Affects of pressure on a speaker

[murphyslabrat]

I've been asking a lot of questions lately, which you guys ave been very helpful with...so here's another. How does pressure affect the performance (in any metric) of a speaker? More specifically, is there a linear relationship to the atmospheric pressure and the frequency response arc of a speaker? Are certain frequencies affected more by higher pressures?

This question is spurred from comparing the Sennheiser pmx 100 and the pmx 200. The only difference appears to be that the former is open-backed, while the latter is a closed design. Then, on the tech-specs of a speaker I used in a project, the decibel rating is at a specific pressure. The actual output of the speakers is very different.

As a followup question, would drilling holes in closed-back headphones improve performance, and would any change in sound be predictable?

 

[CycloWizard]

The pressure will always affect anything to do with sound since sound is really pressure waves. If the pressure is higher, more force is required to move the speaker suspension/spider, effectively because there are more molecules in the way of its motion. As far as drilling holes and such goes, the change in performance is predictable, but not without using some complicated finite element analysis.

 

[Billb2]

Hmmmm...

Whatever the density of the air, the speaker still adds the same amount of energy to it.

As for drilling holes in a non-ported speaker, unexpected results can be expected.

 

[Paperdoc]

The difference you cite in the Sennheiser units is not pressure; the air pressure on either side of the moving element is the same when it's not moving. BUT there is a big difference in what restrains the moving element. On the 100 model the air on both sides can move freely and has unimpeded access to the whole outside world. On the 200 the back side has no free access to the outside, so the air behind it is forced constantly to expand and compress as the moving element moves. This is a minor restraint on the moving element, but the system designers may have taken that into account already. Drilling vent holes will certainly change the performance. As others have said, though, you cannot predict the change, and you might have a lot of trouble undoing your changes.

It's always tempting to think of ways to improve an item with some design change. One of the principles applied in Statistical Process Control and many related areas is to recognize that, while it's important to make adjustments when the process is drifting out of optimal conditions, it is equally important to recognize when to leave it alone. Any change can produce consequences that may be more difficult to control than the current minor issues. In addition, you should not be making a change unless you already have confidence in a model of the process that can predict reasonably the result of the change you propose, so that you know your change actually will improve the system's performance.

 

[KIAman]

Pressure only affects density (increases) of the air, which affects the speed (slows) and propagation (longer).

Denser air won't change a single property of a speaker. It WILL change what you hear from your eardrums. If you are in a room that is pressurized to 2 atmospheres and listen to a tone, it will sound lower pitch than the original sound.

This is also why sucking helium causes your voice to go Micky Mouse Mode. Helium is much lighter than air and the speed of sound is around 3x faster.

 

[bobdole369]

Perhaps a bit too pointed responses, I'll offer up my own knowledge:

Ported speakers (open back) vs. sealed speakers (closed back)

If all else remains the same - the sealed speakers will be 3db lower in volume across the entire frequency range. You will likely see a whole lot more attenuation than that - as the frequency response of a ported speaker is completely different than that of a sealed speaker. You might see up to 10 or even 20db in difference at some points.

THe speaker system works best as designed. drilling holes will likely increase volume at the expense of sound quality. It is totally unpredictable other than the known 3db increase. Anything else is a result of the speaker is now unloaded and free(er) to move.

 

[dawgtuff]

I'm an old audiophile, I used to sell esoteric (expensive) audio equipment. Ported speakers are very efficient (power to decible ratio), but at a cost of distortion. Closed (infinite baffle) designs are more accurate, but require larger voice coils and power(watts) to control the movement of the speaker diaphram(cone). If you want the purest sound without harmonic and intermodulation distortion, go with a closed design. Also, DO NOT drill holes in your speaker(s)! It will eventually tear them to pieces. Relative air pressure does have a factor,but it's minor compared to what I've advised.

 

[CycloWizard]

Originally posted by: KIAman
Pressure only affects density (increases) of the air, which affects the speed (slows) and propagation (longer).

Denser air won't change a single property of a speaker. It WILL change what you hear from your eardrums. If you are in a room that is pressurized to 2 atmospheres and listen to a tone, it will sound lower pitch than the original sound.

That's not really true. Pressure is, by definition, the force exerted per unit surface area. Thus, if the pressure is higher on the cone, it will take more force to push it.

 

[bobdole369]

Thus, if the pressure is higher on the cone, it will take more force to push it.

Aha! Combine dawg and my own post and you realize the "why" there is a 3db drop in a sealed design. Sealed speakers need a LOT more power to sound the same as a ported speaker. (i.e. 300W on a jl audio sealed box sub in my hatch is not nearly as loud as the ported design I had designed earlier)

 

[KIAman]

Originally posted by: CycloWizard

Originally posted by: KIAman
Pressure only affects density (increases) of the air, which affects the speed (slows) and propagation (longer).

