Evaluating loudspeakers

Question

If you don't have a lot of money to spend but you want to estimate the frequency response of different loudspeaker setups what is the best way to do it?

I bought a very nice microphone and built a box with foam on the inside. My idea is to place the loudspeaker and the mic in the box and measure the power level at different frequencies by playing a sine tone at those frequencies.

Or is it better to have the microphone and loudspeaker in an open space where the mic is 10 inches away from the loudspeaker?

I'm sure neither of the 2 methods above are valid for making such measurements (if so please explain why), but how should I do then?

Also, should I use sine tones or is it better to use another test signal like an "impulse", "white" noise or a chirp signal?

Hoping to get some good answers / advice

Thank you!

Answer 1

Not an easy job. A loudspeaker produces a complicated 3 dimensional sound field, which heavily interacts with the environment. Even at a distance of a about 3 feet the reflected energy is significantly bigger than the direct energy at most frequencies. Typical loudspeaker measurements are

1. On axis response in an anechoic chamber
2. Full 3D response in an anechoic chamber
3. Radiated power in the far field in a highly reflective environment.
4. Room responses taken in "real world" setups, deriving a perceptually relevant spectral metric and averaging over a large number of environments and placements

Each of these measurements require specific measurement methods and techniques which are also not trivial.

Anything you can do with a single microphone in real room will most likely be either useless and misleading. The box is also not a good idea since any box has very strong internal acoustic modes and what you will be measuring will be dominated by these modes regardless of what the loudspeaker is doing.

Depending on the physical size of the loudspeaker and the size of your measurement space, you may be able to pull off a near field measurement with careful time gating of the impulse response (loosing a lot of low frequency resolution in the process). Getting this right is a lot of work and in my opinion not worth the bother.

Answer 2

If you have a reference loudspeaker with a frequency spectrum that is known (i.e. measured in an anechoic chamber with calibrated equipment), then you can derive an estimate of the frequency response of any other speaker in an echoic environment (assuming everything in the measurement environment behaves linearly).

1. Set up speaker and microphone positions and keep these fixed.
2. Measure the impulse response of the reference in situ (most reliably achieved using MLS in my experience)
3. Calculate the frequency response of the amplifier/speaker/room/mic/mic-amp by taking the Fourier transform of the impulse response
4. Perform spectral subtraction of your calibrated speaker values and your measurement to give you your "measurement environment response".
5. Obtain the combined frequency response of the speaker under test using steps 1 to 3.
6. Subtract the measurement environment frequency response from this value to give you your estimate.

It is theoretically possible, but there are lots of places where this method can go wrong which is why direct measurement in an anechoic environment is the ideal choice. I don't think the foam box will be suitably anechoic!