OPINION OF THE COURT
HUTCHINSON, Justice.A jury convicted appellant of rape, statutory rape, kidnapping, corruption of a minor and possession of an instrument of crime in a related series of occurrences involving a twelve year old girl.
*312The Superior Court affirmed his judgments of sentence. We granted review on a petition questioning the presence of exigent circumstances to justify a warrantless arrest on forcible entry into defendant’s apartment and alleging reversible error in the refusal to suppress a knife in plain view and a gun found under a mattress in a search incident to the arrest.
Upon such review we find both the arrest without a warrant and the forcible entry were justified under the law then in effect,1 that the suppression court had before it evidence from which it could and did infer the knife was in plain view and that the admission of the gun, though improper, was harmless error because it had a de minimis prejudicial effect on defendant. We therefore affirm.
At the time the police made the warrantless arrest, they had been refused entry to an apartment a lady across the hall had told them was occupied by Clarence Norris and his brother. It had the same number as the apartment the victim had told them she was forcibly taken to at knifepoint. The victim also told the police that her assailant had been called Clarence by a person in the building and the building itself matched her description.
She had given this description to police in an interview the evening of the crime, shortly after she had become hysterical and hung up after appellant had called her at home. The *313appellant had informed her he would wait for her at school the next day and had threatened her for the second time with harm if she reported the rape.
Appellant argues that the forcible entry by the police into his apartment, after an announcement of authority but without an express announcement of purpose, was unlawful.
This contention is devoid of merit. It is well-established that “[a]n announcement of both authority and purpose is required before a door can be broken down, absent exigent circumstances.” Commonwealth v. Newman, 429 Pa. 441, 444, 240 A.2d 795, 797 (1968). In this case, the police arrived at appellant’s door around midnight and noticed that the apartment was lighted and that loud music could be heard coming from inside. The police knocked, identified themselves, and, receiving no response, called headquarters and waited twenty minutes for a back-up unit to arrive before forcibly entering the apartment. It is undisputed, however, that there was no announcement of purpose.
We are satisfied in this case that the officers’ partial noncompliance with the “knock and announce” requirement was justified by the presence of exigent circumstances. We recognize that only a limited number of circumstances can be considered to excuse compliance with this Fourth Amendment protection.2 Sabbath v. United States, 391 U.S. 585, 88 S.Ct. 1755, 20 L.Ed.2d 828 (1967); United States v. Wylie, 462 F.2d 1178, 1186 n. 53 (D.C.Cir.1972); Commonwealth v. Beard, 282 Pa.Super. 583, 423 A.2d 398 (1980). These are (1) “when the officers may in good faith believe that they or someone within are in peril of bodily harm.” Miller v. United States, 357 U.S. 301, 309, 78 S.Ct. 1190, 1196, 2 L.Ed.2d 1332 (1957); (2) when the officers have a basis for assuming that a suspect is “armed or might resist *314arrest.” Sabbath v. United States, 391 U.S. at 585, 88 S.Ct. at 1759; (3) when there is “some affirmative indication to support a belief that evidence is being destroyed.” Commonwealth v. Clemson, 234 Pa.Super.Ct. 191, 194 n. 1, 338 A.2d 649, 650 n. 1 (1975); Miller v. United States; Commonwealth v. Newman, 429 Pa. 441, 240 A.2d 795 (1968); (4) when there are similar indications that “the person to be arrested is fleeing.” Miller v. United States, 357 U.S. at 309, 78 S.Ct. at 1196; Commonwealth v. Newman; or (5) when “the facts known to officers would justify them in being virtually certain that the petitioner already knows their purpose so that an announcement would be a useless gesture.” Miller v. United States, at 310, 78 S.Ct. at 1196; Ker v. California, 374 U.S. 23, 83 S.Ct. 1623, 10 L.Ed.2d 726 (1963); Commonwealth v. Newman; Commonwealth v. Fisher, 223 Pa.Super.Ct. 107, 296 A.2d 848 (1972).
