Is Xmax necessary? (1 Viewer)

[Eric Eash]

most subwoofers, depending on Fs, won't reach Xmax at 50Hz while using a certain amount of power. most around here use there subs for movies, so they only use an amplifier that's rated high enough to get maximum from the driver, but not bottom it out under 20Hz. if you've ever looked at a cone excursion graph, Xmax increases exponentially as the freqency lowers. this is the case with all drivers. depending on the enclosure, you can tune it so a certain frequency will play loud using less Xmax. here is an example.

I have a 550L dual AV15 subwoofer. I am running 450W to each woofer, and the box is tuned at 16Hz. Xmax is reached on the AV15s at 46mm peak to peak. all these numbers are from winISD and indicate playing at full power.
100Hz-------5mm p-p------121dB
50Hz--------16mm p-p-----120dB
40Hz--------23mm p-p-----119.5dB
30Hz--------32mm p-p-----118.5dB
23Hz--------36mm p-p-----118dB
16Hz--------14.5mm p-p---116.5dB
13Hz--------56mm p-p-----111.5dB
10Hz--------93mm p-p-----102dB

as you can see, if i wanted to play a 10Hz tone at 450W, my driver would be destroyed. on the other hand, if all i wanted to do is play a 50Hz tone, i would have to run 3500W to each driver in order to reach Xmax at 50Hz=129dB. as you can see, 3000 more watts got me another 10dB. all these numbers are theoretical. my dB's are higher in my theater room due to room gain of around 10dB. if i'd have made my box different, used a different tuning point, sealed instead of ported, one driver instead of two, all these numbers would be different. since it seems like you are interested in car system with high SPL, that is a totally different beast. car gain is even higher that room gain. as the size of the space the subwoofer is pressurizing lessens, the gain increases, again exponentially. so many factors go in to getting high spl. lastly, to answer your question about the downside to high Xmax with all you want is low bass and high spl. if that is all you want, then high Xmax is a good thing. once again, depending on the freq you want to play. most spl competitions, they only play one note which i think is around 80Hz. keep reading on this forum, there are lots more posts here that will help explain all of this. another thing you need to do is start reading in the DIY forum, you will be able to learn more there.

eric

 

[Matthew Will]

This is the basis of my thinking. Why would you not want a speaker that can handle a 10 Hz note with 450 watts? If you had such a speaker would it make the experience of home theater that much greater? I'm not looking at this specifically from an SPL point of view. I'm thinking maybe a subwoofer with that kind of capabilities would just amaze people for home theater applications. I'm trying to find a downside of such a large xmax. Matt

 

[Brian Bunge]

I guess technically you want a driver that can handle as much power as you can throw at it. If there is a downside to a high excursion/low Fs driver it's the fact that they are very inefficient. In other words, they take more power to get loud.

 

[Matthew Will]

Well is that really so bad? When you look at the kind of power you need to add or how many more speakers you need to add to gain only a few db's, isn't it sort of the same? I would think doubling the xmax doubles the displacement. That is just the same as just adding a second woofer. Except in the case of the increased xmax, the lower frequencies are now much mor eeasily accessible in terms of doing damage to your equipment.

Now here is a follow up question. Why does it take more power for them to get loud and access all that Xmax? Matt

 

[Brian Tatnall]

Greater amplitude in the signal means greater demand for woofer excursion aka more Xmax is used or accessed. Giving more power aka turning up the volume increase signal amplitude.

 

[Matthew Will]

I should probably have been more direct with my question. What determines efficiency? Why can't a subwoofer extend to full xmax on say 100 watts? 50 watts? 10 watts? Is it more on the speaker design or the pressures exerted on the speaker from the air inside and outside of the enclosure?

Matt

P.S. - My copy of the loudspeaker design cookbook 6th edition is in the mail. Will most of my questions be answered in this book or will I need an even more engineer aggressive book for my inquisitions?

 

[Sebastian]

Matt,
Reasons why:
Too expensive- amp, driver
It is not worth it. The chances you will ever use the driver to its full potential is next to nill.
The point of diminishing returns.
Who wants to spend hundreds of dollars just so you can play a 10HZ tone real loud that you can not even hear?

Looks like some of the replies are getting to the meat of your question dealing with Xmax.

 

[Brian Bunge]

Well, the design of the driver itself affects efficiency. A low Fs driver has a heavy cone and may have a stiffer suspension, which both reduce the efficiency. Also, the actual enclosure the sub is used in affects efficiency of the overall system. A large enclosure is more efficient than a small one.

 

[Dennis Gardner]

Xmax ix my favorite day of the year!

More is always better.

 

[Brian Tatnall]

If the the efficiency of the driver is LOW, then either the moving mass is large or the BL is low and if the efficiency is HIGH, then either the moving mass is small or the BL is high. Efficiency, can also be effected as Brian Bunge mentioned by suspension and box.

 

[Michael R Price]

http://www.adireaudio.com/tech_papers/spl.htm

Efficiency is proportional to Fs^3 (meaning reaching lower frequencies with all else equal is a huge compromise); Vas (related to suspension stiffness... high Vas means a soft suspension and necessarily larger enclosures); and 1/Qes (higher motor strength results in lower Qes and values below 0.3 or so tend to cause overdamped response).