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Acoustic delay effect (experimental)
Acoustic delay effect (experimental)
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Old 10th April 2011, 02:48 PM   #11
Dr_EM is offline Dr_EM  United Kingdom
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Long time I know, but I had a bit of an experiment with this idea again recently. I built a transducer to drive the tube from a small speaker I had on hand:

Visaton - Lautsprecher und Zubehör, Loudspeakers and Accessories

Mounted into a small cylindrical MDF enclosure, with the acoustic output via a 5mm brass tube.

Issue with it is very little treble output. See these measured responses, Blue with the microphone directly on the output, Green with 1m of hose:

Acoustic delay effect (experimental)-pwt-output-png

Basically perfect from very low till 800Hz, then a strong resonance at 1kHz before rapidly loosing energy and just nasty peaky response being left at the top. Listening to it, the sound is muffled and the 1kHz peak can be heard.

So, the poor response seems to be inherent in the transducer, so how I can I make a better one? The patent for that audio system used in MRI machine rooms says about loss of high frequencies and required electronic compensation, but that's oweing to the tube itself it seems, so starting off with weak treble isn't ideal!

I will try to add a photo of it but don't have a decent camera with me now.
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Old 10th April 2011, 02:59 PM   #12
Charles Darwin is offline Charles Darwin  United Kingdom
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The first time I flew in a commercial airliner (early '70s) they used a system like that.
The headphones had no electronics in them, just a couple of tubes leading to the ear pieces. A little valve in the armrest let you switch between 'stations'.
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Old 10th April 2011, 05:33 PM   #13
Dr_EM is offline Dr_EM  United Kingdom
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Yes, strangely I've used such a system myself once on a plane! Much more recent than the 70's though as I'm only 23! Must have been using an old model of aircraft for that flight I guess. It sounded alright as I recall, so I don't know why my transducer has such poor high frequency output. There's only so much I can boost it before hiss will become an issue.
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Old 12th April 2011, 09:31 AM   #14
Alex from Oz is offline Alex from Oz  Australia
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Quote:
Originally Posted by Dr.EM View Post
I built a transducer to drive the tube from a small speaker I had on hand...

Mounted into a small cylindrical MDF enclosure, with the acoustic output via a 5mm brass tube.

Issue with it is very little treble output.
Basically perfect from very low till 800Hz, then a strong resonance at 1kHz before rapidly loosing energy and just nasty peaky response being left at the top. Listening to it, the sound is muffled and the 1kHz peak can be heard.

So, the poor response seems to be inherent in the transducer, so how I can I make a better one?
Just thinking out loud...
Perhaps the cylindrical enclosure is acting similar to a compression chamber in a BLH.
Low frequencies are passed up to the cut-off frequency then the high frequencies are attenuated.
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Old 12th April 2011, 12:20 PM   #15
Dr_EM is offline Dr_EM  United Kingdom
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It's quite feasible. I built another one using the Monacor SPH-30X driver and it has a similar response overall. I had hoped that driver's strong peak at 10kHz would help to even out the top end, but it's still weak above 1-2k.

How would on go about minimising the treble loss? Is it unavoidable because lower frequencies are transmitted as pressure waves but higher frequencies have to 'bounce along' the tube? I just find it odd that even with no tube length the high frequencies are minimal. It's not a good starting point really, needing 30dB of correction before it even travels down any hose!
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Old 12th April 2011, 02:02 PM   #16
geraldfryjr is offline geraldfryjr  United States
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I would try using a larger diameter tube.
I had I similar idea one day when I was in home depot and yelled into a rolled up 100' piece of coreagated drain field tubing.

Higher frequency's attenuate at a faster rate than low frequency's do over the same distance.
It is possible that a smaller diameter tube could be imposing a higher impedence to the higher frequency's. jer
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Old 12th April 2011, 08:24 PM   #17
Dr_EM is offline Dr_EM  United Kingdom
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Could also be very true. I can't use any bigger than the 4mm internal diameter tube due to physical size/cost/availability issues though.

Hmm, this project could very well be doomed, if I want a full acoustic delay of 0.5s or so which needs 170 metres of hose, might end up needing an unachievable 100dB of treble boost . A transducer with flat response acoustically to start with would give me some more confidence.
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Old 12th April 2011, 09:57 PM   #18
Alex from Oz is offline Alex from Oz  Australia
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Quote:
Originally Posted by Dr.EM View Post
How would on go about minimising the treble loss? I just find it odd that even with no tube length the high frequencies are minimal.
I wonder if an alternative material for the enclosure would help?
Not sure that MDF is ideal for preserving high frequencies.
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Old 13th April 2011, 02:40 AM   #19
geraldfryjr is offline geraldfryjr  United States
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A compresion driver such as that used on P.A. horns and in the infamous "Talk Box" might give you the added high frequency boost your looking for. jer
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Old 13th April 2011, 10:46 AM   #20
Dr_EM is offline Dr_EM  United Kingdom
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Both good suggestions! Yes, the MDF could well be absorbing much of the high frequency energy. Plastic or metal are the only real alternatives, unless sealing the MDF on the inside surfaces has a really dramatic effect.

A compression driver might work, I expect with <1W input it can cope with signals down to 20Hz without bottoming out? They certainly have great efficiency so it might make it easier to apply any required treble boost without pushing too much power.

Unfortunately looks like any reasonable unit is about £17, which isn't too bad if it works, but like everything in this project it's so experimental that I literally don't know what it'll do until I have physical parts to test and measure (including expensive hose!)
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