Sweep speed, explained
Sweep speed is the time the tuner spends on each channel before moving on: 50 to 350 ms on a P-SB7. A shorter dwell gives smaller, less intelligible fragments and more noise; a longer one lets whole words through and sounds like a radio being tuned. 150–250 ms is the usual choice and 200 ms is the Estes standard.
What the number is
On a P-SB7 the sweep speed is a dwell: the receiver sits on a channel for 50, 100, 150, 200, 250, 300 or 350 milliseconds, then steps to the next. SpiritBox uses the same seven presets, shown on the main display and chosen on the row beneath it. A Frank's Box has no dwell at all — its tuner slides continuously across the band — and the equivalent control is how long one full pass takes, four seconds in the app's Frank's Box mode.
Why it changes everything you hear
A syllable lasts roughly 150 to 250 ms. That single fact explains the whole range.
- 50–100 ms. Each fragment is shorter than a syllable. You hear texture — a vowel, a consonant, a note — but rarely a word. The result is close to continuous noise with colour in it. Any "word" you do perceive is assembled from pieces of several stations, which is why fast sweeps feel more uncanny and less trustworthy at once.
- 150–250 ms. Fragments are syllable-sized. Single words come through intact from talk stations; music gives a beat or a note. This is where most practitioners settle, and where the perceptual effect is strongest.
- 300–350 ms. Fragments carry two or three syllables: whole short words and the beginnings of phrases. It sounds like what it is, a radio being tuned, which is useful when you want to hear clearly where material comes from.
What each device offers
| P-SB7 | 50, 100, 150, 200, 250, 300, 350 ms; forward and reverse; AM and FM. |
|---|---|
| SB7 Pro | 17 speeds from 30 to 350 ms, plus adjustable step size. |
| SB11 | Two tuners, each with its own speed and direction. |
| Frank's Box (linear, 2007) | No dwell: a continuous slide across the band in 3–5 s each way. |
| SpiritBox | The seven P-SB7 presets in SB7 mode; a continuous 4 s pass in Frank's Box mode; 200 ms in the Estes preset. |
Which to use
- Estes method: 200 ms, never below 100. The listener needs syllable-sized fragments to have anything to say aloud.
- A first session: 150 or 200. Fast enough to feel like a sweep, slow enough to hear what the stations are.
- Checking a result: 300 or 350, same band, same position. If a "word" was a presenter, a slower pass over the same station will usually show it.
- Recording for review: whatever you used live. The log carries the dwell, and the WAV can be replayed at any speed afterwards.
Direction
Reverse sweep exists on the P-SB7 and in the app because the order in which stations are passed changes which fragments sit next to each other, and on a stepped sweep a station is approached from its other neighbour, so its bleed sounds different. Investigators alternate directions between passes for the same reason they use reverse audio: to separate what the stations are saying from what the sequence suggests.
Questions
Is 50 ms "too fast to be radio"?
It is still radio; the fragments are just shorter than a syllable. No speed adds anything new — a shorter dwell only cuts the same broadcasts into smaller pieces.
Does a faster sweep use more data?
No. The streams run continuously whatever the dwell; the sweep only decides which one is audible. Data use depends on the number of streams and their bitrate, not on the speed.
Why does the hardware stop at 350 ms?
Beyond about a third of a second the device sounds like someone tuning a radio slowly and the perceptual effect fades. The ceiling is a design choice, not a technical limit.