point of origin of sound in a speaker

This is a question that I am sure must be covered in the basic theory of speakers, but I havent seen it discussed anywhere, at least not in a form I recognize. Basically, does the sound come from the apex of the cone, ie voice coil, or the outside of cone, ie perimeter, or all the points in between ?

It is said that to get sound from two drivers to be in phase, lets say from a woofer and a tweeter, that the voice coils should be vertically aligned, that way the sound from each reaches the listener at the same time. Thus mounting each on a flat panel baffle is not accurate enough since the woofer is deeper than the tweeter, and sound from the woofer voice coil has farther to travel.

But that assumes that the sound is originating at the voice coil of each driver. It seems to me however that if the cone is moving as a piston, which I believe is what is strived for in a well designed driver, then the linear displacement of the cone is everywhere the same. So there is in fact more air dispaced at the outer edges of the cone then at the apex (voice coil end) of the cone, due to the larger circumference.

So why is it assumed that it is the voice coils that must be aligned ? If more sound comes from the outer edges should it not be the outer edges that should be aligned, which is what happens naturally on a flat baffle ?

Instead, is there some sort of center of gravity so to speak for the speaker sound, ie an effective point of origin somewhere between the voice coil and the outer edge ? and this is the point that should be aligned ?

Or is it the case that the sound is distributed, some coming from the voice coil end, some from the perimeter, and some from all points in between, and if so, would it not be the case that a narrow pulse of sound from the speaker will be recieved at a point a few meters away as spread out over a length of time equal to the time it takes for the sound to go from the voice coil to the perimeter of the speaker ? ( ie about .4mS for a 6" deep speaker by my calculation ) Ie sound from the edge gets there first, then .4 mS later the sound from the voice coil gets there ?

And would that mean that trying to phase align the drivers is futile since the sound is spread in out over time any way?

Or have I missed the point completely ?
 

MaVo

Member
2006-01-07 12:40 pm
wavelength is relevant. at 1000hz you have a 34cm long wave. at 100hz it is 3,4 meters long.

other thing is time alignment. woofers have inherent delay additional to their deeper cone, which can be quite big, depending on the woofer and enclosure. in a ms the sound travels 34cm, with a group delay of about 2-5 ms for closed boxes in the bass frequencies, the woofer will be virtually 1-2 meter behind the box.

are you speaking about aligning the drivers on the baffle of the speaker (which is more for aesthetic purposes), or in the depth dimension (which is time alignment)?
 
richie00boy said:
The cone flexes to move the air, different parts of the cone will flex depending on the frequency. Generally the higher frequencies tend to come from towards the centre.

So from the centre right to the circumference is the short answer.

I would disagree with that , speaker designers strive to prevent cone breakup at all costs.

Speakers produce sound by moving air not cone distortion.
 

MaVo

Member
2006-01-07 12:40 pm
Ron E said:
The ear is insensitive to time alignment. We cannot hear waveshapes, only spectrum. If the phase or time alignment issue causes peaks or dips in response, you will hear it.

music is all about time! delay a part of the waveform for a millisecond and the change is minimal. delay it for a second and you will see the problem. only with non transient signals like sine waves and continous noise, time is irrelevant. with transient material, time is crucial.
 

roddyama

diyAudio Moderator Emeritus
2002-01-19 9:25 am
Michigan
Ron E said:
If the phase or time alignment issue causes peaks or dips in response, you will hear it.
This is exactly why time alignment is important, because it will cause peaks and dips in what is heard. However, just locating the so called "origin" of the sound wave is not the only factor to be considered. Assuming you can find the origin (not an easy task) you must also consider the group delays. And not just those caused by the cone breakup patterns and effects of defraction and dispersion but the crossovers and any other frequency dependent element in the sound reproduction path. The goal is not to obtain time alignment just to create the "perfect waveform, but to eliminate the peaks and dips refered to by Ron.
 

roddyama

diyAudio Moderator Emeritus
2002-01-19 9:25 am
Michigan
purplepeople said:
"We cannot hear waveshapes..."

I reject that wholeheartedly. In fact, I'd say that Bob Moog, Ray Kurzweil and Dave Smith would also disagree with you. The only reason we can have sound synthesis is because our ears can distinguish small changes in waveform.

:)ensen.
Technically :)ensen, Ron is right on this point. We don't hear the waveform, we hear two points in the waveform at any point in time. The ear has no way to perceive the entire waveform.
 
Regarding where on the speaker cone a given sound originates from, that is dependent on frequency. That's why a 'whizzer' cone works.

When a high frequency is placed on a general single-cone speaker, high frequencies emanate from the center of the cone, and low frequencies emanate from the entire cone. As frequencies decrees from high to low, more and more of the cone come into play. Meaning that mids, radiate from the center area of the cone; that is, from the voice coil to roughly half way out toward the edge.

The reason is that at high frequency, the sound has come and gone before the forward energy has overcome the inertial mass of the cone. At low frequencies, there might be some slight leading flex in the cone center, but overall the entire cone moves forward.

When a woofer rolls off at the high end, it is because the inertial mass is so high, that the high freq sound pulse has come and gone before the woofer has time to mechanically respond. The high pulse simply doesn't have the time or energy to overcome the woofer mass.

Tweeters have diaphragms with very little mass and therefore very little inertia, and are therefore easily and quickly able to respond to mid and high frequencies.

In it's working frequency range, one can consider the entire woofer cone to be the source of the sound. But that source originates, from a time perspective, at the voice coil.

As to speaker alignment, yes, you would prefer to have the sound source, that is the voice coils, all in a line, but how many speakers have you actually seen that do that? Virtually none. That is a practical design consideration that makes commercial speakers easy to manufacture and affordable, and for most listeners represent an error that very few of them are able to perceive.

Every design compromises something.

steve/bluewizard