Speakers

The transducer is pretty small but there is typically a cavity around it in the phone, with a tuned port (= judiciously placed and sized hole in the phone). 90db at 250hz is asking a lot from a phone though. You might be "hearing" undertones of the frequencies that they do reproduce. If you have a serious requirement then better use an audio spectrum analyzer and calibrated mic.

Th style of dinky little external, enclosed speaker generally sounds better than phone or laptop speakers:

There's lots of sub-2" speakers on adafruit and sparkfun. You could also try something like this, depending on the surrounding device:

Also, I believe that phone speakers are also tuned with the appropriate DSP to make them sound better than they would otherwise sound.

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Yeah, this is to produce informative tones, not speech or music. But there are requirements for 250 Hz and a certain harmonic minimum variation. So it will be driven by square waves at a few different frequencies.

The 90 dB at 10 cm we mostly pulled from our butts, but it will be inside a metal enclosure with openings for other things toward the rear. This need s to be heard across a room so it needs to be loud. The entire front is co vered with a plastic label. We can have openings in the metal behind the l abel.

I'm just wondering what it takes to get some volume in a small speaker. Ce ll phones might not be 90 dB, but they are so freaking tiny too. I did see some units that are in a small can with a port on the end, but $40! I'd b e willing to bet the speakers in the $250 tablet will reach 90 dB. They ha d magnets that weighted more than the battery. It was very end heavy, also very magnet!

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  Rick C. 

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Oh, this sounds like an alarm for your ventilator. First thing I thought of was a Sonalert but those apparently are around 2 khz.

Anyway, maybe try this:

Rather than a 250hz tone you can get a hard-to-ignore effect by pulsing a higher frequency tone at 250 hz.

This looks insane and mentions medical devices as an application area:

Many other ones here:

Why do people post unresponsive advice?

"So it will be driven by square waves at a few different frequencies."

I came onto the project after being a part of another such project which me rged with this one. On that project I used a single frequency sounder for a power state alarm. This project has dug more deeply into requirements an d found that medical equipment uses a series of tones in patterns that indi cate the type of equipment and severity. 250 Hz is the minimum frequency. It is not essential for the fundamental to be produced evenly with the har

armonics of a square wave fall off. So there may need to be some frequency shaping, but I'd like to avoid that other than perhaps a simple enclosure to improve the low end range of the speaker.

Single frequency sounders are not at all useful.

I'm just trying to get some insight into speakers that might be available. Clearly phone and tablet and even PC speakers do a pretty good job of prod ucing such tones. Maybe I should search for laptop speakers.

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Pulse the high frequency sounder on and off at 250 hz, was the suggestion. Does that not get a 250 hz fundamental? I'll try it (in software) when I get home. Similarly with other pulse frequencies.

Lots of small speakers are available on those sites I linked, including some with little enclosures that should acoustically amplify the sound.

If you think a laptop or phone speaker sounds ok, then the first thing I'd do is measure its frequency response. Just recording a sample with a decent microphone and using Audacity's built-in spectrum analyzer feature is a reasonable start. If you can measure the SPL, that is great, but otherwise check whether it's still loud enough from say 20 feet away.

The higher frequency tone is not the spec. The spec is a fundamental with some specific harmonic content that is not pulsing a 2k tone at 250 Hz. Th e goal is not to be hard to ignore. The goal is to provide information tha t quickly conveys information of nature, severity and how quickly the matte r needs to be attended to without having to read the front panel. The atte ndant can then respond appropriately if they are doing something else. I e xpect the effect on the patient was also considered.

This article has lots of good info even if the tables are crap. Some parts are poorly organized, but the info is all there.

This one has audio files of the sounds we will be making.

We have looked at this pretty thoroughly at this point. I just need to fin d a bleeding speaker. I have about two inches square panel space and it lo oks like I won't be getting any more.

When I took my first stab at this I thought "sine waves" and so coded an NC O driving a PWM. When I saw the article I realize sine waves were not righ t and thought saw tooth for the harmonics. So the LUT was out and the phas e accumulator was nakedly feeding the PWM. When I looks at the audio file in an audio editor and I realized they were using square waves, I took just the top bit of the phase accumulator, but still need the PWM to set amplit ude.

I can add some filtering in the analog domain to shape the frequency respon se, but I'd prefer not to. I could do the same in the digital domain, but again, prefer not to. My focus is on finding a speaker that won't require filtering of the signal.

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The centuries old trick how to generate the lowest notes on a small pipe organ is to sound two short pipes simultaneously in which the frequency difference is the same as the low desired note. This may require significant audio levels.

The wavelength at 250 Hz is about 1.3 m, so you must keep the waves generated on opposite side of the cone separated to avoid acoustic "short circuiting". One way is to use a baffle so large that the air wave path from the front to the back side of the cone is at least half a wavelengths. If this is too large, use a sealed box construction to keep the waves separated. The air in the closed box dampens the cone movement, so that more power can be fed into the speaker before hitting the excursion limit.

On Friday, November 6, 2020 at 11:47:28 PM UTC-5, snipped-for-privacy@downunder.com wr ote:

I'm going to hate asking the mechanical engineer to add a baffle. The guy will do all manner of things to make his stuff work, but anything the elect ronic guys want to do he pushes hard on every penny. He's the project lead , btw. I pointed out the speakers can be secured with a fastening plate on the back with a hole to go around the speaker magnet, secure and positions the speaker without issue. It needs to be plexi or other plastic to make sure it doesn't contact the terminals which are very close. His suggestion was to use a double sided sticky washer thing. I'm about ready to punt on this and focus on other work. It's not a job and I'm not going to fight a bout this stuff anymore.