Denser air won't change a single property of a speaker. It WILL change what you hear from your eardrums. If you are in a room that is pressurized to 2 atmospheres and listen to a tone, it will sound lower pitch than the original sound.

That's not really true. Pressure is, by definition, the force exerted per unit surface area. Thus, if the pressure is higher on the cone, it will take more force to push it.

Pfff, in efforts not to argue semantics, I meant the property of the speaker does not change. The speaker still has the same power handling, tensile strength, elastic properties, magnetic flux and physical composition.

"Requires more power" is not an intrinsic property of a speaker.

 

[f95toli]

No, the properties of the speaker certainly changes. It is important to understand WHY the element is mounted in a box: namely that the air in the box essentially acts as a "spring", i.e. it "resists" the motion of the cone as it moves backwards (since there is nowhere for the air to go in a closed box).

This is obviously not the only reason why you need a box, but suffice to say that the interaction between the speaker cone and the box and how it affects the frequency response is a very complicated topic (and the we haven't even touched topics like how the shape of the baffle etc affects the response).
Note also that the closed box speaker is the simplest design; reflex loaded speaker (boxes with holes in them) are even more complicated.
Then there are other types of speakers that are based upon other principles: horns, transmission lines, dipoles (with different types of open baffles) etc....

 

[CycloWizard]

Originally posted by: KIAman
Pfff, in efforts not to argue semantics, I meant the property of the speaker does not change. The speaker still has the same power handling, tensile strength, elastic properties, magnetic flux and physical composition.

"Requires more power" is not an intrinsic property of a speaker.

Pffff, he asked whether it would change the properties of the speaker, which is a system, not whether it would change the material properties of its constitutive parts. And if you want to argue semantics, pressure will always have an effect on the elastic properties of the cone due to the nonlinear relationship between stress and strain in elastomers (see: Mooney-Rivlin constitutive relations).

 

[Howard]

Originally posted by: dawgtuff
I'm an old audiophile, I used to sell esoteric (expensive) audio equipment. Ported speakers are very efficient (power to decible ratio), but at a cost of distortion.

Some types of distortion go up, other types go down.

If you want the purest sound without harmonic and intermodulation distortion, go with a closed design.

No. These types of distortion increase with current and excursion, both of which are higher with a sealed design, all else equal. The vent of a reflex box decreases excursion and increases sensitivity (for any given transfer function).

 

[Rubycon]

Acoustic suspension systems typically sound more accurate throughout their operating range and will overload more gracefully at extremely low frequency inputs. However compared to vented designs they will produce more distortion at a given volume level than a vented design just on sheer efficiency alone. If a vented design is operated within its frequency design, excursion and distortion stays low all the way up to the point of compression due to thermal stresses in the voice coil.

 

[murphyslabrat]

RubyCon, this last post was a little hard to follow. How I read it, is that closed systems are better sounding in general, and have a better bottom-end, but that they will have more distortion at any given point than a vented system?

And what is "excursion" vs distortion?

 

[Howard]

Closed systems may or may not sound better. Sometimes the most important factor is the flatness of the in-room response, which depends on many factors. Also, a poorly-designed vented box will have audible group delay. Generally, non-linear distortion is lower for the vented box compared to the sealed box.

Excursion is the motion of the diaphragm.

 

[Rubycon]

Sealed systems will have a higher distortion than vented/reflex systems for comparable output. That's IF they can match the output without damage!

 

[Fallen Kell]

Originally posted by: Rubycon
Sealed systems will have a higher distortion than vented/reflex systems for comparable output. That's IF they can match the output without damage!

However, most ported or vented designs suffer from distortions and SPL output level fluctuations at harmonic intervals attributed to the ports or vents in the cabinet. A sealed system can much more easily give a flat SPL response over a frequency range than the ported and vented designs can do, especially when dealing with the lower frequency ranges seen in subwoofers. This is why you will see that almost all high end subwoofers are sealed designed (aside from a few IB systems, but those are typically custom built for the location and not something that can be moved from room to room).

Simply adding your own ports to a speaker is NOT a good idea. The design of the control circuits of good speakers takes into account the type of cabinet, size, shape, and if it is sealed or ported/vented. In the case of powered speakers, even the amplifier used was selected to better match the response characteristics of the speaker. Again, I am talking about well designed speakers, things that typically cost more than $100. The cheaper stuff may or may not have had any real design work done on it, and was simply the product of a bunch of parts thrown together and sold.