You put in your straight mute for the muted passage, the conductor brings you in, and the tuning that felt solid two bars ago has drifted sharp. You lip it down, the tone goes dead, and now you're fighting pitch and colour at once. That isn't a personal failing or a bad mute. It's a predictable side effect of putting something in the bell, and once you know your own numbers you can correct before the entrance instead of chasing it during.

Why a mute moves your pitch

When you play a brass instrument, your lips lock onto the resonances of the air column, and the bell is part of what sets those resonances. Put a mute in the bell and you change them. A straight mute also adds a resonance of its own. The result for most straight mutes is a horn that plays sharp, usually most noticeably in the low register, where the bell has the biggest say in the tuning.

How far a mute moves you depends on its shape, its material, and above all how deep it sits in the bell, which is set by its corks. Two straight mutes of the same model can play differently, and the same mute can change when its corks wear. For the acoustics, Murray Campbell, Joël Gilbert and Arnold Myers's article "Put a Sock in It! Mutes for Musical Horns" in Acoustics Today (2021) is a readable start.

Typical ranges, as starting points

These are the typical ranges listed in the Mute Effects section of Virtuosic's trumpet, trombone and French horn guides. They're ranges because mutes and players vary. Use them to know what to listen for, then measure your own.

Trumpet

Straight mute: typically 10 to 25 cents sharp. The most common culprit, and the one most players notice.
Cup mute: a smaller shift than a straight mute; the guide lists 5 to 10 cents sharp.
Harmon mute, stem in: about 5 cents sharp, varying with how far the stem is pushed in. This is the "wah-wah" setting, played with a hand over the stem.
Harmon mute, stem removed: the thin, intimate Miles Davis sound, and usually the sharpest mute in the case: 10 to 25 cents sharp, worst in the low register.
Plunger: close to no change when open; the pitch sags flat (5 to 15 cents) as you close it over the bell.
Bucket mute: little change, slightly flat.
Practice mute: 15 to 30 cents sharp, with enough resistance to change how the horn feels.

Trombone

Straight mute: typically 10 to 20 cents sharp. Bass trombone straight mutes behave the same way, and the notes played on the valves sharpen most.
Cup mute: 5 to 15 cents sharp.
Harmon: 5 to 15 cents sharp with the stem in. With the stem out it varies more than any other mute, so check yours before you move the slide.
Plunger: close to no change when open, sagging flat as it closes.
Bucket: little change, slightly flat.
Whisper or practice mute: sharp, with heavy resistance. A practice tool, not a tuning tool.

Euphonium and tuba straight mutes run sharp in the same way; tuba mutes vary enormously from model to model.

French horn

Hand stopping: closing the bell completely with your hand raises the pitch by about a half step on the F horn (nearer three-quarters of a step on the B♭ side). The standard fix is to finger the note a half step lower, on the F side. The exact amount varies a little from note to note, so stopped passages still need your ear.
Stopping (transposing) mute: behaves like hand stopping, and is brighter and more secure in the low register. Use the same half-step-lower fingerings; for rapid open-stopped alternation, use the hand.
Non-transposing mute: changes the colour without the half-step jump. The guide lists 5 to 20 cents sharp depending on the mute; pull a slide if you'll be in it for a long passage.
Plunger: covering the bell lowers the pitch, like closing your hand, by roughly 5 to 15 cents. Lip up, or open the hand, as it closes.
Your right hand: cupping it more deeply flattens the pitch and darkens the tone; opening it sharpens and brightens. That's your fine-tuning control on open horn, so check where it sits when you play muted.

The fixes

Pull the main tuning slide before sustained muted passages. For a straight mute on trumpet, the guide suggests pulling the main slide about a quarter of an inch (roughly 15 cents) as a starting point. On trombone, extend the main tuning slide. The exact amount depends on your horn and your mute, which is why the routine below matters more than any number here.

On Harmon, set the stem first. The stem position changes the pitch as well as the sound, so settle it before you touch a slide. The stemless Harmon usually needs the biggest pull of any mute, and the low notes, which rise most, still need lipping down.

Use your ear for plunger work. A plunger moves continuously between open and closed, and the pitch sags as it closes, so a fixed slide setting would only trade one out-of-tune state for another. Practise open-closed-open scales against a drone until you lip up through the closed notes without thinking.

Trust the stopped-horn fingerings, then listen. Finger a half step lower on the F side, and adjust from there.

Don't do intonation work in a practice mute. Its resistance changes your embouchure enough that any pitch sense you build won't transfer to your performance mutes.

Check your corks. A mute that sits deeper or shallower than it should plays out of tune. If one mute is always off, a cork adjustment can bring it back in line.

A routine for tuning every mute you own

Twenty minutes, against a drone or a tuner. Do it with the mutes you actually perform with.

Open warm-up (2 minutes). Long tones on a few notes across your range: low, middle and high. Match the drone with the horn open and notice where your slide sits when you're relaxed.
Straight mute baseline (3 minutes). Put the mute in and play the same long tones without moving the slide or changing your embouchure. Listen for beating against the drone, and write down roughly how sharp each register is.
Find your slide position (3 minutes). Pull the main tuning slide until the beating stops on your middle notes. That's your straight-mute position. Mark it, with a piece of tape or just by eye.
Scales, open and muted (8 minutes). Play a one-octave scale open against the drone, then muted at your marked slide position. Listen for the notes that still drift; often they're at the bottom of the range, where the mute's effect is largest.
Every other mute (4 minutes). One long tone per mute (cup, Harmon with and without the stem, plunger, bucket) against the drone. You're not retuning for each one, just learning how far and which way it moves you, so a muted entrance never surprises you.

Write the results on a card in your mute bag. Once you know your numbers, you can set the slide before the passage instead of fixing it during.

"In tune" also depends on the chord

The cents above are measured from equal temperament. In an ensemble chord, your target moves: a major third wants to sit about 14 cents below equal temperament, and a dominant seventh's ♭7 can sit about 31 cents below. So a straight-muted trumpet playing the third of a chord 15 cents sharp is nearly 30 cents above where a pure third sits. The mute adds to the tuning job the section already had. Our post on the 14 cents that separate a good ensemble from a great one covers those targets.

Measure your own mutes

Virtuosic's tuner is free and shows exactly how many cents you are from the target, and the single drone is free too. Put a mute in, play a long tone, and the number on screen tells you where your horn is sitting today. The Mute Effects section of each brass guide (trumpet, trombone, French horn) has the typical ranges for quick reference before a gig.

If your section is still arguing about whether the straight mute is "a little sharp" or "a lot sharp", the answer is now a measurement, not an opinion.

Why Your Straight Mute Makes You Sharp (and How to Tune Every Mute You Own)