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Old 6th May 2004, 12:41 AM   #1
cominup is offline cominup  Viet Nam
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Default question of theory

Dear all,

Please, could you explain something below:

First:
- I know that impedance of loudspeaker is not constant while they are being used. It rates to audio frequency. But I dont know it rates directly or inversely.

- Otherwise, power (Watt) inversely rates to impedance (ex: 8 Ohm ~ 100W; 4 Ohm ~ 200W).

=> So, I am asking the relationship between audio frequency and power.

Second:
- Wide audio frequency bandwidth causes wide loudspeaker impedance bandwidth. But speaker manufactors just tell us only one power (ex: 100W @ 8 Ohm).

So, how to know the loudspeaker impedance bandwidth. What is the marked power on the loudspeaker?

Third:
- The relationship of amplifier power (Watt), loudspeaker sensity (dB) and loudspeaker power (Watt). Are there some equation and/or formula of them? Or it is just only human feeling?


Thanks for your looking.
Regards,

Cominup
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Old 6th May 2004, 06:37 AM   #2
sreten is offline sreten  United Kingdom
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Speakers sensitivity is rated at dB/2.83V.

For an 8 ohm nominal driver / loudspeaker dB/2.83V = dB/W
For an 6 ohm nominal driver / loudspeaker dB/2.83V = dB/1.3W
For an 4 ohm nominal driver / loudspeaker dB/2.83V = dB/2W
For an 2 ohm nominal driver / loudspeaker dB/2.83V = dB/4W

The above are just approximations as real efficiency at each
point of the impedance curve is hardly ever considered.

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Old 6th May 2004, 09:59 PM   #3
Ron E is offline Ron E  United States
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Default Re: question of theory

1.
Ohms Law: Volts = Current * Impedance

Power = Current^2*Impedance
Power = Volts^2/Impedance

Power doubles into a halved impedance only if voltage is constant and source impedance = 0.

2.
Impedance bandwidth (whatever you mean by that) is unimportant. Impedance only matters in most cases if it is too low or too reactive.

The marked power on a speaker is rated by IEC or DIN specifications. THese are a band-limited noise signal that is used to find the thermal (not mechanical) limits of the speaker. Search for "IEC power" or DIN power" in your browser.

3.
efficiency = 9.6e-10*Fs^3*Vas/Qes [Vas in liters]
sensitivity = 112+10*log10(efficiency)

SPL in dB = 10*log10(power) +Sensitivity
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Old 7th May 2004, 04:28 AM   #4
cominup is offline cominup  Viet Nam
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Hi Ron E,

Many thanks for yr help,

Cominup
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