How to check turntable speed with an iPhone
You can check a record player's speed with a stopwatch and a scrap of paper, and an iPhone makes it easier to see small differences. Below are three ways to do it, from no app at all to a live reading, with the arithmetic for each.
I develop Stillspin, the iPhone app used in the second and third methods. The first method and all of the arithmetic work without it.
Before you start
Let the turntable run for a few minutes, and check it under the conditions you play records in. Put a record on the platter if you normally play with one, because a light belt drive can behave differently under load. Fix one small paper marker near the edge of the platter or the record label, somewhere it will not touch the tonearm.
Keep hands, hair and sleeves away from the platter. A strobe can make it look still while it is turning.
Method 1: count turns with a stopwatch
This needs no app. Start the stopwatch as the marker passes a fixed point, count 100 full turns, and stop as it passes the same point again. Then compare your time with the table.
| Speed | Time for 100 turns |
|---|---|
| 33⅓ RPM | 3 min 0.0 s (180.0 s) |
| 45 RPM | 2 min 13.3 s (133.3 s) |
| 78 RPM | 1 min 16.9 s (76.9 s) |
Your actual speed in RPM is 6,000 ÷ the time in seconds. If you are half a second out at either end of a 33⅓ run, that is an error of about 0.3%, so this method catches a large speed error but not a small one. It also needs some patience: losing count means starting again.
Method 2: a strobe and one marker
A marker looks still when the light flashes once each time it comes around. The flash setting for any target speed is:
flash rate in Hz = marks × RPM ÷ 60
| Marks | 33⅓ RPM | 45 RPM | 78 RPM |
|---|---|---|---|
| 1 | 0.556 Hz | 0.750 Hz | 1.300 Hz |
| 3 | 1.667 Hz | 2.250 Hz | 3.900 Hz |
If you use more than one mark, space them evenly. Dim the room so the flash is the main light on the marker, and watch where the marker appears at each flash.
- If it stays in one place, the platter is close to the target.
- If it creeps forward in the direction of rotation, the platter is fast.
- If it creeps backward, the platter is slow.
To put a number on it, time the creep. With one marker, count how long the marker takes to make a full apparent turn. With several, count how long it takes to move on by one mark spacing.
difference in turns per second = apparent turns ÷ (marks × seconds)
For example, take one marker at the 33⅓ setting that makes one apparent turn in 300 seconds. That is 1 ÷ 300 turns per second, or 0.2 RPM. Against 33⅓ RPM it is a difference of 0.6%, which is about 10 cents of pitch.
The free part of Stillspin does this with the iPhone's rear torch. Choose 33⅓, 45 or 78 to set the flash target and time the drift, and the app works out the difference. When your timing is too short to separate it from the target, the app says “Difference not resolved”. The target is a setting, not a measured platter speed.
Method 3: a live camera reading
Timing a drift is awkward while you are turning a speed adjuster. Stillspin's optional one-time unlock adds a turntable screen that keeps reading while you adjust. Rest the iPhone beside the turntable, not on the platter, aim the camera box at the paper marker and lock exposure. Choose the target speed to see the difference in percent and cents, along with the trend.
A steady reading is not the same as an accurate one. No Stillspin reading has been compared against a reference tachometer, and the iPhone's own clock sets the timing. If the methods disagree, or a change matters to you, check with an independent reference. Your deck's manual is the place to look for its adjustment procedure and speed tolerance.
What a speed difference means in pitch
Cents are hundredths of a semitone: cents = 1,200 × log₂(actual speed ÷ target speed).
| Speed difference | Pitch difference |
|---|---|
| 0.1% | 1.7 cents |
| 0.2% | 3.5 cents |
| 0.5% | 8.6 cents |
| 1% | 17.2 cents |
| 2% | 34.3 cents |
What these checks will not tell you
All three methods compare the platter's speed with a target. None of them measures wow and flutter, rumble or audio quality. An off-centre record can still sound wrong when the platter is on speed. If a record sounds unsteady but its average speed checks out, you need a different test, such as a test record and a wow-and-flutter meter.
Try the free strobe
Stillspin is free to download for iPhone with iOS 17 or later. The torch strobe, turntable targets and drift timing are free; a one-time unlock adds the live camera turntable screen, the optical tachometer, sound RPM and Compare. There is no subscription.
Download free on the App Store →