Without question, the police in this case had reason to believe that the suspect was armed and might resist arrest, thus increasing the officers’ peril. See United States v. Scott, 520 F.2d 697 (9th Cir. 1975), cert. denied, 423 U.S. 1056, 96 S.Ct. 788, 46 L.Ed.2d 645 (1976); Gilbert v. United States, 366 F.2d 923 (9th Cir. 1966), cert. denied, 388 U.S. 922, 87 S.Ct. 2123, 18 L.Ed.2d 1370 (1967). Moreover, the police had a legitimate basis for being virtually certain that appellant already knew the purpose of their visit and that an announcement would have been a useless gesture. The police were aware in this instance that just a few hours before, appellant had telephoned the victim and threatened to harm her if she reported the crime to the police. Thus, when the police arrived, knocked on the door, identified themselves and received no response, they were justified in concluding that appellant was inside, was not going to respond and was aware of the officers’ presence and purpose. The failure of the officers to expressly announce their purpose prior to entry was therefore justified. Cf. United States v. Wylie, 462 F.2d 1178 (D.C.Cir.1972). (There, minutes after a purse-snatching, police were directed by eyewitnesses to the suspect’s residence. After five minutes of *315shouting “Police officers, open up,” a forcible entry ensued. The suspect was found hiding within the residence. The court held that an announcement of purpose would have been a useless gesture). The twenty minute delay, considered by the dissent conclusively to preclude a finding of exigency, is indicative of nothing more than an extremely cautious and circumspect approach to a potentially violent situation and in no way vitiated the underlying exigency.
With regard to appellant’s second contention, since the arrest was lawful, it is well settled that police could make a search incident to that arrest, of those areas from which they might expect danger, for the purpose of insuring themselves against such danger. Chimel v. California, 395 U.S. 752, 89 S.Ct. 2034, 23 L.Ed.2d 685 (1969); Commonwealth v. Bess, 476 Pa. 364, 382 A.2d 1212 (1978).
Here the police had every reason to believe a firearm was available to the occupants of the apartment and that one of its usual occupants, appellant’s brother, whom appellant had implicated in his threats of harm to the victim, was unaccounted for in the living room. To expect these officers to turn their backs on the bedrooms is to expect too much. They therefore had a right to enter the bedroom to insure their safety from that quarter. Warden v. Hayden, 387 U.S. 294, 87 S.Ct. 1642, 18 L.Ed.2d 782 (1967). The officers, thus having a right to enter the bedroom, were in a lawful position to see and to seize whatever instruments of crime were in plain view from that position.3 Coolidge v. New Hampshire, 403 U.S. 443, 91 S.Ct. 2022, 29 L.Ed.2d 564 (1971) (dictum). Harris v. United States, 390 U.S. 234, 88 S.Ct. 992, 19 L.Ed.2d 1067 (1968). Commonwealth v. Tome, 484 Pa. 261, 398 A.2d 1369 (1979). The knife was an instrument of the crime. It was in plain view on the bedroom nightstand. The fact that Detective Stasiak, the first offi*316cer to enter the bedroom, did not see it does not make Detective Strunk’s observation of it when he entered other than inadvertent.4
Since the police had a right to enter the bedroom both to insure their own safety and to obtain evidence in plain view they had probable cause to believe would aid in conviction, they had a right to seize the knife one of them saw in plain view when he entered that bedroom. Since the exigencies of the situation had already justified the intrusion into the bedroom without a warrant, the suppression court properly inferred that seizure of the knife from the table, where it was in plain view, did not broaden the scope of the intrusion.
The situation with respect to the gun found under the mattress, however, is different. In fact, that difference nicely points up the distinction between the proper bounds of an exigent warrantless search and its incremental extension into an unreasonable intrusion. The gun could not have been seen without a thorough search of the bedroom. That search occurred after defendant was securely held and after it was apparent there was no one else in the apartment to endanger the officers. The failure to suppress it and its use at trial were, therefore, in error. Indeed, the Superior Court so found and the Commonwealth concedes it here. Nevertheless, we are satisfied such error was harmless under the test announced in Chapman v. California, 386 U.S. 18, 87 S.Ct. 824, 17 L.Ed.2d 705 (1967) and adopted by this court in Commonwealth v. Story, 476 Pa. 391, 383 A.2d 155 (1978)5 *317for all error, both that based on a violation of the constitution, and of law.6
Under the test adopted by this court in Commonwealth v. Story, supra, evidence improperly admitted can be treated as harmless on any one of three grounds, namely, that the evidence of guilt, without regard to the tainted evidence, is so overwhelming that conviction would have followed beyond a reasonable doubt without regard to it, that the tainted evidence was merely cumulative of other proper persuasive evidence on the issue for which it is offered, or that it was so slight or tangential in its effect that its influence on the jury can be determined to have been de minimis.
