The act of listening
What it actually sounds like when a phone finds it
A baby heart through a belly and a microphone is a soft double knock, low and far away, buried under the sound of your own body. Knowing that before you start is the difference between hearing it and scrolling straight past it.
Three real recordings, analysed live in your browser. One is a baby, one is a mother, one is nothing at all.
Firstsong is not a medical device, it is not a doppler and it does not use ultrasound. It cannot tell you that your baby is well and it cannot rule anything out. If you notice fewer movements than usual, call your midwife or maternity unit now. Do not wait to try an app first.
It is a double knock, not a gallop
A heart makes two sounds per beat. Valves closing as the beat starts, then valves closing again once the blood has left. Lub and dub. In a baby those two sit close together and neither one is loud.
A doppler gives you something else entirely. It pushes ultrasound in, listens to the shift in what bounces back off moving blood, and turns that motion into a whoosh. The sound is synthetic on purpose, which is why it gallops and why it is cleaner than anything a microphone will hand you. Firstsong emits nothing, so what reaches your ears is the real event, moved up in volume and stripped of everything above 200 Hz. Duller than the clip in somebody's video. That is what real sounds like.
Why it sounds like it is coming through a wall
Between that small heart and the glass of your phone there are several layers, and every one of them takes the high frequencies away first.
- Fluid and tissue swallow the top end. What survives at the surface of your skin sits mostly between 20 and 200 Hz, below almost anything a phone is normally asked to play.
- The phone touches skin, not sound. A hard flat back against soft skin is a poor acoustic joint. Light steady pressure and a still hand beat any setting in any app.
- Your own body is louder than the baby. Blood in the large vessels of your abdomen, your gut, your breathing, all of it lands in the same band.
The numbers we have, and the one we do not
Those recordings were made with research sensors on the belly, not with an iPhone microphone against real skin. How much survives the move to a phone is the one number we do not have yet, and it is why this site has no launch date. When we have it, it goes here whatever it says.
Headphones are not a preference
The microphone and the speaker sit a few centimetres apart. Play amplified belly sound out loud and it walks straight back into the microphone, which amplifies it again. At best a rising howl, at worst a steady rhythm made by the loop itself, which a naive app will count. A phone speaker also cannot physically produce most of the 20 to 200 Hz band, so out loud you are listening for the one thing it was never built to make.
| How you listen | What it costs you | Verdict |
|---|---|---|
| Wired headphones, or one wired earbud | Nothing. The sound arrives as fast as the maths finishes | Use this |
| Bluetooth headphones | Between 150 and 300 milliseconds of delay before it reaches you | Fine, but you hear the recent past |
| The phone speaker, out loud | Feedback into the microphone, and most of the useful band missing | Do not |
Hear all three before you believe any of them
The tool on this site plays a baby, a mother and a recording with nothing in it, and runs the real engine over each one in front of you. The third clip is the interesting one. It is real audio with nothing in it, the kind of input that produces the arm and sofa reviews.
Runs in your browser. Nothing is uploaded, no account, no cookie.
The four things people mistake for a baby
Your own pulse
The most common mistake here by a distance. At rest a maternal heart at 70 sits at exactly half of a fetal 140, so an app that looks for the strongest repeating thing can double your rate and hand you a beautiful number. The full trap is here.
The placental whoosh
A soft rushing sound in time with your pulse rather than the baby's. It is blood moving through the vessels feeding the placenta, so it runs at your rate. People find it, call it the baby and stop looking.
Your gut
Gurgles are loud and land right in the useful band. What they do not do is repeat at a steady interval, which is why an engine that demands agreement across many seconds throws them away.
The phone moving on skin
The dullest thumps you will hear are your own hand adjusting the phone. Hold still. Thirty seconds of not moving is worth more than five minutes of hunting.
How to actually do it, in order
- Wait for the third trimester. Below week 28 the public evidence behind this engine is four recordings, which is not enough to promise anything.
- Plug in wired headphones. Television off, nobody talking, no fan running.
- Lie back and let your abdomen go soft. A tense abdominal wall is one more layer between you and the sound.
- Start low and off centre, below your navel rather than on top of the bump, because the back and chest usually sit low and to one side by now.
- Press gently and hold still for thirty seconds. A rate appears only after independent windows agree, and moving the phone starts that agreement again from zero.
- If nothing holds, move two fingers' width and stop again. Small deliberate steps. Sweeping across your belly finds nothing anywhere.
When the answer is nothing
Silence means one thing only: the microphone did not find a rhythm it was willing to stand behind. An anterior placenta, the position the baby is lying in, a noisy room, a case on the phone. Any of those is enough, and none of them is a statement about how your baby is doing.
That statement is movement, and it always was. If you notice fewer movements than usual, or a change in the pattern you know, call your midwife or maternity unit straight away. The UK NHS guidance on movements does not mention apps, for a reason.
One email, the day it opens
Firstsong is not on the App Store yet. Leave an address and you get exactly one message when it is, with the price on the first line. No newsletter, no drip, no partner offers. You can reply to that email and a person reads it.
Stored on our own server, never sold, never shared. Your IP is hashed, not kept. Nothing about your pregnancy is asked for or stored.
Common questions about listening
Can I just turn the speaker up instead of using headphones?
No. The speaker feeds back into the microphone that is doing the listening, and it cannot reproduce most of the 20 to 200 Hz band anyway. You would be turning up the one range it cannot make, into the one microphone that must not hear it.
Does Bluetooth change the heart rate the app shows?
No. The rate comes from the recorded samples and those are the same whatever you listen on. Bluetooth adds 150 to 300 milliseconds between the sound happening and you hearing it, which only matters while you are moving the phone to find a spot.
Why does it sound worse than the videos I have seen online?
Because most of those are dopplers, which manufacture a loud clean whoosh out of ultrasound, or recordings sweetened afterwards. Real audio through centimetres of tissue is dull. Anything crisper is either a filter making it pretty or audio that never came from a belly.
How long before something appears on the screen?
At least thirty seconds, often longer. A number is published only when enough independent windows agree and the track holds steady. An app that shows a rate two seconds after you touch your belly is not measuring, it is decorating.
Can I listen through clothes?
Not usefully. Fabric is one more layer of the kind that took the sound away in the first place, and it rustles directly into the microphone. Skin contact, or nothing.
Is there any risk in listening for a long time?
Nothing is sent into your body, because a microphone only receives. The real cost is your attention. Time spent hunting for a sound is time not spent noticing movement. If listening makes you more anxious rather than less, put it down and call your midwife instead.
Keep reading
- Is it my heartbeat or the baby. the single most common question in this category
- The arm test. thirty seconds to catch an app that invents numbers
- iPhone specifics. which microphone you get and what iOS did to the sound first
- Recording what you heard. saving the audio and the verdict together, on your phone
Last checked 2026-08-23