I'm also a bit ticked when I realized that while we have a public document showing our names and images of everyone contributing, in a recent write up about the project it sounded like this was purely his project. I suppose that is more the writer than him, but his is the only truly public face on it. I may have mentioned we are one of very few such projects that actuall y seem to be getting somewhere at all. In fact, other projects are express ing interest in our alarm FPGA design as a common entity.

I just wish I could use the durn VHDL language the way it is intended. VHD L-2008 is now 12 years old and parts are still implemented poorly if at all . I'm trying to use a simple aggregate construct and the simulator I'm usi ng balked at one form of it, now it's balking at every form. It's hard to get work done when you have to fight the tools. Worse, because I'm using f ree tools from an FPGA vendor, Aldec doesn't even want to hear about the bu gs.

Stupid Aldec!

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You don't need much panel space but it would be nice to have some air volume behind the panel space. It doesn't have to be obstruction free. The idea is to use the box as a speaker enclosure. You would make a little hole to act as a bass port. Most small speakers I see use that design. See:

As for the speaker itself, what is wrong with the many available on the sites I linked, and on places like digikey? You say you just need to find a bleeding speaker, but you haven't given any specifications.

Here are some speaker suggestions, first two from Visaton, so proper specs but not the cheapest:

All prices from CPC ( a part of Farnell in UK).

As has been mentioned, you will need a baffle to get a small speaker to work at 250Hz.

If it were my project I would do some tests with the three above and then look for something cheap and similar.

The Visaton 3" is a proper full range speaker and better than you need but properly specified.

After testing these three you would have some points of reference.

An open backed enclosure could almost certainly be tuned to 250Hz which would boost the output at that frequency.

MK

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guy will do all manner of things to make his stuff work, but anything the e lectronic guys want to do he pushes hard on every penny. He's the project lead, btw. I pointed out the speakers can be secured with a fastening plat e on the back with a hole to go around the speaker magnet, secure and posit ions the speaker without issue. It needs to be plexi or other plastic to m ake sure it doesn't contact the terminals which are very close. His sugges tion was to use a double sided sticky washer thing. I'm about ready to pun t on this and focus on other work. It's not a job and I'm not going to fig ht about this stuff anymore.

ent showing our names and images of everyone contributing, in a recent writ e up about the project it sounded like this was purely his project. I supp ose that is more the writer than him, but his is the only truly public face on it. I may have mentioned we are one of very few such projects that act ually seem to be getting somewhere at all. In fact, other projects are exp ressing interest in our alarm FPGA design as a common entity.

VHDL-2008 is now 12 years old and parts are still implemented poorly if at all. I'm trying to use a simple aggregate construct and the simulator I'm using balked at one form of it, now it's balking at every form. It's hard to get work done when you have to fight the tools. Worse, because I'm usi ng free tools from an FPGA vendor, Aldec doesn't even want to hear about th e bugs.

k

Thanks for the info. They are a bit pricey. I think they are a bit too bi g. I don't have a number for depth, but the diameter space available is on ly about 2". The last word I got was they will be mounted on the bottom wh ere the unit is spaced off the table by a fraction of an inch. He doesn't want them mounted on the sides because that is part of the top half of the cabinet (two mating U shapes). There are also requirements for drip tests. Meanwhile there are gaping holes in the sides and plumbing mounted there.

I'm throwing in the towel and not pushing any more. Someone else can deal with this guy.

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After more searching I found this one.

Data page rather than a data sheet and I'll have to beg for pricing info an d delivery. No stocking distributor. I don't think this will need an encl osure but I'll ask to see what can be done. Anything we might provide easi ly would be pretty leaky I think. We'll see. I like the 25 ohm impedance as it matches well to a 5 volt power source and allows the power I need wit hout worrying about the speaker being blown with too much drive.

Availability is going to be a problem though. We custom order a motor, a f ront panel label and sheet metal, but seem to have a problem with any elect ronic component we have to order from the manufacturer even at 10,000 piece quantity. A bit frustrating.

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Wonder if this will work for you:

It's 2.8" x 1.2" so less panel area than a 2" circle or square, but it does exceed 2" in one dimension. It comes with an enclosure. You'd have to test to see if it meets your loudness requirement but you'd also have to test that Kingstate speaker.

Unless you have a powerful audio amp you're not likely to blow out the speaker despite the low impedance. The drive signal itself (if it's coming from a digital pin) will likely have higher impedance. You can also increase the input impedance by adding a small transformer or LC filter, but I don't know if that kind of thing is done any more.

You could add an amplification chip like the one on this board:

I might get one of those enclosed speakers (they are cheap) next time I order from Adafruit since I occasionally also want something like that.

Really though, why not try scrounging a small speaker from a pocket radio or something, to see if it loud enough? That might be enough to stop searching, if a generic speaker suffices.

I have the info below from the german mangazine "Hobby HiFi" . Prices are as given in that magazine, so probably one-off prices.

A small speaker with response to about 90 Hz (in 0.15 liter bassreflex enclosure) is the Visaton BF37 4 ohm. It measures 37x37 mm , depth 24 mm. 11 euros. But the bassreflex channnel must be long (20 mm rond, 400 mm long); you can of course run it without bassreflex.

Another one, even smaller, is the Visaton BF32s 8 ohm. This is only

32x32mm, 10 euro's. 0.1 liter bassreflex gets you below 200 Hz. Quite linear from 0.45 to 15 kHz. If you want more details I'll dig out the old magazine.

Mat Nieuwenhoven