Measuring an oblate-spheroid waveguide, circa 2010
Never embark on a loudspeaker project without decent measuring tool. This is my set of tools to measure speaker performance. My criteria is simplicity and accuracy.
- Sound Card
Soundblaster Live! 24 bit - $60
ARTA. The demo is free and fully functional, except you cannot save files
Alternatives: HOLMImpulse, REW (Room EQ Wizard), Speaker Workshop. All free.
DIY Panasonic WM-61A microphone - US$3 (plenty from ebay)
Alternative: Behringer ECM8000
To utilise the WM-61A is quite easy. Electret mics all needs "phantom power" and the circuit is something like this, just need a 9-volt battery, 2k2 Resistor, and 10uF Capacitor (non polar). Of course if you want the real deal you can create something like SL's measurement mic or something like this.
The output plugs to left channel if USB DAC's "Line-in". On stereo 35mm connector it's the pointy bit. Mic's ground is the one connected to the body.
The finished product! ... I use chopstick to make it more tasty.
Close-up pic of the WM-61A. They're only like $3 a piece but very flat!
Download and install ARTA from this website, it's quite easy to use. For more functionality such as enclosure design etc. You need to use Speaker Workshop which is free, but very difficult to setup. I actually tried and gave up.
What I like to measure using ARTA are:
1. Frequency response. Simply point the mic to the speaker, choose "FR1" and click the play button. There will be pink noise heard and graph should appear. Something like this. In this case there were two measurements - the yellow line shows bass reflex port response.
2. Harmonics distortion - When a speaker is fed a tone, let's say 40Hz it should only sound 40Hz right? Well unfortunately no. There will be other tones heard as well and these are called harmonics. Ideally the harmonics should be none. The harmonics came in multiplies so for 40 Hz. ther will be:
2nd harmonics - 80Hz
3rd harmonics - 120Hz
4th harmonics - 160Hz
People say that 2nd harmonics sounds more pleasant than 3rd and above. To measure these harmonics, choose "SA" or spectrum analyzer then configure the generator to sine wave at the required frequency.
3. Harmonics sweep - this is similiar to the above, only that it is performed in sweeps. To do this quit ARTA and in the start menu there will be a program called "STEPS". Similiar to ARTA, run this and you'd get this diagram. In the example below it is clearly shown that the driver being measured performs best above 70Hz, where its harmonics are low. This is TangBand W3-1364 and with distortion of around 1% like that it's quite impressive.
So when all has been measured will the speaker sound good? NO. I view these measurements as a guide only, a debugging tool. Tranducer performance is much more complex than the above measurements.
But those who do not measure, don't know.
I imported quite a few of these mics, and people have been asking for them. I'm always happy to distribute to fellow diyers, but there is none left now. Please do not ask me to send you the mic. And btw. a group buy at diyAudio or SNA may be the way to go (and do let me know if you organise one!)
The microphone can be easily found in ebay:
In 2015 I was asked to commission a pair of loudspeakers for Princeton University 3D3A project. I thought it was a good opportunity to compare my mic's measurement with purpose-built lab.
Fig 1: Backyard measurement using this microphone
Fig 2: 3D3A anaechoic chamber
Fig 3: Sonogram measurement using this mic
Fig 4: Sonogram measurement by 3D3A
Looking at the comparison it is clear that this seemingly simple microphone is up to the task!