Testing speaker response

Obviously (is that patronising? I hope not...) the secret of success here is measuring things so the we have some idea of how well we're doing. Note the use of the plural first person pronoun here, much like the usual racing driver/rider who likes to blur the effect of his (usually his!) massive ego on the listener.

How to do that? I have

• Measurement software - REW, Java-Based, runs on the MacBook Pro
• Measurement mic - UMK-1 from the miniDSP chaps - a USB mic that plugs into the MacBook Pro
• A DAC (Safire) that will allow the MacBook to drive the amp/speaker under test

There are huge drawbacks to performing speaker measurement in a room, largely because of the effect of the room on the measurement (resonant frequencies, reflections from surfaces) which all distort the response, usually but emphasising or reducing certain (ranges of) frequencies, and introducing phase distortion. So how do we get around all this?

1. Do it in an anechoic chamber - which I don't have
2. Do it outside; requires speaker to be well off the ground, or ground reflections distort the measurement; this is possible, but apparently Altec engineers used to put their speakers 30ft off the ground; er, no
3. Do it inside but in the near-field i.e. stick the mic sufficiently close to the speakers that the room effects are minimised; also do several measurements at different places and integrate them all

This forum entry has lots of info on the 3rd approach, but doesn't use REW for measurements in the first instance, largely because the old version referenced doesn't do integration and dynamic gating (allowing for events within a certain time period, thus reducing room effects). 

Approach:

1. Take on-axis frequency sweep measurements 
1. 3" from woofer
2. 3" from tweeter
3. 1' from woofer
4. 1' from tweeter
5. 3' from tweeter
2. Integrate these measurements to get 3 (3", 1', 3') (how do I do that in REW? Apparently it's the "Average the responses" button!) Note: This destroys the impulse response calculation - you need a single measurement to preserve this

I'm inclined to do both at once - near field and outdoors; how can it hurt?

Here's the test setup in all its glory:

• Speaker raised off ground
• Mic positioned on axis 3" from tweeter
• Laptop running REW
• Safire plugged into Mac and Myst TMA-5 amp, which is wired to speaker

Problems

• Ambient noise - a neighbour has started to mix cement...
• Rain?
• REW's interface allows one to mis-save files with confusing/repeated names
• I'm not very thorough!!

Results

1m response, mic between W/T, on axis

The above graph shows a bit of a droop 2.5-7kHz, interesting. The 2kHz XOver region is not bad. Phase looks a bit biffo. There are some effects from the reflections from the ground, especially around 110Hz - this is probably due to the height from the ground and distance to mic (110Hz 180deg phase is 1.5m approximately, which would be about right).

30 cm on-axis, centred on woofer

This shows a much smoother bass response, with little/no interference effect from the ground. Interesting 2kHz dive - is this interference between woofer/tweeter? The tweeter/woofer are about 25cm apart, and the mic 30cm from the front of the cabinet, which gives a tweeter-mic distance of 39cm, so a difference between W-M and T-M of 9cm, for a wavelength of 18cm which is about 1800Hz. Close enough. It's also in the crossover region which is pretty scary.

30cm on-axis, centred on tweeter

There's a similar thing happening with the on-axis tweeter response, although the frequency is lower - about 1.8kHz interestingly. Did I mess up my calculations above?

30cm on-axis, averaged W/T response

This is the averaged W/T response from 30 cm - note the inter-driver interference dip abut 1800Hz! There's a big suck-out 2-10kHz, or it could be seen as top-end brightness, since it's often good to roll off the top end a bit.

7.5cm on-axis, averaged W/T response

This doesn't look too bad - we aren't getting so much of the W/T interference at this point.

Conclusions

Not sure what to conclude here, apart from

• Overall response not too bad, with reasonable integration between W/T
• Bit of a dip 2-7kHz, with rising 10kHz
• Bass drops off from 80Hz - this speaker is supposed to be used against a wall, which would reinforce that

I'd like to test the drivers separately - oh, I will :-). Hurray.