Time Alignment® Part I

First things first, the term “time alignment”, is a trademarked term owned by renowned speaker designer Ed Long (E.M. Long & Assoc.).  Sometimes referred to as signal alignment, “time alignment” has stood the test of time as the most commonly associated and accepted term when discussing the topic of alignment as it pertains to speakers (or audio in general).  It’s simply more intuitive as we are contemplating the physical alignment of two or more signals in time.  In part I we will discuss exactly what “time alignment” is and how it applies to system design.  In part II we will explore the many ways in which you achieve proper alignment.

Now the topic of “time alignment” as a whole applies the same laws of physics and acoustics to two very different arenas of system design.  In one arena it pertains to the placement of speaker cabinets in relation to one another and their comparative distance from the listener.  In this case you are aligning the arrival times of the sound coming from the various cabinets.  In the second arena it refers to the physical alignment of the individual drivers within a speaker of 2-way design or more (i.e. 3-way, 4-way, etc.).  So in the first scenario you may be dealing with rather large distances and in the second distances may be measured merely in inches (or mm).

It is important to understand that when discussing full range speaker cabinets it is the face of the cabinets that you will be “aligning”.  However in the realm of speaker design (or the use of separate low, mid, high cabinets) it is the voice coils of the various drivers that need to be aligned.  So in the case of a 2-way cabinet with a low frequency driver and a horn we are not concerned with aligning the face (or mouth) of the horn with the front of the woofer.  It is the voice coil within the compression driver at the throat of the horn and the voice coil of the woofer that we will be taking into consideration.

There are multiple ways of achieving proper alignment, in both arenas, and which method is best will depend on the application of the speaker and its’ designer’s intentions.  We will be discussing alignment via passive crossover design, digital signal processing (DSP) and physical placement of either individual drivers or cabinets.  I personally feel it is invaluable to fully understand the concept of and philosophy behind “time alignment” however its’ relevance is a highly debated topic.   The reason it is so passionately debated is that no matter how infallible the laws of physics are there is always one variable that can never be controlled and that is the listener. 

Let’s say for instance we have two listeners and we give them a pair of small 2-way speakers that have been painstakingly engineered for proper “time alignment”.  Listener number one puts his on stands with the tweeters 32” above the floor for use when seated on his couch.  This is ideal, the listener has created an environment in which he will remain on the same vertical axis as the drivers in the speaker.  Listener number two takes his and places them on a bookshelf 6’ above the floor in his office.  Listener number two has essentially negated the alignment of his speakers (assuming they are sitting up-right) because the tweeter and woofer are no longer equidistant from the listener's ears creating a misalignment.  Therefore it’s scenarios like this that make the element of “time alignment” a debatable speaker design factor.  There is a driver configuration that prevents this scenario from occurring known as coaxial design whereas the high frequency driver is located in the acoustic center of the woofer.  The voice coils of each driver are aligned on all three (X, Y & Z) axes.  Of course just as with any other design those of the coaxial family have their shortcomings as well.

Center channel speakers in surround sound setups are a great example of designs which fly in the face of “time alignment”.  Since the majority of center channel speakers have the drivers in a horizontal array the variable is no longer whether the person is standing or sitting, or the height of the speaker but the position of each listener to the speaker relative to the ‘X’ axis.  Unless you have figured out a way of altering the law of physics stating that no two pieces of matter may occupy the same exact space at any given time then the speaker can only be properly aligned for one position in the room.  Therefore; given four viewers, only one gets to enjoy the bliss of the time aligned center channel speaker.


"Time Alignment"®; trademarked by Ed Long of (E.M. Long & Associates)

by Jason Levert