THD of simple speaker - Page 1
Forum section: electronics / beginners

author: zphaze
author_rank: Contributor
timestamp: June 20, 2020, 05:33:36 am
content: Hi,I'm not too sure if I understand clearly what's happening, because I think the manufacturer wouldn't bother sell a speaker that had terrible sound quality but, just to be sure... I must double check here.I'm interested in using a small pcb mount speaker, which has the following datasheet :https://www.cuidevices.com/product/resource/cds-15158-smt.pdfWhat's interesting is that it can withstand reflow. It costs 11.75$.However I noticed the THD curve was quite dramatic between 3kHz and 6kHz (around 80%).I'm I wrong or will this speaker be absolutely awful for most of the midrange?Second, I was looking at this one here :https://www.mouser.ca/datasheet/2/670/cms-151103-16sp-1776947.pdfIt seems to have a much nice frequency response, and THD curve, and it costs 3$. On paper it looks to sound just better. However it must be hand soldered on PCB.But why would it be better sounding if it costs 8.75$ less than the SMT one?Any ideas?Thanks!

author: bob91343
author_rank: Super Contributor
timestamp: June 20, 2020, 05:51:58 am
content: The price of speakers never has correlated with sound quality. One ancient example was the infamous 'sweet sixteen' system. It was a project that required 16 cheap speakers, maybe 4", mounted on a board in a 4 x 4 matrix. It was touted as sounding better than the very expensive speaker systems of the day, back maybe in the vicinity of 1958.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 20, 2020, 06:12:16 am
content: SMT mounting can require quite some compromises for the speaker. This can increase the costs and lower performance. Hand soldering also adds to costs, but it could be worth it.The SMT speaker seem to have some kind of resonance in the 5-7 kHz range, and this can quite naturally amplify harmonics that fall into this range - so more THD in the 1.5-3.5 kHz range.The speaker mounted to a board can also effect the emissions quite a bit, so I would not have too much confidence the curves would look the same on a different board.

author: ratatax
author_rank: Regular Contributor
timestamp: June 20, 2020, 03:06:12 pm
content: I wouldn't bother too much with that THD peak, I think it will increase the perceived brightness since 4Khz is already quite high.For that kind of speaker, there's a lot more things that will make it sound bad rather than THD, first it will have no bass at all and almost no midrange. You also have to build a small enclosure around it or it will suck no matter its performance.Since it's pretty expensive, i suggest you either move to a proper speaker (with a small enclosure like in laptops to make it sound OK) or just take the cheapest you can get and forget about sound quality

author: Zero999
author_rank: Super Contributor
timestamp: June 20, 2020, 03:26:35 pm
content: This speaker clearly isn't designed for hi fidelity. It's meant for speech and sound effects. As mentioned above, there are other things which are often desirable in a speaker, than sound quality. It can be reflow soldered and washed, which is advantageous to another speaker, which needs to be hand soldered.

author: TimFox
author_rank: Super Contributor
timestamp: June 20, 2020, 03:28:53 pm
content: The THD curves are strange. The specs give a very low THD at 2 kHz and a specific power level, and the curve shows a local minimum at that frequency. I wonder if the curve is mislabeled, and first thought it might be the spectrum of the residual harmonics with a suppressed 2 kHz fundamental, as in a traditional THD analyzer, but the content is not exactly the integer-order harmonics. 100% THD is huge.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 20, 2020, 03:36:48 pm
content: The curve is labeled as THD of measured - input , so it could be that they have suppressed most of the ideal signal. This would increase the THD a lot where the subtraction works well and even reduce the THD where the phase is not right. So hard to tell if really meaningful.I have not seen many THD curves in speaker data sheets. So it is odd that it is there at all. The more common curve is the impedance.

author: TimFox
author_rank: Super Contributor
timestamp: June 20, 2020, 04:52:19 pm
content: Both Kleinstein and I seem to agree that the THD graph in the data sheet is after suppression of a 2 kHz fundamental. The data sheet specifies 5% distortion at 0.3 W and 2 kHz (the typical spec for max power) which is not bad for a small cheap device.The graph, therefore, must represent the measured distortion products, and the 100% full scale is not 100% THD.The distortion spectrum is interesting. There is an expected 2nd harmonic at 4 kHz, a stronger peak at 5 kHz, and another at 7 kHz (non-harmonic overtones). These may represent modes of the piston/diaphragm/cone, since vibration modes of 2-dimensional areas are not harmonic.

author: mikerj
author_rank: Super Contributor
timestamp: June 21, 2020, 12:39:18 pm
content: Quote from: TimFox on June 20, 2020, 04:52:19 pmAlso note that all the peaks in the "THD" plot correlate to dips in the frequency response plot, almost certainly caused by modes in the diaphragm constructively/destructively interfering with the fundamental.