Plug bass reflex port... heating problems?

[Davide]

Hi everyone,
I have a question that is baffling me.
I have bookshelf bass reflex speakers, which I don't know if due to proximity to the wall or port leakage, I much prefer with the ports plugged.
From frequency response measurements they are also a bit flatter below 700Hz.
I have two subwoofers perfectly integrated with Dirac Live Bass Control, so I have no bass shortage plugging the ports.
Now, since the speakers are designed as reflex, I doubt that plugging the port could create a motor or crossover overheating problem especially at high volumes (which I prefer).
Are this a mental paranoia or is there some risk in your opinion?

 

[AdamG]

Hi everyone,
I have a question that is baffling me.
I have bookshelf bass reflex speakers, which I don't know if due to proximity to the wall or port leakage, I much prefer with the ports plugged.
From frequency response measurements they are also a bit flatter below 700Hz.
I have two subwoofers perfectly integrated with Dirac Live Bass Control, so I have no bass shortage plugging the ports.
Now, since the speakers are designed as reflex, I doubt that plugging the port could create a motor or crossover overheating problem especially at high volumes (which I prefer).
Are this a mental paranoia or is there some risk in your opinion?

Best to ask this question to the Manufacturer. They may have their own Forum and a means to ask questions about their products. As you indicated you are using electronic bass management. If so, could you not just use a low pass filter on the bookshelf speakers and redirect that content to your subs and leave the ports open as designed to be?

 

[voodooless]

Remember that you’ll have to recalibrate when plugging the ports. If you did not, and prefer still the sound, then most likely your problem is with the calibration.

As for overheating, I think that would be unlikely, especially if you offload anything below 80 or so Hz to subs.

 

[DVDdoug]

Plugging the ports shouldn't affect the power handling. There's even a slight possibility that they can handle more power because the woofers won't move as much... But usually it's the voice coil than burns-out. And hopefully, you aren't THAT close to burning-out the speakers anyway.

Now, what could happen if you have enough power to damage the speakers... You plug the ports and that reduces bass. Then you run Dirac (or other EQ) and the bass gets boosted to compensate for the reduced bass out of the speaker, and now you're pumping more power into it.

But as voodooless says, the subs should are handling the bass so the main speakers probably aren't being pushed that hard. But they are obviously "doing something" in the bass range if plugging the port makes a difference, and it also depends on your bass management settings.

 

[Davide]

My system is Dirac calibrated with the ports plugged, of course.
Unfortunately the effect of the ports is not only on the low frequencies but they also contaminate the frequencies slightly above, up to 700hz in my case.
By ear there is more blurring and from measurements with REW I found more jagged response up to 700hz indeed.

However I asked the manufacturer about the plug on the port and he said it doesn't do any harm. But it seemed like an answer with no technical content... like those things that have never given problems so they don't consider them...

Anyway, what should be the object with the greatest loss and therefore heat? Woofer, tweeter or crossover?

 

[tmuikku]

Hi,
dynamic drivers are quite inefficient, only few percent of electrical power fed in turns into acoustic sound and rest is emitted as heat. You can simplify and assume ~all used amplifier power turns into heat somewhere. Most of it in voice coil if that's the dominating resistance, some could turn into heat on some other high resistance like in a resistor in crossover network, or if you have long thin wiring. Some heat happens in moving parts, perhaps somewhere else, but I think its fine to make a thought experiment assuming that "all power heats voice coil".

Heat makes resistance of the wire go up, which affects impedance of the circuit which has effect on system frequency response and sensitivity. Basically system Q apparently rises and output goes down. Heat from voice coil is transferred to the surroundings, to the magnet and structure of the driver which then radiates the heat into surrounding air, inside a box. If there is ports the air gets at least some heat exchange to outside world increasing temperature difference between magnet and air and helping the system cool bit more efficiently than with closed box. Closed boxes usually have insulation, so heat builds up inside the box with some time span, magnet stays hot, voice coil gets hotter than before and so on.

How problematic this is? depends on your system sensitivity and how loud and how long and what crest factor your music is on. If you have 12" driver and background listening it's not too much, perhaps few watts, heat like a LED light bulb, not much of a problem. Perhaps it heats like a phone charger. But what about if there is hundred watts or more continuously for prolonged periods? A kilowatt? Heat transfer is suddenly a big problem, the voice coil turns into a toaster.

So, plugging ports could be problematic, or not. Hopefully you have now some questimate how big of an issue it is for you! In general, one would want to avoid any significant change in sound, so any significant heat buildup in the system.

If it's a problem just make few more subs, with bigger cones and enclosures to get sensitivity up and more acoustic output per electrical watt. You could mount the drivers magnet out if you wish, but again with high power they likely make mechanical noise so it's better to use biggest bass system you can comfortably fit in your room, for many reasons and heat management could be one.

You can find numerous articles about the stuff, for example:
https://blog.miscospeakers.com/thermal-management-of-speaker-systems

Hot Stuff: Loudspeaker Voice-Coil Temperatures

As Hans Christian Oersted, the Danish physicist and founder of electrodynamics, discovered in 1819, an electric current passed through a wire generates a magnetic field. Place that wire close to a permanent magnet and the interaction of the two fields will generate a force. That, in two...

www.stereophile.com

 

[ZolaIII]

It depends on boxes internal volume (bigger one helps of course) and minimum impedance of drivers used (higher will put less pressure and they will last longer disreging of enclosure). Depending on SPL even port's change their function from outputting a lot of air to almost none.

