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Building a DIY Speaker: Driver Selection

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The driver1.gifsimplest way to determine the proper woofer and required cabinet alignment is to use computer simulation to model the response of several woofers in different enclosures.  Most woofers available to the DIY community have published Thiele/Small parameters that provide the electromechanical properties of the loudspeaker.  The Thiele/Small or T/S parameters are input into software and the acoustic response of a given driver can be modeled in a variety of enclosure sizes and alignments.  For this project, a good 10” driver will meet the required response for this project without requiring a monkey coffin cabinet.  Using SoundEasy, it is pretty simple to input the T/S parameters and generate the expected frequency response and driver excursion.  For a vented enclosure, it is important to find a driver with a Qts between 0.2 and 0.5.  The optimum ratio of the driver’s resonant frequency to Qe for a vented alignment is 100:1 while ratios below 50:1 suggest a sealed enclosure is optimum.  The driver selected for this project was the Peerless XXLS 830843 8 ohm 10” woofer because I happened to have 4 on hand and it meets the project requirements.  After inputting the driver’s T/S parameters from the data sheet into SoundEasy, various cabinet volumes and tuning frequencies were modeled.  The figure below shows the system response for a 2.2 cubic foot vented enclosure tuned to 27Hz in red and a 2.2 cubic foot sealed enclosure in green.

                          03_LF_Response.png

10” Woofer Ported Versus Sealed Response

The 2.2 cubic foot vented cabinet tuned to 27Hz was selected even though it represents a slight misalignment.  Although a tuning frequency of 22Hz would have resulted in flatter response, the cabinet design limited the length of the port tube as lower tuning frequencies require longer ports.  Note here that the sealed alignment in green would yield a pretty flat in-room response and is probably a better overall choice for many listeners.  However, our goal of +/-3dB from 35Hz to 20kHz and high SPL could not be realized with a sealed enclosure.  Most people have heard a woofer or two bottom out and it is important to consider how much abuse a woofer can take before employing it in a design.  Once the T/S parameters are input and the enclosure is modeled, SoundEasy allows a designer to analyze acoustic phase, internal acoustic pressure on the box, port air velocity, group delay, phase and system sound pressure level.  The Peerless woofer used in this design has a linear excursion of +/- 12.5mm.  Below is a graph showing the expected cone excursion with a 150 watt input signal with the vented enclosure in red and the sealed enclosure in green.  Note that this power level yields over 105dB sound pressure level before the driver rolls off for both enclosure types.

 04_Excursion.png

10” Woofer Ported Versus Sealed Excursion

 Clearly, vented enclosures allow significantly more power handling above 20Hz but there is no free lunch here.  Group delay is severely affected by the vented alignment and this means that vented enclosures are sometimes considered to have a less desirable transient response.  Considering the majority of music content is above 40Hz, the group delay for the vented enclosure will suffice for this project.

05_Group_Delay.png 

10” Woofer Ported Versus Sealed Group Delay

At this juncture, it is important to note that the T/S parameters from the loudspeaker manufacturers do not always match the actual T/S parameters.  Many loudspeaker design programs and other tools allow extraction of the T/S parameters.  If precision is key and you do not want to risk cutting cabinets or ports twice then it is important to determine the actual T/S parameters of a loudspeaker before moving forward with the design.  It is often useful to model response using the manufacturer’s data sheet for a ballpark before purchasing a driver and then measure the actual T/S parameters before proceeding with the design using a program such as SoundEasy or Dayton Audio’s WT3 system.

Now that a suitable low frequency driver has been selected, it is typical to find a midrange that is more sensitive.  Generally, the low frequency driver does not require too much response shaping so the critical midrange must have enough sensitivity headroom to allow for response manipulation if needed.  Finding the right combination of woofer and midrange is critical and it is very important to know a few things here.

  • The woofer to midrange crossover frequency can safely be increased as the woofer diameter decreases
  • Higher woofer to midrange crossover frequencies typically equate to less expensive crossover parts such as smaller value capacitors and inductors.
  • Large woofers crossed over too high have bad off axis response at higher frequencies
  • The crossover frequency should be an octave or more away from any driver breakup modes
  • The T/S Parameter for resonant frequency (Fs) should be an octave below the desired crossover frequencydriver2.jpg

Some people love the sound of bookshelf speakers because the dispersion characteristics are difficult to duplicate by speakers with huge drivers.  As with most things in engineering, the art of loudspeaker design is about selecting the set of tradeoffs that make the most sense for a given design goal.  While it would be nice to design a 3-way passive loudspeaker with an 80Hz subwoofer to midrange-woofer crossover frequency, the crossover parts at this frequency are pretty expensive.  This is one application where active crossovers have a huge advantage over passive crossovers.  Since the Peerless XXLS woofer response gets a little funky above 700Hz, we will shoot for a crossover frequency a minimum of an octave below to reduce the amount of response shaping required.  This means that the midrange selected for this project must have enough surface area and excursion to play well below 350Hz at a minimum of 100dB at one meter.  Additionally, the midrange should have a resonant frequency less than 175Hz.  Since I wanted the option to use a 2nd order crossover, a 5 or 6-inch mid-bass was required to meet the target specifications.  The well-reviewed Dayton RS150-8 Reference 6” mid-bass is an 8-ohm driver with the necessary sensitivity, excursion, diameter and low resonant frequency required to meet the design requirements. 

The tweeter selected for ttweeter.jpghis project was the Seas Prestige 27TDF.  This tweeter was selected because it is also well reviewed, has good power handling specifications, is relatively linear and has a low enough resonant frequency.  It is important to select a tweeter with a resonant frequency low enough so that the crossover frequency is a minimum of two times the tweeter’s resonant frequency.  The data sheet recommends usage beginning at 2.5kHz.  Since the Dayton RS150-8 mid-bass driver is pretty large, crossing over as low as possible will provide better off-axis dispersion.  Given this, we will attempt to design a crossover frequency of less than 350Hz for woofer to midrange and 2.5kHz for midrange to tweeter.  Ultimately, the crossover frequencies may change depending on how the drivers measure in the target cabinet.  With the drivers selected and T/S parameters for the low frequency driver, we can begin designing a cabinet.

 

 

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Recent Forum Posts:

utopianemo posts on July 14, 2020 00:06
It’s about time someone revisited this topic. There are so many sites out there now providing great kits! Aside from the previously mentioned Madisound, Parts Express, and others, we have GR-Research, DIYSG, CSS-Audio, Meniscus, and more.
erinpreston27 posts on July 19, 2013 00:27
I just finished a quick read of this.





fuzz092888 posts on July 18, 2013 16:32
It's great software, but don't forget to factor in the cost of LMS as well. You'll need it. If you've never designed a speaker before you might want to go with soundeasy. It's much cheaper and does nearly everything that LEAP does.

audiofox, post: 977984
I always wanted to splurge and get a copy of this software for my speaker building hobby-this is the SW that Madisound uses for their custom crossover and speaker design services.

LEAP Software - Analysis & Design of Loudspeakers, Enclosures, and Crossovers
audiofox posts on July 18, 2013 13:58
I always wanted to splurge and get a copy of this software for my speaker building hobby-this is the SW that Madisound uses for their custom crossover and speaker design services.

LEAP Software - Analysis & Design of Loudspeakers, Enclosures, and Crossovers
Joel Foust posts on July 07, 2013 13:10
I've heard the art USB dual pre works in soundeasy and is decent for the money. I have no direct experience so I can't recommend it but it might be worth researching for < $100 interface. Anyone use it?
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