Phase response of an active bandpass filter - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: December 11, 2014, 05:31:12 pm
content: Hi everyone,I'm working on a filter design project. I decided to build an active wide bandpass filter from 100Hz-5kHz. This filter is essentially an active crossover for a speaker, except it is only for the midrange speaker.So I've simulated it and built it. The gain vs. frequency response is good. I'm curious about the phase vs. frequency response however.For a speaker, I would assume you would want a phase response (ideally) of 0. If I understand the meaning of phase response, it's that certain frequencies are shifted out of phase in relation to the input and thus, this would distort or change the output signal.However, my phase response is obviously not 0. So what effects will this have on an audio circuit and the quality of the sound?Thank you!

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 11, 2014, 06:29:23 pm
content: It depends entirely on the rest of the system. If it's just that one speaker playing all by itself, then you won't notice anything. The oddities arise when you have multiple speakers playing the same note near the crossover frequency with different phase shifts.When it comes to crossover design you can't just treat each speaker independently, you have to design it with the entire system's response taken into account, including the speakers' natural rolloff and impedance changes near resonance and at high frequencies.

author: TimNJ
author_rank: Super Contributor
timestamp: December 11, 2014, 06:33:42 pm
content: Ahh, I suppose that makes sense. Wouldn't there still be a change to the signal passed to the speaker?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 11, 2014, 06:41:59 pm
content: Quote from: TimNJ on December 11, 2014, 06:33:42 pmYes there will be a phase shift between the input to your filter and the output at certain frequencies. With just a single speaker you won't be able to hear it though. Same reason why wiring your speakers backwards (180 deg out of phase) doesn't change anything, it still sounds exactly the same. It's only a problem if you wire one speaker out of phase and one not.

author: TimNJ
author_rank: Super Contributor
timestamp: December 12, 2014, 02:21:17 am
content: Thank you. So the idea would be to have the phase shifts match for the crossover points?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 12, 2014, 02:35:23 am
content: Frankly, those kinds of questions dig into the depths of crossover design that are beyond my level of understanding, so I can't really help you. Speaker crossover design is both a science and an art, it's not something you dive into lightly. If you're just trying to jerry rig something together so it works, that's one thing, but if you're trying to develop a hi-fi system, the crossover network is practically a PhD thesis project in and of itself.More than a few times I've seen the designer intentionally wire the tweeter 180 deg out of phase with respect to the midrange to smooth the crossover, but of course this is completely dependent on the crossover network being used and the behavior of the specific drivers. It's very common to have mis-matched filter properties (eg: 2nd order butterworth highpass for the tweeter with a 4th order linkwitz-riley lowpass for the midrange) depending on the frequency response of the specific drivers and their respective enclosures, in order to flatten the transition with minimal peaks or nulls.It's an area in which I know just enough to know that I don't know nearly enough to be of any use.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 12, 2014, 01:05:33 pm
content: You can get interference (in the wave mechanical sense: a +90 shift on one cancels with a -90 shift from the other) if they overlap in an undesirable way, but as for overall response, the phase is minimally important. Ears are constructed essentially as biological Fourier magnitude detectors, with little care for phase. The ear's response to phase isn't well understood, but obviously it is important at lower frequencies for directional hearing.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: December 12, 2014, 04:38:47 pm
content: Well thanks for your input anyway, suicidaleggroll. I actually was reading some of the preview pages on Amazon for Douglas Self's "The Design of Active Crossovers". Lots and lots of things to consider.Tim, thanks for your input. I presume you mean mechanical sense as in the sound waves destructively interfering with each other. I haven't researched crossovers too much, but from what I can recall, most designs seemed to care more about magnitude than phase. I actually attended a seminar on human sound localization about a year ago. If I remember, for sounds < ~ 1kHz, phase is important for localization, and >~1kHz, the magnitude takes precedence. But that's just for localization, not necessarily whether it sounds correct.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: December 12, 2014, 05:23:55 pm
content: http://www.linkwitzlab.com/is one resource, and it is complicatedthere is a fixed relation between linear analog filters frequency response slope and phase - can be modified with added all-pass filters for limited adjustment of phasethe filter and the driver acoustic frequency response are multiplied, getting the sum relatively flat is only a startbecause there are additional phase differences with driver spacing, angle, box contouras mentioned good listening tests do show human phase sensitivity below 1 kHz - easy with test signals and headphones - much harder with loudspeakers and music - to the point that some persist in in denying phase audibility at allsearch auditory nerve phase locking will show an important physiological mechanism for hearing audio signal phase effects - has been known for a few decades nowcareful researchers have published peer reviewed human listening tests demonstrating positive audio phase discrimination for much longerbut it really doesn't seem that important for music listening

author: magetoo
author_rank: Frequent Contributor
timestamp: December 13, 2014, 01:11:31 am
content: Quote from: TimNJ on December 12, 2014, 04:38:47 pmI have not read it, but if it is anything like "Small Signal Audio Design" it's probably worth getting it and reading the whole thing.