While the untainted evidence against this defendant is, indeed, overwhelming, we prefer not to rest our holding on that ground in the light of the Story analysis because the overwhelming evidence against defendant was contradicted by him, albeit somewhat incredibly, when he took the stand and denied commission of the criminal acts with which he was charged.7
Since the jury had properly before it strong and varied proper evidence of defendant’s use of deadly weapons to overcome his twelve year old victim’s resistance, it would be possible to analyze the effect of the gun in this case as merely cumulative. The victim testified he had threatened her with a gun and a knife, he admitted the knife was his *318and that he had seen the gun which was improperly admitted, although he denied it was his and that it was used to threaten the victim. The victim herself corroborated him in this last denial by testifying it was not the gun she had seen and she pointed out the differences between the two.8
However, on the facts of this case we prefer to ground our holding that the improper admission of the gun was harmless on the de minimis effect it must have had on the jury on all the charges except possession of an instrument of crime. In Commonwealth v. Settles, 442 Pa. 159, 275 A.2d 61 (1971) this court, in an opinion by the present Chief Justice, held the improper admission of a pocketknife in a case where the defendant admitted shooting the victim six times was harmless error. This case is cited as an example of de minimis error in Justice Robert’s opinion for the Court in Commonwealth v. Story, supra, 476 Pa. at 410 n. 18, 383 A.2d at 165 n. 18. In Commonwealth v. Fay, 463 Pa. 158, 344 A.2d 473 (1975), the admission of a gun seized in violation of a defendant’s constitutional rights was also held to be harmless error.
Here the gun was irrelevant on all the charges but possession. In fact the victim, the prosecution’s key witness, repudiated the gun offered at trial and described another gun as the one stuck in her side. Defense counsel utilized this testimony in his closing to attack the credibility of the victim’s testimony.
The improperly admitted gun was largely irrelevant, if not cumulative, entirely tangential to the issues on rape, statutory rape and corruption and apparently inconsistent with the other prosecution evidence. As such it was de minimis on those charges and we so hold.
Finally, appellant, citing Commonwealth v. Walker, 468 Pa. 323, 362 A.2d 227 (1976), alleges that because his *319convictions for rape and corruption of a minor arose from the same act of sexual intercourse, the imposition of separate though concurrent sentences for these distinct crimes constitutes error.9 Appellant thus argues that the sentences are duplicitous and, therefore, violative of the double jeopardy clause of the United States Constitution. This claim is without merit. Clearly, unlike the situation in Walker, the Commonwealth suffered two injuries from appellant’s single act in that appellant not only engaged in forcible intercourse with an individual who was not his spouse,10 but also corrupted the morals of a child under the age of eighteen.11 Nor does the imposition of separate sentences in this instance violate the “same offense” doctrine first articulated in Blockburger v. U. S., 284 U.S. 299, 304, 52 S.Ct. 180, 182, 76 L.Ed. 306 (1932). See Commonwealth v. Tarver, 493 Pa. 320, 426 A.2d 569 (1981).
Judgments of sentence affirmed.