 

[voodooless]

Hi,
dynamic drivers are quite inefficient, only few percent of electrical power fed in turns into acoustic sound and rest is emitted as heat. You can simplify and assume ~all used amplifier power turns into heat somewhere. Most of it in voice coil if that's the dominating resistance, some could turn into heat on some other high resistance like in a resistor in crossover network, or if you have long thin wiring. Some heat happens in moving parts, perhaps somewhere else, but I think its fine to make a thought experiment assuming that "all power heats voice coil".

Heat makes resistance of the wire go up, which affects impedance of the circuit which has effect on system frequency response and sensitivity. Basically system Q apparently rises and output goes down. Heat from voice coil is transferred to the surroundings, to the magnet and structure of the driver which then radiates the heat into surrounding air, inside a box. If there is ports the air gets at least some heat exchange to outside world increasing temperature difference between magnet and air and helping the system cool bit more efficiently than with closed box. Closed boxes usually have insulation, so heat builds up inside the box with some time span.

How problematic this is? depends on your system sensitivity and how loud and how long and what crest factor your music is on. If you have 12" driver and background listening its not too much, perhaps few watts, like a LED light bulb, not much heat and not much of a problem. Perhaps it heats like a phone charger. But what about if there is hundred watts or more continuously for prolonged periods? A kilowatt? Heat transfer is suddenly a big problem, the voice coil turns into a toaster.

So, plugging ports could be problematic, or not. Hopefully you have now some questimate how big of an issue it is for you! In general, one would want to avoid any significant change in sound, so any significant heat buildup in the system.

If it's a problem just make few more subs, with bigger cones and enclosures to get sensitivity up and more acoustic output per electrical watt. You could mount the drivers magnet out if you wish, but again with high power they likely make mechanical noise so it's better to use biggest bass system you can comfortably fit in your room, for many reasons and heat management could be one.

You can find numerous articles about the stuff, for example:
https://blog.miscospeakers.com/thermal-management-of-speaker-systems

Hot Stuff: Loudspeaker Voice-Coil Temperatures

As Hans Christian Oersted, the Danish physicist and founder of electrodynamics, discovered in 1819, an electric current passed through a wire generates a magnetic field. Place that wire close to a permanent magnet and the interaction of the two fields will generate a force. That, in two...

www.stereophile.com

Don’t forget that there is quite a bit of heat transfer to the outside via the basket as well (assuming it’s made from metal).

Unfortunately the effect of the ports is not only on the low frequencies but they also contaminate the frequencies slightly above, up to 700hz in my case.

That may very well be the case. Badly designed ports may leak quite a bit of midrange and can cause what you described.

 

[tmuikku]

Always forget something, or just don't know Volt has product line that improves heat exchange https://voltloudspeakers.co.uk/radial-technology/ making the basket outside of the box. You could implement liquid cooling, drill as deep as you like.

For hifi application, it's just using as many big drivers in big boxes as possible, for the lowest octave(s). Less is likely fine as well. It is not too hard to calculate/measure some numbers for typical SPL one listens at to get more accurate estimation.

edit. volume displacement chart for sealed enclosure attached. It shows how much volume displacement is required to get some SPL output at some frequency with sealed enclosure. Take 30Hz and 100db for example, eyeballing about 250cm3 volume displacement does it. 12" woofer has cone area of ~550cm2 or so, so about 5mm p-p excursion. If you have very small box, then this might take quite much power, if its large box it could be four times less. Two woofers the volume displacement is split in two, excursion halved, power reduced some depending on how its all connected and if you sit at peak or dip of 30Hz mode and so on. It is quite easy to estimate and compare things like this by using imagination, simple calculations.

It is not too complicated to get hold on this subject and be sensible with it. For example, a 15" driver has almost 7x cone area as single 6" driver, so one could assume that if each was in a suitable box it takes 7 times less excursion for the 15" to make same acoustic output, which gives hint that much less power is needed, much less heat, much less excursion and distortion overall. Problems from audio quality are exchanged for problems with wallet and looks and fit, not too much voodoo involved with bass. If your calculations show 6" suffices then there is no need to go bigger. Exotic solutions are required when system needs to be pushed to limits, for example if you must not have your driver bigger than 12" and box must be of some size and it is constantly played at such level heat makes audible difference, then you'd need solutions like the Volt to maintain high performance at extreme. Otherwise just use two 12", better yet four 18", what ever is sensible yet enough capability for your use case so that the system is not required to do miracles.

 

[Looneybomber]

Now, since the speakers are designed as reflex, I doubt that plugging the port could create a motor or crossover overheating problem especially at high volumes (which I prefer).

In a home environment, your speaker drivers and crossovers will produce negligible amounts of heat. Even playing 10w pink noise, while being annoyingly loud, will still produce little heat within your enclosure. To experiment, you could stick a meat thermometer (my cheap digital one reads in one degree increments) into the enclosure and plug the port.

 

[Davide]

Thank you all for the very technical and comprehensive replies. That's what I like about ASR!
I understand that technically it is not possible to quantify how much heat is generated and how much of it is dissipated thanks to the recirculation of the port (assuming that there is recirculation... I'm honestly not sure).
In fact I recognize that the only way to identify a potential problem in this case is to check the temperature difference between the two conditions with a probe applied to the motor.
I'll do some testing as soon as I find some probes and report the results.

 

[voodooless]

And also remember: your amp max power says very little about the average power send to a speaker. A 100W amp will on average do less than 10W running with music at full tilt and more than 99% of that will be turned into heat.