Miniature speaker with ~100 Hz low-end? - Page 1
Forum section: electronics / projects

author: ebastler
author_rank: Super Contributor
timestamp: January 22, 2024, 09:16:52 am
content: I am tinkering with a little handheld model of an ancient video game, Computer Space -- modeling the original discrete electronics in an FPGA and alluding to the funky cabinet with an appropriately styled PCB. Gameplay already works fine on a small TFT screen, and sound is created from a combination of digital signals (like in the original game) and filtered noise (generated digitally instead of the original analog circuits).The original game used a deep "rumble" sound for explosions: white noise filtered by a narrow bandpass centered at ~160 Hz. Using an LM4871 amplifier (with +- 3.3V push-pull output), I can create a pretty convincing rumble when using earbuds. But I would prefer to also have a little speaker on board, and have struggled to find a suitable one.There is room for a speaker with up to 50 mm diameter. I would prefer a flat build, say ~8 mm maximum height. 8 Ohm impedance, driven by 6.6 Vpp (2.3 Veff), hence ~0.7 W peak power would be the maximum the LM4871 can provide.I find that many speakers in this size range are meant for use in headphones, and their specs don't seem to bear any relation to a use as an open-air speaker. (Or at least I can't figure out how to do that...) I have tried a handful of small speakers from my scrapbox and ordered a couple which looked plausible on paper, but found all of them disappointing. I realize that I can't expect overwhelming sound at 160 Hz or below from this small size, but would like to at least hearsomething.If you have come across a little speaker which can render this low-end range (to some extent), many thanks for a lead! -- Other game sounds go up to a few kHz and I have full control over their drive level, so I don't see any challenges on the high end of the spectrum.

author: The Soulman
author_rank: Super Contributor
timestamp: January 22, 2024, 09:27:45 am
content: Either way you need some form of enclosure, how about this one:https://www.esu.eu/en/products/accessories/loudspeakers/speaker-29mm-x-65mm/

author: The Soulman
author_rank: Super Contributor
timestamp: January 22, 2024, 09:35:02 am
content: Ah that one is to thick, they have smaller versions to, and they are all 4 ohms, lm4871 should be able able to drive 4 ohms for short bursts?

author: ebastler
author_rank: Super Contributor
timestamp: January 22, 2024, 09:48:32 am
content: Thanks! I should have mentioned: The idea is to use just a single PCB, with SMD components on the back and the "user interface" on the front. The minimal configuration it would be just the bare PCB; maybe with a piece of clear acrylic cut to the same shape and used as a back panel.So the speaker should ideally be mounted directly to the PCB from behind. Using double-sided adhesive tape or such would be fine; but a flat mounting surface on the speaker's front would be required. The PCB will also serve as a (small) baffle board, which will hopefully help to improve the low-end a bit by reducing the "acoustic short circuit" path around the speaker.

author: The Soulman
author_rank: Super Contributor
timestamp: January 22, 2024, 09:59:09 am
content: Aha, cool design.If you want to eliminate acoustic short circuit at 100Hz you'd need a baffle larger than something hand-held.

author: ebastler
author_rank: Super Contributor
timestamp: January 22, 2024, 10:08:28 am
content: You have a point there. Maybe the baffleboard effect is small beyond "it's a compromise", and rather in the range of "this is meaningless".A closed enclosure might be required, either as part of the speaker itself or as part of my PCB assembly. I will play a bit more with the speakers I have around to see whether a larger box makes for a noticeable difference.

author: jfiresto
author_rank: Frequent Contributor
timestamp: January 22, 2024, 11:40:59 am
content: Air is not so substantial and you will have to move a fair amount of it to produce a decent volume at 100hz, considerably more than you will get out of small driver in a handheld enclosure.What if, instead, you providedhaptic feedbackso that you feel the explosions?

author: mikerj
author_rank: Super Contributor
timestamp: January 22, 2024, 12:16:36 pm
content: You might be able to integrate a speaker from a laptop, these are typically mounted in small ported enclosures to boost low frequency response but even so 100Hz is a really big ask. Even the better sounding laptops don't get much below 200Hz.

author: ebastler
author_rank: Super Contributor
timestamp: January 23, 2024, 08:07:25 am
content: Quote from: jfiresto on January 22, 2024, 11:40:59 amWhat I have tried to do (without convincing results) in an earlier project is to use the PCB itself as the acoustic membrane. I first tried by glueing on a simple piezo disk speaker -- which, not unexpected, did not produce much audible sound at all, since it relies on bending and the PCB is too stiff for that.I also tried a cheap "surface transducer" which has a bit of mass inside and aims to induce translational vibrations of the surface it is attached to. If anyone has come across a type that actually works, that would be a great solution for this project!Quote from: mikerj on January 22, 2024, 12:16:36 pmRight, these are worth another look. The sound modules offered for model trains, which The Soulman had linked to above, are probably similar designs. I was focusing on plain open speakers so far, but some form of resonant enclosure is probably needed. I will look around for surplus offerings, ideally with a spec for their frequency range.

