power distribution fell at 20 db per decade (6 db per octave) above 1 kHz. http://forums.prosoundweb.com/index.php?topic=154583.0"
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Old 11th July 2018, 09:06 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by DougL View Post
I did find a source that claimed that the power distribution fell at 20 db
per decade (6 db per octave) above 1 kHz.
Only for acoustical music. With electronic music, anything goes.
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Old 11th July 2018, 09:32 PM   #6
Ghyduart is offline Ghyduart
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Thanks for the replies. My project is to go full active. So I will have an active crossover and one amp for each driver. For that matter I want to calculate which power I need for each amp. I understand very well the impact of the impedance and the efficiency but not for the frequency. For example, take a speaker that has a efficiency of 93dB for 2,83V with a frequency response of 80-5000Hz. If I want to get 105 dB at 1 meter with a band pass of 80-2560 Hz how much power do I need? If I use a band pass of 160-2560 Hz how much power do I need?

So here is the table from Doug痴 link :
20Hz to 40Hz - power = 512 (relative power)
40Hz to 80Hz - power = 256
80Hz to 160Hz - power = 128
160Hz to 320Hz - power = 64
320Hz to 640Hz - power = 32
640Hz to 1,280Hz - power = 16
1,280Hz to 2,560Hz - power = 8
2,560Hz to 5,120Hz - power = 4
5,120Hz to 10,040Hz - power = 2
10,040Hz to 20,080Hz - power = 1

So if I need 768W to get 105dB with my subwoofer from 20Hz-80Hz, the table above says I would need 248W for mids and 7W for the tweeter.
In the second case (160Hz) I would only need 120W for the mids? Am I right?

All this of course with all the speakers parameters being equal.
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Old 11th July 2018, 09:32 PM   #7
krivium is offline krivium  France
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Hi,
From my experience what Rod Elliott give in the link is not far away from reality, but keep in mind that bass and low bass signals does usually contains heavy transcients ( eg kick drums) so you ll need headroom ( so more overall power requirement for the low/sub):

BiAmp (Bi-Amplification - Not Quite Magic, But Close) - Part 1
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Old 11th July 2018, 09:42 PM   #8
RobWells is offline RobWells  United Kingdom
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Quote:
Originally Posted by Ghyduart View Post
For example, take a speaker that has a efficiency of 93dB for 2,83V with a frequency response of 80-5000Hz. If I want to get 105 dB at 1 meter with a band pass of 80-2560 Hz how much power do I need? If I use a band pass of 160-2560 Hz how much power do I need?

.
If your 2,83V figure is for 1W @ 1M then 16 watts in both cases. For every extra 3dB you double the power.
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Old 11th July 2018, 09:55 PM   #9
charlie2 is offline charlie2  England
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Quote:
Originally Posted by DougL View Post
I found the reference.

The question was why can a "200 Watt" passive speaker have a 40 watt power handling tweeter?

Here was part of that answer.

"I did find a source that claimed that the power distribution fell at 20 db per decade (6 db per octave) above 1 kHz. http://forums.prosoundweb.com/index.php?topic=154583.0"
The crossover implemented can mean that x power goes to woofer and y power goes to tweeter

Watts and RMS figures banded about are meaningless without thiele and small parameters you get to the true nitty gritty

I fell out with someone who argued the point that a 1000 watt mid/woofer with 84db sensitivity would blow away my 600w 96db mid woofer

He needed 1036 watts to reach 114db

Where as I needed 48 watts for 114db and 768 watts to reach 125db

He could not grasp how sensitivity came into it but concerned himself to (watt) figures and rms quoted by the manufacturer
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Old 11th July 2018, 10:27 PM   #10
charlie2 is offline charlie2  England
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Quote:
Originally Posted by Ghyduart View Post
Thanks for the replies. My project is to go full active. So I will have an active crossover and one amp for each driver. For that matter I want to calculate which power I need for each amp. I understand very well the impact of the impedance and the efficiency but not for the frequency. For example, take a speaker that has a efficiency of 93dB for 2,83V with a frequency response of 80-5000Hz. If I want to get 105 dB at 1 meter with a band pass of 80-2560 Hz how much power do I need? If I use a band pass of 160-2560 Hz how much power do I need?

So here is the table from Doug痴 link :
20Hz to 40Hz - power = 512 (relative power)
40Hz to 80Hz - power = 256
80Hz to 160Hz - power = 128
160Hz to 320Hz - power = 64
320Hz to 640Hz - power = 32
640Hz to 1,280Hz - power = 16
1,280Hz to 2,560Hz - power = 8
2,560Hz to 5,120Hz - power = 4
5,120Hz to 10,040Hz - power = 2
10,040Hz to 20,080Hz - power = 1

So if I need 768W to get 105dB with my subwoofer from 20Hz-80Hz, the table above says I would need 248W for mids and 7W for the tweeter.
In the second case (160Hz) I would only need 120W for the mids? Am I right?

All this of course with all the speakers parameters being equal.
How much power you need is in relation to the sensitivity which in turn is how loud do you want to go at what distance

Rule of thumb is 2.83v @ 8ohms @1meter

The lower the sensitivity of any driver in your proposed set up requires more power

Example horn loaded compression drivers can achieve 112db plus @1 watt 350-hz upwards

A sub needs a load of watts because of laws of physics and a horn would be absolutely huge plus power loss
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