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Old 29th November 2003, 02:51 AM   #1
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Default Power, SPL v Vb

Hi

If I force a decrease in a woofer's bass response by changing the Vb what does this do the power handling/spl of the driver?

If it decreases the spl does the excess energy get burned up in the voice coil?

Mos
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Old 29th November 2003, 12:27 PM   #2
sreten is offline sreten  United Kingdom
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Default some basics

Hi,

a bass unit has excursion capabililities beyond which it distorts.

It also has a thermal limit beyond which basically the voice coil
will melt. The temperature of the voice coil is determined by power
and time. e.g. a 50w voice coil will melt if driven at 75W continuously
after a few minutes, but can easily accept a 250W mid-range transient.

Changing Vb does little to the midrange power handling / SPL.

However PH at low frequencies is determined by excursion limits.

Reducing the box volume Vb gives a less extended bass response
consequently reducing bass unit excursion and increases the SPL
the speaker can handle on music program before bass overloads.

The 'excess' energy is really the bass unit having a harder time
compressing the smaller air volume in the smaller box - this is
reflected in the change of the impedance curve of the speaker.

/sreten.
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Old 29th November 2003, 12:54 PM   #3
Mark Kravchenko --- www.kravchenko-audio.com
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Default Typo fix

Sreten is correct about almost all of what he said. The only correction is about a smaller box volume. The smaller volume actually reduces a given spl/watt output because of the driver motor trying to reach higher air compression levels due to the smaller box volume. This will cause the voice coil to heat up and the failure mode is usuallt the coil inter-wire adhesive followed by the coil cone junction. The wire inself can melt but not typically in a home environment.

Two cents on an allready good explanation.

Mark
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Old 29th November 2003, 01:35 PM   #4
sreten is offline sreten  United Kingdom
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Default Re: Typo fix

Quote:
Originally posted by mwmkravchenko
Sreten is correct about almost all of what he said. The only correction is about a smaller box volume. The smaller volume actually reduces a given spl/watt output because of the driver motor trying to reach higher air compression levels due to the smaller box volume. This will cause the voice coil to heat up and the failure mode is usually the coil inter-wire adhesive followed by the coil cone junction. The wire inself can melt but not typically in a home environment.

Two cents on an allready good explanation.

Mark
Just to clarify :

Smaller Vb does reduce SPL/Watt in the low bass, but the SPL
the speaker can handle before the bass level excursion limits
is increased, assuming of course bass is excursion limited.

This is generally why you don't get small drivers in big boxes.

Voice coil melting = failure by heat :
But sometimes it It does ! Once the adhesive melts / chars the
wires begin to seperate and the coil distort - this causes rubbing
noises for an overheated coil - if a wire becomes seperated on its
own without the heatsinking of the surrounding wires it will melt.

Finally - I'm sorry but I cannot agree that the higher air
compression causes heating of the voice coil - it doesn't.
The air in the box is a reactive component - you cannot
do work against it, what changes is the impedance curve.

Of course to move the cone the same amount against against
this stiffer spring, and reach excursion limits, does require more
power, thats why the bass power handling goes up.

But there is a point where you have curtailed bass extension so
much that you reach the thermal limit of the driver before you
reach its excursion limits, but your no longer using the driver as
a bass unit.

/sreten.
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Old 30th November 2003, 02:26 AM   #5
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>Smaller Vb does reduce SPL/Watt in the low bass, but the SPL
the speaker can handle before the bass level excursion limits
is increased, assuming of course bass is excursion limited.

Hi,
This is not correct - a woofer in a smaller box can handle more POWER before the excursion limited output is reached. The sound pressure level it produces will be the same given the same cone excursion - it just takes more power to achieve that excursion since you have made the system less efficient at producing low frequencies.

John
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Old 30th November 2003, 09:07 AM   #6
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I have to disagree about the air inside a speaker box being a (purely) reactive component. When the speaker's cone moves in, the air inside is compressed slightly and that energy is stored as heat. Some of the heat is lost to the surroundings etc. So effectively there is a non-reactive damping component to the air in the box.

CM
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Old 30th November 2003, 02:53 PM   #7
sreten is offline sreten  United Kingdom
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Re CM's point :
yes its true its not purely reactive (what is ?) there will be some
small resistive losses with internal damping - but changing the
box volume doesn't change this situation.


Re JS's point :

Bad wording on my part - should read something like :>

Smaller Vb does reduce SPL/Watt in the low bass, but the
overall program SPL the speaker can handle increases due to
the speaker needing more power to reach bass level excursion
limits, assuming of course overall maximum level is limited by
excursion limits in the bass - the smaller the bass unit the more
likely that this is the case.

/sreten.
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Old 1st December 2003, 03:45 AM   #8
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In trying to answer MOS FETish's original question, I think a reduction in box volume can both reduce cone excursion at very low frequencies, but also increase the excursion slightly at higher frequencies. This might put extra mechanical strain on the suspension/surround (unlikely though unless it was already straining).

When reducing the size of a sealed box the woofer would often waste more of its energy at low frequencies (eg: <50Hz), but becomes more efficient at higher frequencies (eg: 70Hz to 120Hz). Thanks to the impedance curve, the amp is spared from having to operate at full power across the woofer's whole range.

CM
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