I was browsing around the various audio forums when I saw a thread about a Tuba HT, folded horn subwoofer. The builder showed what looked remarkably like an REW screen measuring distortion "live" on his YouTube video. If memory serves he was measuring 22Hz at 122dB! His house and courage must be far stronger than mine!
I wondered why I hadn't tried measuring IB distortion myself. It couldn't be that difficult to produce some meaningful distortion figures of my own for my 8 x 15" IB. I certainly remembered that REW could produce distortion graphs and figures for up to 8 harmonics but the actual method had become vague from lack of practice. Thankfully John Mulcahy's REW help files covered the subject in sufficient detail to make me dangerous. It seemed easy enough to make it work at a basic level even if my understanding of the purpose of the variable controls were still a bit fuzzy.
I have used italics in the paragraph below to indicate REW controls and settings: This may help those unfamiliar with REW but bent on trying to replicate my measurements technique on their own subwoofers or even their speakers:
Having already Calibrated REW using my Galaxy 140 SPL meter and my IB on pink noise (with the SPL meter at the listening position ) I knew I had a safe level to work from. The REW Generator is opened and set to produce a sine wave with Frequency follows cursor for ease of use. Spectrum is selected from the test options just above the graph box. Then the Distortion measurement box down below the graph box is ticked. Once the chosen test tone emerges from the subwoofer the red Record button is pressed. Now a dancing, "boxy" curve is produced in the REW graph space. I played around with the settings then chose RTA 1/6 octave to provide the best looking graph. (IMO of course) The number of sample Averages is also available as an option. This steadies the constantly shifting curve until (eventually) it becomes fairly fixed and "snapshot" can be taken and saved to Windows Documents.
As it is a horribly rainy day my wife (The Head Gardener) is stuck indoors. So no window breaking SP levels were attempted for the moment. Still, I think the results are interesting enough to share even though they only peaked at around 100dB(C). If this figure changes I may have summed up the courage to try even higher levels.
I was actually quite disciplined, for once, and saved the graphs and images at each test frequency. It is quite normal for me to omit recording the blindingly obvious. So being able to resuscitate REW graphs and measurements, at will, is a vital characteristic of this amazing program. I haven't quite grasped how to Copy and Paste the numerical values for instant regurgitation onto a blog so it will mean typing them out longhand. The numerical distortion figures and test conditions all appear just to the left of the REW graph window. So they aren't lost provided the individual graph is saved. I had hoped I could copy and paste the figures for all distortion harmonics straight into a useful format here but it seems not. The time it takes isn't the problem just that I fear inaccuracy creeping in as I carry them across. (this involves maximising the REW page, noting the figures, then maximising the blog page again to enter the details in the table) For the moment I have simply listed frequency, Total Harmonic Distortion and SP Level. I may list the separate harmonics later if I have sudden burst of conscience.
First, an image of the (All) Measured plots for 10Hz upwards @ 75dB until lunch intervened at 50Hz precisely. This is simply all the curves for each test superimposed on one graph to give you an idea of what I was seeing on the individual tests.
BTW: 10Hz is the minimum frequency available for these distortion measurements. It is also the barrier beyond which the cursor's control of frequency will not go. The IB doesn't like it either as distortion goes silly with rising output. A few dBs and distortion doubled.
A disadvantage of using prolonged test tones is rapid fatigue at higher SP levels. Using fewer samples for averaging may help here. As does clicking the red button to freeze the graph followed by a click on the generator button to stop the tone. I can still hear some of the higher level tones long after they have ceased!
Some representative harmonic distortion graphs at different frequencies appear below. Left click for much enlarged images. (only 55kB)
Here you can see a worst case scenario as all 8 drivers reproduce 10Hz at 86dB at the listening position. The door to the IB enclosure is moving about half an inch and output rising only very slowly as I crank the volume control on the preamp beyond half way. The baffle wall and door are obviously limiting output. Probably due to being out of phase with the manifold. At 88dB REW was measuring 33% THD!