*320O’BRIEN, C. J., filed a dissenting opinion. ROBERTS, J., filed a dissenting opinion in which FLAH-ERTY, J., joins.. Commonwealth v. Williams, 483 Pa. 293, 396 A.2d 1177, (1978) cert. denied, 446 U.S. 912, 100 S.Ct. 1843, 64 L.Ed.2d 266 (1980) which established a warrant requirement for arrests in the home in this Commonwealth was held to apply only to arrests occurring after November 18, 1978, when it was handed down. Commonwealth v. Miller, 490 Pa. 457, 417 A.2d 128 (1980), cert. denied, 449 U.S. 1113, 101 S.Ct. 924, 66 L.Ed.2d 842 (1981). Payton v. New York, 445 U.S. 573, 100 S.Ct. 1371, 63 L.Ed.2d 639 (1980) announced the same warrant requirement under federal law prospectively. Appellant was arrested May 29, 1976. At that time, the requirement for a warrant-less arrest in the home was a showing of probable cause. Although it is not necessary for us to address this issue, since appellant has argued only the absence of exigent circumstances under Williams, we note that the facts surrounding the arrest, set forth below in this opinion, clearly amount to probable cause. See Commonwealth v. Flewellen, 475 Pa. 442, 380 A.2d 1217 (1977) (upholding warrantless arrest against a contention that it was made without probable cause).
. The exigent circumstances set forth in Dorman v. United States, 435 F.2d 385 (D.C.Cir.1970) and adopted by this Court in Commonwealth v. Williams, 483 Pa. 293, 396 A.2d 1177 (1978), cert. denied, 446 U.S. 912, 100 S.Ct. 1843, 64 L.Ed.2d 266 (1980) which have been held to justify the failure to obtain an arrest warrant, are not entirely coextensive with those exigencies which justify noncompliance with the “knock and announce” rule.
. They also had a right to seize such evidence they had probable cause to believe would aid in conviction. Warden v. Hayden, 387 U.S. at 307, 87 S.Ct. at 1650. Since there was clear probable cause to believe the clothes, blankets, etc. would aid in conviction, this also disposes of defendant’s arguments on their admissability.
. Stasiak was interested in the clothing and bedding he had seen from the living room. Stasiak asked this be catalogued and bagged before seizure. Such precautions were proper. It was apparently while this was lawfully taking place that Strunk, lawfully present, saw the knife.
. To qualify as harmless the appellate court must be satisfied “beyond a reasonable doubt” that the error did not influence the jury in its finding of guilt. While the use of such a standard might seem more appropriate to apply to a fact finder’s judgment than that of an appellate court judging errors of law, we are, nevertheless, required to follow it, at least in applying the federal constitution Chapman v. California, supra.
. The application of a single test to both constitutional errors and those of law reduces the issues in this case since the admission of the gun is attacked as improper under both the Fourteenth Amendment to the Federal Constitution incorporating the Fourth Amendment prohibition of unreasonable searches and the general law relating to relevancy.
. In the light of our conclusion that the error here was de minimis, we need not decide whether overwhelming evidence can ever overcome an erroneous admission if defendant presents any evidence from which his lack of participation in the criminal conduct charged can be inferred. We have already held the overwhelming evidence test can be applied where the contradictory evidence goes only to mens rea. Commonwealth v. Thornton, 494 Pa. 260, 431 A.2d 248 (1981).
. The victim testified that the gun she was threatened with in the park was white and silver, while the gun introduced at trial was brown and grey.
. The Commonwealth contends that this issue has been waived, since appellant raised it for the first time in the Superior Court. However, because appellant challenges only the legality of the sentences imposed on these convictions, and not the validity of the convictions themselves, the claim has not been waived. Commonwealth v. Walker, supra.
. The crime of rape is defined, in relevant part, as follows:
A person commits a felony of the first degree when he engages in sexual intercourse with another person not his spouse:
(1) by forcible compulsion;
(2) by threat of forcible compulsion that would prevent resistance by a person of reasonable resolution;
18 Pa.C.S.A. § 3121.
. Corruption of minors was defined, in relevant part, as follows:
(a) Offense defined. — Whoever, being of the age of 18 years and upwards, by any act corrupts or tends to corrupt the morals of any child under the age of 18 years, or who aids, abets, entices or encourages any such child in the commission of any crime, or who knowingly assists or encourages such child in violating his or her parole or any order of court, is guilty of a misdemeanor of the first degree
18 Pa.C.S.A. § 3125 which was in effect at the time of defendant’s offense was repealed and reenacted as amended by the Act of July 1, 1978, P.L. 573, No. 104, eff. August 30, 1978, 18 Pa.C.S.A. § 6301.