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The Loudspeaker- Tieing both the electrical and mechanical together.

The cross section of a Loudspeaker

The cross section of a Loudspeaker

Previously, we attempted to “divorce” the electrical from the mechanical portions of a loudspeaker, but we were only semi-successful at doing that. Today we are going to have a reconciliation and get both parts doing their job and working together like a hand in a glove. They really have no choice, but they can work well together and do a good job or less well and do a lousy job.

  • In the last post on loudspeakers,  “The Mechanics of a Loudspeaker” we talked about how speakers have only a limited frequency range where it is able to operate effectively.  This is partly because the low frequency speaker,the wolfer, needs to move a large volume of air and the mass of the cone prevents it from operating at high frequencies. The high frequency speaker, the tweeter, has a small area and cannot move enough air to work well at the low frequencies.  Several speakers are used. Electrical filters are built into the speaker enclosure to cause each speaker to only “see” the frequencies it is designed to use.   The wolfer has a low pass filter connected, the tweeter has a high pass filter connected to it, and the mid-range speaker has a band pass filter ahead of it.  Often this is an inductor in series with the wolfer, a capacitor in series with the tweeter, and some sort of LRC network ahead of the mid-range to clip the high frequencies and not pass the low frequencies to the mid-range.   This is called the “crossover network” in speaker systems.   An introduction to simple filters is in my posts:  “Approximation of RC or RL circuits”  and  “Approximation of RC & RL circuits — Adding Complexity

Now that each loudspeaker is only getting the signals it can handle, we move on to the design of the enclosure.   The enclosure is a lot more than just a simple box to hold all the parts, it is actually part of the mechanical sound producing system and often monitoring the electric properties are used to tune the system.  The main area where most of the speaker design occurs is the low frequency wolfer design.  Much less is written about the tweeter and even less about the mid-range speaker.   For the following discussion refer to the impedance plot of a wolfer found on the MCM electronics website.   The speaker we will look at is an expensive 18″ pro-sound wolfer.  The picture we are interested in on that site is the middle thumbnail just below the picture of the speaker.  The interesting part of the impedance curve is the way it peaks at about 30 Hz.   Notice in the specifications on the speaker the low end of the frequency range is also 30 Hz.  What is happening here is the mechanical system consisting of the weight of the cone and the spring of the spider is resonating at this frequency.   This means the voice coil is moving naturally at this frequency and the movement is creating a greater Counter ElectroMotive Force (C.E.M.F) at this frequency and that causes the impedance to be higher.  If this was mounted on an infinite baffle the speaker would be noticeably louder at this frequency.

What I have just said is a very significant property and really is not intuitively easy to understand.  “Huh, you just said it resonates at this frequency which means it is easy to push at this frequency… So why did impedance increase?”  The impedance increased because the coil is moving more easily and creating a counter voltage to the driving voltage.causing it to be harder to force current through the voice coil at the frequency.   The mechanical system has fed back into the electrical system!   This sounds simple and only took a few lines to write, but it is one of those concepts that will take a while to “sink in”.  It took a little while for it to sink in my head.

The goal for our enclosure would be to design the enclosure in such a way that we minimize that peak and preferably tune the enclosure so the enclosure would want to naturally peak at a frequency somewhat below the peak of the speaker by itself.  This would give us the full speaker range without the low frequency resonance and move the frequency of the whole system a little lower.   This design would be initially calculations of the volume in the enclosure and/or the length of any sound path from the back side of the speaker to a port in the front of the enclosure.  Once the box was build, experiments would be done while tuning and a plot would be created of the final impedance curve by measuring the voltage to the speaker and the current through the speaker and calculating the impedance at various frequencies.  This is long, hard meticulous work and not something I have decided to tackle.  Obviously an enclosure for this loudspeaker would be very large simply because the speaker itself is very large at 18″ (Approximately 45 cm) and also the internal sound path would be very large because of the low frequency involved.  But, IMAGINE THE BASS!

Other fine points in the design is to use very heavy wood or medium density fibreboard (MDF) to build the box and inner baffles. This would minimize the vibrations of the box.  Also corners inside the box should be rounded as much as possible to eliminate pockets that could resonate at a higher frequency and create unwanted sounds.  Sound insulation and diaphragms may be necessary to reduce the sound from the backside of the speaker.  In other words lots of things must be looked at and thought about and that is beyond my ability to present on this blog.  Of course the final test is the actual human ear test.  Does it meet the listener’s expectations?  Does it accurately reproduce the sounds from the recordings?

Once the box is designed, some thought should be given about the placement of the speakers in the room.  Putting the speakers in the corner of the room is usually a bad idea because the corner forms a “horn” and amplifies certain frequencies causing “booming” of those frequencies.   People that really get into all of this also worry about sitting the speakers directly on the floor because the floor may vibrate and cause distortions.   Small rooms are a problem because the room naturally resonates at some frequencies.   This is why many people like to sing in the shower, because the shower stall acts as an organ pipe and makes their voice sound better.

This post was written primarily to talk about the interactions of the electrical system to the mechanical system and as the first baby step into motors.  A loudspeaker was good for that because the system is simple linear motion and we don’t have to deal with rotational motion and various coils being switched off and on.  However, it was not an ideal system to talk about because a loudspeaker has a relatively high resistance in comparison to the amount of CEMF generated by the coil.

For those of you that want to know more about loudspeaker enclosure design, I found several sites by simply doing a Google search with the terms: “loudspeaker enclosure construction”.  There are also books available.  I would start at a good public library and then move on to purchasing books and hopefully you can find some at used bookstore.  No matter what the marketing hype says, revolutionary is not really something that applies in this subject.  Speaker systems have been around for a lot of years and new books on a limited subject such as this are very expensive.  Once when I thought I might want to build some enclosures I purchased some, but I have since disposed of them.  It is an interesting subject, but it is one that is as much art as it is science.

During my research for these posts one of the sites I ran into that seems to be very good is this one  Art Ludwig’s Sound Page

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The Loudspeaker- Tieing both the electrical and mechanical together.” by Create-and-Make.com is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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Gary


 

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