author: Someone
author_rank: Super Contributor
timestamp: January 23, 2024, 09:58:42 am
content: Quote from: mikerj on January 22, 2024, 12:16:36 pmhttps://www.reddit.com/r/macbookpro/comments/qp9j9v/14_macbook_pro_m1_max_frequency_response/But even with that speaker assembly there is some magic in the amplifier and DSP.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 23, 2024, 10:31:32 am
content: I would look into something like a 3D printed speaker enclosure coupled with haptic feedback to give the feeling of deeper bass than the speaker can provide.Smaller speaker datasheets often don't provide thiele-small characteristics, but I've found theDATS speaker testerto work really well.Low frequencies require bulk air movement which smaller speakers struggle with, but if you add in some haptic feedback, the illusion of deeper bass can help.

author: Berni
author_rank: Super Contributor
timestamp: January 23, 2024, 01:56:45 pm
content: Quote from: Someone on January 23, 2024, 09:58:42 amYep exactly my thoughts.Most of the time when you hear decent bass coming from tiny speakers in tablets or laptops and similar, it is the result of a carefully tuned and characterized speaker enclosure that is then corrected using DSP to give a good frequency response. So a lot of the bass comes from the speaker being fed bass at way boosted signal levels to compensate for how inefficient the enclosure is at those low frequencies.This is a difficult nut to crack that needs a lot of engineering resources thrown at it to really make it work well. You can try to find a speaker with a lot of permitted cone movement and then try to experiment with a few 3D printed ported enclosures for it

author: ebastler
author_rank: Super Contributor
timestamp: January 23, 2024, 02:55:49 pm
content: All game sounds, each with its characteristic frequency, are generated separately and mixed digitally within the FPGA: Rumble sound at 160 Hz, various beeps and chirps around 1..2 kHz, pink noise up to 2 kHz or so. I can choose the relative amplitudes freely to compensate for the weak low-end response of the speaker. So I have "poor man's DSP" capabilities for free.

author: mikerj
author_rank: Super Contributor
timestamp: January 23, 2024, 04:28:10 pm
content: Quote from: Someone on January 23, 2024, 09:58:42 amYes this is just like the very compact sub-bass units for cars that can easily be mounted under a seat, you get the low end extension by brute force i.e. driving the speaker harder as it's response starts to roll off. You quickly reach either power or cone excursion limits though.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 23, 2024, 05:21:32 pm
content: Can't vouch for this, I didn't buy one, buthttps://www.aliexpress.com/item/1005005916034243.html

author: ebastler
author_rank: Super Contributor
timestamp: January 27, 2024, 11:00:21 am
content: Quick update on my attempts to get some low-end sound: I revisited the surface transducer idea and obtained a small transducer with some actual specs,https://www.daytonaudio.com/images/resources/295-214--dayton-audio-daex13ct-4-specifications.pdf.Small diameter (26 mm), although slightly thicker than I would like at 9.5 mm; 4 Ohm/3 W spec. It's reasonably inexpensive too, 6.50€ incl. VAT. Stuck onto the back of the PCB, it makes the explosions audible, though not impressive. It also provides a bit of haptic feedback if you hold the game in your hands, which adds to the effect.This would probably work better with a thinner PCB, but I don't want to go below 1.6 mm to get enough mechanical stability -- the PCB is about 25 cm tall. I will try upgrading to a small class D amplifier (PAM8302) and powering it directly from the LiPo battery instead of the regulated 3.3V, but don't expect a big change there.Anyway, I think this is "good enough" for my purpose. But I will keep the headphone connector too, to have the option of hearing some more impressive rumbling. Thanks again to all of you for your input!Edit: Removed the closing "." which got included in the link. My favourite trap...

author: temperance
author_rank: Super Contributor
timestamp: January 27, 2024, 03:56:20 pm
content: A headphone perhaps?

author: ebastler
author_rank: Super Contributor
timestamp: January 28, 2024, 01:35:16 pm
content: Yes, as mentioned in the original post, earbuds and headphones work nicely. That's what the headphone connector is for.But connecting and wearing them is a bit too much fuss for a casual game or a quick show & tell, which is why I want the onboard speaker too.