The Precision Low Frequency Signal Generator produces a User-selected signal from 10Hz to 90Hz with 0.1Hz resolution for testing subwoofer speakers. The frequency is displayed on a 16x2 LCD.

The sine wave is produced by stepping through a table of 64 values and sending them to the MCP4921, a 12-bit serial DAC that uses an SPI interface.

This is how to use the Precision Low Frequency Signal Generator.

INC

- Press and release to increase the frequency

- Press and hold for about 2 seconds to keep increasing the frequency; release to stop

 DEC

- Press and release to decrement the frequency

- Press and hold for about 2 seconds to keep decreasing the frequency; release to stop

MODE

- Press and release to toggle the frequency step between 0.1 Hz and 1.0 Hz

- Press and hold for about 2 seconds (then release) to enter Sweep mode at 0.1 Hz increments

- When in Sweep mode Press and hold for about 2 seconds (then release) to exit Sweep mode

The skill level for stuffing the board is: Intermediate. Make sure you use a temperature-controlled soldering iron. I use a temperature of 340 degrees Celsius. And don’t inhale the solder fumes. If you don’t have a fume hood use a fan to blow the solder fumes away from you.

To compile the source code and download it into the microcontrollers I use Texas Instruments Code Composer Studio. It is free and is on this page: https://www.ti.com/tool/MSP-EXP430G2ET where you can get the versions for Windows, Linux and macOS.

The direct link to the current version for Windows is: https://software-dl.ti.com/ccs/esd/CCSv10/CCS_10_4_0/exports/CCS10.4.0.00006_win64.zip .

The skill level to use Code Composer Studio is: Advanced. (Maybe Really Advanced).

Notes:

1. You can use an MCP4921 in the PDIP package or the SOIC (SMD) version.

2. To program the MSG430G2402 you connect the Precision Low Frequency Signal Generator to the Launchpad with three wires. To make it run standalone remove the three wires and use a header plug to connect the /RES and /Reset pins on the header (on the Signal Generator).

3. The audio outputs are: Line Level, Speaker, and Speaker Test. The Speaker Test has a 100 Ohm resister in series for testing speakers.

The testing of subwoofer speakers and the design of subwoofer cabinets is beyond the scope of this article. I recommend the following:

1. For testing subwoofer speakers to derive the Thiele-Small (TS) parameters: Measuring Loudspeaker Parameters by Rod Elliott

https://sound-au.com/tsp.htm

Basically, you:

a. Connect the Precision Low Frequency Signal Generator to the subwoofer speaker through the onboard 100 Ohm resistor. This makes the output a current source. You will need a sensitive voltmeter with a good low frequency response or you can use my companion project: Woof Meter which contains a low frequency preamp, a precision full-wave rectifier, and a DC filter. It has a voltage output that can be read with any standard DC meter. It also has a current output for use with an analog 1mA meter.

b. Find the free air resonant frequency of the speaker. That is where the voltage across the speaker is at a maximum.

c. Find the higher and lower frequencies where the response is down 3dB. That is 0.707 of the voltage (or current) at resonance.

d. Add a weight of 25 grams to the speaker cone. A U.S. nickel weighs exactly 5.0000 grams so tape 5 nickels onto the cone. (Use masking tape.)

e. Repeat the above tests: find the resonant frequency and the two -3dB frequencies.

f. There are other things for you to measure (with a standard multimeter).

g. Plug your measurements into the proper formulas and you will get the TS parameters. Then you can design the cabinet.

2. For designing subwoofer cabinets for vented, sealed, and custom bandpass cabinets: https://www.ajdesigner.com/speaker/ . Also see: http://www.mh-audio.nl/Loudspeakers.html#top

I live at an altitude of about 6,000 feet. I designed the Precision Low Frequency Signal Generator to see how the altitude affects the TS parameters of a subwoofer. To do that I put the subwoofer in a 55 gallon drum and pressurized it in stages down to sea level.

I have posted more information (which you need) and more pictures on my Web site: www.jmargolin.com/newprojects/woof-tester/woof-tester.htm

This includes the source code. The source code is complete. It does not require any libraries other than the ones that come in Code Composer Studio.

Happy Subwoofer testing and building.

Jed Margolin

Virginia City Highlands

Storey County

Nevada

11/8/2021